Triaxial permeability behaviors and structural damage evolution of deep hot dry rock under different cooling stimulation

Abstract

The geothermal exploitation of hot dry rock assumes a pivotal role in mitigating the global energy crisis and propelling the transformation of the energy structure towards a green and sustainable path. Thermal shock stimulation technology, as one of the fundamental and indispensable means in the development of hot dry rock geothermal resources, is capable of effectively augmenting reservoir permeability and enhancing the exploitation efficiency of geothermal resources by inducing micro-fracture networks. In this research, four distinct cooling methodologies, namely natural cooling, water cooling, liquid nitrogen cooling, and cycle liquid nitrogen cooling, were meticulously employed to conduct an in-depth exploration of the damage characteristics and permeability evolution behaviors of the granite pore structure under varying cooling conditions. The experimental findings clearly demonstrate that the pore structure of granite exhibits a pronounced dual-fractal characteristic under different cooling approaches. Moreover, an increase in the cooling rate is conducive to the formation of a more intricate pore and fracture distribution. Upon high-temperature cooling, the strength and ultrasonic wave velocity of granite are remarkably diminished. Low-temperature impact can significantly elevate the permeability of the reservoir, and notably, liquid nitrogen cycle cooling can enhance the permeability of the samples by several times compared to other methods. This study is poised to offer robust underpinnings for fracture creation and permeability enhancement in deep geothermal reservoirs.