AU2011201486A1 - Extraction of coal seam methane - Google Patents

Abstract

A method of extracting hydrocarbon gas from a coal seam beneath a development area, including the steps of establishing a plurality of peripheral wells spaced around the periphery of a 5 development area and operating the peripheral wells to remove at least water from the coal seam in the development area; and establishing at least one production well within the development area to remove hydrocarbon gas; the hydrocarbon gas being desorbed from the coal seam by a reduced hydrostatic pressure in the coal seam caused by the water extracted from the 10 peripheral wells. Overall Layout, Peripheral Approach E ) Figure 5

Description

P/00/0 11 Regulation 3.2 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Extraction of coal seam methane The following statement is a full description of this invention, including the best method of performing it known to us: 2 Extraction of coal seam methane Field of the invention This invention relates to an improved method of extracting coal seam methane and in particular to a method which reduces field development costs. 5 Background of the invention Coal seam methane (CSM) is known by various names including coal bed methane and coal seam gas. It is extracted by removing some water from the coal seam, so reducing the hydrostatic pressure on a coal seam and allowing methane adsorbed within the coal to desorb and migrate to collection wells. The gas that evolves 10 is largely methane with small quantities of carbon dioxide, some nitrogen, and small quantities of higher hydrocarbons and is typically water vapour saturated. Conventionally, coal seam methane wells tend to be laid out in essentially regular field spacing (for example a square grid pattern). Individual wells might be relocated to move them to locations deemed more suitable (for example out of a creek, away from a 15 dwelling, towards the edge of a paddock rather than in the middle) or an occasional well even deleted from the pattern but from an overall perspective, the field layout remains essentially evenly distributed. This practice potentially started in the USA where individual rights are located on quite small property areas and it is desirable to populate wells to extract gas quickly from a 20 given area - especially as the adjacent holding may be operated by another company. Early coal seam methane development companies in Australia projected gas production of 200 MSCFD (200 thousand standard cubic feet per day) of gas per well, yet many Australian producers are reporting individual well flows in excess of 1 million SCFD and exceptional wells of 5 MMSCFD (5 million standard cubic feet per day). 25 It is observed that there seems to have been little modification to well layouts to take benefit of this high level of permeability present in Australian CSM fields.

3 A conventional coal seam methane development shown in figures 1-4 consists of multitude of wells laid out on an approximately square grid pattern. Figure 1 shows the wells, water piping and gas piping. Note that the paths for water, gas and roadways would normally follow geographical features such as existing roads or ridge lines and as 5 such the figures used in this description is highly simplified but is useful in contrasting the requirements for different approaches. All of the wells will have substantially the same configuration and equipment associated with them. For example, these wells might have 7 %" casing and a 3" central pipe for water flow. For a pumped well, there is normally a progressing cavity pump located near the base of the well which is driven by 10 rotating pump rod assembly within the 3" pipe from a surface-mounted drive assembly. For shallow wells, a submersible electric pump is sometimes used. The water is pumped from the well up the 3" pipe and is directed to a surface mounted gas/water separator. In a commonly encountered configuration, gas is allowed to flow up the annulus formed 15 between the water pipe and the bore casing. This gas, which can contain significant quantities of entrained water if the gas flow is sufficiently high, is also directed to the gas/water separator. Separate gas and water lines go from the water/gas separator to piping networks with gas ultimately being compressed and dewatered before entering the high pressure gas pipeline system. The water goes to dams for collection and may 20 be treated by reverse osmosis or other technologies prior to disposal. After a well has been operating for some time, the water production may decline and it can be converted to what is sometimes referred to in the industry as a "tubing free-flow" configuration. This means that the well has had its pump removed and a mixture of gas and water is allowed to flow up the central pipe. Thus, the gas acts as a gas-lift pump to 25 remove the water coming to the well from the surrounding coal seam. Alternatively for very high gas flows, the gas and water can be allowed to flow up the annulus alone. In this version, the pump might still be installed but may not be operating, and the well can continue to operate with the gas flow in the annulus providing sufficient water transfer to the surface.

4 A further version is possible where both the tubing and the annulus are used for transport of water and gas (water pump removed from the base of the tubing). An even higher flow version is possible which approaches a conventional gas well whereby there is no inner pipe and there is simply a flow of gas and water up the casing. A coal seam 5 methane well can, through its life, change behaviour through different types often starting with pumped operations, moving to higher gas production that ultimately does not need a pump and potentially back to pumped operations late in its life. Each of these changes comes with associated costs to change-out hardware. As each well in a conventional development is initially designed to operate substantially 10 the same, from each wellhead, there is a need to have water and gas piping systems, (figures 3 and 4 respectively), often some form of communications (eg fibre optic, radio link, etc) and access roads (figure 2). Noting again that the diagram is a highly simplified example of potential routes to illustrate the differences between approaches. Each well head, potentially requires an electrical supply (such as grid power, local spark 15 ignition engine or microturbine) to provide power for the down hole water pump, solar panels and back-up batteries for control and communications and a gas-liquid separator. The cost of all this piping, access and these facilities is very substantial as is the impact on the land upon which the CSM development is taking place. The applicant has found that contrary to conventional thinking, production wells can be 20 better positioned to take advantage of the connectivity found in the natural cleat systems found in individual coal seams. The production wells may be positioned in the development area at identified regions corresponding to structures within the coal seam favourable to gas phase transportation and gas extraction from the coal seam. If the production wells are positioned to take advantage of these characteristics, less 25 production wells are required to extract methane from a given development field. Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a 30 person skilled in the art.

5 Summary of the invention According to one aspect of the invention there is provided a method of extracting hydrocarbon gas from a coal seam beneath a development area, including the steps of establishing a plurality of peripheral wells spaced around the periphery of a 5 development area and operating the peripheral wells to remove at least water from the development area; and establishing at least one production well within the development area to remove hydrocarbon gas; the hydrocarbon gas being desorbed from the coal seam by a reduced hydrostatic pressure in the coal seam caused by the water extracted from the 10 peripheral wells. In a preferred form of the invention, the peripheral wells are operated to reduce the prevailing static pressure within the region defined within the peripheral wells and in a region a short distance outside each individual well region. This results in a development area having peripheral wells operating for a period of time to remove at 15 least water around the periphery of the area prior to the production wells being established. In another aspect of the invention there is provided an arrangement of wells for the extraction of hydrocarbon gas from a coal seam beneath a development area including a plurality of peripheral wells arranged around the periphery of the development area to 20 remove at least water from the development area; and at least one production well positioned within the development area and configured to extract hydrocarbon gas desorbed from the coal seam by reduced hydrostatic pressure in the coal seam caused by the water extracted from the peripheral wells, the average volumetric flow per well of hydrocarbon gas from the production well/s being greater 25 than the average volumetric gas flow per well from the peripheral wells. In a preferred form of the invention, arrangement of wells in the development region results in the development region comprising a peripheral well region consisting of the peripheral wells; at least one production region, each production region consisting of at 6 least one production well; and at least one vacant region within the boundary defined by the peripheral wells in which there are no wells. By positioning the wells according to the method of the invention, the development region has a number of production wells gathered together in a number production 5 regions of preferably 2 or more wells and vacant regions or regions of no wells. The configuration of wells and method of extracting gaseous hydrocarbon according to the invention can lead to substantial savings in the cost of facilities required to establish a coal seam methane operation. The core of the concept is to provide two sets of wells - the first set is laid out to largely envelope the area (referred to as peripheral wells) 10 whilst the second set is to provide high gas production wells. In one embodiment the peripheral wells are regularly spaced around the periphery of the development area. The peripheral wells have a major purpose of reducing the prevailing static pressure within the region defined within the peripheral wells though they will also reduce the pressure for a short distance outside the region they define. It is possible to have gas 15 production from these peripheral wells (and there would be locations where the peripheral wells and the high production wells are coincident), but in the most simplified version of the invention, the peripheral wells have no gas production or only very minor gas production (for example perhaps only sufficient to run a local spark ignition engine to provide pumping power). 20 Thus in one preferred form of the invention; the peripheral wells are configured such that the majority of the mass flow rate of fluid from the peripheral well is water and in a most preferred and simplest form of the invention, essentially all of the fluid removed from the peripheral wells is water. In the event that there is no net gas production from the peripheral wells, no gas piping 25 is required from the peripheral wells. Only water piping is required and this can be run directly from the well discharge to the water piping. No surface gas/liquid separators are required as the gas consumption of the spark ignition engines is so low that the bore casing itself can provide more than ample gas/liquid separation performance. Further, because there are no requirements to have a low gas back-pressure on a separator, the 30 water piping pressure drop can be higher allowing the use of smaller diameter piping 7 than otherwise would be the case. Whilst this will put some additional pressure head on the downhole pump, the capital savings can be substantial. The high production wells by contrast are a significantly higher investment. They are likely to be larger diameter wells than the peripheral water pumping wells to cope with 5 the very high design gas production. These wells may have "stimulation" steps taken to ensure they have a high connectivity into the natural cleat system in the coal seam. This can include any of the known flow enhancement techniques such as hydro-fracturing or the interception of horizontal wells. In one embodiment, the stimulation procedure includes the drilling of a series of largely horizontal drill holes radiating from the vertical 10 well into the coal seam or seams. One known technique for this involves the under reaming of the main bore hole to allow a special tool to be inserted that includes a swing arm. A fluid powered drill is lowered through this special tool and drills horizontally into the coal seam. The goal is to drill a series of holes from the bore hole out into the coal seam so natural flaws within the coal (cleats) are intersected so the bore hole is in 15 strong fluid communication with the natural cleats of the coal seam. Whilst this fluid powered drilling technique has been used to drill long holes (up to 300m long), for the purposes of the invention, a greater number of shorter holes (for example 50m long) at different elevations within the coal seams and radiating in different directions is considered preferable as this maximises the prospect of multiple fluid communication 20 paths with the cleat system in the coal. In regions of low permeability coal, the drilling lengths would logically increase to tailor the approach to the prevailing conditions. Ideally the production wells may be positioned in the development area at identified regions corresponding to structures within the coal seam favourable to gas phase transportation and gas extraction from the coal seam. Depending upon factors such as 25 the topography of the coal seam, the production wells may be located central within the area defined by the "peripheral" wells, biased off-centre or may be partly or fully coincident with the peripheral wells. CSM extraction, according to the invention, makes greater use of the natural permeability of the coals for transport of gas and water without having to provide as much of this transport capability via surface facilities and 30 piping installed at the surface.

8 It will generally be desirable to locate the high production wells in the development area within the peripheral wells at a location where the coal seam is higher on average than the balance of the seam within the region defined by the peripheral wells (ie a region that is a natural gas trap). Thus, for a coal area where the coal forms the equivalent of a 5 hill in the middle of the development area defined by the peripheral wells, the high production wells would be closer to the top of this "hill". If the zone of highest coal within the area defined by the peripheral wells is shaped like a ridge running there through, then the high production wells would be biased towards this ridge. If the coal seam had a consistent dip within the area defined by the peripheral wells, then the high production 10 wells would be biased towards the rising edges of the dip and might in fact be coincident with some of the peripheral wells. If the coal seam had the equivalent of a "valley" running through it, the high production wells might be clustered into two groups near the high coal areas either side of the valley. It is also possible that known features within the coal seam may dictate that the high 15 production wells, whilst still unevenly distributed into compact zones, may be in two or more zones. For example, if there is a known fault line across a CSM field where there is expected to be poor connectivity for methane gas flow across the fault, one set of high production wells may be located to one side of the fault with the other set of high production wells at another location in the coal on the other side of the fault. 20 It is important to note that in the context of the invention above, the height of the coal seam is referenced in respect of a fixed height datum such as sea level and is not referring to the depth of the coal from the surface which is influenced by ground surface topography. The hills, valleys and ridges refer to the upper surface of the coal seam itself and do not refer to the shape of the overlaying land surface. 25 One benefit of locating the high production wells near where the coal is at the highest elevation is that if the high production wells do not flow gas immediately, the gas desorbed from the coal, will soon accumulate at the high production wells area and they can be brought on line after there is sufficient gas present to lift the water with the gas flow. This avoids the need to fit water pumps into the high production wells and the 30 need to remove this equipment later. This saves a significant investment in pumping equipment, the provision of power and work-over costs and also makes the wells 9 particularly amenable to operating at variable capacity (or even turn them off) if it is desirable to manipulate the gas production from the field. The ability to manipulate and even shut off gas flow from the wells is particularly desirable within the context of Australian developments where the majority or all of the gas is destined to be liquefied 5 for export. In current CSM developments there are problems when there is a need to seriously restrict CSM well production such as in the event of a liquefaction plant shutdown or during the period leading up to an additional train coming on-line. Even though there is significant investment to make a well high production, by using very high production gas wells, very significant capital savings can be made by having 10 many fewer gas/liquid separators with simpler controls and a reduced field inventory of control equipment (ie fewer, higher capacity control valves, flow meters and the like). The development of a development area according to the invention in most, if not all cases will result in peripheral wells positioned around the periphery of the development field, one or more regions of gas production wells and regions where no wells are 15 positioned or required due to the characteristics of the coal seam. Hence production wells are positioned in an irregular and non symmetrical pattern across the development area. One of the benefits of the invention is the location of the high production wells in a concentrated area rather than the gas production wells being spread out essentially 20 evenly across the development area. The gas piping will be much lower capital cost because it consists of a substantially smaller number of large diameter trunk lines directly to the gas compression facilities. Additionally, the vast array of in-field smaller diameter gas piping, low point drains, valves and associated controls are avoided. Further, because the high production wells are grouped together, it is practical to run 25 distributed power, fibre optic cables, high quality access roads and potentially even compressed air to the wellheads should this be desirable. Similarly, because the peripheral wells are located around the periphery of the area, the total distance of access roads is dramatically reduced compared to a field having an even distribution of wells. This significantly reduces the cost to install the roads, the 10 costs to maintain the roads, the disturbance to the landowner's property and dramatically reduces the cost of distributing power to wellheads. When developing an area in accordance with the invention, there is a large area within the area defined by the peripheral wells that is neither near peripheral wells nor high 5 production wells. This area need not be disturbed by the coal seam methane operations despite coal seam gas being recovered from the coal seams under the area. This allows the deliberate avoidance of areas that are sensitive for one reason or another such as: a local town, farmer's residence, areas of significant cultural heritage, prime cropping land, irrigated pastures, hobby farms etc. 10 The technique particularly comes into its own with large scale developments in excess of 100 peripheral wells. It is of limited benefit for fields comprising 25 wells or less, but is increasingly beneficial as the physical size of the development increases. It is particularly suited to coal seams exhibiting high permeability and high connectivity. Further aspects of the present invention and further embodiments of the aspects 15 described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings. Brief description of the drawings Figure 1 is a highly stylised combined schematic layout of a conventional grid layout of the prior art; 20 Figure 2 is a schematic layout of the access roads shown in figure 1; Figure 3 is a schematic layout of the water piping shown in figure 1; Figure 4 is a schematic layout of the gas piping shown in figure 1; Figure 5 is a highly stylised overall schematic layout of the arrangement of wells according to the invention; 25 Figure 6 is a schematic layout of the access roads according to the figure 5; Figure 7 is a schematic layout of the water piping according to the figure 5 11 Figure 8 is a schematic layout of the gas piping according to the figure 5 Figure 9 is a schematic layout of the gas piping where there is a gas flow from the peripheral wells; Figure 10 (a) and 10 (b) are sectional views of a production well according to one 5 embodiment of the invention. Detailed description of the embodiments A CSM development configured in accordance with the present invention is shown in Figure 5. In this example, seismic studies indicate the coal seams are 10 continuous through the field with no fault lines that would suggest an inability to conduct gas through the coal seams. The topography of the coal is such that a region of the coal seam towards the mid-line of the field is elevated compared to the coal either side of it forming a potential gas-trap. The development is planned in accordance with this patent. A plurality of peripheral 15 wells are positioned around the development area C. The wells are drilled and completed as CSM wells, but initially do not have gas/water separators installed. Because power is not available to the field at the outset, each bore annulus is fitted with a gas off-take for the connection of fuel gas to an engine driven generator. The generator delivers power for use primarily in the drive head for the downhole 20 progressing cavity pump. Some power is also supplied to the wellhead controls that manipulate the speed of the pump to avoid a "pump off" condition and to operate control and radio communication equipment. A diesel-fuelled alternator is used temporarily at start-up of the well (if required) until sufficient gas is available from the well to operate the gas fuelled generator set. 25 The peripheral wells are operated such that the majority of the fluid and preferably substantially all the fluid removed is water. The water discharge connection from the pump at each peripheral well is connected to underground polyethylene pipes 25 (in figure 7) that carry this water to a buffer dam (not shown) servicing the development. From the dam, water is sent to a water treatment facility (not shown) prior to release.

12 The water piping operates at significantly higher velocities and pressure drops than is typically the case for CSM operations. This allows the use of smaller diameter piping which may be amenable to laying using ripping equipment rather than trenching. Initially, there is no gas gathering piping provided to the peripheral wells (figure 8), 5 although peripheral gas piping 27A can be added later in the project (shown in figure 9) to assist with exhaustion of the gas within the coal. Note that the actual gas piping route may differ, but the figure serves to estimate the total piping requirement. At least one production well 20 is positioned within the development area C. No specific equipment is installed to extract water from the production wells though water will 10 commonly be co-produced with the gas. Once the hydrostatic pressure in the coal seam is reduced by water extracted from the peripheral wells, hydrocarbon gas desorbed from the coal seam by the reduction in hydrostatic pressure flows from the production well. seam If the production wells are positioned to take advantage of the characteristics of the coal seam, less production wells are required to extract methane 15 from a given development field. In the example shown in figure 5, the production wells 20 are established largely coincident with the "ridge" in the topography of the coal. These wells 20 preferably have larger diameter casing than the peripheral wells so they can handle the larger gas flows (and any associated water) without exceeding accepted erosion velocities. 18" casing is provided in this example which is much larger than 20 nominally 8" casing used for conventional CSM wells. Further, specific measures are undertaken to ensure good fluid communication between these high gas flow production wells and the coal seams. Figure 10(a) and 10(b) shows a cross sectional view of an embodiment of a production well 20 used in the invention. In this example, the well may be under-reamed in the vicinity of the coal seams producing an expanded region 24 in 25 the coal seam 23. A special device (not shown) may be lowered which allows the deflection of a fluid-powered cutting tool to be used to drill a multitude of holes 21 out into the coal seam 23. The holes drilled are nominally 50m long and at each elevation within the main well 25, a number of bores are drilled radiating out from the main well 25. This drilling exercise is then repeated at a further two depths within the coal to 30 provide fluid communication within the one or more seams 23.

13 The high gas production wells 20 are not fitted with water pumps in this example. It is estimated that 4 weeks after dewatering commences in the peripheral wells, gas will free-flow from the high production wells 20 without the need for pumping. Very large capacity gas/water separators are provided for the high gas production wells to cope 5 with the large gas volumes and associated water. The separators servicing the high gas production wells 20 can be rated for high pressure so they can be operated in a choked-in fashion should it be desirable to limit the gas production from the field - for example during times when the methane liquefaction plant may be off-line. Whilst not an essential part of this example, the use of high pressure separator (or pre-separator) 10 allows the choke valves to operate on largely single phase fluids rather than 2 phase fluids which can be very damaging to valves and piping. The high gas production wells 20 are connected to very large gas piping and water piping systems (D and E in figures 8 and 7 respectively ) to cope with the large flows. The wells are arranged on a major link to a compressor station servicing the field C. In 15 this example, because the production wells 20 are arranged along the central coal seam ridge , for much of the initial life of the field, there is limited initial gas piping 27 provided because it is short and high capacity. This is in stark contrast to a conventional CSM development. In this example, the high gas production wells 20, are located along a single corridor, 20 and so are easily accessible by road 26. (Figure 6). The high gas production wells make use of high reliability fibre optic communications rather than relying upon radio links. It is also practical to supply compressed air to the wells so high pressure air is available to actuate the large valves rather than relying upon instrument gas as is typical in the industry. Further, electrical power might be distributed from the gas compressor station 25 to the high production wells to provide a secure control system power supply compared to solar powered wells typically used in the industry where power for pumping is not required. Road access is also provided to the peripheral wells by road 28. At some later stage in the field life, power may be distributed to the peripheral wells in order to avoid the use of spark ignition engines. This is easy to provide compared to a 30 conventional CSM development because the wells are all located along the periphery of 14 the pattern making the total installed length of cable significantly shorter. This greatly reduces the cost of electrifying the field. It is possible to fit separators and gas piping 27A to the peripheral wells either initially or later in the field life. It is desirable to defer this if possible as it defers capital. It is also 5 possible to provide some in-fill bores later in the field's life to provide additional gas extraction capability or to provide water removal means from the high gas production wells without departing from the intent of this patent. Table 1 shows a comparison of piping and roads needed to service a 10km x 10 km development site. Note that whilst the layouts used for the piping and roads are highly 10 stylised, the comparison serves to exemplify the differences through the approach of the present invention versus conventional CSM developments. Table 1 Grid Well Approach Peripheral Well Approach 220km - Part Boundary Access Roads: road 50km - Full boundary road 240km - Full boundary road Gas Piping: 220km 8.8km Water Piping: 220km 50 km In the above example, a conventional development for a development area of this size 15 requires a 10km x 10km grid layout, with the wells spaced 500m apart. This utilises 441 wells. In the arrangement according to the invention, even if the production wells are spaced 200m apart only 44 gas collection wells are required as well as 80 peripheral wells spaced 500 m apart. Excluding the distance between major collection piping and 15 gas compression unit, the above calculations illustrate that considerably less roadway, water piping and gas piping is required with the arrangement of the present invention compared to a conventional grid arrangement of wells used currently. While the above example is a simplified illustration of the development of a field 5 according to the method of the invention, it would be more common for a development field to have peripheral wells positioned around the periphery of the development field, one or more regions where the gas production wells are grouped together and regions where no wells are positioned or required due to the characteristics of the coal seam or development requirements. 10 Thus invention is able take advantage of the connectivity of the natural cleat system in the coal seam to provide the following advantages over conventional well drilling arrangements; * The total number of wells to be bored is reduced. " The wells to be drilled are located along corridors providing easy access and 15 minimising disturbance to the land. * The number of gas separators required is substantially reduced and this saves considerable capital. * The total cost of gas piping within the field is dramatically reduced and stays lower than a conventional development even if the peripheral wells are 20 connected to the gas system and some in-fill wells are provided. * The costs to provide electricity to the field are dramatically reduced because the bores are arranged along corridors. This reduces the cost and complication of providing distributed power. * The water piping costs are reduced because it is possible to use pump discharge 25 pressure to push the water along at a higher velocity than is the case if gas pressure in the separator is used for this purpose.

16 * The number of control valves is dramatically reduced as the gas flow is from a smaller number of high production wells. Whilst the valves would be larger, this significantly reduces the effort to inspect and service these components. * The field is capable of having the capacity modulated more aggressively than is 5 the case for conventional wells because the majority of the gas flows from free flowing wells. This eases the operations where the gas end user might be subject to large swings in demand (such as if a liquefaction train drops off line). " The total distance of road ways within the field is dramatically reduced making servicing the wells simpler and less labour intensive. The cost of roadways and 10 rights of way are significantly reduced. * It is possible to withdraw gas from under sensitive areas without the need to locate equipment as close as would be the case in conventional CSM. " Because there are fewer wells and the nature of the wells changes less (ie peripheral water wells remain pumping wells for longer and high production gas 15 wells do not need pumps fitted) there are less requirements for work-over rigs to change a well's mode of operation. * The disruption to an area being developed is less and this simplifies the granting of approvals for access ways, rights of way, disturbance, surveying and pipe laying. In some cases, particular properties may not be entered at all. 20 0 The noise associated with fewer pumping wells and fewer higher production wells is expected to be less because of the lower number and the enhanced practicality of providing noise reduction measures to the lesser number of wells. It will be understood that the invention disclosed and defined in this specification 25 extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims (14)

1. A method of extracting hydrocarbon gas from a coal seam beneath a development area, including the steps of establishing a plurality of peripheral wells spaced around the periphery of a 5 development area and operating the peripheral wells to remove at least water from the coal seam in the development area; and establishing at least one production well within the development area to remove hydrocarbon gas; the hydrocarbon gas being desorbed from the coal seam by a reduced hydrostatic pressure in the coal seam caused by the water extracted from the 10 peripheral wells.

2. The method of claim 1 wherein the peripheral wells operate for a period of time to remove at least water prior to the production wells being established.

3. The method of claim 1 wherein the production wells are established in the development area at identified regions corresponding to structures within the coal seam 15 favourable to gas phase transportation and gas extraction from the coal seam.

4. The method of claim 3 wherein the production wells are established in a location which is central within the development are, biased off-centre or partly or fully coincident with the peripheral wells.

5. The method of claim 3 wherein the production wells are established in the 20 development area at a location where the coal seam is higher on average than the balance of the seam within the region defined by the peripheral wells.

6. The method of any one of the preceding claims wherein the production wells are established in at least two distinct zones depending on the characteristics of the coal seam. 25

7. The method of any one of the preceding claims wherein a fault line exists in the coal seam in the development area, the production wells being established in at least two zones where one zone is located to one side of the fault and a second zone is established on a second side of the fault. 18

8. The method of any one of the preceding claims wherein the peripheral wells reduce the prevailing hydrostatic pressure within the development area defined within the peripheral wells.

9. The method of claim 1 wherein the peripheral wells are initially fitted with pumps to 5 remove fluids and no pumps are initially fitted to the production wells.

10. The method of claim 1 wherein the production wells are substantially vertical and created to provide high connectivity with the coal seam using augmentation techniques, the augmentation techniques being selected from the group consisting of underreaming, the drilling of horizontal wells from the vertical shaft, the drilling of interconnecting wells, 10 the use of hydrofracing and the use of cavitation.

11. The method of any one of the preceding claims wherein the peripheral wells are fitted with water/gas separators and gas piping either initially or after a period of operation.

12. An arrangement of wells for the extraction of hydrocarbon gas from a coal seam 15 beneath a development area including a plurality of peripheral wells arranged around the periphery of the development area to remove at least water from the development area; and at least one production well positioned within the development area and configured to extract hydrocarbon gas desorbed from the coal seam by reduced hydrostatic pressure 20 in the coal seam caused by the water extracted from the peripheral wells, the average volumetric flow per well of hydrocarbon gas from the production well/s being greater than the average volumetric gas flow per well from the peripheral wells.

13. The arrangement of claim 12 wherein the development region comprises 25 a peripheral well region consisting of the peripheral wells; at least one production region, each production region consisting of at least one production well; and 19 vacant region within the boundary defined by the peripheral wells in which there are no wells.

14. The arrangement of claim 13 wherein each production region comprises at least 2 production wells.