Numerical simulation of hydrothermal convection within discretely fractured porous media with application to the seafloor.

摘要

Fluid circulation driven by thermal gradients is an important physical phenomenon characteristic of the subsurface of the earth. My thesis contributes to investigating the physical behaviour and computational techniques of heat transport and fluid flow in explicitly fractured porous media. Both analytical and numerical methods are developed to study problems related in particular to hydrothermal convective systems in the mid-ocean ridge environment.; An analytical solution of the heat transport process in a single fracture is first derived for the impermeable host rock. For general hydrothermal convective systems, a numerical algorithm is developed. The algorithm is validated by comparing numerical results with analytical solutions. It is indicated discrete fractures not only initiate and maintain hydrothermal convection but also greatly change an established convection pattern.; The effect of anisotropic permeability fields on hydrothermal convection is studied both analytically and numerically. A more useful form of the Rayleigh number is defined, which contains the geometric mean of the vertical and horizontal permeabilities. The critical value for the onset of stable convection is derived analytically. It is shown that anisotropy resists the initiation of hydrothermal convection.; An alternative theory is developed to explain the origin of small-scale seafloor heat-flow variations over a mid-ocean ridge flank where basement relief is not clear. It is indicated that inclusion of explicit fractures in upper oceanic crust promotes and maintains hydrothermal convection within the upper basalts. Consequently, the small-scale heat-flow variations are produced on the seafloor.; A 3-D hydrothermal system within the TAG-like sulfide mound is examined. A range of models with different physical parameters and boundary conditions is investigated. Numerical results reveal that a central high-temperature zone is coincident with a non-magnetic zone inferred from magnetic data. Upflow fluid motion is along the vertical high-permeability column surrounding a central pipe. The maximum fluid velocity in the latter is in the order of the observed values.