Objective This study aims to reveal the microscopic damage characteristics and permeability evolutionary patterns of high-temperature granite reservoirs fractured under cyclic thermal shock produced by low-temperature fluids like drilling fluids and circulating heat recovery media in the exploitation process of the enhanced geothermal systems (EGSs).
Methods Using experiments on the fracturing characteristics of high-temperature granites under the conditions of varying cooling methods, numbers of thermal shock cycles, and cooling temperatures, this study determined microstructural characteristic parameters such as compressional wave (P-wave) velocity, pore volumetric fraction, and fractal dimension of granites across a temperature range of 25 to 700 ℃ under cooling via cyclic water injection and natural cooling. Using the CT scan-based 3D reconstruction technology and the interactive joint modeling technology based on Avizo and Comsol, this study built a model describing the evolutionary characteristics of microscopic pore permeability. Employing this model, this study revealed the streamline distribution patterns of the seepage, pressure, and velocity fields during pore fluid flow and calculated the absolute permeability in the X, Y, and Z directions.
Results and Conclusions Key findings are as follows: (1) The heat treatment temperature and the number of thermal shock cycles were negatively correlated with the P-wave velocity. Specifically, more thermal shock cycles corresponded to a more significant decrease in the P-wave velocity and more severe rock damage. Furthermore, the water cooling led to a more significant overall reduction in the P-wave velocity than air cooling; (2) In the case of granite temperature T≤300 ℃, CT scan slices revealed a small number of microcracks in granites, indicating poor connectivity. In contrast, at T≥400 ℃, microcracks and isolated pores occurred rapidly within granites, gradually forming an interconnected fracture network. Moreover, water cooling caused more significant internal damage-induced fracturing of granites; (3) The heterogeneity of microscopic pore structures resulted in differences in the calculation results of seepage in the X, Y, and Z directions. Consequently, the pore pressure decreased and the flow rate increased along the flow direction, with the flow rate surging sharply at locations where fracture channels narrowed. However, fluid retention or backflow was inevitable in some complex pore structures. The results of this study reveal the damage mechanisms and permeability evolutionary patterns of high-temperature granites under cyclic thermal shock, providing reliable parameters for the thermal stimulation of hot dry rock (HDR) reservoirs.
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