Study on the pore-fracture system of low-rank coalbed methane reservoirs in the Bayanhua Sag, Erlian Basin
Abstract
The development potential of low-rank coalbed methane (CBM) resources in the Erlian Basin, Inner Mongolia, is enormous. However, current research on pore-structure systems in coal reservoirs remains insufficient to accurately characterize the distribution patterns of pore heterogeneity and clarify the intrinsic controlling mechanisms of multi-scale pore evolution. This directly leads to significant deviations between the quantitative evaluation results of coal reservoir physical parameters and actual geological conditions, thus limiting the effective development of CBM. In this study, scanning electron microscopy, low-temperature nitrogen adsorption, high-pressure mercury porosimetry, and nuclear magnetic resonance techniques were comprehensively applied to systematically analyze the pore morphology, full-scale pore structure, and heterogeneity of coal reservoirs in the Bayanhua Sag of the Erlian Basin. Finally, the controlling factors governing the development of the pore-fracture system were elucidated. The main findings are as follows: ① The pore space is mainly composed of plant tissue pores, with closed and semi-closed pores being predominant. Micropores mainly contribute to the specific surface area, whereas transition pores and macropores mainly contribute to the total pore volume. The connectivity between adsorption pores and seepage pores is relatively weak. ② Both adsorption pores and seepage pores exhibit significant fractal characteristics, forming a complex and highly heterogeneous pore structure, with seepage pores showing particularly pronounced heterogeneity. ③ The development degree and complexity of the pore system in coal reservoirs are controlled by the coupled effects of coal seam burial depth, coal quality parameters, and organic maceral composition. This study provides accurate geological parameters for CBM reservoir evaluation, and supports the efficient development of low-rank CBM in the Erlian Basin.