Frictional stability and permeability evolution of fractures subjected to repeated cycles of heating-and-quenching: granites from the Gonghe Basin, northwest China

Abstract

Fluid injection into enhanced geothermal system (EGS) reservoirs can reactivate subsurface fractures/faults and trigger earthquakes—requiring that frictional stability and permeability evolution characteristics are adequately evaluated. This behavior potentially becomes more complicated when the impacts of temperature and cycled thermal stresses, and the resulted damage accumulation on both stability and transport characteristics are getting involved. We conducted coupled shear-flow experiments on saw-cut fractures recovered from an analog surface outcrop representative of a reservoir at 2450 m in the Gonghe Basin of northwestern China. The rocks were subjected to variable numbers of repeated heating-quenching (25-180-25 °C) cycles for shear-flow experiments at an effective stress of ~ 3 MPa and with velocity stepped between 10-1-10-1-10 μm/s. The smooth fractures return frictional coefficients in the range ~ 0.69 to 0.72 and are little affected by the thermal cycling. The frictional stability parameter (a–b) decreases and the instantaneous permeability increases with an increase in the number of heating-quenching cycles, during which intergranular and intragranular microcracks were generated in fracture surface. The above results indicate that the heating-quenching cycles during hydraulic fracturing of geothermal reservoir could affect both the fracture frictional instability and permeability evolution.