Coal seam gas (CSG) also known as coalbed methane is an important source of natural gas in Australia. Natural gas feedstock for LNG production units, also known as trains, in eastern Australia is predominantly sourced from coal seam gas (CSG) reservoirs. Generally, gas production from coal seams entails extracting water from coal seams known as dewatering operation. Extraction of water reduces the pressure, triggers gas detachment (desorption) from coal matrix and allowing gas to flow into a connected fracture network that naturally exists in coal seams. Figure.1a shows a natural fracture system, known as cleat system, in a coal sample. Figure 1.b is the micro-CT scan images of a coal sample recovered from a CSG well located in the Fairview Field, a prolific CSG reservoir in the Queensland. The micro-CT images clearly demonstrate the cleats system through the sample. In this example, we have a fossilized cleat system meaning that cleat system has been preserved because of deposition of some materials (Calcite) in the fractures.
is the schematic of a CSG well with a downhole pump.
The cleat system consists of two sets of fractures, face and butt cleats, which are perpendicular together. This fracture network provides an avenue for gas and water flow during production from a coal seam. Coal seams are generally shallow and can be accessed by drilling vertical or horizontal wells. Figure.2 is the schematic of a CSG well completion with a down-hole pump.
Figure.3 shows a typical production profile for a CSG well. The term negative decline refers to the increasing gas production until peak of production is observed. The peak of production is followed by a decline afterward. At its peak of gas production, this well can supply gas to more than 60000 houses for cooking and heating purposes (calculated based on average daily consumption rate of 39 cubic feet per day per house).
Hydraulic connectivity assessment
Water production from coal seam gas wells in a CSG field significantly varies from only a few barrels per well per day up to thousands of barrels per well per day. The produced water, as a co-product of CSG production, can be used in agriculture, aquifer recharge, industry and domestic uses after treatment.
Obtaining an equitable balance between development of coal seam gas resources that has important economic benefits and protection of fresh water aquifers, which is a matter of national environment significance entails in-depth study of hydraulic connectivity. This is achieved by conducting hydraulic connectivity characterization to ensure fresh water aquifers remain intact. The hydraulic connectivity can occur due to intrinsic factors (associated with geological setting of the area), extrinsic factors (related to well completion practices), or a combination of both intrinsic and extrinsic factors.
The intrinsic factors such as permeable faults or unconformities can promote the likelihood of hydraulic communication. Permeable faults can provide potential avenues for fluid exchange among different geological formations and unconformities can remove impermeable barriers and create a contact zone bringing coal seam at immediate contact with a permeable layer. The extrinsic factors involve well completion practices such as hydraulic fracturing that stimulate target coal seams and potentially adjacent formations. Figure.4 is a conceptual model representing cross-formational flow scenarios due to intrinsic and extrinsic factors.
Conducting hydraulic connectivity assessment by operators allows mitigating cross-formational flow issues and benefits both regional communities and the CSG industry.