Research and Practice of Flexible Reservoir Stimulation Technology for Hot Dry Rock Geothermal Reservoirs

Hot dry rock masses are characterized by four characteristics: high temperature, high hardness, high in-situ stress, and a high differential horizontal in-situ stress, and they face technical challenges such as high breakdown pressure, difficulty in forming complex fracture networks, small stimulated reservoir volume, and induced strong microseisms. Therefore, existing unconventional oil and gas fracturing technologies are not applicable. To address this problem, physical simulation experiments were conducted using large-scale hot dry rock outcrop samples to study the effects of different geological and engineering conditions on the breakdown pressure, fracture propagation complexity, and the number and energy level of induced micro-seismic signals. Based on the obtained insights, a flexible reservoir stimulation technology for hot dry rock, which integrated reducing the breakdown pressure, improving the fracture complexity, and preventing the induction of strong microseisms, was proposed, and field practices were conducted in areas such as Gonghe of Qinghai, Xinghua of Jiangsu, and Haikou of Hainan. The results indicate that hot dry rock fracturing first opens natural fractures and propagates along the natural fractures; injecting CO₂ reduces the breakdown pressure by 30% and the AE energy level by about 15%; cyclic alternating injection easily triggers and induces the formation of complex fractures; natural fractures, large thermal differential effects, and alternating injection of slickwater and supercritical CO₂ are the main controlling factors for forming complex fractures in hot dry rock. Practices indicate that this technology shows good field effects. Study and promotion of the application of this technology can promote the development of stimulated reservoir volume technologies for hot dry rock and support the utilization of hot dry rock resources in China.