AU2021200590A1

AU2021200590A1 - A method for the extraction of hydrocarbon

Info

Publication number: AU2021200590A1
Application number: AU2021200590A
Authority: AU
Inventors: Erik Christopher Dunlop; David Sword Warner
Original assignee: Deep Coal Tech Pty Ltd
Current assignee: Deep Coal Technologies Pty Ltd
Priority date: 2020-01-31
Filing date: 2021-01-29
Publication date: 2021-08-19
Also published as: US20230074077A1; WO2021151140A1

Abstract

This disclosure relates to a method to facilitate the extraction of hydrocarbon from a coal seam gas formation. In one aspect, the method comprises steps of: (a) drilling a wellbore using a drill string to access the coal seam gas formation; (b) perforating a section of the drill string; (c) abandoning a section of the drill string in the wellbore; (d) setting a temporary plug to isolate the abandoned section of the drill string and the wellbore; (e) installing a production tubing string above the temporary plug, so as to create a fluid passageway to facilitate the extraction of hydrocarbon; (f) displacing a fluid above the temporary plug, so as to create a high pressure differential; (g) removing the temporary plug, so as to create an initial stimulated reservoir volume to extract hydrocarbon from the coal seam gas formation and, resultantly, at least partially filling an annulus formed between the abandoned section of the drill string and the wellbore wall with coal fragments; and (h) continuing to extract hydrocarbon from the coal seam gas formation via the fluid passageway, at a high pressure drawdown, so as to create an expanding stimulated reservoir volume. 1/11 300 220 230 700 210 210 220 211 110 100 100' 221 223 222 224 120 225 226 Fig. 1

Description

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A METHOD FOR THE EXTRACTION OF HYDROCARBON PRIORITY DOCUMENTS

[0001] The present application claims priority from Australian Provisional Patent Application No. 2020900274 titled "A METHOD FOR EXTRACTION OF HYDROCARBON" and filed on 31 JANUARY 2020, the content of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates to a method to facilitate the extraction of hydrocarbon from a geological formation. In a particular form, the present disclosure relates to a method to facilitate the extraction of hydrocarbon from a coal seam gas formation.

BACKGROUND

[0003] Unconventional hydrocarbon reservoirs are commonly referred to as those that require greater than industry standard levels of technology or investment to commercialise. Declining production from conventional hydrocarbon reservoirs, coupled with increasing demand for energy worldwide, has led to a major shift towards the commercialisation and viability of unconventional hydrocarbon resources. This paradigm shift has been facilitated by a combination of higher prices and key technological breakthroughs over the past few decades.

[0004] When considering unconventional hydrocarbon reservoirs, unconventional gas and associated gas-condensate liquids are at the forefront relative to unconventional heavy oil (for example "tar sands"), due to the geographical abundance of gas, and the fact that its use as a fuel has less environmental impact than the combustion of heavy oil. Some of the common unconventional gas reservoir play types are low-permeability (tight) sandstones, thermogenic shale source rock formations, biogenic and thermogenic coal seams, and methane hydrate accumulations within shallow marine sediments. Some of the most challenging unconventional reservoirs from which to extract commercial hydrocarbon volumes are coal seam gas reservoirs, also referred to as coal seam gas formations.

[0005] Presently available drilling, wellbore completion and reservoir stimulation methodologies typically face four key challenges when it comes to extracting and commercialising hydrocarbon from coal seam gas formations, and in particular deep and ultra-deep coal seam gas formations (for which the pre-existing permeability available for gas extraction is typically very low). These challenges mainly apply in the context of the well-established practice, in most unconventional reservoirs, of drilling high-angle or horizontal wellbores. High-angle or horizontal wellbores are proven to be the best-practice approach for drilling and completing unconventional gas reservoirs, since they achieve the high surface area reservoir contact that is essential for the commercial extraction of hydrocarbon. The four main challenges associated with the extraction and commercialisation of hydrocarbon from coal seam gas formations and, in particular, deep and ultra-deep coal seam gas formations, are:

1. Maintaining a structurally stable wellbore that does not deform or collapse whilst drilling the coal seam gas formation. Deep and ultra-deep coal seam gas formations may be overpressured and highly stressed, so they become structurally unstable when exposed to presently utilised drilling methods and techniques; 2. Installation of a production/completion string after drilling the coal seam gas formation. Retrieving the drill string after drilling the coal seam gas formation generates a "swabbing" (hydraulic suction) effect that promotes structural instability of the wellbore, or a tendency for the wellbore to deform or collapse (leading to a stuck drill string). A deformed/collapsed wellbore containing coal fragments from the coal seam gas formation is likely to prevent, or at least inhibit, the production/completion string from being installed to the desired depth; 3. Effectively stimulating the coal seam gas formation to flow gas at a commercial rate of extraction. For deep and ultra-deep coal seam gas formations, a unique combination of "coal-like" geomechanical properties and "shale-like" reservoir rock properties may exist, which may cause the coal seam gas formation to exhibit characteristics that relate to both coal reservoirs and shale reservoirs, which reduces the effectiveness of presently available reservoir stimulation techniques; and 4. Difficulty, for the deeper coal seam gas formations, in achieving a similar technical and/or operational outcome when compared to shale reservoirs, at a lower cost.

[0006] In addition to the above challenges, the most significant obstacle inhibiting the commercialisation of coal seam gas formations and, in particular, deep and ultra-deep coal seam gas formations, is the high initial reservoir confining stress and the extreme sensitivity of the very limited coal fabric apertures to increasing effective stress during extraction of the contained hydrocarbon.

[0007] Based on the challenges associated with the extraction of hydrocarbon from specific types of coal seam gas formations, and the commercialisation thereof, there is a need to provide a new, fit for-purpose method for the extraction of hydrocarbon from such coal seam gas formations that may ideally utilise presently available conventional (standard) oilfield drilling and coiled tubing equipment, whilst being simple, low-cost and repeatable.

[0008] It is against this background, and the problems and difficulties associated therewith, that the present invention has been developed.

[0009] Certain objects and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying drawings, wherein by way of illustration and example, an embodiment of the present invention is disclosed herein.

[0010] Throughout this description of the present invention, the term "abandon", "abandoned", "abandoning", or "abandonment" when referring to a drill string, a lower portion of a drill string, a drilling assembly, a drill bit system, a mud motor, or a drill bit, refers to the act or operation of disconnecting any one of the above from being in connection to a drilling rig, a coiled tubing unit, or any other equipment at the surface that may be used to rotate, run, retrieve or manipulate any one of the drill string, the lower portion of the drill string, the drilling assembly, the drill bit system, the mud motor, or the drill bit. It will also be appreciated, by those skilled in the art, that any one of the terms "abandon", "abandoned", "abandoning", or "abandonment" may refer to one or more of oilfield equipment positioned at a depth below the surface that is currently not connected to any one of the drilling rig, or any other equipment at surface that may be used to rotate, run, retrieve, or manipulate any one or more of the oilfield equipment.

[0011] Throughout this description of the present invention, the term "perforation" may refer to any one or more equivalent term(s), which may include, but not be restricted to terms such as "hole", "aperture", "port", "opening", "slot", or any other term that may describe a fluid pathway that may be created in a drill string, a casing string, or any other form of production conduit, that facilitates the extraction of hydrocarbon from a geological formation.

SUMMARY

[0012] Embodiments of the present disclosure relate to a method to facilitate the extraction of hydrocarbon from a coal seam gas formation, the method comprising the steps of: (a) drilling a wellbore using a drill string, to a depth to access at least a portion of the coal seam gas formation; (b) perforating a section of the drill string; (c) severing and abandoning a section of the drill string in the wellbore, comprising the perforated section; (d) setting a temporary plug to isolate the severed and abandoned section of the drill string; (e) installing a production tubing string above the temporary plug and the severed and abandoned section of the drill string, so as to create a fluid passageway to facilitate the extraction of hydrocarbon from the coal seam gas formation; (f) displacing a fluid above the temporary plug, so as to create a high pressure differential; (g) removing the temporary plug, thereby creating an initial stimulated reservoir volume to extract hydrocarbon from the coal seam gas formation and, resultantly, at least partially filling an annulus, formed between the severed and abandoned section of the drill string and the wellbore wall, with coal fragments of the coal seam gas formation; and (h) continuing to extract hydrocarbon from the coal seam gas formation, via the fluid passageway, at a high pressure drawdown, so as to create an expanding stimulated reservoir volume.

[0013] According to a first aspect, there is provided a method to facilitate the extraction of hydrocarbon from a geological formation, the method comprising the steps of: (a) drilling a wellbore below one or more casing string(s) using a drill string, to a depth to access at least a portion of the geological formation, wherein a lower portion of the drill string comprises a drilling assembly and one or more stabilising means; (b) perforating a section of the drill string, with access to the portion of the geological formation using coiled tubing, wherein the perforated section of the drill string comprises the lower portion of the drill string; (c) severing and abandoning a section of the drill string in the wellbore, with access to the geological formation, so as to create an abandoned section of the drill string having an open-ended stub disposed within the one or more casing string(s); (d) setting a temporary plug above the open-ended stub, so as to isolate the abandoned section of the drill string, the wellbore, and the geological formation below the temporary plug; (e) installing a production tubing string within the one or more casing string(s), to a depth above the temporary plug and the open-ended stub, thereby creating a fluid passageway to facilitate the extraction of hydrocarbon from the geological formation through the abandoned section of the drill string; (f) displacing a fluid above the temporary plug, so as to create a high pressure differential between the one or more casing string(s) above the temporary plug and the wellbore and the geological formation below the temporary plug; (g) removing the temporary plug, so as to allow the wellbore to produce through the fluid passageway, thereby creating an initial stimulated reservoir volume to facilitate the extraction of hydrocarbon from the geological formation and, resultantly, at least partially filling an annulus, formed between the abandoned section of the drill string and the wellbore wall, with fragments of the geological formation; and (h) continuing to extract hydrocarbon from the geological formation, at a high pressure drawdown, so as to create an expanding stimulated reservoir volume.

[0014] In one form, the geological formation from which hydrocarbon is extracted is a coal seam gas formation.

[0015] In one form, the wellbore produces through the fluid passageway, in response to a pressure differential between a flowline at a surface location of a drilling rig and the geological formation.

[0016] In one form, the expanding stimulated reservoir volume grows larger and more permeable over production time.

[0017] In one form, the abandoned section of the drill string functions as a production conduit to facilitate the extraction of hydrocarbon from the geological formation.

[0018] In one form, the production conduit provided by the abandoned section of the drill string is accessible from the fluid passageway created by the production tubing string, thereby facilitating the extraction of hydrocarbon from the geological formation.

[0019] In one form, the fragments of the geological formation are coal fragments that bulk the annulus formed between the abandoned section of the drill string and the wellbore wall.

[0020] In one form, the drilling assembly of the drill string comprises a drill bit system, comprising a mud motor and a drill bit.

[0021] In one form, a diameter of the drill bit is selected to maximise the size of the annulus formed between the abandoned section of the drill string and the wellbore wall in the geological formation.

[0022] In one form, the one or more stabilising means comprises one or more reamer(s), and/or stabiliser(s), and/or centraliser(s), so as to support the lower portion of the drill string in the wellbore.

[0023] In one form, the one or more stabilising means facilitate(s) the removal of any large fragments of the geological formation that may be present in the annulus formed between the abandoned section of the drill string and the wellbore wall, thereby minimising the risk of the drill string becoming stuck during the construction of the wellbore.

[0024] In one form, a drill pipe severing tool may be used to abandon a section of the drill string in the wellbore, with access to the geological formation.

[0025] In one form, one or more types of coiled tubing conveyed drill pipe severing tool(s) may be used to abandon a section of the drill string in the wellbore, with access to the geological formation.

[0026] In one form, the abandoned section of the drill string is left in situ with respect to the geological formation.

[0027] In one form, the abandoned section of the drill string is left unsecured with respect to the geological formation.

[0028] According to a second aspect, there is provided a method to facilitate the extraction of hydrocarbon from a geological formation, the method comprising the steps of: (a) drilling a wellbore below one or more casing string(s) using a drill string, to a depth to access at least a portion of the geological formation, wherein a lower portion of the drill string comprises a drilling assembly, one or more stabilising means, and one or more temporarily sealed, pre-perforated drill pipe segment(s); (b) removing, opening or breaking one or more temporary seal(s) of the one or more temporarily sealed, pre-perforated drill pipe segment(s), thereby creating one or more open perforation(s) along the lower portion of the drill string, so as to facilitate the extraction of hydrocarbon from the geological formation; (c) parting and abandoning a section of the drill string in the wellbore, with access to the geological formation, by "backing off' (unscrewing) a joint between adjoining drill pipe segments, at a "tensile free point" (or "free point") in the drill string, so as to create an abandoned section of the drill string having an open-ended stub disposed within the one or more casing string(s); (d) setting a temporary plug above the open-ended stub, so as to isolate the abandoned section of the drill string, the wellbore, and the geological formation below the temporary plug; (e) installing a production tubing string within the one or more casing string(s), to a depth above the temporary plug and the open-ended stub, thereby creating a fluid passageway to facilitate the extraction of hydrocarbon from the geological formation through the abandoned section of the drill string; (f) displacing a fluid above the temporary plug, so as to create a high pressure differential between the one or more casing string(s) above the temporary plug and the wellbore and the geological formation below the temporary plug; (g) removing the temporary plug, allowing the wellbore to produce through the fluid passageway, thereby creating an initial stimulated reservoir volume to facilitate the extraction of hydrocarbon from the geological formation and, resultantly, at least partially filling an annulus formed between the abandoned section of the drill string and the wellbore wall with fragments of the geological formation; and (h) continuing to extract hydrocarbon from the geological formation, at a high pressure drawdown, so as to create an expanding stimulated reservoir volume.

[0029] In one form, the geological formation from which hydrocarbon is extracted is a coal seam gas formation.

[0030] In one form, the wellbore produces through the fluid passageway, in response to a pressure differential between a flowline at a surface location of a drilling rig and the geological formation.

[0031] In one form, the expanding stimulated reservoir volume grows larger and more permeable over production time.

[0032] In one form, the abandoned section of the drill string advantageously functions as a production conduit to facilitate the extraction of hydrocarbon from the geological formation.

[0033] In one form, the production conduit provided by the abandoned section of the drill string is accessible from the fluid passageway created by the production tubing string, thereby facilitating the extraction of hydrocarbon from the geological formation.

[0034] In one form, the fragments of the geological formation are coal fragments that bulk the annulus formed between the abandoned section of the drill string and the wellbore wall.

[0035] In one form, the one or more temporarily sealed, pre-perforated drill pipe segment(s) is/are temporarily sealed using one or more blanked-off shear-pin stub(s), one-way valve(s), one-way ball seal(s), or differential pressure-activated burst disk(s).

[0036] In one form, activation of one or more of the blanked-off shear-pin stub(s), the one-way valve(s), the one-way ball seal(s), or the differential pressure-activated burst disk(s) removes, opens or breaks one or more temporary seal(s) of the one or more temporarily sealed, pre-perforated drill pipe segment(s), thereby creating one or more open perforation(s) along the lower portion of the drill string, so as to facilitate the extraction of hydrocarbon from the geological formation.

[0037] In one form, the drilling assembly of the drill string comprises a drill bit system, comprising a mud motor and a drill bit.

[0038] In one form, a diameter of the drill bit is selected to maximise the size of the annulus formed between the abandoned section of the drill string and the wellbore wall in the geological formation.

[0039] In one form, the one or more stabilising means comprises one or more reamer(s), and/or stabiliser(s), and/or centraliser(s), so as to support the lower portion of the drill string in the wellbore.

[0040] In one form, the one or more stabilising means facilitate(s) the removal of any large fragments of the geological formation that may be present in the annulus formed between the abandoned section of the drill string and the wellbore wall, thereby minimising the risk of the drill string becoming stuck during the construction of the wellbore.

[0041] In one form, the abandoned section of the drill string is left in situ with respect to the geological formation.

[0042] In one form, the abandoned section of the drill string is left unsecured with respect to the geological formation.

BRIEF DESCRIPTION OF DRAWINGS

[0043] Embodiments of the present disclosure will be discussed with reference to the accompanying drawings, wherein:

[0044] Figure 1 is a schematic plan diagram illustrating a first embodiment of a method to facilitate the extraction of hydrocarbon from a geological formation, wherein a wellbore is drilled below one or more casing string(s) using a drill string;

[0045] Figure 2 is a schematic plan diagram illustrating the method of Figure 1, wherein a lower portion of the drill string is perforated;

[0046] Figure 3 is a schematic plan diagram illustrating the method of Figure 2, wherein a lower portion of the drill string is severed and abandoned in the wellbore;

[0047] Figure 4 is a schematic plan diagram illustrating the method of Figure 3, wherein a temporary plug is set, a production tubing string is installed, and a fluid above the temporary plug is displaced;

[0048] Figure 5 is a schematic plan diagram illustrating the method of Figure 4, wherein the temporary plug is removed and an annulus is filled with fragments of the geological formation;

[0049] Figure 6 is a schematic plan diagram illustrating a second embodiment of a method to facilitate the extraction of hydrocarbon from a geological formation, wherein a wellbore is drilled below one or more casing string(s) using a drill string comprising one or more temporarily sealed, pre perforated drill pipe segment(s);

[0050] Figure 7 is a schematic plan diagram illustrating the method of Figure 6, wherein one or more temporary seal(s) of the one or more temporarily sealed, pre-perforated drill pipe segment(s) is/are removed, opened or broken;

[0051] Figure 8 is a schematic plan diagram illustrating the method of Figure 7, wherein a lower portion of the drill string is parted and abandoned in the wellbore by "backing off' (unscrewing) at a "tensile free point" (or "free point") in the drill string;

[0052] Figure 9 is a schematic plan diagram illustrating the method of Figure 8, wherein a temporary plug is set, a production tubing string is installed, and a fluid above the temporary plug is displaced;

[0053] Figure 10 is a schematic plan diagram illustrating the method of Figure 9, wherein the temporary plug is removed and an annulus is filled with fragments of the geological formation; and

[0054] Figure 11 is a schematic cross sectional diagram illustrating a number of reservoir stimulation effects around the lower, perforated portion of the drill string, at a depth to access at least a portion of the geological formation.

[0055] In the following description, like reference characters designate like or corresponding parts throughout the Figures.

DESCRIPTION OF EMBODIMENTS

[0056] In the embodiments of the present disclosure that follow, although the described method may particularly be applicable to coal seam gas formations, as well as other formation types, the present disclosure is specifically designed to be a fit-for-purpose solution to the severe, but not insurmountable, technical and commercial challenges imposed by coal seam gas formations that are deeper than the well-established "permeability depth limit" of conventional shallow coal seam gas (CSG) formations (approximately 6,000 feet / 1,830 metres). Such "deep" and "ultra-deep" coal seam gas formations represent a very different play type, having reservoir characteristics that more closely resemble those of a shale gas play. A new set of geological and environmental properties exists, for which a very different drilling, wellbore completion and reservoir stimulation approach is required. The current technical and commercial practices applied to shallow coal seam gas reservoirs are not applicable in "deep" and "ultra-deep" geological settings. As an example, "deep" and "ultra-deep" coal seam gas formations typically have no commercially significant pre-existing coal fabric (i.e. cleat) permeability, and contain an insignificant amount of mobile formation water. For this reason, the present invention disclosed herein deliberately excludes the use of any form of conventional (standard) artificial lift system, such as a downhole "electrical submersible pump" (ESP), for de watering the coal seam gas formation, so as to reduce pressure within the coal seam gas formation and initiate desorptive gas production. In essence, the present invention disclosed herein describes a contrarian (or "disruptive") holistic drilling, wellbore completion and reservoir stimulation process, which may be achieved by only a single, "one-way trip" of the drill string into the coal seam gas formation, whilst maintaining continuous contact of the drill bit with the cutting face. Current gas extraction techniques applied to shallow coal seam gas formations require multiple trips into the coal seam gas formation, which typically include; 1) drilling of the initial pilot hole, which invariably requires the drill bit to be removed from the cutting face on multiple occasions during "drill pipe connections", "wiper trips", and finally when the drill string is retrieved to surface, 2) installation of a production casing string and/or some form of wellbore completion string, and 3) installation of an artificial lift system, so as to de-water the shallow coal seam gas formation, which typically involves the use of a downhole electrical submersible pump.

[0057] Referring to any one of the Figures, there is illustrated a method to facilitate the extraction of hydrocarbon from a geological formation (100). The method generally relates to the extraction of hydrocarbon, such as gas from coal seams, and in particular "deep" and "ultra-deep", very low permeability coal seams, that do not respond to presently available hydrocarbon extraction techniques (such as, for example, hydraulic fracture stimulation, hydraulic jetting, open-hole cavitation, or under reaming techniques).

[0058] Particularly, the present invention relates to a method that may entirely utilise, but not necessarily be restricted to, presently available conventional (standard) oilfield drilling and coiled tubing equipment, methods, and techniques to: a) enable the consistent, non-problematic drilling of long-reach high-angle wellbores and long-reach horizontal wellbores within inherently unstable geological formations (100); b) effectively stimulate those geological formations (100) to flow hydrocarbon to the surface; and c) advantageously optimise the drilling, wellbore completion, and reservoir stimulation phases by reducing the number of steps required in comparison to presently available hydrocarbon extraction techniques.

[0059] More particularly, and referred to herein, the present invention relates to a method to facilitate the extraction of gas from a coal seam gas formation (100'). Thus, the reference to "gas" may be interchangeably used with "hydrocarbon", the reference to "geological formation" may be interchangeably used with "coal seam gas formation", or "coal seam formation", or "coal seam reservoir", and the terms "extract/extraction" may be interchangeably used with "produce/production". However, it will be appreciated, while the following method relates primarily to gas and coal seam formations, the method may also be applicable to the extraction of other hydrocarbon types (such as oil and gas-condensate liquids) from other geological formation types.

[0060] With reference to coal seam gas formations (100') in this description of the present invention, and the extraction of gas therefrom, the discussion below relates to background and theory with respect to i) the impact of stress on the permeability of coal seam gas formations and how this may be modulated by the prevailing geomechanical reservoir boundary condition, and ii) Pressure Arch Theory.

i) Impact of Stress on Coal Seam Permeability

[0061] Stress is generally accepted to be a dominant variable that ultimately controls the permeability and hence flow rate capacity of all coal seam formations. For shallow coal seam gas (CSG) reservoirs (also commonly referred to as coalbed methane (CBM) reservoirs), gas extraction efficiency is primarily controlled by the initial stress state of the reservoir. Reservoir confining stress is generally relatively low, so shallow coal seams are typically permeable and flow freely. Thus, in many shallow coal seam gas formations, reservoir stimulation treatments may not be required. Over the life of a wellbore, production performance is typically modulated by the stress path, as desorption induced coal matrix shrinkage interacts with the prevailing geomechanical reservoir boundary condition during the depletion/extraction of gas from the coal seam gas reservoir. That is to say, the permeability of coal seam gas reservoirs becomes dynamic.

[0062] The lower the native coal seam gas reservoir permeability, the more critical is the stress path to maintaining gas extraction. For the specific type of coal seam gas reservoir being targeted by the present invention, the native stress state is too high for pre-existing commercial coal fabric permeability to exist. The stress path itself, when combined with a compatible up-front reservoir stimulation strategy, becomes the primary control on gas extraction efficiency from the coal seam gas reservoir. In this way, in order to achieve a commercial gas flow rate and ultimate gas recovery (extraction), a large, complex domain of enhanced permeability must be artificially created by engineering a stress path that leads to stress-reduction.

[0063] The fabric permeability of all coal seams during gas extraction is dynamic and very sensitive to reservoir confining stress. It is strongly controlled by the large-scale physical response of the coal seam and the surrounding host rock framework to the increasing effective stress generated by production pressure drawdown. Whether permeability increases, decreases, or does both over time, as gas is extracted (produced) from the coal seam gas reservoir, depends upon the complex, competitive interaction that occurs between ongoing desorption-induced coal matrix shrinkage and the prevailing geomechanical reservoir boundary condition. Hence, to understand the reservoir stimulation requirements of low-permeability coal seam gas formations, it is essential that the correct geomechanical reservoir boundary condition be identified. Owing to the generally extreme depths of the coal seam gas formations (100') specifically being targeted by the present invention, typical geomechanical reservoir boundary conditions applicable to shallow coal seam gas reservoirs cannot be assumed by default. The method of the present invention harnesses and optimises the phenomenon of desorption-induced coal matrix shrinkage, which counteracts the tendency for reservoir compaction by promoting the dilation of coal fabric apertures, to thereby increase permeability of the coal seam gas formation.

ii) Pressure Arch Theory

[0064] One of the most significant obstacles inhibiting the commercialisation of coal seams below the "permeability depth limit" of conventional shallow coal seam gas (CSG) formations is the high initial reservoir confining stress and the extreme sensitivity of the very limited coal fabric apertures to increasing effective stress during gas extraction. One solution to this obstacle may be the application of Pressure Arch Theory (or simply "pressure arching"), which is a well-established, widely recognised, macro-scale geomechanical concept, originally conceived in the context of the underground coal mining industry.

[0065] Pressure Arch Theory demonstrates that a proven mechanism may exist for dynamically enhancing the permeability of coal seam gas formations at extreme depth and stress. In principle, once desorptive gas production has been initiated around a wellbore by some form of up-front reservoir stimulation treatment, and the isolated fracture network stimulated reservoir volume (SRV) domain is exposed to continuous, high production pressure drawdown, pressure arching causes an "expanding reservoir boundary and decreasing confining stress" condition to be generated that locally neutralises the pre-existing reservoir confining stress and shields the production pressure transient region from the compaction effect caused by increasing production pressure drawdown-induced effective stress.

[0066] Pressure Arch Theory may not yet have been evaluated, modelled, and/or field-tested by the oil and gas industry as a potential tool for assisting in the commercialisation of coal seam gas reservoirs. As such, an opportunity exists for Pressure Arch Theory to be harnessed by the method of this present invention, so as to neutralise the detrimental effects of high initial reservoir confining stress and increasing effective stress during production. If these effects can be significantly reduced within the time frame of gas extraction, ongoing desorption-induced coal matrix shrinkage may generate an isolated "self-fracturing reservoir" domain, within which the coal fabric planes of weakness open, dilate and extend outwards, away from the wellbore, as gas is produced from the coal seam gas reservoir.

[0067] In essence, pressure arching with respect to geological formations is caused by a non uniform areal distribution of reduced pore pressure and/or geomechanical competence. This, in turn, leads to a non-uniform areal distribution of increased effective stress and reduced reservoir confining stress. In this way, the amount of reservoir confining stress reduction is a function of pore pressure reduction and the effectiveness of pressure arch formation. Pressure arching is most effective when the region of reduced pore pressure and/or geomechanical competence is small and geomechanically compliant compared to the scale and rigidity of the surrounding host rock framework of the geological formation.

[0068] There are four main factors controlling the generation of pressure arch effects and their effectiveness as stress shields for underlying isolated voids, and they are (assuming, for simplicity and conceptual understanding, that the maximum stress direction is vertical):

1. Void width - which controls pressure arch dimensions;

2. Void depth - which controls the maximum size to which a pressure arch may grow, before it reaches the surface and becomes breached, which then allows compaction, down warping and surface subsidence to occur;

3. Geomechanical competence of the host rock (geological formation) framework - which controls pressure arch structural stability; and

4. Geomechanical contrast with the host rock (geological formation) framework - an isolated, laterally discontinuous domain of easily compactible rock mass of low structural integrity, and low compressive strength, such as a chalk formation, which does not transmit stress well, also exhibits the stress deflecting behaviour of a macro-scale void sensu stricto, and is therefore capable of generating a pressure arch in response to a localised increase in production pressure drawdown-induced effective stress.

[0069] Based on the discussion above regarding Pressure Arch Theory, the novel reservoir stimulation technique disclosed herein employs the creation of an initial, up-front subsurface excavation, or cavity, as this represents the most "incompetent" member of the geomechanical spectrum, and has maximum stress transmission contrast with respect to the host rock (geological formation) framework. In this way, the method disclosed herein may achieve optimal pressure arch development, and invoke the resultant de-stressing effect as a mechanism for allowing desorption induced coal matrix shrinkage to increase the aperture width (i.e. permeability) of coal seam fabric planes of weakness, in defiance of the rapidly increasing effective stress during the extraction of hydrocarbon from the geological formation (100).

[0070] The method disclosed herein requires two presently available, conventional (standard) oilfield drilling and wellbore completion systems, and/or equipment, consisting of; a standard drilling rig (200), and a standard coiled tubing unit (300). In this way, it will be appreciated, by those skilled in the art, that the method advantageously may not require any specialised drilling, wellbore completion or reservoir stimulation equipment. Thus, standard, generic oilfield equipment may be used, or re purposed, so as to achieve any one of the embodiments of the method disclosed below.

[0071] Referring now to any one of Figures 1 to 5, there is illustrated an embodiment of the method to facilitate the extraction of hydrocarbon from the geological formation (100). The method may comprise the following Steps (a) to (h):

Step (a)

[0072] The drilling rig (200), illustrated by any one of Figures 1 to 10, may be used to drill a wellbore (110) below one or more casing string(s) (210) using a drill string (220), to a depth (120) to access at least a portion of the geological formation (100). In this way, the drilling rig (200) may be used to construct a wellbore. It will be appreciated, by those skilled in the art, that the one or more casing string(s) (210) may typically comprise at least one of a conductor casing, and a surface casing. In this way, as is well known to those skilled in the art, the one or more casing string(s) (210), typically comprising at least one of a conductor casing and a surface casing, will generally be concentric casing strings (210). The drilling rig (200) may comprise entirely conventional (standard) oilfield equipment, such as a drilling fluid circulation system (not shown), whereby a suitable drilling fluid (commonly referred to as "drilling mud") is circulated under high pressure through the drill string (220), to a point at or near the advancing face (120) of the wellbore (110)being constructed, and then back to the drilling fluid circulation system at the drilling rig (200) via an annulus (211) formed between the drill string (220) and the wellbore wall (110). During the construction of the wellbore

(110) by the drilling rig (200), the drilling fluid passes through a large valve that is capable of sealing and isolating the wellbore contents when hazardous situations arise that may potentially result in the uncontrolled flow of hydrocarbon and/or other fluids (such as formation water) from the geological formation (100). The large valve may be (and generally is) a blowout preventer (230), commonly referred to as a "BOP". Functions of the blowout preventer (230) are well known, to those skilled in the art, and may be utilised at this Step (a) to ensure the wellbore (110) being constructed is maintained under control, and safely drilled by the drilling rig (200), by various methods and techniques that are well known to those skilled in the art.

[0073] The wellbore (110) may be drilled by the drilling rig (200) under pressure-overbalanced conditions, whereby the drilling fluid utilised may be selected to have an adequate mudweight, such that the resultant combination of drilling fluid hydrostatic pressure and drilling fluid circulation pressure is significantly greater than the pore pressure within the geological formation (100) being drilled. Pressure-overbalanced drilling conditions may advantageously serve to provide structural integrity to the wellbore (110), particularly in the coal seam gas formation (100'). This structural integrity may be achieved by counteracting the lithostatic and tectonic stresses that would otherwise promote the deformation or eventual collapse of the wellbore (110) in the coal seam gas formation (100'). In this way, maintaining high drilling fluid hydrostatic pressure, combined with high drilling fluid circulation pressure within the wellbore (110), may inhibit the release of large coal fragments (140) of the coal seam gas formation (100') into the annulus (211) where, if not efficiently expelled from the wellbore (110) to surface, the large coal fragments (140) may impede drilling of the wellbore (110), and may potentially cause the drill string (220) to become stuck in the wellbore (110), thereby potentially preventing its ability to drill further into the coal seam gas formation (100'). Pressure overbalanced drilling may be further advantageous in high-angle or horizontal wellbores (110) (as illustrated in any one of Figures 1 to 10), where drilling in such orientations may inherently exacerbate structural instability of the wellbore (110).

[0074] The drill string (220) may comprise a lower portion (221). The lower portion (221) of the drill string (220) may comprise a drilling assembly (222), which may often be referred to as a bottomhole assembly (BHA), and one or more stabilising means (223).

[0075] The drilling assembly (222) of the drill string (220) may comprise a drill bit system (224) at the point at or near the advancing face (120) of the wellbore (110)being constructed. The drill bit system (224) may optionally comprise a motor (225), and a drill bit (226).

[0076] In one embodiment, the optional motor (225) may be a mud motor, comprising a rotor and a stator (both not shown), whereby the drill bit (226) may be driven by the circulation pressure of the drilling fluid. The use of the mud motor (225) in the drill bit system (224) and thus the drill string (220) to drill the wellbore (110), advantageously does not require the drill string (220) to be rotated in order for the drill bit (226) to drill the wellbore (110), and thereby the coal seam gas formation (100'). Hence, the use of the mud motor (225) may reduce or eliminate possible eccentric rotation (otherwise referred to as "whipping") of the drill string (220) that may cause it to collide against and destabilise the wellbore wall (110), thereby potentially releasing large coal fragments (140) of the coal seam gas formation (100') into the annulus (211).

[0077] In an alternative embodiment, the drill bit system (224) may comprise a rotary drilling system (not shown) and the drill bit (226). In this embodiment, the drill bit (226), the rotary drilling system (not shown), and thus the drill bit system (224), may be driven by rotational torque transmitted from the drilling rig (200) via the drill string (220) to thereby drill the wellbore (110).

[0078] In one embodiment, a diameter of the drill bit (226) may be selected, so as to maximise the size of the annulus (211) formed between the drill string (220) and the wellbore wall (110) in the geological formation (100). Maximising the size of the annulus (211) via the larger diameter drill bit (226) advantageously contributes to the overall success of the method, by correspondingly maximising the size of the initial stimulated reservoir volume (i-SRV) (800 in Figures 5, 10 and 11). Additionally, smaller diameter drill pipe (not shown) may be selected for use in the drill string (220) to also aid in maximising the size of the annulus (211). It will be appreciated that further methods of optimising components of the drill string (220) are also envisaged, so as to maximise the size of the annulus (211). The method disclosed herein, as it will become apparent to those skilled in the art, with reference to detail below, deliberately aims to maximise the size of the annulus (211) between the drill string (220) and the wellbore wall (110), so as to provide the largest possible void space volume for the surrounding one or more coal seam(s) of the coal seam gas formation (100') to subsequently "bulk into", during the reservoir stimulation phase of the method herein, and thereby de-stress. In this way, as will become apparent, the method provides advantages, which are disclosed herein.

[0079] In one embodiment, the one or more stabilising means (223) may comprise one or more reamer(s), and/or stabiliser(s), and/or centraliser(s) to support the lower portion (221) of the drill string (220) in the wellbore (110), as well as facilitating the removal (via a grinding action) of any large fragments (140) of the geological formation that may be present in the annulus, and which may obstruct the drilling process, thereby minimising the risk of the drill string becoming stuck during the construction of the wellbore. In an alternative embodiment, it will be appreciated that the one or more stabilising means (223) may also be positioned anywhere along the drill string (220), so as to support the drill string (220) in the wellbore (110), as well as providing the aforementioned wellbore cleaning function. In either of these embodiments, the one or more stabilising means (223) may be positioned in the drill string (220), and the lower portion (221) of the drill string (220), so as to be located in the high-angle and horizontal sections of the wellbore (110). In this way, the one or more stabilising means (223) may advantageously function to centralise the drill string (220), and the lower portion (221) of the drill string (220), thereby reducing drag whilst drilling the wellbore (110), and assist in the drilling of the coal seam gas formation (100') by grinding through any large coal fragments (140) that may have collapsed from the wellbore wall (110) and may now reside in the annulus (211) along the high-angle and horizontal sections of the wellbore (110), thereby reducing the risk of the drill string (220) becoming stuck prematurely during drilling.

[0080] In one embodiment, referring now to any one of Figures 6 to 10, the lower portion (221) of the drill string (220) may further comprise one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227). The one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227) may advantageously provide the lower portion (221) of the drill string (220) with access to the geological formation (100), in order to facilitate the extraction of hydrocarbon from the geological formation (100), without the need for direct perforation of the drill string (220), using coiled tubing (310) conveyed drill pipe perforating devices, which may include abrasive hydraulic jets, plasma jets, or explosive charges. It will be appreciated, by those skilled in the art, that the one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227) may be positioned in the lower portion (221) of the drill string (220), so as to access the geological formation (100).

[0081] In the above embodiment, referring particularly to Figure 6, the one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227) may be temporarily sealed (228a) using one or more blanked-off shear-pin stub(s), one-way valve(s), one-way ball seal(s), or differential pressure activated burst disk(s) (all not shown). It will be appreciated that activation of one or more of the blanked-off shear-pin stub(s), the one-way valve(s), the one-way ball seal(s), or the differential pressure-activated burst disk(s) may remove, open or break (228b) one or more temporary seal(s) (228a) of the one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227), which thereby provides the lower portion (221) of the drill string (220) with access to the geological formation (100), thereby facilitating the extraction of hydrocarbon from the geological formation (100), without the need for direct perforation of the drill string (220), using coiled tubing (310) conveyed drill pipe perforating devices, which may include abrasive hydraulic jets, plasma jets, or explosive charges. One or more temporary seal(s) (228a) of the one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227) may be removed, opened or broken (228b), so as to allow hydrocarbon, or any other fluid, to flow into the drill string (220), by any one or more of a coiled tubing (310) conveyed device (not shown) (such as a blanked-off shear-pin stub-breaking device), the reversal of differential pressure to open a one-way valve, the reversal of differential pressure to unseat or seat a ball, or the breaking of a differential pressure-activated burst disk (all not shown). It will be appreciated that further ways are envisaged by which to temporarily seal (228a) and then subsequently open (228b) the pre-perforated drill pipe segment(s) (227), and these may be apparent to those skilled in the art. The one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227) are temporarily sealed (228a) (prior to removing, opening or breaking the seal (228b)) to advantageously prevent the loss of drilling fluid circulation during the drilling of the wellbore (110).

[0082] In any one of the above embodiments, referring to any one of Figures 1 to 10, whilst drilling the wellbore (110), the drill string (220) and hence the drill bit (226) may be maintained at the point at or near the advancing face (120) of the wellbore (110), particularly while drilling the coal seam gas formation (100'). The maintenance of the drill bit (226) at the point at or near the advancing face (120) of the wellbore (110) may be achieved by ensuring the drill bit (226) is not pulled off bottom (that is, away from the point at or near the advancing face (120) of the wellbore (110)) when making a connection in the drill string (220). As the wellbore (110) may potentially reach a vertical depth of 10,000 feet or more, and be at a high-angle or horizontal orientation within the coal seam gas formation (100'), a significant amount of drill string (220) stretch (typically 1 foot of stretch per 1,000 feet of vertical depth below the drilling rig (200)) may need to be overcome before there is sufficient tension in the drill string (220) to pull the drill bit (226) away from the advancing face (120) of the wellbore (110). Thus, if a new drill pipe segment (not shown) is connected that is of a length less than or equal to the amount of stretch in the drill string (220), the drill bit (226) may advantageously remain in contact with the advancing face (120) of the wellbore (110). Advantageously, a conventional (standard) "pup joint" (not shown), having a length (for example 10 feet) that is approximately equal to the drill string (220) stretch at the depth of the coal seam gas formation (100') (for example 10,000 feet), with connections matching that of the drill string (220), may be incorporated into the drill string (220), so as to compensate for the stretch, and thereby maintain the drill bit (226) at the point at or near the advancing face (120) of the wellbore (110) (i.e. the drill bit (226) is not pulled off bottom). Similarly, one or more conventional (standard) "slip joint(s)" (not shown), well known to those skilled in the art, may also be incorporated into the drill string (220), so as to achieve the same objective of maintaining the drill bit (226) at a point at or near the advancing face (120) of the wellbore (110). As will be appreciated, by those skilled in the art, the purpose of maintaining the drill bit (226) at the point at or near the advancing face (120) of the wellbore (110), is to optimise the structural stability of the wellbore (110) in the coal seam gas formation (100') during the drilling phase (but not the reservoir stimulation phase) by eliminating the "swabbing effect", and this maximises the length of the wellbore (110) that may be drilled through the inherently unstable coal seam gas formation (100').

[0083] Still referring to any one of the above embodiments, and Figures 1 to 10, whilst drilling the coal seam gas formation (100'), the likelihood of the drill string (220) becoming stuck in the coal seam gas formation (100') progressively increases and, potentially, retrieval of the drill string (220) and its components may not be possible. It will be appreciated, by following the subsequent steps of the disclosed method, that this is not an undesirable situation according to the method, as the drill string (220), the lower portion (221) of the drill string (220), and the drilling assembly (222) may advantageously function as a production conduit to facilitate the extraction of hydrocarbon from the geological formation (100). At the target depth (120) of the wellbore (110), or when the drill string (220) becomes prematurely stuck in the coal seam gas formation (100'), it may be considered that the drilling of the wellbore (110) is complete, or at least sufficient in length for the commercial extraction of hydrocarbon. Consequently, hydraulically activated pipe rams (not shown) of the blowout preventer (230) may then be closed, so as to isolate the annulus (211) formed between the drill string (220), the wellbore wall (110), and the innermost casing string (210). In this way, the closed hydraulically activated pipe rams of the blowout preventer (230) ensure that a competent, high-pressure seal is formed, so as to ensure the wellbore (110) is maintained under control (for well control purposes), and that the drill string (220) is immobile at the drilling rig (200), thereby permitting the subsequent operational steps of the method as follows:

Step (b)

[0084] Referring now to any one of Figures 1 to 5, in one embodiment, a coiled tubing unit (300) may be installed on the drilling rig (200). The coiled tubing unit (300) may include a coiled tubing string (310) having a diameter less than an inner diameter of the drill string (220), the lower portion (221) of the drill string (220), and the drilling assembly (222). In this way, the coiled tubing string (310) may be pushed down the inside of the drill string (220), and subsequently the lower portion (221) of the drill string (220), and the drilling assembly (222), such that an end (320) (best illustrated in Figure 2) of the coiled tubing string (310) may be positioned, so as to be at a depth within the geological formation (100).

[0085] In this embodiment, a pressure seal device (not shown) may be installed with the coiled tubing unit (300), which is well known, to those skilled in the art, as a "lubricator" or "stripper", so as to allow the coiled tubing string (310) to be pushed down the inside of the drill string (220), without the drill string (220), the lower portion (221) of the drill string (220), or the drilling assembly (222) being exposed to atmospheric pressure, thereby maintaining well control.

[0086] In one embodiment, still referring to Figures 1 to 5, a coiled tubing (310) conveyed drill pipe perforating device (not shown) may be positioned at a point at or near the end (320) of the coiled tubing string (310). The drill pipe perforating device may perform a perforating mechanism, which may include, but not be limited to, abrasive hydraulic jetting, plasma jetting, or explosive charges, and is capable of being positioned and operated within any one of the drill string (220), the lower portion (221) of the drill string (220), or the drilling assembly (222), so as to create one or more perforation(s) (130) through the wall of the drill string (220), at any location thereof, with access to at least a portion of the geological formation (100). In this way, the one or more perforation(s) (130) may be created, so as to create a fluid pathway between the geological formation (100), the wellbore (110), and the lower portion (221) of the drill string (220). Thereby, the one or more perforation(s) (130) provide a perforated section of the lower portion (221) of the drill string (220), so as to facilitate the extraction of hydrocarbon from the geological formation (100).

[0087] In an alternative embodiment, whereby the drill string (220) includes one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227), referring now to any one of Figures 6 to 10, one or more temporary seal(s) (228a) of the one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227) may be removed, opened or broken at this stage, so as to create one or more open perforation(s) (228b) along the lower portion (221) of the drill string (220), in such a way as to facilitate the extraction of hydrocarbon from the geological formation (100). In this way, the one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227) may create the fluid pathway between the geological formation (100), the wellbore (110), and the lower portion (221) of the drill string (220). The one or more temporary seal(s) (228a) of the one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227) may be removed, opened or broken (228b) by any one of a coiled tubing (310) conveyed blanked-off shear-pin stub-breaking device, the reversal of differential pressure to open a one-way valve, the reversal of differential pressure to unseat or seat a ball, or the breaking of a differential pressure-activated burst disk (all not shown), used by the one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227).

[0088] In this embodiment, whereby the lower portion (221) of the drill string (220) comprises one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227), and the coiled tubing unit (300) is installed on the drilling rig (200), the coiled tubing (310) conveyed drill pipe perforating device may be positioned at a point at or near the end (320) of the coiled tubing string (310), and may perform the perforating mechanism, so as to create one or more additional direct perforation(s) (not shown). In this way, additional fluid pathways between the geological formation (100), the wellbore (110) and the lower portion (221) of the drill string (220) may be created in the drill string (220) that already comprises one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227).

[0089] In any one of the above embodiments, referring now to any one of Figures 1 to 10, the drill string (220), the lower portion (221) of the drill string (220), and the drilling assembly (222) may advantageously function as the production conduit to facilitate the extraction of hydrocarbon from the geological formation (100), via any one or more of the perforation(s) (130), or via any one or more of the one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227), whereby the one or more temporary seal(s) (228a) is/are removed, opened or broken (228b), thereby allowing hydrocarbon to flow into the drill string (220), and then to a flowline (700) of the drilling rig (200).

Step (c)

[0090] Referring now to any one of Figures I to 5, in one embodiment, a section of the drill string (220) may be abandoned in the wellbore (110), with access to the geological formation (100), so as to create an abandoned section of the drill string (220) having an open-ended stub (229) disposed within the one or more casing string(s) (210). In this embodiment, as illustrated by Figure 3, the coiled tubing unit (300) and the coiled tubing string (310) may further include a drill pipe severing tool (330), positioned at a point at or near the end (320) of the coiled tubing string (310). The coiled tubing (310) conveyed drill pipe severing tool (330) is able to perform the function of severing, at a location therealong, any one of the drill string (220), the lower portion (221) of the drill string (220), or the drilling assembly (222). The coiled tubing (310) conveyed drill pipe severing tool (330) may be any one or more of the conventional (standard) specialised cutting device(s) (not shown) presently available, and well known to those skilled in the art, for use with coiled tubing units (300). In this way, the severed and detached section of the drill string (220) may be abandoned in the wellbore (110), with access to the geological formation (100) by the actuation, action or function of the coiled tubing (310) conveyed drill pipe severing tool (330). Additionally, in this embodiment, it will be appreciated, by those skilled in the art, that the coiled tubing string (310) of the coiled tubing unit (300) may include not only the coiled tubing (310) conveyed drill pipe severing tool (330), but also the coiled tubing (310) conveyed drill pipe perforating device (not shown) used to perform the perforating mechanism of Step (b). In this way, advantageously, both the perforation mechanism utilising the coiled tubing (310) conveyed drill pipe perforating device (not shown) and the abandonment mechanism of the section of the drill string (220) utilising the coiled tubing (310) conveyed drill pipe severing tool (330) may be performed by the coiled tubing unit (300) optimally during a single "trip" of the coiled tubing string (310) into the wellbore (110). This reduces the time and cost associated with Steps (b) and (c).

[0091] In an alternative embodiment, referring now particularly to Figure 8, the section of the drill string (220) may be abandoned in the wellbore (110), with access to the geological formation (100), by "backing off' (unscrewing) a joint between adjoining drill pipe segments, at a "tensile free point" (or "free point") (240) in the drill string (220), so as to create the abandoned section of the drill string (220) having the open-ended stub (229) disposed within the one or more casing string(s) (210). It will be appreciated, by those skilled in the art, that multiple procedures exist for "backing off' (unscrewing) a joint between adjoining drill pipe segments, at the "tensile free point" (or "free point") (240), to thereby abandon the section of the drill string (220), so as to create the open-ended stub (229), such as mechanically "backing off' (unscrewing) by transmitting torque down the drill string (220) via rotation from the drilling rig (200) to the calculated "tensile free point" (or "free point") (240). Alternative "backing off' (unscrewing) procedures exist, which may include the use of a mechanical "back off' tool (not shown), located at the desired "tensile free point" (or "free point") (240) along the drill string (220), so as to create the open-ended stub (229), and further procedures are envisaged to achieve the same desired result of creating the open-ended stub (229). It will be appreciated, by those skilled in the art, that the "tensile free point" (or "free point") (240) in any one of the embodiments, whereby the drill string (220) is "backed off' (unscrewed), may be calculated/positioned/designed so as to locate the open-ended stub (229) within the one or more casing string(s) (210). Advantageously, in this embodiment, "backing off' (unscrewing) at the "tensile free point" (or "free point") (240) in the drill string (220), so as to create the open-ended stub (229), may reduce the time and cost spent in comparison to the previous embodiment comprising the coiled tubing unit (300), the coiled tubing string (310), and the coiled tubing (310) conveyed drill pipe severing tool (330).

[0092] In either one of the above embodiments, as illustrated by Figures 3 or 8, the abandoned section of the drill string (220), comprising the lower portion (221) of the drill string (220), and the drilling assembly (222), is left in situ with respect to the geological formation (100). In this way, the abandoned section of the drill string (220), the lower portion (221) of the drill string (220), and the drilling assembly (222) remain unsecured with respect to the geological formation (100). It will also be appreciated that, in either one of the above embodiments, the abandoned section of the drill string (220) may advantageously function as the production conduit to facilitate the extraction of hydrocarbon from the geological formation (100). In this way, any one of, via any one or more of the perforation(s) (130), or via any one or more of the one or more temporarily sealed (228a), pre perforated drill pipe segment(s) (227), whereby the one or more temporary seal(s) (228a) is/are removed, opened or broken (228b), may permit the abandoned section of the drill string (220) to advantageously function as the production conduit to facilitate the extraction of hydrocarbon from the geological formation (100). Furthermore, in either one of these embodiments, a free upper section (241) of the drill string (220) is created, resultant of the abandonment of the drill string (220) in the wellbore (110), with access to the geological formation (100). The free upper section (241) of the drill string (220), from above the open-ended stub (229), is recovered by the drilling rig (200), and thereby removed from the wellbore (110).

[0093] Additionally, in either one of the above embodiments, the section of the drill string (220) comprising the lower portion (221) of the drill string (220), and the drilling assembly (222), advantageously functioning as the production conduit to facilitate the extraction of hydrocarbon from the geological formation (100), may function in a similar manner to a typical "conventional" wellbore completion string, such as those used in presently available wellbore completion methods. It will be appreciated, by those skilled in the art, that advantageously, the method disclosed herein does not require any of the typical "conventional" wellbore completion strings used in presently available wellbore completion methods, rather, the drill string (220) utilised to drill the wellbore (110), with access to the geological formation (100), advantageously functions as the production conduit to facilitate the extraction of hydrocarbon from the geological formation (100). This reduces the time, cost and operational challenges that would otherwise be associated with the installation of typical "conventional" wellbore completion strings by presently available wellbore completion methods.

[0094] Prior to the next Step (d) of the method, optionally, the drilling rig (200) may be utilised to repeat Steps (a) to (c), so as to drill additional wellbores (not shown), by sidetracking procedures, that are well known to those in the art, originating from the same one or more casing string(s) (210). These additional wellbores (not shown) may be drilled, so as to be within or immediately adjacent to the coal seam gas formation (100'), or so as to target additional coal seam gas formations (not shown). This results in a multilateral well design (not shown). In this way, the multilateral well design (not shown) may include one or more additional open-ended stub(s) (not shown), associated with one or more additional abandoned drill string section(s) (not shown), abandoned within the additional wellbores (not shown) that may have access to the coal seam gas formation (100'), or target additional coal seam gas formations (not shown). It will also be appreciated that each of the subsequent one or more additional open-ended stub(s) (not shown), resultant from the one or more additional abandoned drill string section(s) (not shown), resultant from sidetracking procedures from the original one or more casing string(s) (210), may be positioned shallower in relation to the previous open-ended stub (229) (not shown). Thereby, each of the one or more additional open-ended stub(s) (not shown), associated with the one or more additional abandoned drill string section(s) (not shown), may provide one or more corresponding additional production conduit(s) (not shown), so as to further facilitate the extraction of hydrocarbon from the geological formation (100), or the coal seam gas formation (100'). Additionally, in this way, the repetition of Steps (a) to (c), so as to drill the additional wellbores (not shown), resulting in the multilateral well design (not shown), advantageously enables the consistent, non-problematic, and cost-effective drilling of long-reach high-angle wellbores (110) and long-reach horizontal wellbores (110) within inherently unstable geological formations (100).

Step (d)

[0095] Referring now to any one of Figures 4 or 9, in one embodiment, a temporary plug (400) may be set at an optimal depth that is immediately above the open-ended stub (229), so as to isolate the abandoned section of the drill string (220), the lower, openhole section of the wellbore (110), and the geological formation (100), which all lie below the temporary plug (400). It will be appreciated that, in the instance that the multilateral well design (not shown) exists, the temporary plug (400) may be set and positioned at an optimal depth that is immediately above the shallowest of the one or more open-ended stub(s) (229) (not shown), within the one or more casing string(s) (210). In this embodiment, the temporary plug (400) forms a seal that temporarily isolates the abandoned section of the drill string (220), the lower, openhole section of the wellbore (110), and the geological formation (100), which all lie below the temporary plug (400). Temporary plugs (400) that function so as to create the seal to isolate therebelow are well known to those skilled in the art, and are often referred to as wellbore packers, wellbore isolation plugs, casing packers, bridge plugs and the like. The temporary plug (400) may be selected based on its sealing properties, the type of hydrocarbon that may be present in the wellbore (110), the temperature of the wellbore (110), the type of drilling fluid used to drill the wellbore (110), and other properties, well known to those skilled in the art, of selecting a temporary plug (400), so as to isolate therebelow. The temporary plug (400) may also be selected so as to withstand high differential pressures that it may be exposed to from the wellbore (110), and the geological formation (100). It will be appreciated that further temporary isolation measures, other than the temporary plug (400), are envisaged, so as to isolate the abandoned section of the drill string (220), the lower, openhole section of the wellbore (110), and the geological formation (100) therebelow.

[0096] In the embodiment wherein the multilateral well design (not shown) is drilled by the drilling rig (200), so as to be within or immediately adjacent to the coal seam gas formation (100'), or additional "target" coal seam gas formations (not shown), the temporary plug (400) may be positioned within the one or more casing string(s) (210) at an optimal depth that is immediately above the shallowest positioned one or more additional open-ended stub(s) (not shown) associated with the one or more additional abandoned drill string section(s) (not shown). In this way, the temporary plug (400) may isolate the one or more additional abandoned drill string section(s) (not shown), the lower, openhole section of the one or more associated additional wellbore(s) (not shown), and the geological formation (100), or the coal seam gas formation (100'), which all lie below the temporary plug (400).

Step (e)

[0097] Still referring to any one of Figures 4 or 9, in one embodiment, a production tubing string (600) may be installed within the one or more casing string(s) (210), to an optimal depth that is immediately above the temporary plug (400), and thence the open-ended stub (229) of the abandoned section of the drill string (220), thereby creating a fluid passageway (610) to facilitate the extraction of hydrocarbon from the geological formation (100) through the abandoned section of the drill string (220). In this way, the production conduit provided by the abandoned section of the drill string (220) may be accessible from the fluid passageway (610) created by the production tubing string (600), thereby facilitating the extraction of hydrocarbon from the geological formation (100).

[0098] In this embodiment, the production tubing string (600) may be installed via procedures that are well known to those skilled in the art, by way of either the drilling rig (220), or the coiled tubing unit (300), so as to position the production tubing string (600) to an optimal depth that is immediately above the temporary plug (400), and thence the open-ended stub (229) of the abandoned section of the drill string (220).

[0099] Also in this embodiment, whereby the production tubing string (600) may be installed within the one or more casing string(s) (210), a production tubing packer (620) may be positioned in an annulus (630) formed between the production tubing string (600) and the innermost casing string (210). The production tubing packer (620) may also be referred to as a production packer (620), to those skilled in the art, the function of which is to isolate zones within the wellbore (110), and direct extracted hydrocarbon from the geological formation (100), without loss of pressure or fluid to the annulus (221).

[00100] In any one of the above embodiments, in the instance that the multilateral well design (not shown) exists, the production tubing string (600) may be installed via procedures that are well known to those skilled in the art, so as to position the production tubing string (600) to an optimal depth that is immediately above the temporary plug (400), and thence the one or more additional open-ended stub(s) (not shown) of the one or more additional abandoned drill string section(s) (not shown) that may exist in the multilateral well design (not shown).

Step (f)

[00101] Still referring to any one of Figures 4 or 9, in one embodiment, following the setting of the temporary plug (400), and the production tubing string (600), both at an optimal depth that is immediately above the shallowest open-ended stub (229), a fluid (500) (which may be a low-density wellbore completion fluid including any one of a gas, such as nitrogen, or even air) may be introduced into the section of the wellbore (110) above the temporary plug (400), to displace the drilling fluid (which is generally of higher density when drilling the wellbore (110)), so as to create a high pressure differential between the one or more casing string(s) (210) above the temporary plug (400) and the section of the wellbore (110) below the temporary plug (400), comprising the abandoned section of the drill string (220). In this way, the high pressure differential provides the initial production pressure drawdown required for "priming" the wellbore (110) for the flowback process, which harnesses the stored reservoir energy of the coal seam gas formation (100'), so as to create an initial stimulated reservoir volume (i-SRV) (800 in Figures 5, 10 and 11). It will be appreciated, by those skilled in the art, that the fluid (500) may be selected based on its properties in comparison to the drilling fluid used to drill the wellbore (110), such that the properties of the fluid (500) are sufficient to create the required high pressure differential (drawdown). It will further be appreciated, that the temporary plug (400) maintains the geological formation (100) below the temporary plug (400) in a state of pressure equilibrium with respect to the wellbore (110) below the temporary plug (400), so as to maintain well control with respect to the geological formation (100).

[00102] In this embodiment, the blowout preventer (230) may be removed, so as to permit the installation of a wellhead (not shown), the functions of which are known, to those skilled in the art, to permit completion of the wellbore and allow the safe, controlled extraction of hydrocarbon from the geological formation (100). The wellhead is typically installed so as to be compatible with the one or more casing string(s) (210). Optionally, the drilling rig (200) may be demobilised, so as to drill one or more additional wellbore(s) (not shown) elsewhere, as a part of a drilling program, whereby, prior to the creation of one or more initial stimulated reservoir volume(s) (i-SRV) (800 in Figures 5, 10 and 11) in Step (g), the drilled wellbore(s) (110) is/are maintained with adequate well control with respect to the geological formation (100) by a temporary plug (400).

Step (g)

[00103] Referring now to any one of Figures 5 or 10, in one embodiment, the temporary plug (400) may be removed, so as to allow the geological formation (100) to immediately, or optionally later, be exposed to high pressure drawdown, when the entire wellbore (110) is opened to flow at a flowline (700) of the drilling rig (200), which thereby creates the initial stimulated reservoir volume (i SRV) (800 in Figures 5, 10 and 11) that is required to facilitate the extraction of hydrocarbon from the geological formation (100) and, resultantly, at least partially fill the annulus (211) between the abandoned section of the drill string (220) and the wellbore wall (110) with fragments (140) of the geological formation (100). The temporary plug (400) may be removed, typically with the assistance of a coiled tubing unit (300), by any one of a variety of conventional (standard) retrieval tools, grinding/milling/jetting tools, or differential pressure-activated burst disk mechanisms (all not shown), that are well known to those skilled in the art.

[00104] In this embodiment, the initial stimulated reservoir volume (i-SRV) (800 in Figures 5, 10 and 11) may be experienced along a length, to an entire length, of the abandoned section of the drill string (220), comprising the lower portion (221) of the drill string (220), and the drilling assembly (222). The initial stimulated reservoir volume (i-SRV) (800 in Figures 5, 10 and 11) may optimally be created, subsequent to the removal of the temporary plug (400), by rapidly opening the entire wellbore (110) to flow at a flowline (700) of the drilling rig (200). The resultant large, rapid production pressure drawdown may cause the entire section of the wellbore (110) accessible to at least a portion of the geological formation (100) to experience mixed-mode (i.e. tensile/shear/compressional) geomechanical rock failure. This may further result in the annulus (211) between the abandoned section of the drill string (220) and the wellbore wall (110) completely filling with fragments (140) of the geological formation (100). It will be appreciated that, where the geological formation (100) is the coal seam gas formation (100'), the fragments (140) of the geological formation (100) are coal fragments that fill the annulus (211) formed between the abandoned section of the drill string (220) and the wellbore wall (110). This at least partially fills the annulus (211), wherein the fragments (140) of the geological formation (100) may "bulk" or be "bulking" the annulus (211) with coal fragments (140) of the coal seam gas formation (100'). The annulus (211), now bulked with coal fragments (140) of the coal seam gas formation (100'), being resultant of the initial stimulated reservoir volume (i SRV) (800 in Figures 5, 10 and 11), may partially protect the wellbore (110) in the coal seam gas formation (100') from effective stress-induced compaction. Additionally, a pressure arch "stress shield" (not shown) is generated within the surrounding native coal seam and host rock strata, around the bulked annulus (211), and this provides further, more effective protection against the compaction effect. It will be appreciated, by those skilled in the art, that the at least partial filling of the annulus (211), by bulking the annulus (211) with coal fragments (140) of the coal seam gas formation (100'), may be a vigorous process, by which the highly fragmented coal rapidly fills the annulus (211) during the creation of the initial stimulated reservoir volume (i-SRV) (800 in Figures 5, 10 and 11). Thus, resultant of the annulus (211) becoming bulked with coal fragments (140) of the coal seam gas formation (100'), a significantly de-stressed, permeable domain of highly fragmented coal may rapidly form around the abandoned section of the drill string (220) within the wellbore (110), extending significantly into the surrounding coal seam gas formation (100'). It will be apparent, to those skilled in the art, that the bulking of the annulus (211) between the abandoned section of the drill string (220) and the wellbore wall (110) is deliberate and desirable, as it advantageously causes the one or more coal seam(s) (not shown) of the coal seam gas formation (100') to effectively collapse into, and thereby de-stress into, the pre-existing void space of the annulus (211) that has also been deliberately maximised in size by the selection of the larger diameter drill bit (226), and the smaller diameter drill string (220). Thus, the maximised size of the annulus (211) contributes to the overall success of the method herein, by in turn maximising a void space volume, into which the one or more coal seam(s) of the coal seam gas formation (100') may collapse and de-stress, thereby optimising the size of the initial stimulated reservoir volume (i-SRV) (800 in Figures 5, 10 and 11).

[00105] Additionally, in this embodiment, although the abandoned section of the drill string (220) is left in situ and unsecured with respect to the geological formation (100), it will be appreciated, by those skilled in the art, that a by-product of the bulking of the annulus (211) between the abandoned section of the drill string (220) and the wellbore wall (110) may advantageously function to secure, and provide structural support, to the section of the drill string (220) abandoned within the wellbore (110), with access to the coal seam gas formation (100'). To those skilled in the art, the by-product advantage provided by the bulking of the annulus (211) may function similar to (without the inherent restrictions, but comprising similar structural benefits of) cementing the abandoned section of the drill string (220) in place, by the typical "conventional" cementing process. Furthermore, it will be appreciated, by those skilled in the art, that it is not the intention, and it would be counterproductive of the method disclosed herein, to introduce any one or more of a cement, a slurry, a gravel pack, and indeed casing and screens, which form the basis for more "conventional" wellbore completions.

[00106] Furthermore, in this embodiment, optionally, one or more follow-up reservoir stimulation event(s) may be performed, as described in this Step (g), at any time during the production life of the wellbore (110), so as to potentially enhance the size and permeability of the initial stimulated reservoir volume (i-SRV) (800 in Figures 5, 10 and 11), by repeatedly shutting in the wellbore at the wellhead, by procedures that are well known to those skilled in the art, allowing pressure within the annulus (211) to increase, and then opening the wellbore to the high pressure drawdown. It will be appreciated, by those skilled in the art, that this approach may advantageously increase the size and flow rate capacity of the created initial stimulated reservoir volume (i-SRV) (800 in Figures 5, 10 and 11), thus potentially increasing the production performance and ultimate hydrocarbon recovery of the wellbore (110).

Step (h)

[00107] Still referring to any one of Figures 5 or 10, in one embodiment, the continuation of the extraction of hydrocarbon from the geological formation (100), at a high pressure drawdown, creates an expanding stimulated reservoir volume (e-SRV) (900 in Figures 5, 10 and 11) that may be experienced along a length, to an entire length of the abandoned section of the drill string (220), comprising the lower portion (221) of the drill string (220), and the drilling assembly (222). The expanding stimulated reservoir volume (e-SRV) (900 in Figures 5, 10 and 11) advantageously may increase in size and permeability over the production life of the wellbore (110), in response to ongoing confining stress reduction provided by the combined, mutually sustaining actions of progressive coal matrix shrinkage-induced coal fabric tensile dilation, and sympathetic pressure arch stress deflection around the evolving expanding stimulated reservoir volume (e-SRV) (900 in Figures 5, 10 and 11).

[00108] In this embodiment, it will be appreciated, by those skilled in the art, being apparent due to the method disclosed herein, that the creation of the expanding stimulated reservoir volume (e SRV) (900 in Figures 5, 10 and 11) at this Step (h), is achieved by simply allowing the drilled wellbore (110) to continue extracting hydrocarbon from the coal seam gas formation (100'). The rate at which the expanding stimulated reservoir volume (e-SRV) (900 in Figures 5, 10 and 11) increases in size and permeability may be optimised by varying the flowing bottom-hole pressure of the wellbore (110), via methods well known to those skilled in the art.

[00109] In any one of the above embodiments, referring now to Figure 11, which illustrates a) the spatial distribution of the aforementioned pressure drawdown-induced reservoir stimulation effects around the lower portion (221) of the drill string (220), with access to the geological formation (100), b) the one or more perforation(s) (130) in the drill string (220) in the embodiment whereby the one or more temporarily sealed (228a), pre-perforated drill pipe segment(s) (227) is/are not comprised within the drill string (220), c) the initial stimulated reservoir volume (i-SRV) (800 in Figures 5, 10 and 11) created by the removal of the temporary plug (400) and opening the entire wellbore to flow at surface, d) the fragments (140) of the geological formation (100) bulking the annulus (211), and e) the expanding stimulated reservoir volume (e-SRV) (900 in Figures 5, 10 and 11) propagating away from the wellbore (110). Arrows in Figure 11 are for illustrative purposes only, depicting the advantageous effect of the expanding stimulated reservoir volume (e-SRV) (900 in Figures 5, 10 and 11), which increases in size and permeability over the production life of the wellbore (110), in response to ongoing confining stress reduction provided by the combined, mutually sustaining actions of progressive coal matrix shrinkage-induced coal fabric tensile dilation, and sympathetic pressure arch stress deflection. It will be appreciated, by those skilled in the art, that the schematic representation of the spatial distribution of the pressure drawdown-induced reservoir stimulation effects around the lower portion (221) of the drill string (220) depicted by Figure 11 is for illustrative purposes only, and not to scale, whereby the size and shape of any one of the drill string (220), the wellbore (110), the annulus (211), the perforations (130), the fragments (140) of the geological formation (100), the initial stimulated reservoir volume (i-SRV) (800 in Figures 5, 10 and 11), and the expanding stimulated reservoir volume (e-SRV) (900 in Figures 5, 10 and 11) may vary, dependent on numerous factors that may or may not be anticipated during the drilling of the wellbore (110) to the depth (120) to access at least a portion of the geological formation (100), nor during the subsequent reservoir stimulation and extended production phases.

[00110] Referring to any one of the above embodiments, and any one of Figures 1 to 10, it will be appreciated that the method disclosed herein comprises 8 distinct steps, comprising Steps (a) to (h), so as to achieve the desired objective of the present invention, which is to facilitate the extraction of hydrocarbon from the geological formation (100), particularly in the case of the coal seam gas formation (100') comprising one or more low-permeability coal seam(s). It will also be appreciated that the method does not require (is not dependent upon) the use of any presently available reservoir stimulation equipment, methods, or techniques per se (for example the hydraulic fracture stimulation process), or personnel specialised therein, to achieve the extraction of hydrocarbon from the geological formation (100), whereas the use of presently available reservoir stimulation equipment, methods, and techniques would involve additional time, risk, and cost. Instead, the method disclosed herein harnesses the inherent stored reservoir energy of the coal seam gas formation (100'), as well as the combined stress-reduction capacity of desorption-induced coal matrix shrinkage and pressure arching, which thereby allows the one or more coal seam(s) of the coal seam gas formation (100') to be induced to progressively "self-fracture" naturally, in response to ongoing high production pressure drawdown, so as to generate an overall stimulated reservoir volume (shown in Figure 11), which is the combination of both the initial stimulated reservoir volume (i-SRV) and the expanding stimulated reservoir volume (e-SRV) (800 and 900 respectively in Figures 5, 10 and 11) that steadily increases in size and hydrocarbon extraction capacity within the geological formation (100) over production time.

[00111] It will additionally be appreciated, that the method disclosed herein, comprising Steps (a) to (h), advantageously utilises presently available conventional (standard), generic oilfield equipment, primarily comprising the use of the drilling rig (200) and the coiled tubing unit (300). Thereby, the method disclosed herein provides for the use of presently available oilfield drilling and coiled tubing equipment, methods, and techniques, to facilitate the extraction of hydrocarbon from one or more coal seam(s) of the coal seam gas formation (100'), particularly those coal seams containing gas in deep and ultra-deep, very low permeability settings that do not respond to presently available hydrocarbon extraction and commercialisation methods and techniques. Advantageously, the method disclosed herein, comprising Steps (a) to (h), overcomes the four key challenges associated with the extraction and commercialisation of hydrocarbon from coal seal gas formations (100'), thereby providing a simple, low-cost, repeatable solution, which enables the commercialisation of coal seam gas formations (100') on a full-cycle standalone basis.

[00112] It will further be appreciated that the method disclosed herein advantageously achieves a commercial hydrocarbon flow rate and ultimate hydrocarbon recovery by "engineering" an artificial path of stress reduction within the coal seam gas formation (100') that leads to the creation of a large, complex domain of enhanced coal fabric permeability, comprising the initial stimulated reservoir volume (i-SRV) and the expanding reservoir volume (e-SRV) (800 and 900 respectively in Figures 5, and 11). The method achieves this commercial hydrocarbon flow rate and ultimate hydrocarbon recovery by both; a) the inherent and deliberate bulking of the annulus (211) by fragments (140) of the coal seam gas formation (100') between the abandoned section of the drill string (220) and the wellbore wall (110) in the geological formation (100), and b) the harnessing of the phenomenon of desorption-induced coal matrix shrinkage that counteracts the tendency for the coal seam gas formation (100') to compact (when exposed to production pressure drawdown-induced effective stress), by the dilation of coal fabric apertures therein, so as to increase the permeability of the coal seam gas formation (100').

[00113] It will be apparent, to those skilled in the art, that the method to facilitate the extraction of hydrocarbon from the geological formation (100) disclosed herein, advantageously provides a new, contrarian (or "disruptive") method for drilling and completing wellbores (110), that involves only a single, "one-way trip" of the drill string (220) into the geological formation (100), which is then deliberately abandoned in situ and left unsecured with respect to the geological formation (100). That is, those skilled in the art will identify that the method disclosed herein is contrary to currently accepted drilling, wellbore completion, and reservoir stimulation practices, whereby drill strings/casing strings/liner strings are ultimately cemented or secured in the wellbore (110) prior to performing operations that specifically facilitate the extraction of hydrocarbon. Advantageously, the method disclosed herein does not require the step of cementing, or securing in any other way, the abandoned section of the drill string (220) in the wellbore (110), with access to the geological formation (100), in order to facilitate the extraction of hydrocarbon. Indeed, such a "conventional" approach to wellbore completion would be deleterious to the method disclosed herein.

[00114] It will be appreciated, by those skilled in the art, that the geological formation (100) and the coal seam gas formation (100') that are the targets of the method disclosed herein, may be independent of conventional geological hydrocarbon-trapping structures sensu stricto (e.g. anticlines). Those skilled in the art will appreciate that thermogenic source rock reservoir types, such as deep, high-temperature shale formations, and deep, high-temperature coal seams, do not require a structural trapping mechanism for hydrocarbon (particularly gas) to accumulate. Hence, the geological target of the method disclosed herein may justifiably be referred to as a geological formation (100) and/or a coal seam gas formation (100').

[00115] Throughout the specification and the claims that follow, unless the context requires otherwise, the words "comprise" and "include", and variations such as "comprising" and "including", will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

[00116] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.

[00117] It will be appreciated, by those skilled in the art, that the disclosure is not restricted in its use to the particular application described. Neither is the disclosure restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the disclosure is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions, without departing from the scope of the disclosure, as set forth and defined by the following claims:

Claims

1. A method to facilitate the extraction of hydrocarbon from a geological formation, the method comprising the steps of: (a) drilling a wellbore below one or more casing string(s) using a drill string, to a depth to access at least a portion of the geological formation, wherein a lower portion of the drill string comprises a drilling assembly and one or more stabilising means; (b) perforating a section of the drill string, with access to the portion of the geological formation, using coiled tubing, wherein the perforated section of the drill string comprises the lower portion of the drill string; (c) severing and abandoning a section of the drill string in the wellbore, with access to the geological formation, so as to create an abandoned section of the drill string having an open-ended stub disposed within the one or more casing string(s); (d) setting a temporary plug above the open-ended stub, so as to isolate the abandoned section of the drill string, the wellbore, and the geological formation below the temporary plug; (e) installing a production tubing string within the one or more casing string(s), to a depth above the temporary plug and the open-ended stub, thereby creating a fluid passageway to facilitate the extraction of hydrocarbon from the geological formation through the abandoned section of the drill string;

(f) displacing a fluid above the temporary plug, so as to create a high pressure differential between the one or more casing string(s) above the temporary plug and the wellbore and the geological formation below the temporary plug; (g) removing the temporary plug, allowing the wellbore to produce through the fluid passageway, thereby creating an initial stimulated reservoir volume to facilitate the extraction of hydrocarbon from the geological formation and, resultantly, at least partially filling an annulus formed between the abandoned section of the drill string and the wellbore wall with fragments of the geological formation; and (h) continuing to extract hydrocarbon from the geological formation, at a high pressure drawdown, so as to create an expanding stimulated reservoir volume.

2. The method of claim 1, wherein the geological formation, from which hydrocarbon is extracted, is a coal seam gas formation.

3. The method of any one of claims 1 or 2, wherein the wellbore produces through the fluid passageway, in response to a pressure differential between a flowline at a surface location of a drilling rig and the geological formation.

4. The method of any one of the preceding claims, wherein the expanding stimulated reservoir volume grows larger and more permeable over production time.

5. The method of any one of the preceding claims, wherein the abandoned section of the drill string advantageously functions as a production conduit to facilitate the extraction of hydrocarbon from the geological formation.

6. The method of claim 5, wherein the production conduit provided by the abandoned section of the drill string is accessible from the fluid passageway created by the production tubing string, thereby facilitating the extraction of hydrocarbon from the geological formation.

7. The method of any one of the preceding claims, wherein the fragments of the geological formation are coal fragments that bulk the annulus formed between the abandoned section of the drill string and the wellbore wall.

8. The method of any one of the preceding claims, wherein the drilling assembly of the drill string comprises a drill bit system, comprising a mud motor and a drill bit.

9. The method of claim 8, wherein a diameter of the drill bit is selected to maximise the size of the annulus formed between the abandoned section of the drill string and the wellbore wall in the geological formation.

10. The method of any one of the preceding claims, wherein the one or more stabilising means comprises one or more reamer(s), and/or stabiliser(s), and/or centraliser(s), so as to support the lower portion of the drill string in the wellbore.

11. The method of claim 10, wherein the one or more stabilising means facilitate(s) the removal of any large fragments of the geological formation that may be present in the annulus formed between the abandoned section of the drill string and the wellbore wall, so as to minimise the risk of the drill string becoming stuck during the construction of the wellbore.

12. The method of any one of the preceding claims, wherein a drill pipe severing tool may be used to abandon a section of the drill string in the wellbore, with access to the geological formation.

13. The method of any one of claims 1 to 11, wherein one or more types of coiled tubing conveyed drill pipe severing tool(s) may be used to abandon a section of the drill string in the wellbore, with access to the geological formation.

14. The method of any one of the preceding claims, wherein the abandoned section of the drill string is left in situ with respect to the geological formation.

15. The method of any one of the preceding claims, wherein the abandoned section of the drill string is left unsecured with respect to the geological formation.

16. A method to facilitate the extraction of hydrocarbon from a geological formation, the method comprising the steps of: (a) drilling a wellbore below one or more casing string(s) using a drill string, to a depth to access at least a portion of the geological formation, wherein a lower portion of the drill string comprises a drilling assembly, one or more stabilising means, and one or more temporarily sealed, pre perforated drill pipe segment(s); (b) removing, opening or breaking one or more temporary seal(s) of the one or more temporarily sealed, pre-perforated drill pipe segment(s), thereby creating one or more open perforation(s) along the lower portion of the drill string, so as to facilitate the extraction of hydrocarbon from the geological formation; (c) parting and abandoning a section of the drill string in the wellbore, with access to the geological formation, by "backing off' (unscrewing) at a "tensile free point" (or "free point") in the drill string, so as to create an abandoned section of the drill string having an open-ended stub disposed within the one or more casing string(s); (d) setting a temporary plug above the open-ended stub, so as to isolate the abandoned section of the drill string, the wellbore, and the geological formation below the temporary plug; (e) installing a production tubing string within the one or more casing string(s), to a depth above the temporary plug and the open-ended stub, thereby creating a fluid passageway to facilitate the extraction of hydrocarbon from the geological formation through the abandoned section of the drill string; (f) displacing a fluid above the temporary plug, so as to create a high pressure differential between the one or more casing string(s) above the temporary plug and the wellbore and the geological formation below the temporary plug; (g) removing the temporary plug, allowing the wellbore to produce through the fluid passageway, thereby creating an initial stimulated reservoir volume to facilitate the extraction of hydrocarbon from the geological formation and, resultantly, at least partially filling an annulus formed between the abandoned section of the drill string and the wellbore wall with fragments of the geological formation; and (h) continuing to extract hydrocarbon from the geological formation, at a high pressure drawdown, so as to create an expanding stimulated reservoir volume.

17. The method of claim 16, wherein the geological formation, from which hydrocarbon is extracted, is a coal seam gas formation.

18. The method of any one of claims 16 or 17, wherein the wellbore produces through the fluid passageway, in response to a pressure differential between a flowline at a surface location of a drilling rig and the geological formation.

19. The method of any one of claims 16 to 18, wherein the expanding stimulated reservoir volume grows larger and more permeable over production time.

20. The method of any one of claims 16 to 19, wherein the abandoned section of the drill string advantageously functions as a production conduit to facilitate the extraction of hydrocarbon from the geological formation.

21. The method of claim 20, wherein the production conduit provided by the abandoned section of the drill string is accessible from the fluid passageway created by the production tubing string, thereby facilitating the extraction of hydrocarbon from the geological formation.

22. The method of any one of claims 16 to 21, wherein the fragments of the geological formation are coal fragments that bulk the annulus formed between the abandoned section of the drill string and the wellbore wall.

23. The method of any one of claims 16 to 22, wherein the one or more temporarily sealed, pre perforated drill pipe segment(s) is/are temporarily sealed using one or more blanked-off shear-pin stub(s), one-way valve(s), one-way ball seal(s), or differential pressure-activated burst disk(s).

24. The method of claim 23, wherein activation of one or more of the blanked-off shear-pin stub(s), the one-way valve(s), the one-way ball seal(s), or the differential pressure-activated burst disk(s) removes, opens or breaks one or more temporary seal(s) of the one or more temporarily sealed, pre-perforated drill pipe segment(s), thereby creating one or more open perforation(s) along the lower portion of the drill string, so as to facilitate the extraction of hydrocarbon from the geological formation.

25. The method of any one of claims 16 to 24, wherein the drilling assembly of the drill string comprises a drill bit system, comprising a mud motor and a drill bit.

26. The method of claim 25, wherein a diameter of the drill bit is selected to maximise the size of the annulus formed between the abandoned section of the drill string and the wellbore wall in the geological formation.

27. The method of any one of claims 16 to 26, wherein the one or more stabilising means comprises one or more reamer(s), and/or stabiliser(s), and/or centraliser(s), so as to support the lower portion of the drill string in the wellbore.

28. The method of claim 27, wherein the one or more stabilising means facilitate(s) the removal of any large fragments of the geological formation that may be present in the annulus formed between the abandoned section of the drill string and the wellbore wall, so as to minimise the risk of the drill string becoming stuck during the construction of the wellbore.

29. The method of any one of claims 16 to 28, wherein the abandoned section of the drill string is left in situ with respect to the geological formation.

30. The method of any one of claims 16 to 29, wherein the abandoned section of the drill string is left unsecured with respect to the geological formation.

31. A method to facilitate the extraction of hydrocarbon from a coal seam gas formation, the method comprising the steps of: (a) drilling a wellbore using a drill string, to a depth to access at least a portion of the coal seam gas formation; (b) perforating a section of the drill string; (c) severing and abandoning a section of the drill string in the wellbore comprising the perforated section; (d) setting a temporary plug to isolate the severed and abandoned section of the drill string; (e) installing a production tubing string above the temporary plug and the severed and abandoned section of the drill string, so as to create a fluid passageway to facilitate the extraction of hydrocarbon from the coal seam gas formation; (f) displacing a fluid above the temporary plug, so as to create a high pressure differential;

(g) removing the temporary plug, thereby creating an initial stimulated reservoir volume to extract hydrocarbon from the coal seam gas formation and, resultantly, at least partially filling an annulus formed between the severed and abandoned section of the drill string and the wellbore wall with coal fragments of the coal seam gas formation; and (h) continuing to extract hydrocarbon from the coal seam gas formation via the fluid passageway provided by the production tubing string, at a high pressure drawdown, so as to create an expanding stimulated reservoir volume.