Evolution of anisotropic permeability of fractured sandstones subjected to true-triaxial stresses during reservoir depletion

Abstract

Reservoir rock masses contain numerous fractures caused by regional tectonic movements or hydraulic fracturing operations. An accurate understanding of the evolution of permeability of fractured reservoir rocks during depletion is thus crucial for evaluating the long-term performance of producing wells. Although extensive study on this issue has been conducted, the anisotropic permeability of fractured reservoir rock under in-situ true-triaxial stress conditions remains unclear. In this work, the anisotropic permeability behaviour of fractured sandstones in three principal stress directions is experimentally investigated. The tight gas reservoir sandstone is sampled from Tarim oilfield. An artificial fracture with different inclination angles is introduced into the cubic specimen. The true-triaxial stress path simulating the depletion process is first calculated using poroelastic theory and the initial geological stress data. During the true-triaxial testing, the permeability in three principal directions is simultaneously measured at each loading step. The intact sandstone specimen is also tested for comparison purpose. Experimental results show that directional permeability exponentially decreases with the increase of the effective stress and exhibits remarkable anisotropy along the principal stress axes. Moreover, the directional permeability for the fractured specimens is about one order of magnitude higher than that of intact rock at the same stress state, and the permeability variation obviously relies on the fracture geometry. Furthermore, the permeability along the fracture plane shows fluctuations as the effective stress increases, which is probably induced by the variation of the fracture aperture due to micro-slipping and the cracking of the matrix skeleton enclosing the fracture surface.