Fluid Flow in a Fracture Network Under Changing Stresses: A Laboratory Study

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ABSTRACT: Fluid flow in fractured rocks is ubiquitous in natural and human-induced subsurface processes. Subsurface technologies, such as in-situ mining, CO2 storage and geothermal exploitations, require large contact areas between the flowing fluid and the rock to facilitate efficient heat and mass transport and, therefore, ensure the effectiveness of these technologies. Much of the experimental research on fluid flow in fractures has focused on a single fractures in a rock specimen, which leaves the fluid flow in fracture networks under changing stresses largely unexplored. Here, we study fluid flow in a fracture network under changing stresses using a triaxial system. We create the fracture network composed of five orthogonal fractures in rock specimens by assembling six saw-cut rock parts. We measure the overall permeability of the fracture networks under changing vertical and horizontal stresses. We find that the fracture network permeability decreases when the vertical and horizontal stresses both increase. A greater reduction of permeability is observed with increasing horizontal stress than that with increasing vertical stress. We show that this phenomenon results from the larger total area of the vertical fractures than that of the horizontal fractures, making the fracture network more sensitive to horizontal stress changes. Our study shows the importance of the relative orientations between the fractures and the stresses in determining the permeability of the fracture network. This insight can be incorporated to improve the discrete and continuum modeling of fracture networks. 1 INTRODUCTION Fracture networks are the "highways" for the fluid flow in subsurface rock-fluid systems. They provide a fast path for contaminant transport in fractured aquifers (Tsang and Tsang, 1987; Grisak and Pickens, 1980; Grisak et al., 1980). They could increase the permeability of a geothermal reservoir and, at the same time, reduce the heat extraction efficiency by inducing short-circuiting (Gee et al., 2021). They can also localize the dissolution of porous rocks to the fracture along and induce larger cavities earlier (Li et al., 2021). The overall permeability of a fracture network is affected by the fracture orientation, stress conditions, and fluid pressure. Understanding and predicting the the fracture network permeability as the stresses and pore pressure changes are important for many engineering applications in the subsurface rock-fluid systems.