Coal seam gas 'basics'
What is coal seam gas and how is it extracted?
Coal seam gas of commercial interest comprises mostly methane gas formed within coal seams that are usually saturated with water. The water pressure serves to hold the methane within the coal.
CSG is extracted from coal by means of wells that are drilled down into and (in some cases) along the coal seams. When the water is pumped out of the coal the CSG is desorbed and released from the coal.
CSG is produced at low pressure. Small, in-field screw type compressors are therefore typically used to push produced gas to a central gas processing facility.
At the central processing facility the CSG is dried and/or compressed as necessary for injection into a transmission pipeline.
Water that is pumped from the coal seam to stimulate gas production is either conveyed to an evaporation pond for disposal or processed for use in a variety of applications, including agricultural irrigation or as potable water.
Differences between CSG and conventional gas
CSG and conventional gas are very similar. Both are odourless natural gases suitable for domestic, commercial and industrial uses including power generation. CSG may have a slightly lower energy content than conventional gas in circumstances where the conventional gas also contains longer chain hydrocarbons (ethane, propane and butane).
CSG is absorbed to the internal surfaces of the coal within which it was formed, whereas conventional gas is stored in the pore spaces between grains of sandstone or similar rock. The micropore surface area of coal is very large and, for similar depths of burial, has the potential to hold significantly more methane on a per unit volume basis than most conventional gas reservoirs.
CSG project development considerations
CSG projects tend to be capital intensive in the initial years of development, mainly due to the large number of wells (over 100 in many projects) that need to be drilled. There are significant lead times, as long as two years for some projects, before revenue can be generated from gas sales.
CSG projects generally progress via three stages: exploration, pilot appraisal and full development. Once suitable coals have been identified at the exploration stage, pilot appraisal is carried out to identify optimal coals, refine extraction (i.e. completion and stimulation) processes and, often, generate some revenue prior to undertaking full development. This staged development process allows capital expenditure to be staggered and costly development expenditure on suboptimal opportunities to be avoided.
Production techniques
In general, production from a CSG well must be continuous. If production is halted water will typically re-enter the coal seam and the dewatering process must be undertaken again.
Gas can often be extracted from multiple coal seams via a single well thus substantially improving economic viability as fixed costs are spread over a greater accessible reserve base.
Companies have experimented with various drilling techniques and both horizontal and vertical drilling is now being used in active CSG developments. Horizontal wells are technically more challenging to drill and can be more expensive but, for ESG's PEL 238 CSG project, have the advantage that they can be oriented to maximise reservoir connectivity, thereby giving rise to significantly enhanced production rates relative to vertical wells.
