Numerical Modeling of Discrete Fractures in a Field-Scale FORGE EGS Reservoir

摘要

The response of the Fallon FORGE geothermal reservoir to hydraulic stimulation is modeled numerically to aid the conceptual design of the well geometries and completions. The numerical modeling approach is based on (a) a full hydro-mechanical coupling;;and (b) explicit representation of a discrete fracture network (DFN). The reservoir-scale model has a Core region of 1500 ×1200 ×1200 m~3 volume around the positions of conceptual sub-horizontal well designs at the Fallon FORGE site. It contains relevant geological structure and a fracture network constructed and simplified using borehole data. Stimulation was modeled in six stages/zones along the horizontal well using both openhole (low injection rate at 5 kg/s) and cased borehole (combination of high, 80 kg/s, and low injection rates) completions to test the effectiveness of hydraulic fracturing combined with hydro-shearing. The reservoir stimulation in the two models was analyzed and quantitatively assessed. Results indicate that the Fallon FORGE site is well suited for an EGS by multi-staged hydraulic stimulation using sub-horizontal wells. The conclusion is based on the evaluation of potential for fracture slip and aperture increase as functions of induced pressure changes and the orientations of the natural joints relative to the regional principal stresses. Advantages and limitations of explicit representation of the DFN in the numerical models are presented and discussed.