Study on Elastoplastic Damage Coupling of Reservoir Mudstone Considering Permeability Change

Mudstone, a common complex medium in oil and gas reservoirs and with widely distributed micro-pore and micro-fissures, is liable to produce significant damage evolution and plastic deformation under high buried depth stress environments. Based on the analysis of the physical characteristics, the elastoplastic damage coupling mechanical characteristics of mudstone in a high buried depth reservoir for oil and gas engineering are discussed. Firstly, conventional triaxial compression tests under different confining pressures were performed to calculate the damage variable and obtain the damage evolution. The damage evolution included the elastic damage stage, the plastic-dominated elastoplastic damage coupling stage and the damage-dominated elastoplastic damage coupling stage. Secondly, a coupled elastoplastic damage mechanical model for mudstone was proposed, which was based on the degradation of the damage stiffness and plastic flow caused by the plastic and damage internal variables and considered the elastic damage coupling and elastoplastic damage coupling during the loading process. Thirdly, the elastoplastic damage coupling mechanical characteristics of mudstone were simulated. The simulation results are in good agreement with the experimental results, which reflects well the mechanical characteristics of mudstone, including the transition from volume compression to expansion, plastic hardening, damage softening and residual strength, etc. Finally, based on the relevant research results, a permeability evolution model of mudstone based on the damage was proposed, and the secondary development was carried out based on ABAQUS. UMAT and USFLD subroutines were compiled, and seepage–stress coupling simulation verification was carried out. The relevant results provide a reliable basis for engineering theory research and stability analysis of deep mudstone reservoirs.