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Geothermal energy has been widely proposed as a potential renewable energy to replace traditional fossil fuel energy. Hot dry rock (HDR) reservoir which contains abundant geothermal energy widely distributes in China. The Gonghe Basin in Northwest China is chosen to develop the Chinese first HDR field operation project. HDR is a low-permeability, high temperature and hard granite without fluid. Developing HDR requires water cyclically flowing between injection and production wells to extract heat energy. Hydraulic fracturing, as a key reservoir stimulation technology, can create the path of fluid cyclically flowing. However, few studies have investigated hydraulic induced artificial fractures in HDR geothermal formations. This paper investigated HDR geothermal reservoir stimulation characteristics and fracture patterns during hydraulic fracturing. Reservoir stimulation was conducted with a true triaxial hydraulic fracturing apparatus which could establish a real HDR formation environment in the laboratory. The factors affecting breakdown pressure and fracture creation were investigated via experiments and numerical simulations. This study could be used to evaluate and design reservoir stimulation in field HDR geothermal operation. [ABSTRACT FROM AUTHOR]

The total flow cycle (TFC) has a great potential to improve efficiency of geothermal energy utilization. However, it can not fully utilize the heat source due to the limited expansion ratio of the two-phase expander. To solve the problem, a coupled system of TFC and partially evaporated organic Rankine cycle (TFC-PEORC) is proposed. The thermodynamic model considering the pinch point location migration is developed, and the effects of operating parameters on the temperature matching and system performance are investigated. On this basis, system optimization is carried out. At the same time, the performance of TFC-PEORC was compared with the single-stage TFC, the coupled system of TFC and organic Rankine cycle (TFC-ORC), the coupled system of trilateral cycle and organic Rankine cycle (TLC-ORC), and the coupled system of TFC and Kalina cycle (TFC-KC) under optimal operating conditions. The results show that the net output power of TFC-PEORC has a 14.26 % increase compared to TFC, and that TFC-PEORC can provide excellent the temperature matching of the high temperature (HT) stage and the low temperature (LT) stage working fluids, thus providing the maximum net output power (84.36 kW) that is up to 1.93 % higher than TFC-ORC. [ABSTRACT FROM AUTHOR]

The abundant geothermal energy in hot dry rock (HDR) formations is an attractive renewable energy resource with great potential. China will develop its first HDR geothermal formation in the Gonghe Basin. HDR is a hard and low-permeability granite containing very few fluids. Development requires fluids to cyclically flow between injection and production wells to extract geothermal energy in the artificial heat transfer zone. Hydraulic fracturing is the main technology for creating flow paths. But few studies have investigated fractures in HDR geothermal formations. This paper investigated fractures as flow paths in HDR geothermal formations during hydraulic fracturing. Hydraulic fractures were simulated using a custom true-triaxial hydraulic fracturing test system in a realistic formation environment, in which a scaled wellbore was used that was built in outcrop granite rock from the Gonghe Basin. Fracture creation in granite was investigated via experiments, as well as influence factors, and what experience could be achieved. This study can be used to design and evaluate hydraulic fracturing projects in potential HDR geothermal formations. • Hydraulic fracturing was simulated in high temperature via experiment. • Fracture creations were studied in outcrop granite of HDR geothermal formation. • In-situ stress and weak planes affects fracture propagation. • Custom designed pumping procedures can enlarge the heat transfer area. [ABSTRACT FROM AUTHOR]

Abstract Hot dry rock geothermal energy is a type of renewable energy with great development prospects in deep strata. However, it is quite difficult to construct an artificial reservoir. In this paper, deep large-dip-angle fault zone in the Yangbajing geothermal field in Tibet of China is used as a natural artificial reservoir, and a set of hot dry rock heat extraction schemes for such fault modes is proposed. A three-dimensional thermo-hydro-mechanical coupling model is established for the scheme to study the distributions of temperature, stress and seepage during the process of mining fault-mode hot dry rock geothermal energy. The temperature of #1 production well remains 445 °C after 22-year operation and it decreases from 445 °C to less than 200 °C after 15-year operation in #2 production well. The initial vertical stress near the injection well is 199 MPa, which decreases to 193 MPa after 1 year and remains unchanged. The specific water flow in the fault zone between the wells increases negatively and exponentially with the extraction time. The seepage resistance of the fault rock mass gradually decreases. The total effective heat production is 13130 MWa after 22-year operation. Highlights • Fault zone can be used to easily construct artificial reservoir in EGS. • Seepage resistance of fault rock mass gradually decreases with mining time. • Extracted water temperature exponentially decreases with mining time. • A geothermal power plant with capacity of 500 MW has more than 25-year lifespan. [ABSTRACT FROM AUTHOR]