Experimental study of the permeability of fractured sandstone under complex stress paths

Abstract

Oil and gas are main storage resources in the pores and fractures of rocks. Under a complex stress environment, the permeability evolution of the reservoir rock fracture directly affects the flow of oil and gas, which is important in oil and gas exploration and development. The permeability test of a single sample in complex stress paths was performed on intact sandstone, single-fractured sandstone, and double-fractured sandstone using high-precision hydro-mechanics-coupled triaxial experimental equipment to study the permeability evolution of fractured sandstone. The experimental schemes of the permeability tests are as follows: (a) under increasing confining pressure; (b) under increasing liquid pressure; (c) under cyclic loading and unloading deviatoric stress; and (d) under increasing confining pressure and deviatoric stress synchronously. Results show that liquid flow on fractured sandstone can be regarded as laminar flow with low velocity. The permeability and stress sensitivity levels of the fractured sandstone are higher than those of the intact sandstone. The permeability decreases in a negative exponential manner when the confining pressure increases but increases in a positively exponential manner when the liquid pressure increases. The increase in the deviatoric stress decreases the permeability, whereas unloading increases the permeability. The entire evolution of permeability is irreversibly reduced. As the confining pressure and deviatoric stress increase synchronously, the permeability decreases, and the decrease rate tends to be stable. These results can provide an important basis for prediction of the permeability evolution of fractured sandstone and for efficient oil and gas exploitation.