AU7160900A - Process for production of methane and other hydrocarbons from coal - Google Patents

Description

P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990 t

S

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "PROCESS FOR PRODUCTION OF METHANE AND OTHER HYDROCARBONS FROM COAL" The following statement is a full description of this invention, including the best method of performing it known to me/us: 2 TITLE PROCESS FOR PRODUCTION OF METHANE AND OTHER HYDROCARBONS FROM COAL The invention relates to a process of producing methane, other gases and liquid hydrocarbons from in-situ coal seams.

BACKGROUND TO THE INVENTION Coal has been used as a source of energy production for many decades. Coal has traditionally been retrieved either from open cut or underground mines. Open cut mines are much more efficient in extraction of the resource, but have the disadvantage of requiring a high level of ooooo 10 environmental rehabilitation after mining is completed. Underground mines tend to be far less efficient in terms of resource utilisation and necessarily involve a higher degree of danger, with personnel working the coal face. As a result techniques are being developed to access methane and other ooooo hydrocarbons from in-situ coal seams from the surface, without the need for a traditional open cut or underground mining.

~In coal-bed methane deposits, methane gas is adsorbed onto and held within the coal grain and micropores and held there by the hydraulic pressure of the water present in the coal seam. The coal seam therefore acts as the source, reservoir and seal for this type of gas deposit.

Because of the microporous nature of coal, the coal can potentially hold much more methane per unit of volume than most other typical natural gas reservoirs. The methane stored can be released by removing some of the water from the coal seam, thus creating a depressurised or dewatered zone, allowing the methane gas to be released and flow from the coal seam. Apart from the removal of the methane and some of the water, the coal seam remains unaffected. This technique is referred to as coal bed methane production.

In-situ coal seam gasification is a process employed to convert an in-situ coal seam into useable energy forms. Coal gasification processes involve the introduction of oxygenating gas(es), such as air, oxygen, air/oxygen mixed with water vapour and carbon dioxide, into the coal seam rr_ which has been previously ignited. Ignition of the coal seam is typically achieved by the addition of burning coke to the bore hole. This procedure 10 has been employed in Russian coal fields and is discussed, by way of general background, in US 4,185,692 in the name of R. C. Terry, discussed in more detail below. The oxygenating gas reacts with the coal to partially gasify it, resulting in the production of a gaseous mixture, containing largely ooooo nitrogen, water vapour, carbon dioxide, carbon monoxide, hydrogen and some hydrocarbons, which are taken to the surface, typically, via a second borehole. If the intention is to ultimately manufacture pipeline quality gas, it may be preferable to use oxygen-rich gases for the gasification, rather than air, as the resulting gas is of a higher calorific value and more easily upgraded into pipeline quality gas.

US 4,014,721 in the name of G Pusch and R. Gedenk, describes the use of an ignition mixture comprising an initiating agent, selected from unsaturated hydrocarbons or fatty acids, an initiator of organic peroxide and an activator of heavy metal salt. This composition is injected into a borehole to ignite a coal seam and commence coal gasification 4 reactions.

US 4,036,298 in the name of E. V. Kreinin and K. N.

Zvyaginstev, described the linking of boreholes through the successive ignition of boreholes by the injection of an oxidizing gas into a borehole as the burn front of the coal seam approaches from a first borehole to a subsequent borehole.

US 4,089,374 in the name of R. C. Terry, describes the production of methane from a coal seam having two stratifications, wherein oooo the water is pumped from the coal seam creating a dewatered zone in each of the coal stratifications and allowing adsorbed methane present in the dewatered zones to be desorbed and extracted via the bore. US 4,089,374 goes on to describe a process of igniting the coal to promote a gasification process when the methane production of the lower coal stratification diminishes. Ignition of the coal as described in US 4,089,374 is designed to make the diameter of the borehole bigger within the coal seam to allow for greater desorption of methane. This technique is commonly referred to as reaming. There is no teaching of the use of this technique for multiple boreholes or the pressure conditions required for the injection of oxidising gas into the coal seam for the optional gasification reaction.

R. C. Terry is also the author of US 4,185,692 and US 4,544,037. US 4,185,692 describes a method of drilling of two or more L shaped bores with the lower portions directed towards one another and creating a coal gasification zone between the lower portions of the L shaped bores by injecting oxidising gas into one or both of the bores. US 4,544,037 describes initiating production of methane from underground wet coal seams by injecting high pressure gas down a borehole to the coal seam to drive the water away from that section of the coal seam, before stopping the gas injection and allowing the methane to be desorbed from the coal and flow to the surface, via the borehole. As water returns to the dewatered zone, it is pumped from the coal seam to sustain methane production.

US 4,313,499 in the name of S. C. Tsai et al, describes the use of heated air in the gasification process. The heating of the air softens any plugs that may have formed in the initial forward burn of the coal, allowing •••Qo the reverse burn front to proceed without being driven in another direction or being extinguished.

US 4,317,492 in the name of D. A. Summers et al, describes a method and apparatus for boring horizontal holes from a main vertical ooooi borehole. In in-situ coal gasification vertical boreholes, generally spaced 1 00ft apart, are linked by reverse combustion of a horizontal hole in the coal ~seam. The technique described in US 4,317,492, of linking the vertical boreholes by drilling horizontal holes through the coal seam, allows for gasification of coal to occur over a larger area.

US 4,436,153 in the name of F. M. Carlson, describes the controlled thermal linking of an injection bore and a production bore by creating an L shaped injection bore into which oxidant is injected. The coal seam between the injection bore and the production bore is coked and gases extracted through the production bore.

One of the disadvantages of the known coal gasification 6 processes and coal bed methane processes is that the processes have a finite life before the methane or gas production drops off. At the end of the process life there is often unburnt coal and/or adsorbed methane left in the seam. Thus the currently known processes do not maximise the possible yields of methane and gasification gases.

Other known disadvantages specific to coal gasification processes is the requirement for oxidising gas to be injected at sufficiently high pressures to keep water out of the combustion zone. The pressurisation of the oxidising gases adds significantly to the cost of 10 production of methane and other gasification gases.

OBJECT OF THE INVENTION It is an object of the present invention to provide a process for production of methane and other hydrocarbons from in-situ coal seams which overcomes one or more or the disadvantages of the prior art.

Further objects will be evident from the following description.

DISCLOSURE OF THE INVENTION In one form, although it need not be the only or indeed the broadest form, the invention resides in a method of producing methane, other gases and/or liquid hydrocarbons from in-situ coal seams comprising the steps of: drilling an array of extraction boreholes into an in-situ coal seam and drilling a number of ignition boreholes within a boundary of the array; fitting fluid extraction apparatus to one or more of the extraction boreholes and/or the ignition borehole; creating a zone of reduced hydraulic pressure in an underground 7 region in the vicinity of the array by removing water from the extraction boreholes with the fluid extraction apparatus, and allowing adsorbed methane and other gases to be desorbed from the in-situ coal seam surfaces; interconnecting ignition boreholes through the coal seam; injecting an oxidising gas, at a gauge pressure determined by the depth of the in-situ coal seam, into at least one of the ignition boreholes; igniting the in-situ coal seam; forming other gases and/or liquid hydrocarbon fractions from the 10 reaction of the oxidising gas and in-situ coal seam, by desorption and o.

pyrolysis of the coal; removing methane, other gases and/or liquid hydrocarbon fractions from one or more of the extraction boreholes and/or ignition boreholes; and upgrading the methane, other gases and/or liquid hydrocarbons.

Preferably the array of extraction boreholes are arranged in a square, rectangular or circular arrangement. The ignition holes are within the boundary of the extraction boreholes and are preferably in a similar arrangement as that of the extraction boreholes or in a single line in the centre of the array.

Preferably the fluid extraction apparatus comprises two concentric pipes, the inner most pipe being connected to a pump and the outermost pipe or casing being adapted to extract gases and/or liquid hydrocarbons.

The zone of reduced hydraulic pressure is suitably formed in 8 the coal seam created in the vicinity of each extraction borehole, such that the zone of reduced hydraulic pressure of one extraction borehole may overlap with that of one or more of the neighbouring extraction boreholes.

The interconnections between the ignition boreholes are formed by fracturing, drilling horizontal holes from each borehole and/or the burning of the coal seam between the ignition holes through the addition of a heat source, such as burning coke, and injection of oxidising gas.

Preferably the ignition boreholes are interconnected to one or more of the extraction boreholes.

10 Suitably the oxidising gas is selected from air, oxygen enriched air, oxygen, a mixture of oxygen and steam, a mixture of 90% nitrogen and inert gases and steam, or a mixture of oxygen, nitrogen, steam and carbon dioxide.

The gauge pressure of the oxidising gas is suitably determined as approximately 1 OkPa per metre depth of the in-situ coal seam. Preferably the oxidising gas is injected into the ignition boreholes at a gauge pressure of less than 1000kPa. More preferably the gauge pressure is less than 500kPa. Suitable the gauge pressure is less than 300kPa. More suitable the gauge pressure is between 50-200kPa.

The upgrading of methane and other gases may include or be selected from gross solids removal; gross liquids removal; fine solids removal; fine liquids removal; flow monitoring; compression; addition of steam; removal of sulfur containing species; catalytic shift reaction; catalytic methanation; removal of water vapour; compression to pipeline pressure; 9 and/or sweetening by removal of hydrogen sulfide.

The process of the invention further comprises the step of extracting liquid hydrocarbon fractions from one or more extraction boreholes. Preferably the liquid hydrocarbons can be further processed and purified using gravity decantation, hydrogenation, distillation and other suitable separation and/or purification techniques.

The process of the invention further comprising the steps; drilling one or more carbon dioxide addition boreholes; and injecting carbon dioxide into the carbon dioxide addition 00.10 boreholes.

Preferably the carbon dioxide addition boreholes are situated outside the boundary of the extraction boreholes.

Preferably the carbon dioxide is obtained from the purification of methane and other gases.

The process of the invention allows for increased generation of gas from the existing boreholes making better use of the capital investment spent on the drilling of the boreholes.

The current invention has found that great synergy and higher gas production can be achieved through the creation of a zone of reduced hydraulic pressure within a coal seam through the use of an array of extraction and ignition boreholes, arranged so as to maximise the flow of methane, other gases and liquid hydrocarbons to the extraction and/or ignition boreholes. The creation of a zone of reduced hydraulic pressure has been found to significantly increase the economic return achievable from gas and/or liquid production from an in-situ coal seam.

The process of the invention allows for the injection of the oxidising gas to be carried out at lower gauge pressures than those currently being utilised. As a result oxygen or oxygen rich air can be produced by cheaper processes such as a low-pressure, pressure swing adsorption plant or a vacuum pressure swing adsorption plant. The lower operating pressure of the gasification operations allows more volatiles to be released from the in-situ coal seam and swept to the surface with the gas stream. Higher pressure operations may keep significant quantities of recoverable condensates such as liquid hydrocarbons in the in-situ coal seam where they are ultimately lost by coking and cracking reactions.

A further benefit is that the recovery of liquid hydrocarbons can be maximised above and beyond that recovered by operating the injection oo: of the oxidising gases at currently utilised gauge pressures of 1000kPa or greater. Liquid hydrocarbons produced by the pyrolysis of the coal will tend to migrate along the curved water surface to the bores where it is extracted.

The existing water bores can be used to pump a mixture of water and liquids to the surface for subsequent separation.

BRIEF DETAILS OF THE DRAWINGS To assist in understanding the invention preferred embodiments will now be described with reference to the following figures in which FIG I is a flowchart of the methane production process steps; FIG 2 is a schematic diagram of an array of extraction and injection boreholes; 11 FIG 3 is a cross-sectional schematic diagram of a fluid extraction apparatus; FIG 4 is a cross-sectional schematic diagram showing interconnections between an extraction borehole and two ignition boreholes.

DETAILED DESCRIPTION OF THE DRAWINGS In the drawings, like reference numerals refer to like parts.

FIG 1 is a flowchart of the process steps. A series of ~boreholes are drilled, including a number of extraction, ignition and carbon 10 dioxide addition boreholes. An example of borehole arrangement is described below, with reference to FIG 2. A description of the fluid extraction apparatus is described below with reference to FIG 3. One or more of the extraction and/or ignition boreholes is fitted with a fluid extraction apparatus. Water is pumped from one or more of the extraction boreholes to form a depressurised zone in the coal seam. Removed water is collected in an evaporation pond. The change in hydraulic pressure in the coal seam creates a change in the partial pressures within the in-situ coal seam allowing for the desorption of the methane and other gases from the coal. The methane and other gases are removed from the coal seam by the fluid extraction apparatus.

The ignition boreholes are interconnected either by fracturing, horizontal drilling and/or strategic ignition of the coal seam and is described below in further detail with reference to FIG 4.

The oxidizing gas mixture is prepared and injected, at a gauge 12 pressure of less than 1000 kPa, into one or more of the ignition boreholes.

The oxidizing gas mixture may be air or preferably a mixture of oxygen enriched air. Steam and carbon dioxide may be added to the oxidizing gas mixture to alter the temperature and chemistry of the reaction between the coal and oxidizing gas.

The in-situ coal seam is ignited through the addition of a heat source, such as burning coke, and injecting a volume of oxidising gas to one or more ignition boreholes.

The oxidizing gas reacts primarily with the in-situ coal seam 10 near the ignition borehole. The heat from the gases is conducted through the coal and leads to drying, desorption of adsorbed gases and the pyrolysis of the in-situ coal seam, forming other gases, such as gasification gases and pyrolysis gases, and liquid hydrocarbons. The gasification gases including gases, such as methane, carbon monoxide, carbon dioxide, water vapor and hydrogen are produced. Any high molecular weight gases which are produced may be liquified in subsequent operation, if desired. Heavy tars are also produced in the coal pyrolysis reaction, some of which remain in the coal seam. Both the liquid fractions and the gases can be removed through one or more of the extraction and/or ignition boreholes. In the case that liquid fractions are to be removed one of the extraction and/or ignition boreholes will need to be equipped with fluid extraction apparatus able to withstand high temperatures.

Depending on the final use of the gases produced they can be purified. Both the desorbed methane and other gases may be subject to one 13 or more of the following purification processes; gross solids removal; gross liquids removal; fine solids removal;-fine liquids removal; flow monitoring; compression; addition of steam; removal of sulfur containing species; catalytic shift reaction; catalytic methanation; removal of water vapour; compression to pipeline pressure; and/or sweetening by removal of carbon dioxide and hydrogen sulfide.

Liquid hydrocarbon fractions may be separated from water by any suitable means such as gravity decantation and subsequent separation techniques carried out as required. The recovery of liquid hydrocarbon S 10 fractions can be maximized if there is a greater distance between the area of reaction between the coal and the oxidizing gas and the extraction borehole. A greater distance allows the hot gases to pass through more coal which over time is pyrolysed to liberate heavier fractions of hydrocarbons.

It will be appreciated by the person skilled in the art that the steps of the process may be performed in a different order to that described above, depending on the characteristics and features of the in-situ coal seam to be processed.

FIG 2 is a schematic view of an array of extraction and ignition boreholes. Extraction boreholes 1 to 10 are drilled in a rectangular arrangement. Ignition boreholes 11 to 13 are drilled within the boundary of the extraction boreholes. In this case the ignition boreholes are arranged to direct gasification gases from the middle of the extraction borehole array to an extraction borehole for extraction of other gases and liquid hydrocarbons.

Carbon dioxide addition boreholes may be drilled and are generally placed

I

14 outside the boundary formed by the extraction boreholes, shown as 14 to 17.

The carbon dioxide injected into the carbon dioxide addition boreholes is preferentially adsorbed onto the surfaces and micropores of the coal, thus desorbing methane. Additional methane desorbed from the addition of carbon dioxide flows through the in-situ coal seam towards the extraction boreholes.

The pumping of water from the extraction boreholes creates a zone of reduced hydraulic pressure in the coal seam in the vicinity of each .s extraction borehole. The zone of reduced hydraulic pressure formed by one 10 extraction borehole may overlap with one or more of the zones of reduced :°ooo o hydraulic pressure formed by neighboring extraction holes, creating an ~enlarged region of reduced hydraulic pressure within the in-situ coal seam.

FIG 3 is a schematic cross-sectional view of a fluid extraction apparatus when fitted to an extraction borehole. A borehole 21 is drilled through the overburden 22 and into the coal seam 23. The borehole 21 is fitted with the fluid extraction apparatus 24 and held into place by the addition of cementitious material 25. The cementitious material is added to a maximum depth of the overburden 22. The fluid extraction apparatus comprised two concentric pipes. The inner most pipe 26 has connected to it in the in-situ coal seam a pump 27 for the extraction of water to the surface via the inner most pipe 26. The outermost pipe or casing 28 allows the passage of methane and other gases to the surface, prior to being sent to a compressor.

FIG 4 is a cross-sectional schematic diagram showing interconnections 18 between an extraction borehole 10 and two ignition boreholes 11 and 12. The interconnections 18 between the boreholes, 11 and 12 are shown in FIG 4 as being on an angle between boreholes and located at the approximately the same depth within the in-situ coal seam.

The person skilled in the art would appreciate that these interconnections may be at differing depths. The ignition boreholes, such as 11 and 12 may be connected to at least one of the extraction boreholes, such as 10, also by either horizontal drilling and/or strategic ignition of the coal seam. In the event that the ignition boreholes are not connected to one or more extraction boreholes, then one or more of the ignition boreholes is fitted with a fluid extraction apparatus.

EXAMPLES

Example 1 ooooo ~For ease of description the below example will make reference to the borehole arrangement of FIG 2.

An array of 10 extraction bores, 1 to 10, are developed on a square grid of side 250m. A series of ignition holes 11 to 13 are drilled into the coal seam along the midline of the extraction borehole array. Extraction boreholes. 1 to 10 are fitted with bore pumps and gas take off assemblies so they can both reduce the water level within the local in-situ coal seam and capture gases that evolve. Low pressure and low temperature piping, such as polythene piping, is used to collect gas from bore 1 to 9 and take it to a manifold and flow monitoring point. Water from the bores is sent to a surface evaporation pond.

16 Borehole 10 is fitted with a higher temperature rated piping as gases will come out in a hot state. Borehole 11 is fitted with a "nodding donkey" pump which is capable of handling hot oil and tar. FIG 4, shows directional drilling or interconnection 18 is used to link ignition boreholes 11 and 12 with extraction borehole 10. A similar interconnection can inturn be made between ignition boreholes 13 and 12. Borehole 11 is used as a liquids extraction point. The drilling between bores 13 and 12 and then 11 i preferably slope downhill to allow for the drainage of liquid fractions. In o* addition the connection from borehole 10 to 11 is reasonably higher in the 10 in-situ coal seam than the drilling from 12 to 11, so liquids are dropped into the borehole 11 and recovered by pumping to the surface.

The liquid from borehole 11 is sent to a separate dam where water and hydrocarbon liquids are separated. The liquid from borehole ooo.oi "I is also sent to this special dam in case it is also contains hydrocarbons.

15 Oxygen (typically 90 vol%) is produced in a pressure swing adsorption plant and introduced into borehole 13 together with steam. The molar ratio of oxygen to steam would be typically 1:2, but the exact quantity is at the discretion of the operator. Ideally the pressure swing adsorption plants operates at mild gauge pressure of 80kPa during the production cycle and perhaps -50kPa during the regeneration cycle. This is in contrast with conventional gasification-only operations that may have to operate at gauge pressures of 1000kPa or greater to keep water in the in-situ coal seam away from the reaction zone.

The in-situ coal seam is ignited by adding to the borehole a 17 heat source such as burning coke, and injecting a volume of oxygen or other oxidising gas into one of the ignition borehole. After ignition has commenced injection of oxygen is recommenced and continues whilst gasification reactions occur in the in-situ coal seam.

Gasification of the coal occurs near the base of borehole 13 with hot gases progressing towards borehole 10 through the horizontal boreholes 18. The gasification region operates at a temperature of approximately 7500C for a feed of 90vol% oxygen:0lvol% nitrogen with i steam subsequently mixed to achieve an oxygen to steam ratio of 1:2. The gases from the gasification zone would have a composition of approximately 0.8vol% methane, 46.9vol% carbon monoxide, 12.5vol% carbon dioxide, 31.1vol% hydrogen, 6.6vol% water vapour and 2.2vol% nitrogen.

The gases cool in their passage through the coal seam, due to the heat transfer to the surrounding coal. This heat drives off adsorbed methane from the coal and pyrolyses some of the coal leading to the production of liquid or liquefiable hydrocarbon fractions. The liquid fractions migrate to well 11 where most of them are recovered. The gasification gases cool to approximately 350 0 C which represents the pyrolysis temperature of the in-situ coal seam. The gases emerge from bore 10 cooler than this owing to additional heat loss to, for example, the central water pipe and the bore casing.

The gases from borehole 10 are further cooled on the surface to recover liquid hydrocarbons. This liquid hydrocarbon removal can be enhanced by compression/expansion, use of absorbent liquids, refrigeration 18 or any of the established technologies. Then the gas is sweetened by the removal of hydrogen sulfide. Next the gases flow to a gas upgrading section where steam is added, then it is heated to ensure it is at least 250 C, then they go to a sulfur trap consisting of a bed of zinc oxide before being heated to around 450°C for the shift reactor. A shift reaction is used to swing the relative proportions of hydrogen and carbon monoxide and then the gas goes to a methanation step where hydrogen and carbon monoxide are 6reacted to give methane and carbon dioxide.

o0o.

The gases then go to join the other gas flow from boreholes 1 to 9 and go through stages of compression and final gas dehydration and sweetening. It then flows through a gas flow monitoring station for sale into the main gas pipeline.

A series of distant boreholes, shown in FIG 2 as boreholes 14 to 17 are for the introduction of carbon dioxide or other gas to augment the 15 desorption of coal bed methane from the in-situ coal seam. This carbon oeo° ~dioxide is available at low cost from the gas sweetening operations.

The operating pressure of the gasification system is not set by the hydraulic pressure of the surrounding coal seam but by the local zone of low pressure produced by removal of water by pumps, prior to and during the production of methane, other gases and liquid hydrocarbon. Gasification is nominally conducted at 1.5 bar absolute, though any pressure can be used up to that corresponding to the hydraulic pressure within the coal seam.

Example 2 19 A series of 10 extraction bores are drilled into a coal seam, in two parallel rows spaces 250m apart, as per example 1. Each bore is fitted with a water pump and water is pumped out of the bores. The water withdrawn from the bores is sent to an evaporation pond.

A target gasification region between the bores is established as this is.a large region having a low level of water within the coal seam. A series of ignition bores are established leading away from one of the *:extraction bores at 50 metre intervals toward the centre line of the existing oooo *e*borehole pattern, not dissimilar to the arrangement described in FIG 2.

A package pressure swing adsorption oxygen plant is installed on the surface to produce oxygen of nominally 90% purity. Initially oxygen s is pumped into the extraction boreholes closest to the ignition boreholes.

This extraction borehole has been isolated from the methane collection network of piping. Hot material is poured into the nearest most ignition well and the surrounding coal mass ignited as oxygen permeates through the coal. This burning pattern will automatically migrate towards the oxygen source. As it does, the oxygen supply pressure falls indicating an easier passage to the next borehole.

Once the burning has migrated right through to the existing bore, oxygen addition is ceased there and it is added through the next bore.

The fire will migrate back towards the oxygen source again until the fire exists at the bottom of that bore. This step is repeated until a linkage is formed between all the ignition boreholes and the extraction boreholes.

The extraction bore is reconnected to the gas collection network and a flow of oxygen is added essentially continuously from the most distant ignition borehole. The intermediate ignition boreholes are blanked off but can be used for temperature monitoring. Combustion occurs underground, but tends to remain close to the oxygen addition point. This gives a long run of hot gases through the tortuous path to the extraction bore. Along the path, the gas surrenders heat to the surrounding coal which dries and pyrolyses to produce a range of gases with molecular weights from 00 tars through to light liquidfiable products and gases.

Equipment to recover condensates can be installed after the 10 methane compressor. This might involve simply cooling the compressed S9000 gases and separating the condensate or it might involve the use of an 000 *00 o adsorbing fluid, refrigeration, turbo-expander or any of the known liquids 0000 separation systems. Additionally, it is likely that, given time, liquids 0 produced by pyrolysis of the coal that does not leave in the gas phase will migrate towards the water extraction point and can be pumped to the surface 0:69 *000 for separation.

Depending on the gas purity requirements, the gases can be upgraded by processes such as water gas shift reaction to achieve roughly a 3:1 mole ratio of hydrogen to carbon monoxide; methanation to methane and carbon dioxide; and purification stages such as removal of hydrogen sulfide, carbon dioxide and water vapour may occur.

Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features.

Claims (24)

1. A method of producing methane, other gases and/or liquid hydrocarbons from in-situ coal seams comprising the steps of: drilling an array of extraction boreholes into an in-situ coal seam and drilling a number of ignition boreholes within a boundary of the array; fitting fluid extraction apparatus to one or more of the extraction boreholes and/or the ignition boreholes; creating a zone of reduced hydraulic pressure in an underground region in the vicinity of the array by removing water from the extraction boreholes with the fluid extraction apparatus, and allowing adsorbed methane and other gases to be desorbed from the in-situ coal seam; eeeee interconnecting ignition boreholes through the in-situ coal 15 seam; 0: 0 0. injecting an oxidising gas, at a gauge pressure determined by the depth of the in-situ coal seam, into at least one of the ignition boreholes; igniting the in-situ coal seam; forming other gases and/or liquid hydrocarbon fractions from the reaction of the oxidising gas and in-situ coal seam, by desorption and pyrolysis of the coal; removing methane and/or other gases and/or liquid hydrocarbon fractions from one or more of the extraction boreholes 22 and/or ignition boreholes; and upgrading the methane, other gases and/or liquid hydrocarbons.

2. The method of claim 1, wherein the zone of reduced hydraulic pressure is formed in the in-situ coal seam.

3. The method of claim 2, wherein the zone of reduced hydraulic pressure is created in the vicinity of each extraction borehole, and the zone of reduced hydraulic pressure of one extraction borehole overlaps with that of one or more of the neighbouring extraction 10 boreholes.

S4. The method of claim 1, wherein the array of extraction boreholes are arranged in a square, rectangular or circular arrangement. oooo

5. The method of claim 1 or 4, wherein the ignition boreholes are drilled oooo• in a similar arrangement to the extraction boreholes or in a single line in the centre of the array.

6. The method of claim 5, wherein the extraction boreholes are in a rectangular arrangement and the ignition boreholes are in a single line in the centre of the array.

7. The method of any one of the proceeding claims, wherein the fluid extraction apparatus comprises two concentric pipes, the inner most pipe being connected to a pump and the outermost pipe or casing being adapted to extract gases.

8. The method of claim 1, wherein the interconnections between the ignition boreholes are formed by fracturing, drilling horizontal holes 23 from each borehole and/or by burning of the coal seam between the ignition holes through the addition of a heat source, such as burning coke, and injecting oxidising gas.

9. The method of claim 1, wherein at least one of the ignition boreholes is interconnected to one or more of the extraction boreholes.

The method of claim 9, wherein the interconnection between the ignition boreholes and the extraction boreholes is by drilling horizontal holes from each borehole or by burning of the in-situ coal oO.e seam between the ignition holes through the addition of a heat .o.o source, such as burning coke, and injecting oxidising gas.

11. The method of claim 1, wherein the oxidising gas is selected from air, oxygen enriched air, oxygen, a mixture of oxygen and steam, a ooeo• mixture of 90% oxygen: 10% nitrogen and inert gases and steam, or a mixture of oxygen, nitrogen, steam and carbon dioxide. i.

12. The method of claim 1, wherein the gauge pressure of the oxidising gas is determined as being approximately 10OkPa per metre depth of the in-situ coal seam.

13. The method of any one of the proceeding claims, wherein the oxidising gas is injected into the ignition boreholes at a gauge pressure less than 1000kPa.

14. The method of any one of the proceeding claims, wherein the oxidising gas is injected into the ignition boreholes at a pressure less than 500kPa.

The method of claim 11, wherein the oxidising gas is injected into the 24 ignition boreholes at a pressure is less than 300kPa.

16. The method of claim 12, wherein the oxidising gas is injected into the ignition boreholes at a pressure between 50-200kPa.

17. The method of any one of the proceeding claims, wherein the upgrading of methane and/or other gases may include or be selected from, gross solids removal; gross liquids removal; fine solids removal; fine liquids removal; flow monitoring; compression to pipeline i pressure; addition of steam; removal of sulfur containing species; catalytic shift reaction; catalytic methanation; removal of water vapour; compression to pipeline pressure; and/or sweetening by removal of carbon dioxide and hydrogen sulfide. oee*

18. The method of any one of the proceeding claims, further comprises ooooo the step of extracting liquid hydrocarbon fractions from one or more extraction boreholes and/or ignition boreholes. 15

19. The method of claim 18, further comprising the step of further processing and purification of the liquid hydrocarbons.

The method of claim 19, wherein the step of further processing and purification of the liquid hydrocarbons is carried out by one or more of the following techniques gravity decantation, hydrogenation, distillation and the like.

21. The method of any one of the proceeding claims, further comprising the steps; drilling one or more carbon dioxide addition boreholes; and injecting carbon dioxide into the carbon dioxide addition boreholes.

22. The method of claim 21, wherein the carbon dioxide addition boreholes are situated outside the boundary of the extraction borehole array.

23. The method of claim 22, wherein the added carbon dioxide is obtained from the purification of methane and other gases.

24. The method of claim 1 as herein described, with reference to the examples. DATED this fourteenth day of November, 2000 QUEENSLAND GAS COMPANY By their Patent Attorneys oo* *o FISHER ADAMS KELLY soooeo 0oo *0000