Estimation of Fracture Energy from Hydraulic Fracture Tests on Mortar and Rocks at Geothermal Reservoir Temperatures

An Enhanced Geothermal System (EGS) can be thought of as an underground heat exchanger designed to extract geothermal energy. The performance of these systems can be improved by increasing permeability with hydraulic fracturing, following the same technique used for hydrocarbon reservoirs. To understand hydraulic fracturing, whether it is implemented in an EGS or in a hydrocarbon reservoir, it is important to know the fracture parameters of the rock at stake, e.g., the fracture energy. We report here the use of a method based on an energy balance during hydraulic fracture tests. Specimens were prepared and they have been mechanically and hydromechanically characterized at 20 oC and 100 oC, a temperature representative of actual reservoir conditions. The fracture energy is obtained from a balance of kinetic, potential and pressure energies involved in the hydraulic fracture tests. The method provides fracture energies that are consistent with the literature data on similar materials. It is also found that the fracture energy increases upon heating.