Numerical computation of hydrothermal £uid circulation in fractured Earth structures

Hydrothermal £uid circulation through porous Earth materials is an important physical phenomenon occurring in both submarine and continental environments. Irregularly interconnected discrete fractures are pervasive in nearly all Earth materials, providing preferential paths for £uid £ow and controlling the circulating £uid patterns. Most mathematical algorithms addressing hydrothermal convection problems treat rocks as piecewise continuous media. The representation of local, large changes in permeability requires a high level of discretization for accurate results and a corresponding large number of unknowns. The alternative is to incorporate fractures discretely through special adaptation of the numerical code.We adopt this approach to solve the coupled, time-dependent heat and £uid transport di¡erential equations using the ¢nite element method. The ¢nal algorithm is validated against both an analytical solution and numerical solutions from a complementary but less general ¢nite di¡erence scheme. Case studies of some simpli¢ed fractured models indicate that fractures can induce and maintain hydrothermal £uid circulation in media which would otherwise be passive. Fracture location can control both convection pattern and vigour in a closed system. Discrete fractures can also signi¢cantly change an established convection pattern. Multiply fractured porous media are comparable with the homogeneously aniso-tropic media in the numerical solutions if the e¡ective average horizontal and vertical permeabilities are kept the same. Key words: anisotropic medium, discrete fracture, ¢nite di¡erence method, ¢nite element method, fractured porous medium, hydrothermal circulation.