Reservoir characteristics and gas production potential of deep coalbed methane: Insights from the no. 15 coal seam in shouyang block, Qinshui Basin, China

Considering the large burial depth and unclear coalbed methane (CBM) development potential of the No. 15 coal seam in the Shouyang block in the northern Qinshui Basin, this study investigated the characteristics and gas production capacity of deep CBM reservoirs in the No. 15 coal seam based on the analysis of geological characteristics of CBM and the simulation of CBM production capacity. The No. 15 coal seam has a large thickness, strong adsorption capacity, medium gas content, low critical desorption pressure, low reservoir pressure, and a low ratio of critical desorption pressure to reservoir pressure (also referred to as the ratio). Based on the reversible relationship between the adsorption and desorption of CBM, we established an equation expressing the relationship between the gas content and burial depth of the No. 15 coal seam using massive data from gas content tests, isothermal adsorption experiments, and well tests. The analyses of main factors controlling the permeability of deep reservoirs of the No. 15 coal seam and their differences with those of shallow reservoirs indicate that deep CBM reservoirs have significantly low permeability owing to their large burial depth and high in-situ stress despite their favorable coal structure and well-developed fractures. Moreover, this study predicted the gas production characteristics of the No. 15 coal seam through numerical simulations of CBM drainage. The simulation results were similar to the actual gas production results, indicating that the No. 15 coal seam has great gas production potential. The results of this study will provide geological support for predicting the gas content and assessing the development potential of the No. 15 coal seam in the deep part of the northern Qinshui Basin.