Effect of Heating-Cooling Cycles on the Friction-Permeability Evolution of Granite Fractures Under Shearing
Authors: Cui Li , Zhang Fengshou , An Mengke , Zhuang Li , Wang Hengdong 
Published: September 2023

ABSTRACT
Fluid injection into geothermal reservoirs can enhance reservoir permeability and thus improve thermal recovery efficiency, but large-scale fluid injection may reactivate underground faults and induce earthquakes. The relationship between friction and permeability of natural faults/fractures under shearing, a critical factor for seismic potential, has not been well understood yet. It is of great importance to explore the friction-permeability relationship of deep geothermal reservoir fractures for the optimization of reservoir stimulation and the mitigation of induced seismicity during geothermal exploitation. In this study, we conduct velocity step shear-flow experiments on granite fractures collected from the Gonghe Basin, Northwest China, under repeated heating-cooling (25-180-25°C) cycles, to investigate the friction-permeability relationship of granite artificial fractures. In addition, the microstructure of the samples treated by different heating-cooling cycles is characterized. Experimental results show that the frictional coefficient of granite cracks ranges from 0.69 to 0.72. The frictional stability parameter (a − b) &gt; 0 under all conditions, indicating the velocity strengthening behavior. However, the value of (a − b) decreases and the fracture permeability increases with the increasing number of heating-cooling cycles. Microstructural characterization results show that the number, length and width of microcracks in the granite samples increase rapidly with the increasing number of heating-cooling cycles, and the pattern of microcracks develops from intergranular dominated to the mix of intergranular and intragranular. Our study shows that the frictional stability and permeability of granite fractures are affected by the heating-cooling cycles, which are also reflected by the change of microstructure on the fracture surfaces.

INTRODUCTION
Geothermal energy is a kind of renewable energy from the earth interior, and it is one of the most promising resources (Barbier, 2002; Moya et al., 2018). With the concern of energy shortage and the increasing demand of clean energy around the whole world, the exploitation and utilization of geothermal resource is of great importance for future development (Singh et al., 2020; Watanabe et al., 2020). At present, the enhanced geothermal system (EGS) is the common method to acquire the deep geothermal energy. This technology involves the cold-fluid injection to enhance the permeability of the geothermal reservoir and the hot water extraction from the deep reservoir (Brown et al., 2012; Wood, 2009). However, the fluid injection could greatly elevate the pore fluid pressure of deep strata, leading to the sudden decrease of effective stress in pre-existing fractures/faults and promoting the fracture/fault failure, even inducing earthquakes. (An et al., 2021; Candela et al., 2018; Li et al., 2022; Majer et al., 2007; Verdon & Bommer, 2021). The EGS process involves the sudden temperature change from the cold fluid injection into the geothermal reservoir and this temperature change could induce thermal stress and cause damage to granite fractures. The stronger heating-cooling cycles during hydraulic fracturing in geothermal reservoirs could be a predominant factor for fracture stability and permeability evolution.