AU2006201101A1 - Method for accessing and producing from an underground coal seam - Google Patents

Description

Regulation 3.2 Revised 2/98

AUSTRALIA

Patents Act, 1990

ORIGINAL

COMPLETE SPECIFICATION TO BE COMPLETED BY THE APPLICANT NAME OF APPLICANT: ACTUAL INVENTOR: ADDRESS FOR SERVICE: INVENTION TITLE: DETAILS OF ASSOCIATED PROVISIONAL APPLICATION NO: A J Lucas Group Limited (ACN 060 309 104) Duncan Charles Macdonald Peter Maxwell and Associates Level 6 Pitt Street SYDNEY NSW 2000 METHOD FOR ACCESSING AND PRODUCING FROM AN UNDERGROUND COAL SEAM 2005 901 283 16 March 2005 Australian The following statement is a full description of this invention including the best method of performing it known to us:m:\docs\20061 01 7\096204.doc ;This invention relates to methods for accessing and producing gas from an underground coal seam.

Underground coal seams contain substantial quantities of methane gas which can be extracted for commercial use. Underground coal seams also 0 5 contain water which must be drained from the coal seam to enable and/or the production of methane.

I Hitherto, horizontal drilling techniques have been used to remove water Sand to extract gas from coal seams. During the process of drilling, fluid or "mud" is pumped down the drill string comprised of connected lengths of drill pipe and 0 out through the drill bit. As it flows out of the bit, the mud picks up drill cuttings and transports them up through the annulus between the drill string and the wellbore to the surface. When the cuttings reach the surface they are removed from the mud and the fluid is re-circulated. With conventional drilling, the mud will exert a hydrostatic pressure on the formation. This pressure is a function of the vertical height of the fluid, its density and the frictional pressure losses as the fluid circulates throughout the wellbore.

In permeable formations, where formation pore pressure is less than mud pressure, the drilling fluid will enter the formation and this is referred to as "overbalanced" drilling. Overbalanced drilling is undesirable as the mud which is mixed with the cuttings tends to block the coal pores causing restrictions in gas flows when producing.

To prevent overbalancing from occurring, the pressure conditions in the wellbore must be managed. This is done by injecting air/gas into the drilling fluid whilst drilling to achieve a pressure in the wellbore close to or below the pressure in the coal pores. This is generally referred to as "underbalanced" drilling (UBD).

Existing methods for introducing air/gas into the drilling fluid include methods whereby the gas/air to be injected is pumped directly down the drill string to the drill bit. However such methods have disadvantages. A particular disadvantage results from the fact that the pumping of the air/gas mixture will often have to be interrupted during the addition of lengths of jointed pipe to the drill string. These interruptions cause spikes in the pressure exerted on the formation at the bottom of the wellbore which can damage the pores in the formation and thereby reduce the efficiency of production. Another disadvantage in pumping the air/gas mixture directly to the drill bit is that the temperature and pressure conditions that exist at the drill face, combined with the presence of hydrocarbons, result in an inability to use air due to the increased risk of fire. Using other gases such as methane or nitrogen requires greater amounts of surface equipment and infrastructure and thus greatly increases the costs of drilling over systems that utilise air.

Another disadvantage is that, where existing methods use an outer production casing and an inner temporary casing to facilitate the injection of air into the wellbore, the air/gas injection process will cause the temporary casing to shake if it is not held in place. Due to this shaking, the temporary casing will unscrew itself downhole.

The object of this invention is to ameliorate some of the problems associated with the prior art and provide an improved method for accessing and producing gas from an underground coal seam.

According to one aspect of the invention there is provided an apparatus for maintaining the downhole ends of at least two concentrically spaced apart casings in a fixed position relative to each other during underbalanced drilling, the apparatus comprising: a first sleeve adapted to engage the downhole end of an outer casing and including an inwardly directed spacing member, a second sleeve comprising an outwardly directed spacing member which engages upon the inwardly directed spacing member, 16/03/06 whereby the second sleeve spaces the outer casing concentrically apart from an inner casing.

Preferably the second sleeve is welded to the inner casing.

In a preferred embodiment the inwardly directed spacing member may be an inwardly inclined surface and the outwardly directed spacing member may be an outwardly inclined surface, the surfaces being adapted to abuttingly interengage with pressure.

It is preferred that the outer casing is a production casing having a threaded downhole end adapted to screwably engage a threaded end of the first sleeve.

In a preferred form the inner casing is a temporary casing.

According to another aspect of the invention there is provided a method for accessing and producing gas from an underground coal seam, comprising the steps of:drilling a wellbore through to below the coal seam, introducing a casing assembly for internally lining the wellbore, providing a casing wedge within the casing assembly at a predetermined location corresponding to the coal seam, providing a drill string through the casing assembly, the drill string including a drill bit at its downhole end, manipulating the drill string within the casing assembly so that the drill bit exits the casing assembly at the predetermined location so as to enter the coal seam, whereby the downhole end of the casing assembly provides a location at which water draining from the coal seam can collect, pumping the collected water to the surface, and collecting gas that is produced from the coal seam.

According to yet another aspect of the invention there is provided a sub-surface support assembly for a wellbore within which a drill string is 16/03/06 located, comprising an outer surface casing, an inner production casing concentrically spaced apart from the outer surface casing to define an annular passageway communicating between a top entry location for pressurised gas and a bottom exit location for the gas, the exit location comprising one or more openings through the inner production casing, whereby the gas exiting the or each opening can rise through the wellbore between the drill string and the production casing.

Preferably the gas is air.

It is preferred that the or each opening is an elongated slot. Where there are a plurality of elongated slots, the slots may be in a regular, staggered and non overlapping arrangement.

According to a still further aspect of the invention there is a provided a method of constructing a sub surface support assembly for a wellbore, comprising the steps of:drilling a wellbore to a depth at above or below a coal seam, introducing through the wellbore an outer surface casing and an inner production casing concentrically spaced apart from the outer surface casing to define a first annular passageway therebetween, introducing cement through the production casing to a downhole end thereof and causing the cement to fill a second annular passageway between the wellbore and the production case, concurrently with step causing pressurised water to enter the first annular passageway whereby the cement is prevented by pressure of the water from filling the first annular passageway and thereby sets only in the second annulus.

According to another aspect of the invention there is provided a sub surface support assembly for a wellbore, within which a drill string is located, comprising a production casing apparatus lining the wellbore, the apparatus 16/03/06 including an upper portion comprising a larger diameter casing and a smaller diameter casing interconnected at their respective lower ends by the upper ends of a tapered sub, the tapered sub being connected at its lower end to a lower portion of the apparatus comprising a smaller diameter production casing, the smaller diameter casing located above the tapered sub having one or more openings for passage of pressurised gas therethrough from between the smaller and larger diameter casings to an annular passageway between the drill string and the smaller diameter casing located above the tapered sub.

In order that the invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings in which:- Fig. 1 is a part sectional schematic diagram of a system for accessing and producing gas from an underground coal seam according to one embodiment of the invention, Fig. 2 is a sectional schematic diagram of a system for accessing and producing gas from an underground coal seam according to a second embodiment of the invention, Fig. 3 is a sectional schematic diagram of a system for accessing and producing gas from an underground coal seam according to a third embodiment of the invention, Fig. 4 is a sectional schematic diagram of a system for accessing and producing gas from an underground coal seam according to a fourth embodiment of the invention, Fig. 5 is a front elevational view of a milling and cleaning tool for use with the invention, Fig. 6 is a front elevational view of a drilling tool for use with the invention, 16/03/06 Fig 7 is a cross sectional view of an internal sleeve, Fig. 8 is a cross sectional view of an external sleeve, Fig. 9 is a perspective view of the external sleeve assembled with a length of temporary casing, Fig. 10 is a perspective sectional view of the sleeve assembly for use with the invention, and Fig. 11 is a perspective view of the slotted portion of a production tube for use with the invention.

According to one embodiment of the invention a concentric casing method is provided to facilitate underbalanced drilling as depicted in Fig. 1 and Fig. 2. This method has the advantage of being able to limit the injection point to a depth that can be easily handled by a single compressor without the use of boosters to increase pressure. Further as it does not involve sending the gas/air down the drill string an underbalanced pressure state can be maintained even during the addition of drill pipes to the drill string. Also a sleeve assembly has been invented to allow the temporary casing to be seated securely on the production casing whilst drilling. This is useful as the air/gas injection process will cause the temporary casing to shake if it is not held in place. Due to this shaking the casing will unscrew itself downhole. A cleaning and milling tool has been provided which matches with the sleeve assembly.

This tool is used to clean out the sleeve assembly and when connected to the drilling tool allows for a lead hole to be drilled (of approximately 1-3m) when the cement job has been completed.

In this method a substantially vertical wellbore is drilled using a drill string that includes a downhole motor and bit. The downhole motor is used to control the orientation and direction of the wellbore. A measurement while drilling (MWD) device is positioned in the string to allow information on the 16/03/06 position of the drill string to be transmitted to personnel on the surface. A substantially vertical well should have an inclination close to 0 0 The inclination of the wellbore is the measurement of the angle of deviation from the vertical.

As the shallow section of most wells is generally unconsolidated and in order to prevent the sides of the wellbore from caving in, a sub-surface support assembly for a wellbore is constructed.

Firstly, a conductor pipe 10, having a length of 10-15 metres is set in this section with cement 12. Drilling is continued in order to set a surface casing 14. Once the desired depth is reached, the drill string is removed and the surface casing 14, is then inserted into the wellbore. The surface casing is then cemented in place using conventional techniques resulting in a layer of cement 16. The surface casing, once set in place, is used to protect fresh water aquifers and to provide pressure integrity so that a diverter or blowout preventer (BOP) (not shown) may be connected to it. The surface casing 14 also provides structural strength so that additional casing strings and other drilling devices may be suspended within it.

After sufficient time has been allowed for the cement to cure, drilling of the substantial vertical wellbore is continued. At point 18, the inclination of the wellbore is increased or the "build section" started to define a lateral wellbore portion at an appropriate rate according to predetermined parameters and predrilling calculations. Drilling of the wellbore continues along the desired drill path which is specified by a measured depth, inclination and azimuth or easting, northing and true vertical depth (TVD).

If the system is to target a single coal seam the desired drill path will terminate some 2-5 metres vertically above the coal seam as depicted in Fig.

1. If multiple coal seams are to be targeted or a deviated sump utilised (to be described later), it will extend through the seam(s) as depicted in Fig. 2.

16/03/06 Once the desired drill path to the target seam has been created the production casing 20, is installed down the length of the drill path. The production casing 20 will have an internal sleeve 22 (see Fig. 7) threadably attached to its bottom for the purpose of supporting the smaller, temporary casing 24 which is inserted after the production casing 20 is cemented into the wellbore.

After cementing, a milling and cleaning tool (Fig. 5) used in conjunction with a drilling tool (Fig. 6) connected to the downhole end of the milling and cleaning tool is used to clean the bottom of the production tube and internal sleeve, to mill cement plugs and to drill a 1 3m lead hole which allows any extension of the soon to be inserted temporary casing to pass through and into.

An external sleeve 26 is welded onto a lower portion of the temporary casing 24 which is held abuttingly with pressure against the internal sleeve 22 of the production casing 20 when in use. Fig 9 depicts a length of temporary casing which has had the external sleeve connected to it. The connection can be made by various means including welding, or alternatively the sleeve can be preformed at the time the casing is created. The length of temporary casing that extends beyond the external sleeve can be modified to suit the particular purpose, and to suit the length of any extension drilled with the drilling tool described above. Should there be a requirement that no extension be drilled during the milling and cleaning run (for example for use with a deviated sump to be described below) the length of production tube that extends beyond the external sleeve 26 can be minimised to an extent that it fits the space defined by the internal sleeve 22 as depicted in Fig. There are also slots 28 in the temporary casing 24 at depths which will enable a single compressor to be used to inject the requisite amount of gas/air under sufficient pressure. Figure 11 depicts the relative size and spacing of 8 16/03/06 slots which extend in a staggered fashion around a 6 metre length of temporary casing. The slots depicted in Fig. 11 are approximately 100-150 mm in length and approximately 2-5 mm in width. This particular arrangement of slots 28, has been found sufficient to provide a UBD system where air injection pressure is approximately 250psi and the depth at which the slots 28 are located approximately 150-170 metres underground (depending on the density of the drilling fluid). The shape, size and orientation of the holes were arrived at so as to minimise any reduction in structural integrity. In particular the feature of regular, staggered and non-overlapping orientation of the slots is particularly important to maintain structural integrity.

A single compressor will inject air into the annulus 30 between the production casing 20 and the temporary casing 24. The air passes through the slots 28 into an annulus 32 between the temporary casing 24 and the drill string 34.

Drilling continues into the seam and thereafter substantially horizontally inseam in an underbalanced manner. In the embodiment of the invention depicted in Fig. 1 the drilling continues inseam until it reaches and intersects with a vertical sump 36, which is created by pre-drilling a vertical wellbore at least 70m (horizontal displacement) from the point where the lateral wellbore portion first encounters the coal seam. This vertical sump 36, is drilled to a depth below the seam of coal 38, providing space for the positioning of a downhole pump (not shown). The downhole pump is used to remove water from within the coal seam before production can begin.

The preferred embodiment shown in Fig. 2 is substantially similar to that shown in Fig. 1 except that it utilises a deviated sump 40, located beneath the coal seam 38. The advantages in using a deviated sump include eliminating the need for a second shaft. This then allows the drilling operator to maintain a 16/03/06 very small footprint on the surface thereby facilitating production on sites where there is uneven terrain, environmental sensitivity or limited access.

The method involved in this embodiment is substantially the same as was described with respect to the first embodiment. The most significant differences occur during the build section of the method. When the inclination of the wellbore reaches 70 to 80 degrees, the angle is held for a predetermined length, or until the coal seam is completely drilled through and a region below the coal seam 38 reached. After this stage of the drilling has been completed the production tube 20 and internal sleeve 22 are put into place and cemented as in the previous example. However after cementing, only the milling and cleaning tool depicted in Fig. 5 is lowered into the wellbore as the drilling tool is not required for the purposes of extending past the end of the internal sleeve 22. Once the wellbore and internal sleeve have been cleaned the temporary casing 24 and external sleeve 26 assembly are introduced and seated abuttingly with pressure against the internal sleeve 22. After this has occurred the drill string is put into the point where the angle of 70-80 degrees is first achieved. A casing wedge 44 located within the temporary casing is then used to deflect the drill string so it can exit the casing assembly and then continue building through the coal seam 38, as depicted in Fig. 2. After drilling into the seam is completed the drill string 34 and the temporary casing 24 and external sleeve 26 assembly are removed. A downhole pump (not shown) is then inserted into sump 40 so that it can remove water draining down from the coal seam thereby facilitating production therefrom.

Another preferred embodiment provides for the safe, efficient, cost effective method of air injection during drilling, as depicted in Fig. 3. The advantages of this system are to be found during the setting up for UBD drilling. In particular this system does not rely on the addition of a temporary 16/03/06 12 casing inside the production casing. This method is therefore quicker and easier to set up which results in a reduction of costs associated with drilling.

As in the previous example the conductor pipe 10, and surface casing 14 are installed to a depth determined by pre-drilling design and are cemented into the wellbore using conventional cementing techniques. The surface casing 14 is then installed to the predetermined depth and cemented into the wellbore.

A production casing 20 which has slots 28 at positions determined by predrilling calculations (see Fig. 11) and a stab in float shoe attached to its bottom (not shown) is then introduced into the wellbore.

A cementing assembly consisting of a drill string threadably connected to a stab-in latch-in stinger and, optionally, drillpipe centralisers, is sent down the wellbore. The cementing assembly is lowered inside the production casing until it reaches bottom. It is then connected to the float shoe. Connection of the stinger and shoe is confirmed by pulling on the drill string. If it disengages connection is re-attempted. Once connection is achieved cement mixture is pumped down the drill string and through the float shoe and up the annulus 45. At the same time water is pumped under pressure into annulus 46 formed between the production casing and the surface casing. The production casing has slots as depicted in Fig. 11 which allows water to flow from the annulus 46 up and out of the wellbore through annulus 32.

A pre-flush mixture containing sodium acid pyrophosphate (SAAP) may be circulated just before the cement. SAAP is used to breakdown any mud in the borehole. Once the cement has been fully displaced from the drill string, pressure is increased in the drill string to hold the cement plug in place which was used to push the cement into annulus 45. After 3 minutes the cementing assembly is removed by unscrewing the stab-in latch-in stinger, and the cement is allowed to set.

16/03/06 13 The end result is an annulus between the surface casing 14 and the production casing 20 which extends to at least the slots and may extend below the slots, in which event the annulus 46 below the slots is filled with cement 47.

A compressor is connected to inject air into the annulus 46 between the surface casing 14 and the production casing 20. The air will circulate through the slots 28 in the production casing 20 and back up to the surface through the annulus 32 between the production casing 20 and the drill string 34.

The embodiment depicted in Figure 3 features a deviated sump as described with respect to the embodiment shown in Fig. 2. However as with all of the embodiments described herein the drilling operator is free to incorporate the separate vertical shaft as sump approach depicted in Fig. 1 or indeed any sump or structure which has the effect of lowering the water levels to below that of the coal seam.

A further embodiment of the invention is shown in Fig. 4. This embodiment is similar to the embodiment depicted in Fig. 3 however it requires even less casing and a less complex cementing method over the previous example and therefore has added cost benefits.

The conductor pipe 10 and surface casing 14 is installed are a depth determined by pre-drilling calculations and is cemented into the wellbore through conventional means. A production casing assembly is comprised of a larger diameter casing, 54 connected to a smaller diameter casing 28 and a tapered sub 56. The length and size of each casing section will be specified during the engineering phase The production casing assembly is then cemented into the wellbore using conventional techniques resulting in the concrete structure 48. Once the cementing is completed a string of casing 58 of the same size as the smaller casing section 28 is put into the wellbore inside the bigger section and connected to the tapered sub at point 60. This smaller 16/03/06 14 length of casing will have slots 62. The depth of the slots 62 will be determined by the amount of air or gas required to be injected into the wellbore.

A compressor (not shown) is connected to inject air into the annulus between the bigger production casing and the inner casing length. The air will circulate through the slots 62 in the smaller casing 34 and back up to the surface through the annulus 52 between the smaller casing 58 and the drill pipe 34.

As in the case of the previous embodiment the present embodiment has been described with a deviated sump however it works equally as well with the previously described vertical shaft as sump approach.

Various modifications may be made in details of design and construction without departing from the scope or ambit of the invention.

16/03/06

Claims (9)

1. An apparatus for maintaining the downhole ends of at least two concentrically spaced apart casings in a fixed position relative to each other during underbalanced drilling, the apparatus comprising: a first sleeve adapted to engage the downhole end of an outer casing and including an inwardly directed spacing member, a second sleeve comprising an outwardly directed spacing member which engages upon the inwardly directed spacing member, whereby the second sleeve spaces the outer casing concentrically apart from an inner casing.

2. The apparatus of claim 1 wherein the second sleeve is welded to the inner casing.

3. The apparatus of claim 1 wherein the the inwardly directed spacing member may be an inwardly inclined surface and the outwardly directed spacing member may be an outwardly inclined surface, the surfaces being adapted to abuttingly interengage with pressure.

4. The apparatus of claim 3 wherein the outer casing is a production casing having a threaded downhole end adapted to screwably engage a threaded end of the first sleeve.

The apparatus of claim 4 wherein the inner casing is a temporary casing.

6. A method for accessing and producing gas from an underground coal seam, comprising the steps of:- 16/03/06 drilling a wellbore through to below the coal seam, introducing a casing assembly for internally lining the wellbore, providing a casing wedge within the casing assembly at a predetermined location corresponding to the coal seam, providing a drill string through the casing assembly, the drill string including a drill bit at its downhole end, manipulating the drill string within the casing assembly so that the drill bit exits the casing assembly at the predetermined location so as to enter the coal seam, whereby the downhole end of the casing assembly provides a location at which water draining from the coal seam can collect, pumping the collected water to the surface, collecting gas that is produced from the coal seam.

7. A sub-surface support assembly for a wellbore within which a drill string is located, comprising an outer surface casing an inner production casing concentrically spaced apart from the outer surface casing to define an annular passageway communicating between a top entry location for pressurised gas and a bottom exit location for the gas, the exit location comprising one or more openings through the inner production casing, whereby the gas exiting the or each opening can rise through the wellbore between the drill string and the production casing.

8. A method of constructing a sub surface support assembly for a wellbore, comprising the steps of:- drilling a wellbore to a depth at above or below a coal seam, 16/03/06 I introducing through the wellbore an outer surface casing and an inner production casing concentrically spaced apart from the outer surface casing to define a first annular passageway therebetween, introducing cement through the production casing to a downhole end thereof and causing the cement to fill a second annular passageway between the wellbore and the production case, concurrently with step causing pressurised water to enter the first annulas whereby the cement is prevented by pressure of the water from filling the first annulus and thereby sets only in the second annulus.

9. A sub surface support assembly for a wellbore, within which a drill string is located, comprising a production casing apparatus lining the wellbore, the apparatus including an upper portion comprising a larger diameter casing and a smaller diameter casing interconnected at their respective lower ends by the upper ends of a tapered sub, the tapered sub being connected at its lower end to a lower portion of the apparatus comprising a smaller diameter production casing, the smaller diameter casing located above the tapered sub having one or more openings for passage of pressurised gas therethrough from between the smaller and larger diameter casings to an annular passageway between the drillstring and the smaller diameter casing located above the tapered sub. Dated this 16 day of March 2006 AJ Lucas Group Limited Patent Attorneys for the Applicant PETER MAXWELL ASSOCIATES 16/03/06