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Currently, water fracturing under deep geothermal conditions remains poorly understood because the reservoir rocks are usually high-strength crystalline rocks characterized by high temperatures. The aim of this study is to investigate the effects of rock properties, injection rates, and temperatures on hydraulic fracturing behavior and the induced crack characteristics through experimental investigations. A series of hydraulic fracturing experiments was conducted on two Indosinian granite types to investigate the differences in hydraulic fracturing behavior caused by rock properties. Among others, six samples were tested under a room-temperature condition at different injection rates from 1 to 30 mL/min to clarify the effect of the injection rate and three samples were tested under a high-temperature condition (150 • C) to simulate specific geothermal environments. The results indicated that granites with different rock properties have different injection rate thresholds. When the injection rate is below the threshold, the injection pressure finally reached a constant value without fracturing. For rocks with the same properties, the temperature effect can lead to a high injection rate threshold due to the occurrence of thermally-induced cracks. The number of acoustic emission events recorded during the room-temperature experiments increased linearly with increasing injection rate, while high-temperature tests increased sharply. The investigation results imply that a complex hydraulically-induced crack network is expected to be achieved in geothermal reservoirs by a high injection rate or high temperature differences (between injected fluid and rock). Additionally, the characteristics of the hydraulically-induced cracks were investigated by cutting through the sample blocks and measuring the residual pressure. The results indicated that the induced crack aperture can maintain a fluid conductivity of 0.1-0.8 mm/s at a closure pressure of 12 MPa.

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Hot dry rock (HDR) geothermal resources are renewable energy source. Many of the findings of HDR resource evaluations have been used in energy planning and EGS design. However, to assess the amount of HDR resources in different locations, a consistent classification scheme and evaluation methods are still lacking. Considering geological credibility and economic feasibility, HDR resources are separated into three categories: vision, reserve, and exploitable. Vision and reserve are stationary resources that can be evaluated using the volume technique, and the exploitable resources can be evaluated using the numerical simulation approach. The HDR vision resource of the Gonghe Basin is evaluated to be 4.076 × 1022 J, and the reserve resource of the Qiabuqia HDR mass is evaluated to be 2.11 × 1020 J. At the Qiabuqia HDR development site, a discrete fracture network (DFN) model is applied for numerical simulation computations, which is based on the notion of local thermal nonequilibrium. The K1 and K2 wells produce varying amounts of heat due to the heterogeneous features of the fractured medium model, which is primarily due to differences in fracture density, heat exchange area, and fluid migration pattern. The categorization system and assessment technique can be used as a guide for evaluating HDR resources in the future.