Comparative experimental investigation on shale fracture propagation induced by high-temperature water and carbon dioxide

High-temperature convective heating is a promising in situ conversion method that has the potential to improve the productivity of oil and gas reservoirs substantially. Thermal fluid can efficiently fracture rocks and enhance the permeability of reservoirs. A self-developed in situ high-temperature convective thermal simulation experimental equipment was utilized to fracture shale in this research. The patterns of fracture propagation in shale fractured by high-temperature water and CO₂ were systematically investigated. The experimental results indicated that high-temperature convective heat may significantly decreased the fracture pressure and increased the complexity of fracture propagation. In comparison to conventional hydraulic fracturing, the fracture pressure of shale decreased by 50.75% with 450℃ water fracturing and by 62.23% with 450℃ CO₂ fracturing. The decrease in shale fracture pressure mainly resulted from the permeation of low viscosity fluid into minuscule pores or fractures, reducing the effective stress. Elevated temperature fluid induced the formation of many bending fractures, and the width of these fractures enlarged. As the temperature of the injected fluid rose, the intensity of thermal shock, the number of microcracks, and the severity of rock damage all increased. CO₂ fractured shale had more branch fractures and rough fracture surfaces than water. These characteristics facilitated the formation of complex fracture networks inside the shale. The fracture properties of thermal fluid fracturing shale were better understood through this work, which offered references for implementing in situ high-temperature convective thermal recovery in shale reservoirs.