Effect of Stress and Thermal Deformation Modes on Correlation of Permeability of Naturally Fractured Sandstone and Shale Formations

Summary Proper correlations of the variations of petrophysical properties like the permeability of naturally fractured porous subsurface formations are developed under different effective stress and thermal deformation modes. The threshold or critical stress and temperature conditions splitting the different deformation modes are determined by analyzing the experimental data. The stress and thermal deformation mode changes occurring at the critical stress and temperature are referred to as the stress and thermal shock phenomena. The improved correlations are developed using the experimental permeability data of the naturally fractured porous sandstone and shale formations based on the theoretically sound modified power-law equation. The critical effective stress and temperature values are determined at the slope discontinuity observed in the variation of the parameters of the modified power-law equation. The parameters of the modified power-law correlations are also correlated as functions of the effective stress and temperature. It is demonstrated that correlating the data separately over the different portions of the full data set divided by the critical stress and temperature values yields more accurate and representative correlations of the naturally fractured porous formations than the correlations obtained using the full-range data. The improved analytical and theoretically sound methodology and approaches presented in this paper can be applied effectively for the development of the improved correlations of the permeability of subsurface porous rock formations deforming by variation of the effective stress and temperature conditions, such as in geothermal and thermally stimulated petroleum reservoirs, and the subsurface storage processes considered for energy transition.