Dynamic Model of Discrete Fracture Opening under Fluid Injection

Abstract

A new, dynamic, fully-coupled, Thermal-Hydraulic-Mechanical-Chemical (T-H-M-C) numerical model is developed for simulating flows and transport in Enhanced Geothermal System (EGS) reservoirs. The model assumes the presence of a single, pressure-conducting planar fracture of unknown aperture and size or a system of such fractures in a geologic medium before the onset of fluid injection. The shape of each planar fracture both in aperture and lateral extension is determined by the dynamic balance of the hydrodynamic fluid pressure distribution over the fracture plane and the elastic compression resistance of the geologic rockmass surrounding the fracture. The non-isothermal, and time-dependent, planar, flow, pressure, temperature, and chemical species concentration distribution in the fracture is simulated with a Computational Fluid Dynamics (CFD) element in MULTIFLUX. The fracture aperture at each surface grid is adjusted iteratively, allowing for: (a) elastic deformation in the fracture system by hydrodynamic pressure; (b) thermal dilatation of the rock; and (c) geochemical precipitation and/or dissolution. The CFD model-element in MULTIFLUX is coupled to the model of the host geothermal formation by importing the numerical, non-isothermal, time-dependent results from TOUGH2 and/or TOUGHREACT. Coupling of the T-H-M-C model of the fracture flow to the model of host rockmass applies the NTCF (Numerical Transport Code Functionalization) technique, a modeling accelerator of the iterations in MULTIFLUX. A model validation example is given comparing simulation results with published data for the Fenton Hill EGS experiments. The results prove the capabilities of the new model in dynamically controlling fracture shape including the development of fracture opening as well as lateral-transversal size evolution.