Review of the Hydrogeological Controls on Coalbed Methane (CBM) and Development Trends

Hydrogeological conditions can control the generation, preservation, enrichment, and production of coalbed methane (CBM) in the field; however, research on these impacts is insufficient, resulting in the limitation of the development of coalbed methane. This paper summarizes the current research status and development trends of the effect of hydrogeology on CBM using methods such as mathematical statistics, literature analysis, well logging, and hydrochemical analysis. The results indicate that it is beneficial for the generation of secondary biogenic gases in low-rank coal seams under the situations like active hydrodynamic conditions with a salinity less than 1000 mg/L, a pH range from 5.9 to 8.8, or a range of oxidation-reduction potential from -540 mV to -590 mV. The abnormally high temperature due to the magmatic-hydrothermal fluids accelerates the metamorphism of coal rocks, leading to the promotion of the generation of thermogenic gases. When the coalbed structural conditions of one area are similar to the depositional conditions in that area, the CBM is accumulated if the conditions of that area meet the following criteria: the water type is NaHCO3, the salinity is greater than 1500 mg/L, the desulfurization coefficient is less than 1, and the sodium-chloride coefficient is less than 10. The stable isotope analysis of CBM well-produced water shows that the δD values in the groundwater shift to the left of the global meteoric water line, indicating that the produced water comes from atmospheric precipitation. In the CBM enrichment zone, the area with a relatively high salinity and a low sodium-chloride coefficient is the high-production area. Based on our study, three high CBM-production patterns are summarized: coalbed structure-hydraulic trapping, fold limb-fracture development, and syncline core-water stagnation. Additionally, four development trends of the control of hydrogeology on CBM are proposed: transformation from qualitative evaluation to quantitative evaluation, from a singular evaluation standard to multiple evaluation standards, from static evaluation to dynamic evaluation, and from pure theoretical research to theoretical guidance on production practices.