“FRAC” — A simulation code for forced fluid flow and transport in fractured, porous rock

The 3D Finite Element Program FRACTure was developed with the specific aim of studying the coupling of interactive mechanisms in geoscience and in particular those relevant to the long term behaviour of a Hot Dry Rock reservoir. The flexible modular structure facilitates the addition of further processes and elements to the existing library and the handling of linear and non-linear constitutive laws and the calculation of their interactions. The Hot Dry Rock applications involve essentially forced fluid flow of cool fluid injected into a hot fractured rock matrix. A study of the relevant processes required finite element solutions for the hydraulic, thermal and elastic fields and especially their interactions. Particular attention has been paid to modelling the perturbations arising from poro-elastic and thermo-elastic effects in the rock matrix and of a non-linear, stress dependent joint closure law. The fracture network is designed in a way to represent nature as realistically as possible: fracture flow takes place along planes or in pipes (fracture intersections), porous matrix flow in 3D bodies. In devising the coupling scheme careful thought has been given to the different time constants for each physical process. Apart of simulating the coupled Hot Dry Rock behaviour, the code has been successfully practised in a variety of applications in geoscience: analysis of geoelectric measurements, radon transport, solute tracer tests, simulations of heat probe operations and hydrogeologic system modelling.