Numerical simulation study on microseepage and displacement mechanism of coal based on digital core technology

The combination of the primary pore–fracture structure in coal and the spatial distribution of minerals significantly influences the complex seepage–displacement behavior of coal seams, which is a critical issue that requires close attention in CO2 geological storage in coal seams. This study conducts microcomputed tomography scanning and three-dimensional (3D) reconstruction technology, permeability testing, and numerical simulations of the seepage and displacement process, aiming to reveal the heterogeneous characteristics of coal and their influence mechanisms on the microscopic seepage and the CO2 displacing water effect. Results show that the pores and fractures in the samples are highly heterogeneous spatial distribution. The distributed dense pores and microfractures are mainly distribute in sample SX01, two axially distributed penetrating large fractures and accompanied isolated pores are developed in sample SX02, and large-scale penetrating large fractures and confined dense pores and microfractures are embedded in sample SX03. The displacement efficiency and displacement equilibrium time show a trend of first significantly decreasing and then slightly increasing with the increase in the demineralization index. The CO2 displacing water process evolves into three typical stages: the uniform weak flow stage, the strong preferential flow stage, and the weak preferential flow stage. Adjusting the CO2 injection position, CO2 injection rate, CO2 injection pressure, and CO2 injection holding time, it is possible to achieve microscale uniform flow in the coal seam, thereby improving the injectivity and storage capacity of coal seams with mineral rich coal seams.