Experimental Investigation of Water-Cooled Thermal Shock Effects on Hydraulic Fracturing of High-Temperature Tight Sandstone

Water-cooled thermal shocks degrade the physical and mechanical properties of rocks and lower their fracturing pressure. The effects of heating–watercooling (H–WC) cycles on the physical and mechanical properties of sandstone and the hydraulic fracture distribution characteristics are critical for the efficient development of geothermal and deep tight oil and gas resources. Herein, experiments were conducted to explore changes in hydraulic fracture following real-time high-temperature true triaxial fracturing at different temperatures under the thermal shock stress induced by cyclic water injection. High-resolution computed tomography was employed to identify hydraulic fractures after such injections. Results revealed that mechanical parameters, including uniaxial compressive strength, modulus of elasticity, and tensile strength, decreased gradually with rising temperature and H–WC cycle count, while Poisson’s ratio remained nearly constant. Permeability exhibited a positive correlation with both temperature and H–WC cycle count. Microscopic imaging revealed that surface pore density and size gradually increased with increasing temperature and H–WC cycle count. Newly formed pores around the sandstone mineral grains gradually interconnected, forming cracks after 10 H–WC cycles at 150 and 180 °C. At 120 °C, increasing the H–WC cycle count increased the pore area and pore distribution complexity. Furthermore, the magnitude of change increased when a threshold cycle count was reached, increasing the permeability and degrading macroscopic mechanical properties. Hydraulic fracture volume gradually increased with increasing H–WC cycle count, although the volume of the main fracture surface remained nearly constant. This expansion was attributed to the combined effects of enhanced microporosity and increased fracture volume of the hot rock around the fracture surface. The mechanism of the deterioration of the physical and mechanical properties of high-temperature tight sandstone subjected to H–WC treatment was elucidated. These findings provide experimental evidence and theoretical insights to support the long-term, reliable development of geothermal and deep tight oil and gas resources.