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Hydraulic fracturing has been used as an indispensable method of improving energy productivity in oil, gas and geothermal reservoirs. Current experience of hot dry rock reservoir fracturing shows that new cracks are hard to create in intact hard rocks, and therefore, hydraulic fracturing in rock mass with pre-cracks is more efficient. However, there is a lack of easily applicable theoretical methods to predict the propagation direction of pre-cracks during hydraulic fracturing. In this study, a new method based on the hyperbolic failure envelope is proposed to qualitatively predict the crack-initiation direction of pre-cracks and to evaluate the failure mode of the rock adjacent to the crack tips during hydraulic fracturing. Results show that the failure mode of the rock and the crack-initiation direction at the pre-crack tips are dominated by the initial in-situ stress state and the mechanical properties of the rock. Four scenarios involving different crack-initiation directions and rock failure modes are discussed in this study, and corresponding crack propagation direction and failure mode are given. Numerical modeling results based on the extended finite element method show good agreement with theoretical prediction results. Comparisons with an experimental study from literature further verify the theoretical prediction and numerical modeling results. The proposed method can provide guidance for the hydraulic fracturing stimulation of not only geothermal energy reservoir, but also oil and gas reservoirs with pre-existing natural fractures.

The hydraulic and heat transfer properties of artificial fracture networks are key to the efficiency of energy production from geothermal reservoirs. To date, no conclusive view exists of the evolution in fracture permeability and heat transfer coefficient when arbitrary stresses and temperatures are applied. This work examines the evolution of hydraulic and heat transfer properties during simulated geothermal energy extraction using a novel fluid flow-through test device accommodating large single artificial fractures in granite. Experiments are conducted in two contrasting modalities: at constant temperature with increasing confining pressures, and at constant confining pressure with increasing temperature. At constant temperature, as the confining pressure increases from 4 to 20 MPa, both hydraulic and heat transfer properties decrease, with permeability decreases by 46–63% and heat transfer coefficient decreases by 13–67%. Permeability decreases by 28–37% as temperature increases at constant confining pressure larger than 10 MPa, but permeability may first decrease and then increase at low constant confining pressure of 5 MPa. As the temperature increases from 100 to 200 °C at constant confining pressures, heat transfer coefficient increases by 25–45%. Results show that confining pressure impacts hydraulic properties more strongly than heat transfer properties, while reservoir temperature affects the heat transfer properties more strongly than hydraulic properties. These new findings on the evolution of permeability and heat transfer rate for different paths of temperature and confining pressure are critically important to the understanding of heat production from real geothermal reservoirs.