A COUPLED THERMAL-HYDROLOGICAL-MECHANICAL-CHEMICAL-BIOLOGICAL EXPERIMENTAL FACILITY AT DUSEL HOMESTAKE

Most natural and engineered earth system processes involve strong coupling of hydrological, thermal, mechanical, chemical, and sometimes biological processes in rocks that are heterogeneous at a wide range of spatial scales. Fluids - primarily water, but also CO₂, hydrocarbons and volcanic gases - exert a pervasive influence on processes in the Earth’s crust where their transport is moderated by flow through fractured heated rock under stress. A preliminary design has been formulated for a large-scale subsurface experimental facility at the 4850 foot level depth of the Homestake Mine to investigate coupled Thermal-Hydrological-Mechanical-Chemical-Biological (THMCB) processes in fractured rock at depth. The experiment will be part of the proposed Deep Underground Science and Engineering Laboratory (DUSEL) in the Homestake Mine, South Dakota. Geochemical, isotopic, microbiological, mechanical and transport experiments and numerical modeling are being used to guide the experimental design and to evaluate the anticipated time and spatial scales of the coupled THMCB processes. In particular, the experimental facility will probe the nonlinear feedbacks between processes that can dynamically modify physical and chemical properties – those that have taken place and those that will take place, as analogs of natural and engineered processes. Initial conditions and history are only known roughly at best, and the boundary conditions have likely varied over time as well. Processes such as multicomponent chemical and thermal diffusion, multiphase flow, advection, and thermal expansion/contraction, are taking place simultaneously in rocks that are structurally and chemically complex—heterogeneous assemblages of mineral grains, pores, and fractures—and visually opaque. The only way to fully understand such processes is to carry out well-controlled experiments at a range of scales (grain/pore-scale to decimeter-scale) that can be interrogated and modeled. The THMCB experimental facility is also intended to be a unique laboratory for testing hypotheses regarding effects of heat and chemical reactions on microbial communities. We describe an array of investigations that may be completed in such a facility related to the evolution of natural and engineered processes.