Evaluation of gas contents for a multi-seam deep coalbed methane reservoir and their geological controls: In situ direct method versus indirect method

Abstract

Gas content is one of the most important parameters determining the potential resources and recovery of coalbed methane (CBM). Extensive field work has been conducted for a perspective CBM field in Qinshui Basin with multi-targeted producing coal seams. The gas content was estimated by both direct and indirect methods based on the canister desorption results and methane sorption isotherms. Within the studied CBM field, coal seams are well developed at greater burial depth (>1000 m). The #2, #3, #8, #12, and #15 coal seams in Shanxi and Taiyuan formations are the primary and targeted CBM producing formations. By applying the USBM direct method, the in situ gas contents of the main coal seams are then estimated from the analysis of desorption data. It was found that the indirect method depending on sorption isotherm overestimate the gas content in the studied field because it assumes the gas saturated CBM formation. We found that the coal seams are primarily undersaturated and located under CBM weathered and oxidized zone (CH4 > 90%) with good preservation conditions. The in situ gas contents of the #2, #3, #8, #12, and #15 coals range from 5.62 to 19.36 cm3/g, 11.84 to 31.24 cm3/g, 2.96 to 29.28 cm3/g, 10.96 to 31.19 cm3/g, and 7.91 to 27.32 cm3/g, respectively, with increase trends from the marginal to central areas for all tested seams. Coalification directly determines gas adsorption capacity and it significantly influences the in situ gas content. Coupled contributions of higher gas adsorption capacity and reservoirs pressure lead to higher gas content occur at depth between 1300 and 1550 m. Moreover, high in situ gas content is always associated with roof lithologies with stronger sealing capacity. However, normal faults and groundwater recharge area could promote gas release from the coals. Thus, the internal area of Block II with poor faults development and weak groundwater runoff is a target area for deep CBM preservation.