Study on logging evaluation of key parameters of “five properties” of deep coalbed methane reservoirs

Deep coalbed methane (CBM) resources are vast. Addressing the complex geological characteristics of deep CBM reservoirs, which include the coexistence of adsorbed and free gas, complex occurrence mechanisms, significant heterogeneity, and elevated ground stress, accurately identifying and assessing the critical geological parameters of these deep coal reservoirs through logging technology is essential for optimizing the development of deep CBM reservoirs. This study focuses on challenges in deep CBM reservoirs, such as the coexistence of free and adsorbed gas, and complex configurations between coal seams and associated roof/floor strata. Taking the Ordos Basin as an example, a logging evaluation methodology for the “five properties” of deep CBM was established by comprehensively considering the coal-rock lithology, reservoir characteristics, gas content, fracturing ability, and the sealing properties of the roof and floor. The results indicate that high-quality reservoirs of deep CBM are primarily characterized by bright coal and primary structural coal in terms of coal lithology, exhibiting low ash content, high vitrinite content, and significant thickness. Regarding reservoir properties, these reservoirs are distinguished by elevated porosity and permeability. In terms of gas content, both free gas and adsorbed gas are present, with total gas content exceeding 20 m3/t. The optimal combination consists of roof limestone and floor mudstone; furthermore, an increased stress differential between the roof and floor correlates with a reduced degree of fracture development, thereby enhancing sealing effectiveness. The results derived from gas testing and production wells demonstrate that the developed “five-property” logging evaluation method for deep CBM reservoirs is effective.