3D Modelling of Fracture Injection Tests Using Nonlinear Joint Elements with Application to Sigma-V

Abstract

Heat extraction from geothermal reservoirs relies on water circulation in natural and hydraulically-induced fractures. Cold water is injected into the reservoir to extract the rock’s heat upon contact as it flows through fractures. The circulation involves fracture deformation, fluid flow inside the fractures, leakoff to the matrix, fracture propagation, and heat exchange. A three-dimensional displacement discontinuity model is used along with a fluid flow model to investigate the response of fractures to different injection/extraction scenarios. Nonlinear contact elements are used to represent closed fracture elements in this study. A fracture propagation scheme is implemented in this model to capture the tensile propagation of natural fractures. Several numerical examples pertaining to the SIGMA-V project are presented to analyze the natural fracture behavior under different injection scenarios including simultaneous injection-extraction and “huff and puff”. Our results indicate that both pre and post shut-in behaviors are affected by the fracture stiffness parameters. Fluid leakoff, on the other hand, controls the rate of propagation as well as the pressure variations inside the fracture.