US20170044887A1

US20170044887A1 - Multi purpose drilling system and method

Info

Publication number: US20170044887A1
Application number: US15/304,136
Authority: US
Inventors: Jan Benjamin Pieter Coetzee
Original assignee: Peabody Energy Australia
Current assignee: Peabody Energy Australia Pty Ltd; Peabody Energy Australia
Priority date: 2014-04-14
Filing date: 2015-03-24
Publication date: 2017-02-16
Also published as: AU2015246646A1; AU2015246646B2; WO2015157812A1

Abstract

A method of forming a multi purpose drilling for degassing a coal seam that is to mined, the method including the steps of drilling a primary borehole from a surface into a zone above the coal seam, such that at least a portion of the primary borehole extends substantially parallel to the coal seam, and drilling at least one secondary borehole, wherein the least one secondary borehole extends from the primary borehole into the coal seam.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Stage Application under 35 U.S.C. §371 of PCT Application No. PCT/AU2015/050130, filed, March 24, 2015, which is incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to a multi purpose drilling system and method. The invention relates in particular, although not exclusively, to a method of forming a multi purpose drilling for degassing a coal seam that is to be mined.

BACKGROUND OF THE INVENTION

Before coal seams can be mined, they are typically degassed to remove coal seam gases which would otherwise be lost to the atmosphere and/or pose a danger to the mining operation. Additionally, the coal seam is typically degassed during development, production and after mining (goaf degassing). One prior art solution is to initially drill surface to in seam holes to pre drain the coal seam.
At a later stage, as longwall panels are being developed, underground to in seam holes are drilled from a maingate or a tailgate into a longwall panel to achieve effective drainage of the coal seam gases before and during production. These holes are typically connected to pipes in the maingate or the tailgate to convey the coal seam gases to the surface.
Prior and during production, vertical goaf gas drainage wells are drilled from the surface into a position where the wells can drain the goaf as the longwall is mined, these vertical wells are typically spaced 50 to 200 meters apart.
Although the above mentioned prior art solution is effective at draining coal seam gases prior to, during development, production and after mining, it requires a large amount of capital, effort and time, particularly over the life of a mine.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

OBJECT OF THE INVENTION

It is an object of the invention to overcome or at least alleviate one or more of the above problems and/or provide the consumer with a useful or commercial choice.
Other preferred objects of the present invention will become apparent from the following description.

SUMMARY 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 forming a multi purpose drilling for degassing a coal seam that is to be mined, the method including the steps of:
drilling a primary borehole from a surface into a zone above the coal seam, such that at least a portion of the primary borehole extends substantially parallel to the coal seam; and
drilling at least one secondary borehole;
wherein the at least one secondary borehole extends from the primary borehole into the coal seam.
Preferably the step of drilling the primary borehole involves drilling the primary borehole such that the majority of the primary borehole in the zone above the coal seam extends substantially parallel to the coal seam.
Normally the step of drilling the primary borehole is performed prior to the step of drilling the at least one secondary borehole. A skilled addressee will understand that in one embodiment, the at least one secondary borehole can be drilled before the primary borehole. For example a secondary borehole is drilled from the surface through a surface casing and into the coal seam, and a primary borehole is then drilled through the surface casing such that it extends from the secondary borehole into the zone above the coal seam (although the secondary borehole was drilled first, it still extends from the primary borehole).
Preferably the zone is a primary caving zone. More preferably the zone is up to 2.5 times the height of the coal seam, above the coal seam. For example, if the height of the coal seam is 6 meters, the zone is up to 15 meters above the coal seam. In a preferred embodiment, the zone is between 5 to 40 meters above the coal seam. Preferably the zone is between 5 to 25 meters above the coal seam. More preferably, the zone is between 10 to 15 meters above the coal seam.
Preferably at least a portion of the primary borehole extends parallel or at least at a slight angle to horizontal. For example, if a coal seam extends horizontally, at least a portion of the primary borehole may extend parallel to −5° to the coal seam. A benefit of at least a portion of the primary borehole extending parallel or at a slight angle to horizontal is that any water or other liquids in that portion of the primary borehole may be better controlled.
Preferably the primary borehole is not drilled into an upper coal seam (i.e. a coal seam above the coal seam that is to be mined) in a direction substantially parallel to the upper coal seam.
Preferably a portion of the primary borehole is drilled such that it has a larger diameter than the average diameter of the at least one secondary borehole. For example, the average diameter of the at least one secondary borehole may be in the range of approximately 180 mm (˜7″) to 300 mm (˜12″) and the diameter of the larger portion of the primary borehole may be in the range of approximately 300 mm (˜12″) to approximately 610 mm (˜24″).
Preferably the primary borehole is drilled such that the portion of the primary borehole extending substantially parallel to the coal seam is larger in diameter than a portion of the primary borehole adjacent to the surface. In one embodiment, the step of drilling the primary borehole involves drilling a pilot hole and reaming at least a portion of the pilot hole. A skilled addressee will understand that either an advance or retreat reaming method may be used to ream at least a portion of the pilot hole.
In one embodiment, the step of drilling at least one secondary borehole involves drilling one secondary borehole into the coal seam such that a majority of the secondary borehole in the coal seam extends substantially parallel to the coal seam. Preferably the method includes drilling branching holes from the secondary borehole that extend into the coal seam. A skilled addressee will understand that the branching holes extending from the secondary borehole into the coal seam can be in the form of any scatter branching as desired, such as, but not limited to, fan, parallel and/or herringbone patterns or the like or a combination thereof at predefined intervals.
In another embodiment, the step of drilling at least one secondary borehole involves drilling a plurality of secondary boreholes extending from the primary borehole into the coal seam. Preferably, the plurality of secondary boreholes are drilled such that they extend from the portion of the primary borehole that extends substantially parallel to the coal seam, and into the coal seam. Preferably, the plurality of secondary boreholes are drilled at predefined intervals. Preferably, the plurality of secondary boreholes are drilled in a pattern. For example, the plurality of secondary boreholes may be drilled in any scatter branching as desired, such as, but not limited to, fan, parallel and/or herringbone patterns or the like or a combination thereof at predefined intervals when viewed from above.
In another embodiment, the step of drilling at least one secondary borehole involves drilling at least one secondary pilot hole and reaming each of the at least one secondary pilot hole.
Preferably the method includes the step of providing a casing in the primary borehole. Preferably the step of providing a casing in the primary borehole involves providing a casing that is at least partially perforated in the portion of the primary borehole that extends substantially parallel to the coal seam. Preferably the casing is a fibreglass casing. Alternatively, the casing may be a steel or PVC casing. The casing may be a fire retardant and/or anti static casing. A benefit of providing a casing is that borehole stability is improved.
Preferably the method includes the step of providing at least one casing in the at least one secondary borehole. Preferably the at least one casing is at least one fibreglass casing. Alternatively, the at least one casing may be at least one steel or PVC casing. The at least one casing may be fire retardant and/or anti static. Preferably the at least one casing is at least partially perforated.
Preferably the method includes the step of drilling at least one well from the surface, each of which intersects with the primary borehole. Preferably each well is a vertical well. A skilled addressee will understand that a vertical well includes a well that is substantially vertical. Preferably there are a plurality of wells. More preferably the plurality of wells are spaced from each other. For example, the plurality of wells are spaced from each other by about 1000 meters. Preferably each well is adapted to convey coal seam gas to the surface.
In one embodiment, the step of drilling at least one well from the surface, each of which intersects with the primary borehole, involves drilling the at least one well such that each well also intersects with one of the at least one secondary borehole.
It will be appreciated that the step of drilling at least one well from the surface can be carried out before, during or after the primary borehole and/or the at least one secondary borehole is drilled.
Preferably the method includes the step of determining the location of the coal seam. Preferably, the method includes the step of determining the coal seam floor. More preferably, the method includes the step of determining the coal seam roof Preferably the method includes the step of determining the location of the zone above the coal seam into which the primary borehole is drilled.
In another form, the invention resides in a multi purpose drilling system for degassing a coal seam that is to be mined, the multi purpose drilling system comprising:
a primary borehole extending from a surface into a zone above the coal seam; and
at least one secondary borehole extending from the primary borehole into the coal seam;
wherein at least a portion of the primary borehole extends substantially parallel to the coal seam.
In one embodiment, a plurality of multi purpose drilling systems are used to degas a coal seam.
Further features of the invention will become apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

To assist in understanding the invention and to enable a person skilled in the art to put the invention into practical effect, preferred embodiments of the invention will be described by way of example only with reference to the accompanying drawings, wherein:

FIGS. 1a-1e show schematic views of various stages of a multi purpose drilling system according to an embodiment of the invention;

FIG. 2 shows a schematic plan view of part of the multi purpose drilling system of FIGS. 1a -1 e;

FIG. 3 shows a schematic view of a multi purpose drilling system according to an embodiment of the invention;

FIG. 3a shows a schematic plan view of a multi purpose drilling system according to an embodiment of the invention;

FIG. 4 shows a schematic view of two back to back multi purpose drilling systems;

FIG. 5 shows a schematic view of two facing multi purpose drilling systems;

FIG. 6 shows a schematic view two multi purpose drilling systems originating from the same location; and

FIG. 7 shows a schematic view of two overlapping multi purpose drilling systems.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1a -2 show a multi purpose drilling system 30. In particular, FIGS. 1a-1c show a sequence of drillings to create a multi purpose drilling system, FIGS. 1d-1e show the multi purpose drilling system 30 as the coal seam 12 is progressively mined and FIG. 2 shows a plan view of a portion of the multi purpose drilling system 30.
With reference to FIG. 1 a, a primary borehole 32 is drilled from the surface 10 to above the coal seam 12. The primary borehole 32 transitions to being substantially parallel to the coal seam 12 as it extends in a primary caving zone 14, which is located above the coal seam 12.
A perforated casing (not shown) is inserted into the primary borehole 32 (typically during the drilling of the multi purpose drilling system 30).
In FIG. 1 b, a number of secondary boreholes 36 are drilled such that they extend from the primary borehole 32 into the coal seam 12. The secondary boreholes 36 extend downwardly (as seen in FIG. 1b ) and outwardly (as seen in FIG. 2) from the primary borehole 32.
In use, the plurality of secondary boreholes 36 pre drain the coal seam 12 so that longwall panels (22 as seen in FIG. 2) can be developed. During development and production, the plurality of secondary boreholes 36 continue to drain the coal seam gases.
In FIG. 1c a vertical well 40 is drilled from the surface 10 such that it intersects with the primary borehole 32. The vertical well 40 is adapted to convey coal seam gases to the surface 10.
FIGS. 1d and 1e show the coal seam 12 being mined by a shearer 18, the shearer 18 and the hydraulic jacks 20 advance along the coal seam 12, creating a goaf 16. The coal seam gases from the goaf 16 are drained by the primary borehole 32.
It will be appreciated that the secondary boreholes 36 will no longer drain coal seam gases once the corresponding portion of the coal seam 12 has been mined by the shearer 18.
A benefit of having a perforated casing in the primary borehole 32 is that the primary borehole 32 remains operational for a longer time in the goaf 16 (compared to not having a casing in the primary borehole 32) as the primary caving zone 14 collapses into the goaf 16. A benefit of draining the goaf 16 using the primary borehole 32 is that not as many vertical wells 40 are required (compared to the prior art solution of drilling vertical wells every 50-200 meters).
FIG. 2 shows a schematic plan view of part of the multi purpose drilling system 30 in relation to a developed longwall panel 22, defined by a tailgate 24 and a maingate 26.
It will be appreciated that the multi purpose drilling system 30 is typically drilled before the tailgate 24 and maingate 26 are formed and before the longwall panels 22 are developed and the coal seam 12 is mined. The location of the maingate 26 and tailgate 24 are typically predetermined and the drilling system 30 is drilled such that the ends of the secondary boreholes 36 stop short of the respective maingate 26 or tailgate 24. The secondary boreholes 36 typically stop 5-10 m short of the respective maingate 26 or tailgate 24. A benefit of stopping short of the respective maingate 26 or tailgate 24 is that this prevents gas from escaping from the drilling system 30 into the maingate 26 or tailgate 24.
FIG. 3 shows a further embodiment of a multi purpose drilling system 130. The multi purpose drilling system 130 has a primary borehole 132 that is drilled from the surface 10 to above the coal seam 12. It will be appreciated that drilling can include reaming and that drilling the primary borehole 132 may include reaming a pilot hole.
The primary borehole 132 transitions to being substantially parallel to the coal seam 12. It will be understood that in the present specification, substantially parallel to the coal seam 12 includes ±5° to the coal seam 12. The portion of the primary borehole 132 that extends substantially parallel to the coal seam 12 is preferably 5 to 40 meters above the coal seam 12 depending on the strata and the permeability of the strata above the coal seam.
Similar to the embodiment shown in FIGS. 1A-1E, in the embodiment shown in FIG. 3, a perforated casing (not shown) is inserted into the primary borehole 132. A benefit of having a perforated casing is that the primary borehole 132 remains operational for a longer time after the coal seam 3 has been mined below it.
A secondary borehole 136 extends from the primary borehole 132 into the coal seam 12. Branching holes 142 extend from the secondary borehole 136 into the coal seam 12. A casing (not shown) is inserted into the secondary borehole 136. A benefit of casing the secondary borehole 136 is that the casing maintains borehole stability.
Typically the primary borehole 132 is drilled before the secondary borehole 136 is drilled. A skilled addressee will understand that in one embodiment, the secondary borehole 136 can be drilled before the primary borehole 132. An advantage of drilling the secondary borehole 136 first is that the coal seam roof can be accurately located (using for example gamma detection from the secondary borehole 136) such that a portion of the primary borehole 132 can be drilled so that it can extend substantially parallel to the coal seam 12.
The primary borehole 132 is typically drilled such that the portion of the primary borehole 132 that extends substantially parallel to the coal seam 12 has a larger diameter than the secondary borehole 136. For example, the average diameter of the secondary borehole 136 is approximately 205 mm (˜8″) and the larger diameter portion of the primary borehole 132 has a diameter of approximately 432 mm (˜17″). The larger diameter portion of the primary borehole 132 aids in draining the goaf (not shown) once the coal seam 12 below the primary borehole 132 has been mined.
Vertical wells 140 are drilled from the surface 10. The vertical wells 140 are spaced approximately 800-1000 meters apart. The vertical wells 140 intersect the primary borehole 132 and the secondary borehole 136. The vertical wells 140 assist in conveying the coal seam gases to the surface 10 as well as draining water and other liquids from the primary borehole 132 and the secondary borehole 136.
FIG. 3a shows a schematic plan view of part of the multi purpose drilling system 130 in relation to a developed longwall panel 22, defined by a tailgate 24 and a maingate 26. In this embodiment, it can be seen that the branching holes 142 extend from the secondary borehole 136 in a ‘herringbone’ pattern.
FIGS. 4, 5, 6 and 7 show various combinations of multi purpose drilling systems used to degas a coal seam 12. It will be appreciated that it can become increasingly difficult to push casing longer distances. For example it may be difficult to push casing further than 1800 meters. As some longwall panels are longer than 1800 meters, it may be necessary to provide more than one multi purpose drilling system to degas a coal seam 12.
FIG. 4 shows a tandem configuration, where two multi purpose drilling systems 230, 330 are drilled to degas a portion of the coal seam 12. The portion of the coal seam 12 to be degassed is a proposed longwall panel that extends from a main road 160 to an install road 162. The arrow 164 indicates the direction of mining, from the install road 162 towards the main road 160.
The first multi purpose drilling system 230 has a primary borehole 232 that is drilled from the surface 10 to above the coal seam 12 and a secondary borehole 236 that is drilled from the same location as the primary borehole 232 from the surface 10 into the coal seam 12. As can be seen, the secondary borehole 236 extends from the primary borehole 232. The primary borehole 232 and the secondary borehole 236 extend towards the install road 162.
The second multi purpose drilling system 330 has a primary borehole 332 that is drilled from the surface 10 to above the coal seam 12 and a secondary borehole 336 that is drilled from the same location as the primary borehole 332 from the surface 10 into the coal seam 12. The primary borehole 232 and the secondary borehole 236 extend towards the install road 162 from an area close to the ends of the primary borehole 232 and the secondary borehole 236 of the first multi purpose drilling system 230.
Vertical wells 240, 340, 344 are drilled from the surface 10 so that they intersect with a corresponding primary borehole 232, 332 and a corresponding secondary borehole 236, 336. The vertical wells 240, 340, 344 are spaced approximately 800-1000 meters from corresponding adjacent vertical wells 240, 340, 344. The vertical wells 240, 340, 344 convey coal seam gasses to the surface 10.
FIG. 5 shows a facing configuration, where two multi purpose drilling systems 430, 530 are drilled towards each other to degas a portion of the coal seam 12. The portion of the coal seam 12 to be degassed is a proposed longwall panel that extends from a main road 160 to an install road 162. The arrow 164 indicates the direction of mining, from the install road 162 towards the main road 160.
The first multi purpose drilling system 430 has a primary borehole 432 that is drilled from the surface 10 to above the coal seam 12 and a secondary borehole 436 that is drilled from the same location as the primary borehole 432 from the surface 10 into the coal seam 12. The primary borehole 432 and the secondary borehole 436 extend towards the install road 162 and intersect a vertical well 544.
The second multi purpose drilling system 530 has a primary borehole 532 that is drilled from the surface 10 to above the coal seam 12 and a secondary borehole 536 that is drilled from the same location as the primary borehole 532 from the surface 10 into the coal seam 12. The primary borehole 532 and the secondary borehole 536 extend towards the main road 160 and intersect the vertical well 544.
Additional vertical wells 440, 540 are drilled from the surface 10 so that they intersect with a corresponding primary borehole 432, 532 and a corresponding secondary borehole 436, 536. The vertical wells 440, 540, 544 are spaced approximately 800-1000 meters from corresponding adjacent vertical wells 440, 540, 544. The vertical wells 440, 540, 544 convey coal seam gasses to the surface 10.
FIG. 6 shows a combined configuration, where two multi purpose drilling systems 630, 730 are drilled away from each other to degas a portion of the coal seam 12. The portion of the coal seam 12 to be degassed is a proposed longwall panel that extends from a main road 160 to an install road 162. The arrow 164 indicates the direction of mining, from the install road 162 towards the main road 160.
The first multi purpose drilling system 630 has a primary borehole 632 that is drilled from the surface 10 to above the coal seam 12 and a secondary borehole 636 that is drilled from the same location as the primary borehole 632 from the surface 10 into the coal seam 12. The primary borehole 632 and the secondary borehole 636 extend towards the main road 160.
The second multi purpose drilling system 730 is drilled from the same location as the first multi purpose drilling system 630 and has a primary borehole 732 that is drilled from the surface 10 to above the coal seam 12 and a secondary borehole 736 that is drilled from the same location as the primary borehole 732 from the surface 10 into the coal seam 12. The primary borehole 732 and the secondary borehole 736 extend towards the install road 162.
Vertical wells 640, 644, 740, 744 are drilled from the surface 10 so that they intersect with a corresponding primary borehole 632, 732 and a corresponding secondary borehole 636, 736. The vertical wells 640, 644, 740, 744 are spaced approximately 800-1000 meters from corresponding adjacent vertical wells 640, 644, 740, 744. The vertical wells 640, 644, 740, 744 convey coal seam gasses to the surface 10.
FIG. 7 shows an ‘overlapping’ configuration (when viewed side on), where two multi purpose drilling systems 830, 930 are drilled towards each other and overlap to degas a portion of the coal seam 12. The portion of the coal seam 12 to be degassed is a proposed longwall panel that extends from a main road 160 to an install road 162. The arrow 164 indicates the direction of mining, from the install road 162 towards the main road 160.
The first multi purpose drilling system 830 has a primary borehole 832 that is drilled from the surface 10 to above the coal seam 12 and a secondary borehole 836 that is drilled from the same location as the primary borehole 832 from the surface 10 into the coal seam 12. The primary borehole 832 and the secondary borehole 836 extend towards the install road 162.
The second multi purpose drilling system 930 has a primary borehole 932 that is drilled from the surface 10 to above the coal seam 12 and a secondary borehole 936 that is drilled from the same location as the primary borehole 932 from the surface 10 into the coal seam 12. The primary borehole 932 and the secondary borehole 936 extend towards the main road 160.
Vertical wells 840, 844, 940, 944 are drilled from the surface 10 so that they intersect with a corresponding primary borehole 832, 932 and a corresponding secondary borehole 836, 936. The vertical wells 840, 844, 940, 944 are spaced approximately 800-1000 meters from corresponding adjacent vertical wells 840, 844, 940, 944. The vertical wells 840, 844, 940, 944 convey coal seam gasses to the surface 10.
Although it is counterintuitive, as the present invention involves more cost and a greater amount of work upfront compared to known prior art solutions, the applicant has found that the present invention can save costs, production time and require less work in the long run for pre drainage and drainage of coal seam gases during development, production and after mining (goaf degassing).
The foregoing embodiments are illustrative only of the principles of the invention, and various modifications and changes will readily occur to those skilled in the art. The invention is capable of being practiced and carried out in various ways and in other embodiments. For example, it will be understood that the invention can be used for multi seam gas extraction. It is also to be understood that the terminology employed herein is for the purpose of description and should not be regarded as limiting.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Claims

1. A method of forming a multi purpose drilling for degassing a coal seam that is to be mined, the method including the steps of:

drilling a primary borehole from a surface into a zone above the coal seam, such that at least a portion of the primary borehole extends substantially parallel to the coal seam; and

drilling at least one secondary borehole;

wherein the at least one secondary borehole extends from the primary borehole into the coal seam.

2. A method as claimed in claim 1, wherein the step of drilling at least one secondary borehole involves drilling one secondary borehole into the coal seam such that a majority of the secondary borehole in the coal seam extends substantially parallel to the coal seam.

3. A method as claimed in claim 2, further including the step of drilling branching holes from the secondary borehole that extend into the coal seam.

4. A method as claimed in claim 1, wherein the step of drilling at least one secondary borehole involves drilling a plurality of secondary boreholes such that they extend from the portion of the primary borehole that extends substantially parallel to the coal seam, and into the coal seam.

5. A method as claimed in any one of the preceding claims, wherein the step of drilling the primary borehole involves drilling the primary borehole such that the majority of the primary borehole in the zone above the coal seam extends substantially parallel to the coal seam.

6. A method as claimed in any one of the preceding claims, wherein a portion of the primary borehole is drilled such that it has a larger diameter than the average diameter of the at least one secondary borehole.

7. A method as claimed in any one of the preceding claims, wherein the primary borehole is drilled such that the portion of the primary borehole extending substantially parallel to the coal seam is larger in diameter than a portion of the primary borehole adjacent to the surface.

8. A method as claimed in any one of the preceding claims, wherein the step of drilling the primary borehole involves drilling a pilot hole and reaming at least a portion of the pilot hole.

9. A method as claimed in any one of the preceding claims, wherein the step of drilling at least one secondary borehole involves drilling at least one secondary pilot hole and reaming each of the at least one secondary pilot hole.

10. A method as claimed in any one of the preceding claims, further including the step of drilling at least one well from the surface, each of which intersects with the primary borehole.

11. A method as claimed in claim 10, wherein the step of drilling at least one well from the surface, each of which intersects with the primary borehole, involves drilling the at least one well such that each of the at least one well also intersects with one of the at least one secondary borehole.

12. A method as claimed in claim 10 or claim 11, wherein each of the at least one well is a vertical well.

13. A method as claimed in any one of the preceding claims, further including the step of providing a casing that is at least partially perforated in the portion of the primary borehole that extends substantially parallel to the coal seam.

14. A method as claimed in any one of the preceding claims, further including the step of providing a casing that is at least partially perforated in the at least one secondary borehole.

15. A multi purpose drilling system for degassing a coal seam that is to be mined, the multi purpose drilling system comprising:

a primary borehole extending from a surface into a zone above the coal seam; and

at least one secondary borehole extending from the primary borehole into the coal seam;

wherein at least a portion of the primary borehole extends substantially parallel to the coal seam.

16. A multi purpose drilling system as claimed in claim 15, wherein a majority of the secondary borehole in the coal seam extends substantially parallel to the coal seam.

17. A multi purpose drilling system as claimed in claim 16, wherein branching holes extend from the secondary borehole into the coal seam.

18. A multi purpose drilling system as claimed in claim 15, wherein a plurality of secondary boreholes extend into the coal seam from the portion of the primary borehole that extends substantially parallel to the coal seam.

19. A multi purpose drilling system as claimed in any one of claims 15 to 18, wherein the majority of the primary borehole in the zone above the coal seam extends substantially parallel to the coal seam.

20. A multi purpose drilling system as claimed in any one of claims 15 to 19, further comprising at least one well from the surface, each of which intersects with the primary borehole.

21. A multi purpose drilling system as claimed in any one of claims 15 to 20, further comprising a casing that is at least partially perforated in the portion of the primary borehole that extends substantially parallel to the coal seam.

22. A multi purpose drilling system as claimed in any one of claims 15 to 21, further comprising a casing that is at least partially perforated in the at least one secondary borehole.

23. A multi purpose drilling system as claimed in any one of claims 15 to 22, wherein the portion of the primary borehole extending substantially parallel to the coal seam is larger in diameter than a portion of the primary borehole adjacent to the surface.

24. A method or multi purpose drilling system as claimed in any one of the preceding claims, wherein the zone is a primary caving zone.

25. A method or multi purpose drilling system as claimed in any one of the preceding claims, wherein the zone is between 5 to 40 meters above the coal seam.

26. A method or multi purpose drilling system as claimed in any one of the preceding claims, wherein the zone is between 5 to 25 meters above the coal seam.

27. A method or multi purpose drilling system as claimed in any one of the preceding claims, wherein the zone is between 10 to 15 meters above the coal seam.