IMPACT OF RESERVOIR CHARACTERISTICS ON WATER PRODUCTION IN ENHANCED COALBED METHANE OPERATIONS

Coalbed methane formations deliver a considerable amount of the US natural gas production and have the potential of storing significant amounts of carbon dioxide through enhanced gas recovery operations. Enhanced coalbed methane (ECBM) recovery by injection of carbon dioxide or a mixture of carbon dioxide and nitrogen 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 methane 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 or injecting a mixture of gases. Disposal of the produced water is an environmental challenge since harmful impurities must be removed. Consequently, reduction of the water production is desirable. In this paper we present a detailed numerical investigation of the potential reduction in water production during ECBM operations while increasing the methane production. We use a two‑ and a three‑dimensional coalbed models with an aquifer located on the bottom to investigate the amounts of gas and water produced in ECBM operations per volume of coal seam as a function of aquifer support, cleat spacing and sorption characteristics of the coal. The amount of gas/water produced varies significantly depending on the aquifer strength. We demonstrate that injection of carbon dioxide in some settings reduces the water handling problem. CBM is becoming 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 and carbon dioxide storage capacity provides valuable new guidelines for future operations.