| Description |
An enhanced geothermal system (EGS) is an engineered reservoir that has been stimulated to efficiently extract heat from low permeability geothermal resources. Generating efficient hydraulic pathways for heat exchange is crucial. Due to inadequate permeability, hydraulic stimulation is required in order to circulate water through a conductive fracture network between the injection and production wells. Developing long-term enhanced permeability might depend on one of two conductivity development mechanisms: 1) hydraulic fracturing (HF), and 2) hydro-shearing (HS). This research is aimed at understanding the effects and collaboration of hydraulic shearing in stimulation operation in EGS systems. Laboratory fracture conductivity tests were conducted to investigate the effects of temperature, time, and sliding (shearing) of natural fractures on the conductivity of a single fracture under FORGE (Frontier Observatory for Research in Geothermal Energy) reservoir conditions. Conductivities of these sheared fractures were compared to the same un-sheared fractures under the same testing conditions. The average conductivity of the fractures increased significantly after shearing. Increasing the temperature and the time of experiment, the fracture conductivity decreases. The conductivity of the fractures drops significantly over a 1-week period of experimental program. In one of the experiments, the conductivity drops to less than 30% of the value obtained at ambient temperature during the first 177 hours of testing. A 3-dimensional Discrete Fracture Network (DFN) model was developed for simulating and determining the optimum hydraulic fracture network for the proposed phase 3 injection and production wells at the FORGE system. Numerical simulations are used to study hydrogeothermal transport in fractured media. The effects of natural fractures on hydraulic fracture growth were studied considering changes in natural fracture intensity, flow rate, hydraulic pathways, permeability, and spacing. The study performed serves as a benchmark for developing an improved understanding of the effects of the existing FORGE natural fracture systems. The simulations suggest an optimized pumping schedule that generates conductive poststimulation fracture networks between hypothetical phase 3 injection and production wells and establishes an effective hydrological thermal model under various fluid flow conditions. It is often believed that fracture slip (shearing) is a primary stimulation mechanism in impermeable fractured reservoirs. However, determining the extent of fracture slip during stimulation is not clear. In this study, pressure analysis of the DFIT tests and FMI logs were used in the FORGE project to study the importance of the shearing mechanism during hydraulic facture stimulation. Comparing the FMI logs from first and second runs, before and after DFIT testing, shows that there are either shearing or mixed mode I and II mechanisms (tensile and shearing fracturing). In addition, pressure analysis of the DFIT tests and the numerical simulations of the DFIT tests suggest that pressure-dependent leakoff (PDL) occurs during the DFIT tests. In the case of DFIT tests in well 58-32, the shearing mechanism led to underestimation of the minimum principal stress. |
