Pore Structure and Porosity–Permeability Evolution Characteristics of Coal/Rocks under the Action of High-Viscosity Slickwater

The pore structure, porosity, and permeability of coal/rocks are affected by the action of high-viscosity slickwater (HSW) during coalbed methane (CBM) development, which are key factors influencing CBM extraction. The high-pressure and high-temperature (HPHT) coupling system is adopted to conduct experiments on the interaction between slickwater and coal/rock samples from the Qinshui Basin. The variation characteristics of mineral constituent and solution of coal/rocks under the action of HSW are analyzed, and the evolution characteristics of the pore structure, porosity, and permeability of coal/rocks and their controlling mechanism are revealed. The results show that pores within coal/rock continuously expand with increasing pressure and time. The expansion pores are micropores and transition pores in sandy mudstone and coal, mainly, while those in siltstone, fine sandstone, and limestone mainly are mesopores. As the reaction progresses, the infiltration path of water is increased, but the contact area between water and coal/rocks is reduced and the pore expansion rate is slowed. The surface of coal/rocks primarily undergoes the dissolution of cement and soluble minerals, increasing the ion concentration, electric conductivity (EC), and potential of hydrogen (pH) in the solution. The pore types in the interior of coal/rocks gradually evolve from the initial intergranular and intercrystalline pores to later dissolution pores and interlamellar cracks, leading to increased porosity and permeability of the coal/rock. Hydration expansion intensity is found to be positively correlated with the contents of montmorillonite, kaolinite, and illite. The expansion strain in the parallel bedding direction is smaller than that in the vertical bedding direction, and the expansion of clay minerals significantly increased after 50 h. This work supplies a theoretical foundation for the efficient development of the CBM and reservoir reconstruction.