Monitoring Slip Along Saturated Fractures in Granite at Elevated Temperatures
Authors: Han K. , Pyrak-Nolte L. J. , Bobet A. 
Published: August 2024

ABSTRACT:
Enhanced geothermal systems (EGS) require assessment of the mechanical instability of rock discontinuities at high temperatures, as fluid injection may change the loading conditions of the surrounding rock. A key challenge in monitoring slip along rock fractures is whether the same geophysical methods used to detect precursory signatures at room temperature are applicable at elevated temperatures. Laboratory direct shear tests were performed on tension-induced fractures in Sierra White granite in a custom-built chamber. The shear tests were conducted under an effective normal stress of 6 MPa at a temperature of 50°C, to test whether geophysical methods can detect slip at elevated temperatures. During the tests, the effective normal stress was applied first and the temperature in the chamber was increased to the desired value. Afterwards, a shear load was applied at a constant displacement rate. The normal and shear loads, and the normal and shear displacements were measured, as well as the chamber pressure (pore pressure) and temperature. During the tests, compressional (P) and shear (S) waves were generated and propagated across the fracture. The recorded full waveforms included both the transmitted and converted wave modes. The amplitudes of both modes showed a distinct peak prior to the shear failure, followed by a decrease. A peak in wave amplitude was attributed to a change in fracture specific stiffness caused by a change in the contact area between the two fracture surfaces undergoing shear. This peak is taken as a seismic precursor to the shear failure/slip of the rock fracture. The success of the tests at 50°C was necessary to continue the research at temperatures beyond 100°C. This work is ongoing and will be the focus of a future publication.

1. INTRODUCTION
Geothermal energy is heat energy that is extracted from deep underground and is considered a renewable energy that is an alternative to fossil fuels. Geothermal systems can be categorized into two types: (1) Naturally driven geothermal systems (hydrothermal system); and (2) Engineering-driven (or human-made) geothermal systems (enhanced geothermal system). A hydrothermal system uses the heat energy extracted from hot water and/or steam in porous or fractured rocks. In contrast, an enhanced geothermal system (EGS) creates artificial fractures and/or reopens pre-existing fractures in hot rock by injecting fluid into the rock mass. This technique, i.e., hydraulic fracturing, is used to increase the conductivity of low-permeability crystalline rocks (e.g., granitoids). Hence, EGS has the advantage of tapping more energy by extracting hot fluid circulating through engineering-induced fractures in crystalline rocks. However, fluid injection during hydraulic fracturing could alter pre-existing loading conditions in nearby rock discontinuities (e.g., magnitudes of in-situ effective stress and/or pore pressures), leading to induced seismicity (Eyre et al., 2019; Schultz et al., 2020). Thus, monitoring of slip along rock discontinuities (e.g., fractures, joints, and faults) is essential to the assessment of the mechanical stability of rock during and after hydraulic fracturing, which can be used to prevent or at least minimize induced seismicity.