Investigation of stress sensitivity differences between shale and tight sandstone based on pore morphology and multi-scale pore oil recovery

Current research on the stress sensitivity lacks an explanation of the mechanisms underlying differences in stress sensitivity across reservoirs with different lithologies. Therefore, stress sensitivity experiments were conducted on shale and tight sandstone cores. These experiments were combined with pore structure analysis derived from core scanning electron microscope (SEM) images and multiscale pore recovery data obtained via nuclear magnetic resonance (NMR). By employing elastic mechanics to simulate pore deformation and accounting for recovery discrepancies among different pore types, dominant mechanisms governing stress sensitivity in shale and tight sandstone were clarified. The experiments revealed that while the porosity decay coefficients of shale and tight sandstone are similar, shale exhibits a stronger permeability sensitivity. This occurs primarily because 8.63% of large pores in shale have a major-to-minor axis ratio greater than six, and oil recovery from these large shale pores is 25.53% higher than that in tight sandstone.Numerical simulations confirmed that pores with substantial displacement but minor deformation, along with pores featuring both large displacement and deformation, are the primary contributors to the stress sensitivity of shale and tight sandstone. The deformation ratio of pores is positively correlated with pore area and negatively correlated with pore shape factor.