The injection of large volumes of natural gas into geological formations, as is required for underground gas storage, leads to alterations in the effective stress exerted on adjacent faults. This increases the potential for their reactivation and subsequent earthquake triggering. Most measurements of the frictional properties of rock fractures have been conducted under normal and shear stresses. However, faults in gas storage facilities exist within a true three-dimensional (3D) stress state. A double-direct shear experiment on rock fractures under both lateral and normal stresses was conducted using a true triaxial loading system. It was observed that the friction coefficient increases with increasing lateral stress, but decreases with increasing normal stress. The impact of lateral and normal stresses on the response is primarily mediated through their influence on the initial friction coefficient. This allows for an empirical modification of the rate-state friction model that considers the influence of lateral and normal stresses. The impact of lateral and normal stresses on observed friction coefficients is related to the propensity for the production of wear products on the fracture surfaces. Lateral stresses enhance the shear strength of rock (e.g. Mogi criterion). This reduces asperity breakage and the generation of wear products, and consequently augments the friction coefficient of the surface. Conversely, increased normal stresses inhibit dilatancy on the fracture surface, increasing the breakage of asperities and the concomitant production of wear products that promote rolling deformation. This ultimately reduces the friction coefficient.

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Keywords: Sandstone fracture, Friction coefficient, Lateral stress, Normal stress, Shear rate, Rate-state friction model