WO2011000022A1

WO2011000022A1 - A method of extracting methane from a coal deposit

Info

Publication number: WO2011000022A1
Application number: PCT/AU2010/000310
Authority: WO
Inventors: Scott Thomson
Original assignee: Methane Technologies Pty Ltd
Priority date: 2009-07-01
Filing date: 2010-03-18
Publication date: 2011-01-06
Also published as: AU2010268745A1

Abstract

The present invention relates generally to a method of extracting methane from a coal deposit involving the following steps: 1. Drilling generally vertically into a coal bed methane (CBM) deposit (10) to form a vertical well (12); 2. Drilling directionally off the vertical well (12) into a coal seam (14) to form two (2) in-seam bore holes (16) and (18) respectively; 3. Depositing pre-treatment materials, nutrients and possibly microbial matter (depicted by the arrow (20)) into one of the bore holes (16) via the vertical well (12); 4. Extracting biogenically enhanced methane (depicted by the arrow (22)) from the coal deposit (10) via the other bore hole (18) and the vertical well (12).

Description

A METHOD OF EXTRACTING METHANE FROM A COAL DEPOSIT

FIELD OF THE INVENTION

The present invention relates broadly to a method of extracting methane from a coal deposit.

BACKGROUND TO THE INVENTION

The use of directional drilling for coal seam gas production is reasonably well established. Most of this work has been conducted in the relatively high rank black coals of Australian Permian basins. This technique has not been applied to lower rank coals such as lignites. Bio enhancement of coal seams is in its infancy but some trial work has taken place in the Powder River Basin in the USA, and the process is known to occur in geological time, namely over many thousands of years. Bio enhancement - the production of methane through the action of methanogens and nutrients introduced by groundwater - is common in Australian and international coal seams. Some of the major successful coal seam gas plays have been predicated on at least some bio enhancement of coal.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a method of extracting methane from a coal deposit, said method comprising the steps of:

drilling substantially vertically into a coal deposit to form one or more vertical wells;

drilling directionally off said one or more vertical wells into one or more coal seams of said deposit to form one or more in-seam boreholes;

depositing of microbial matter into at least one of the in-seam boreholes via at least one of the vertical wells thereby enhancing methane production;

extracting biogenically enhanced methane from the coal deposit, said extraction being via said one or more of the vertical wells.

According to another aspect of the present invention there is provided a method of extracting methane from a coal deposit, said method comprising the steps of:

drilling directionally off said one or more vertical wells into one or more coal seams of said deposit to form one or more in-seam boreholes; depositing of nutrients into at least one of the in-seam boreholes via at least one of the vertical wells thereby enhancing methane production;

extracting biogenically enhanced methane from the coal deposit, said extraction being via said one or more of the vertical wells. According to a further aspect of the present invention there is provided a method of extracting methane from a coal deposit, said method comprising the steps of:

depositing of pre-treatment materials into at least one of the in-seam boreholes via at least one of the vertical wells thereby enhancing methane production;

Preferably the method comprises sequential steps of depositing the pre-treatment materials and the nutrients into the in-seam boreholes. More preferably the method also comprises the step of depositing the microbial matter following the sequential deposition of the pre-treatment materials and the nutrients. Preferably the method also comprises the step of promoting the passage of the pre- treatment materials, nutrients or microbial matter across the coal seam between a pair of adjacent in-seam boreholes. More preferably this step of promoting the passage of this injected material is effected by reducing the pressure in one of the pair of said in-seam boreholes. Preferably the steps of drilling vertically and drilling directionally are performed

continuously in the form of directional surface to in-seam drilling.

Preferably the step of extracting the biogenically enhanced methane involves pumping from at least one of the vertical wells to provide a pressure gradient for diffusion of the methane from the adjacent coal seams. More preferably or additionally the extraction step also involves sequestering CO₂ to the coal seams to aid development of the biogenically enhanced methane. Even more preferably the extraction step involves a preparatory step of dewatering the coal deposit by pumping water from the deposit.

Generally the microbial matter includes methanogens such as bacteria and archaea. Generally the pre-treatment materials include caustic materials.

Generally the coal deposit includes lignite, peat or sub-bituminous coal and oil shale.

BRIEF DESCRIPTION OF THE FIGURES

In order to achieve a better understanding of the nature of the present invention, a preferred embodiment of a method of extracting methane from a coal deposit will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic sectional view of one embodiment of a preferred methodology for extracting methane from a coal deposit; and Figure 2 is a schematic plan view of another embodiment of extracting methane from a coal deposit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in Figures 1 and 2, a preferred methodology of extracting methane from a coal deposit involves the following general steps: 1. drilling generally vertically into a CBM deposit 10 to form a vertical well 12;

2. drilling directionally off the vertical well 12 into a coal seam 14 to form two (2)

in-seam boreholes 16 and 18 respectively;

3. depositing pre-treatment materials, nutrients and possibly microbial matter

(depicted by the arrow 20) into one of the boreholes 16 via the vertical well 12; 4. extracting biogenically enhanced methane (depicted by the arrow 22) from the coal deposit 10 via the other borehole 18 and the vertical well 12.

In this particular embodiment, the coal deposit 10 is a lower rank coal such as a lignite deposit. The invention may extend to other coal deposits such as a coal bed methane deposit. The coal deposit may also extend to peat or sub-bituminous coals and oil shale. The deposition steps in step 3 are generally performed separately and sequentially. The pre-treatment materials include caustic materials. The microbial matter includes methanogens such as methanogenic bacteria and archaea which provides enhanced biogenic treatment in the lignite deposit 10.

In this example the two in-seam boreholes 16 and 18 are positioned relative to one another to exploit the natural permeability anisotropy of the lignite deposit 10. The boreholes 16/18 of Figure 1 are thus lined up vertically to maximise the methanogenic process using natural vertical pathways (such a joints depicted by arrows 24). In the embodiment of Figure 2, the boreholes 16/18 are lined up horizontally to maximise production along natural horizontal pathways (bedding planes depicted by arrows 26). For ease of reference the same reference numerals have been used to designate

corresponding "components" of the alternate embodiments of Figures 1 and 2.

In this embodiment the deposition of the pre-treatment materials and optionally the methanogenic bacteria into the borehole 16 is effected as a pre-treatment and injection steps via the vertical well 12. The borehole 16 thus functions as a pre-treatment and injection well. The injection of these materials into the borehole 16 may involve a gravity feed system alone or a combination of gravity and pumping from the vertical well 12 head. The injected materials then travel directionally across the coal seam such as 14 via gravity to the other adjacent in-seam borehole 18. The materials migrate directionally with the assistance of gravity in the general direction of maximum permeability of the lignite deposit 10. It will be appreciated that the lignite deposit 10 and associated coal seam 14 of Figure 1 has maximum vertical permeability whereas the coal seam depicted in

Figure 2 has "seam dip" with maximum horizontal permeability.

In each of the examples of Figures 1 and 2, drilling of the pre-treatment and injection borehole 16 and the capture borehole 18 is provided in order to maximise the natural permeability and anisotropy of the lignite deposit 10. The borehole 16 provides the pre-treatment and injection pathway and the other borehole 18 provides the capture pathway. The migration of the injected materials between the pair of boreholes 16 and 18 is promoted by reducing the pressure in the capture borehole 18. This pressure reduction is most typically effected by using a vacuum pump installed at the head of the vertical well 12.

In this embodiment extracting the biogenically enhanced methane from the lignite deposit involves the following steps:

1. de-watering of the coal seam such as 14 where water which is formed as a

by-product of coalification is pumped from the lignite deposit 10; 2. biogenically enhanced methane is extracted by continued pumping to create the required pressure gradient for diffusion of methane from the coal seam such as 14 into the capture borehole 18 and the vertical well 12. This extraction of biogenically enhanced methane may also involve pumping or sequestering carbon dioxide to the coal seam such as 14 to aid development of the biogenically enhanced methane.

Now that a preferred embodiment of the invention has been described, it will be apparent to those skilled in the art that the method of extracting methane from a coal deposit has the following advantages:

1. the methodology provides an effective way of recovering biogenically enhanced methane from lignite and other coal deposits using the natural permeability anisotropy of the deposit; 2. effective methane extraction can be provided by combining directional in-seam drilling with biogenic enhancement of the coal deposit;

3. the process may utilise pre-existing vertical wells which are effective coupled by directional in-seam drilling of sub-parallel directional boreholes;

4. the process is relatively fast, compared to natural bio enhancement, by injecting for example methanogens and nutrients into coal seams to promote this otherwise natural reaction to occur in real time;

5. the process integrates the highly specialised discipline of directional drilling

technology and presents it as a delivery and capture mechanism suitable for a relatively fast bio enhancement process.. Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. For example, the orientation and disposition of the wells and boreholes may vary from that described and illustrated provided effective deposition and migration of the injected materials and subsequent extraction of biogenically enhanced methane can be effected. All such variations and modifications are to be considered within the scope of the present invention, the nature of which is to be determined from the foregoing description.

Claims

1. A method of extracting methane from a coal deposit, said method comprising the steps of:

2. A method of extracting methane from a coal deposit as defined in claim 1 also comprising the step of depositing nutrients into the in-seam boreholes.

3. A method of extracting methane from a coal deposit as defined in claim 2 wherein the steps of depositing the pre-treatment materials and the nutrients into the in-seam boreholes are performed sequentially.

4. A method of extracting methane from a coal deposit as defined in claim 3 wherein the step of depositing the microbial matter into the in-seam boreholes follows the sequential deposition of the pre-treatment materials and the nutrients.

5. A method of extracting methane from a coal deposit, said method comprising the steps of:

depositing of nutrients into at least one of the in-seam boreholes via at least one of the vertical wells thereby enhancing methane production;

6. A method of extracting methane from a coal deposit, said method comprising the steps of:

drilling substantially vertically into a coal deposit to form one or more vertical wells; drilling directionally off said one or more vertical wells into one or more coal seams of said deposit to form one or more in-seam boreholes;

7. A method of extracting methane from a coal deposit as defined in any one of the preceding claims also comprising the step of promoting the passage of the pre-treatment materials, nutrients or microbial matter across the coal seam between a pair of adjacent in-seam boreholes.

8. A method of extracting methane from a coal deposit as defined in any one of the preceding claims wherein this step of promoting the passage of this injected material is effected by reducing the pressure in one of the pair of said in-seam boreholes.

9. A method of extracting methane from a coal deposit as defined in any one of the preceding claims wherein the steps of drilling vertically and drilling directionally are performed continuously in the form of directional surface to in-seam drilling.

10. A method of extracting methane from a coal deposit as defined in any one of the preceding claims wherein the step of extracting the biogenically enhanced methane involves pumping from at least one of the vertical wells to provide a pressure gradient for diffusion of the methane from the adjacent coal seams.

11. A method of extracting methane from a coal deposit as defined in any one of the preceding claims wherein the extraction step also involves sequestering CO₂ to the coal seams to aid development of the biogenically enhanced methane.

12. A method of extracting methane from a coal deposit as defined in any one of the preceding claims wherein the extraction step involves a preparatory step of dewatering the coal deposit by pumping water from the deposit.

13. A method of extracting methane from a coal deposit as defined in claim 1 wherein the microbial matter includes methanogens including bacteria and archaea.

14. A method of extracting methane from a coal deposit as defined in claim 6 wherein the pre-treatment materials include caustic materials.

15. A method of extracting methane from a coal deposit as defined in any one of the preceding claims wherein the coal deposit includes lignite, peat or sub-bituminous coal and oil shale.