Evaluation of multistage characteristics for coalbed methane desorption-diffusion and their geological controls: A case study of the northern Gujiao Block of Qinshui Basin, China

Based on the diverse geologic data and on-site desorption experimental results of No. 8 coal seam in northern Gujiao Block, a systematic investigation was conducted on multistage characteristics of coalbed methane (CBM) desorption-diffusion behaviors. Meanwhile, the comprehensive effects of geologic control factors and its implication for CBM production process were also studied. The results indicate that No. 8 coal seam has mainly experienced three deformation phases during early Yanshanian-Himalayan orogeny. Hereinto, the tectonic deformation occurred with superposition of various stages and scales dominate the regional distribution of tectonically deformed coals in the study area. A mathematical method was established to identify the turning time and decay time, which quantitatively divides the CBM desorption process into three stages. Moreover, the on-site CBM desorption data can be well fitted by the multiporous diffusion model, indicating that the CBM diffusion process also has obvious three-stage characteristics. Due to synthetic effects of coal destruction degree and the heterogeneous distribution of gas content/burial depth, CBM desorption-diffusion characteristics demonstrate the unique regional distribution in the deep buried area near Malan syncline axis, the graben structure region of central Tunlan area and the south-central Dongqu area. On this basis, three typical modes of CBM desorption-diffusion behaviors were distinguished in northern Gujiao Block. It specifies the comprehensive influence of geologic control factors on CBM desorption-diffusion process and its impact on gas production characteristics of CBM wells. Therefore, these investigations can provide support for the preliminary forecasting and simulation of CBM production.