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

Abstract

Summary Proper correlations of the variation of petrophysical properties like the permeability of naturally fractured porous subsurface formations by deformation are developed under different effective stress and thermal conditions. The fractures and matrix may have different deformation trends. The matrix-fracture dual-porosity nature of the naturally fractured porous formations is proven to be the key factor on the variation of the modes of their petrophysical properties. The threshold or critical stress and temperature conditions splitting the fracture and matrix deformation modes are determined by analyzing the experimental data. The stress and thermal deformation mode changes occurring at the critical stress and temperature are called the stress and thermal shocks phenomena. The improved correlations are developed using the experimental permeability data of the naturally fractured sandstone and shale based on the theoretically sound modified power-law equation. The critical values of the effective stress and temperature are determined based on the slope discontinuity observed in 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 dual-porosity characteristics 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 also for development of the improved correlations of the other petrophysical properties of subsurface porous rock formations deforming by variation of the effective stress and temperature conditions, such as in geothermal and thermally stimulated petroleum reservoirs.