Coalbed methane reservoir fracture evaluation through the novel passive microseismic survey and its implications on permeable and gas production

Fracture networks in coalbed reservoirs serve as the primary gas pathway and thus determine the gas production potential for coalbed methane (CBM) recovery. However, the characterization of the fracture network is extremely challenging due to the complexity of both the induced and natural fracture system. A microseismic event analysis can be used to locate the fracturing, and determine the orientation, length, complexity, and temporal growth of the induced fracture by using the focal mechanism. In this study, the fracture system of a coal-bearing formation covering 1.2 km² in the Luan mining area of China is probed via passive microseismic imaging. Focal mechanisms of individual events are used to characterize the gas production potential for the 10 CBM wells in this area. Fracture reactivation modes are of three types - strike slip, dip slip, and extensional modes – with strike slip the most common followed by dip slip and then extensional type as the least likely. In addition, the location of different types of fractures are different, which indicates the difference of the in-situ stress regime. The 10 CBM wells were hydraulically stimulated in December 2017 then dewatered and allowed to produce for 14 months. We show that the microseismic data have a general positive correlation with gas production with a few exceptions - the higher the event count, the higher the gas production. This result is a best embodiment of the mutual control of reservoir fractures, stress regime, permeable and gas production in CBM development. We suggest passive microseismic imaging as an effective technique in evaluating the potential for gas production.