Characteristics of Water Occurrence in Coal/Rock under the Action of High-Viscosity Slickwater and its Mechanical Properties

During coalbed methane (CBM) extraction, the micropore structure and macro mechanical properties of coal/rock mass change significantly under the action of external fluid. This study focuses on the main coal seams and the roof sandy mudstone in the Qinshui Basin. Based on multiscale experimental methods, a systematic analysis is conducted on the pore evolution law and the mechanical property weakening mechanism of coal/rock under different moisture content (MC) conditions. The results indicate that micropores in coal/rock are the main sites for water–rock interactions, accounting for 67.94 (coal) and 69.07% (rock) of the total porosity. The expansion speed of the pore structure is exponentially related to time. The dissolution of hydrophilic minerals drives the transformation from micropores to mesopores, resulting in a significant increase in the porosity. Montmorillonite and other clay minerals absorb water and expand, causing pore closure. Based on this, a dynamic model of pore evolution during the coal/rock water absorption process was constructed. The compressive strength and tensile strength of coal/rock samples have a negative exponential relationship with the moisture content. From dry to saturated, the compressive strength and tensile strength of coal samples decreased by 64.6 and 83.3%, and the rock samples decreased by 79.7 and 65.55%. Quantitative analysis based on computed tomography (CT) scanning revealed a significant increase in the coal/rock porosity after water action. The coal sample fractures are mainly distributed in the range of 6000–10000 μm. The rock samples are concentrated in the range 100–300 μm. This work supplies a theoretical basis for the efficient development and reservoir stimulation of CBM wells.