The effects of mineral distribution, pore geometry, and pore density on permeability evolution in gas shales

We explored the ensemble effects of pore density, pore geometry, and pore stiffness on the permeability evolution of an ellipsoidal pore under both applied uniaxial stress and relative pore pressure change. We found that rocks undergoing identical compressional strain and pore pressure can undergo significantly different magnitudes of pore closure or dilation based on the concomitant influence of these three variables. This is especially important in gas shales, where nano-porosity is challenging to characterize and heterogeneity at all scales has led to disparate permeability responses in both the field and laboratory. Simulations were carried out using a finite element solver, a method that allowed each variable to be studied in isolation and concomitantly. We found that the aspect ratio is the most sensitive parameter influencing pore compressibility. The pore density becomes important when external stress is applied, but it has no significant effect when pore pressure is varied in the absence of external stress. To capture the effects of mineral precipitation and transformation in pore walls, we simulated mismatches between the mineral stiffness of the pore and the surrounding matrix. We determined that for a given strain, mineralogically soft pores (soft relative to the bulk material) experience higher increase in permeability than pores that are mineralogically stiff relative to the surrounding matrix. While soft pores experience greater closure than stiff pores for a given applied stress, they also experience a greater amount of dilation when pore pressure increases.