Infiltration mechanism of clay minerals in coalbed methane wells using a compound acid plugging removal agent

Clay minerals obstruct the gas production channels of coalbed methane wells, leading to reduced production and severely limiting overall gas extraction. Acid corrosion-based plugging is an effective technique for efficiently cleaning clay minerals. This study investigates the infiltration mechanism and wetting effects of a compound acid unplugging removal agent on different clay minerals, using samples from the Wuxiang block. Molecular simulation was employed to optimize the surfactant composition within the compound acid system. The results reveal that, compared to fatty alcohol polyoxyethylene ether and dodecyl glucoside surfactants, dodecyl dimethylaminoethyl lactone (BS-12) exhibits the smallest molecular orbital energy difference, the largest dipole moment, and the strongest molecular polarity. Additionally, BS-12 demonstrates the highest electrostatic potential extremum, forming more hydrogen bonds with stronger bond energy in the compound acid system. Consequently, BS-12 was identified as the optimal surfactant component due to its superior wettability. Equilibrium analyses of different mineral systems revealed that surfactant molecules adsorb horizontally onto mineral surfaces. The interactions between surfactants and minerals influence the aggregation and dispersion of acids within the compound solution. In muscovite, almandine, and corundum systems, acetic acid concentrated at the solid–liquid interface, while hydrofluoric and hydrochloric acids were dispersed at lower concentrations. In the goethite system, all acids were uniformly distributed at low concentrations. Upon the addition of surfactant, the diffusion coefficients of water molecules decreased, improving the wettability of mineral surfaces to varying extents. The migration and diffusion capacities of water molecules, ranked by mineral system, were in the order of goethite > quartz > corundum > almandine > muscovite. These findings align with interaction energy analyses between the liquid phase and the respective minerals. The compound acid system exhibited the strongest wetting effect on goethite compared to other minerals.