Influence of true-triaxial stress and elevated temperature on three-directional permeability of a purple sandstone

Abstract

The reservoir rocks exist within geological environments characterized by elevated true-triaxial stress, pore pressure, and temperature. Understanding the coupled effects of temperature and true-triaxial stress on three-directional permeability is crucial for the efficient extraction of underground resources. Therefore, this study employs a self-developed true-triaxial geophysical imaging cell equipped with a temperature-controlled module to conduct three-directional helium permeability tests on purple sandstone. A stepwise loading path is adopted in this study to examine the influence of temperature (from 20 to 80 °C) and true-triaxial stress on three-dimensional permeability. As a result of inherent structural heterogeneity, the three-directional permeability exhibits significant anisotropy even under isotropic stress conditions. Under the coupled effects of thermal expansion and stress compaction, the three-directional permeability decreases with increasing temperature and principal stresses, and its sensitivity exhibits a negative correlation with both temperature and stress levels. This study introduces anisotropy ratios to quantitatively characterize the anisotropy degree in three-directional permeability (\({{k_{y} } \mathord{\left/ {\vphantom {{k_{y} } {k_{x} }}} \right. \kern-0pt} {k_{x} }}\), \({{k_{z} } \mathord{\left/ {\vphantom {{k_{z} } {k_{x} }}} \right. \kern-0pt} {k_{x} }}\), \({{k_{z} } \mathord{\left/ {\vphantom {{k_{z} } {k_{y} }}} \right. \kern-0pt} {k_{y} }}\)). Owing to the differences in sensitivity of three-directional permeability to temperature and principal stresses, the anisotropy ratio exhibits complex nonlinear evolution with changing temperature and true-triaxial stress states. A new three-directional permeability model is developed by introducing variable temperature-stress sensitivity coefficients into exponential permeability formulation. This model can effectively characterize the evolution of three-directional permeability and corresponding anisotropy ratio under varying temperature and true-triaxial stress conditions.