Water production in enhanced coalbed methane operations

Coalbed methane (CBM) formations provides a considerable amount of the US natural gas production and have the potential of storing significant amounts of carbon dioxide (CO2) through enhanced gas recovery operations. Enhanced coalbed methane (ECBM) recovery by injection of CO2 or a mixture of CO2 and nitrogen (N2) has been proven to recover additional natural gas resources. However, since coalbeds are normally saturated with water and can be in communication with an aquifer, a large amount of water is often co-produced during the natural gas extraction.The conventional approach for CBM production relies on the reduction of the gas partial pressure in the coal seam. This can be accomplished by either pumping the formation water to the surface and/or by injecting gases such as N2 and CO2. Disposal of the produced water is an environmental challenge as harmful impurities must be removed by appropriate purification techniques. Consequently, a reduction of water production in CBM operations is desirable.In this paper we present a numerical investigation of the potential reduction in water production during ECBM operations that are commonly used to increase methane (CH4) recovery. We use a three-dimensional coalbed model with an aquifer located at the bottom to investigate the amounts of gas and water produced in ECBM operations per volume of coal seam as a function of aquifer strength and sorption characteristics including sorption induced strain. The amount of gas/water that is produced varies significantly depending on the aquifer strength and injection gas composition. We demonstrate that injection of CO2 and/or N2 in some settings reduces the water handling problem substantially.CBM is an important worldwide energy source with a large number of formations being excellent candidates for ECBM recovery processes. Our analysis of the interplay between coal characteristics, aquifer support and the resultant behavior in terms of gas/water production provides valuable input for optimization of future planning and operations.

► ECBM production and water production are sensitive to injection gas composition.
► CO2 ECBM may only be feasible with moderate aquifer support.
► Flue gas provides the best ECBM performance with low to moderate aquifer support.