Thermal-shock-induced surface softening and conductivity evolution of hydraulic fracture in enhanced geothermal system

Abstract

To reveal the thermal shock effect on micromechanical properties and conductivity evolution of hydraulic fracture in enhanced geothermal system, nanoindentation tests and fracture conductivity tests were performed on granite samples after different thermal treatments. Experimental results show that the hardness and the Young's modulus of fracture surface decreased with the increasing thermal treatment temperature. This degradation of micromechanical properties indicates the softening of fracture surface due to thermal shock effect which further influence the conductivity evolution. For propped fractures, the higher thermal treatment temperature, the lower fracture conductivity under the same closure stress. Particularly, at the thermal treatment temperature of 300°C, the dominant mechanism of quick fracture conductivity loss transformed from the proppant crushing under high closure stress (>50 MPa) to the enhancement of proppant embedment under low closure stress (<30 MPa). For self-propped fracture with small roughness, the thermal shock effect enhanced the conductivity under in-situ closure condition, while reduced that in case of shear slip. When the rough fracture was characterized by a local hump, it increased the flow resistance of in-situ closed fracture under high closure stress and reduced the fracture conductivity, but may form an effective flow channel with ultra-high conductivity once shear slip occurred.