Mechanical Properties and Permeability Evolution of Red Sandstone Subjected to Hydro-mechanical Coupling: Experiment and Discrete Element Modelling

Abstract

This paper presents the hydro-mechanical behaviors of a red sandstone from the Three Gorges reservoir area using laboratory testing and numerical modelling. A series of laboratory triaxial compression tests were performed on the red sandstone to understand the effect of confining stress and seepage pressure on the rock mechanical properties and permeability. The laboratory results show that the initial elastic modulus, Poisson’s ratio and peak strength of the red sandstone increase with confining stress, and decrease with the increase in seepage pressure. The change in the permeability of the red sandstone is slight during the initial and elastic deformation stages, but increases rapidly when approaching to the peak stress. Micromechanical insights on the sandstone’s hydro-mechanical behaviors were unveiled by implementing an improved pipe network flow model in the discrete element method. The improved pipe network model can well capture the stress–strain response and the permeability changes of the red sandstone that were consistent with the laboratory test results. The numerical results reveal that the distribution of flow rate, pore pressure, and force chains in rock is closely related to microcracking, and the permeability increases accordingly with the increase in the number of microcracks and their coalescence. The number of microcracks and the rock fracture angle with respect to the axial axis at peak stress increase with the effective confining stress. The damage process of the red sandstone during the hydro-mechanical loading is divided into three distinct stages based on the mechanical response and permeability evolution.