NUMERICAL COMPUTATION OF HYDROTHERMAL FLUID CIRCULATION IN FRACTURED EARTH STRUCTURES

Hydrothermal fluid circulation through porous Earth materials is an im portant physical phenomenon occurring in both submarine and continenta l environments. Irregularly interconnected discrete fractures are perv asive in nearly all Earth materials, providing preferential paths for fluid flow and controlling the circulating fluid patterns. Most mathem atical algorithms addressing hydrothermal convection problems treat ro cks as piecewise continuous media. The representation of local, large changes in permeability requires a high level of discretization for ac curate results and a corresponding large number of unknowns. The alter native is to incorporate fractures discretely through special adaptati on of the numerical code. We adopt this approach to solve the coupled, time-dependent heat and fluid transport differential equations using the finite element method. The final algorithm is validated against bo th an analytical solution and numerical solutions from a complementary but less general finite difference scheme. Case studies of some simpl ified fractured models indicate that fractures can induce and maintain hydrothermal fluid circulation in media which would otherwise be pass ive. Fracture location can control both convection pattern and vigour in a closed system. Discrete fractures can also significantly change a n established convection pattern. Multiply fractured porous media are comparable with the homogeneously anisotropic media in the numerical s olutions if the effective average horizontal and vertical permeabiliti es are kept the same.