This project will develop a preliminary design and work-breakdown-structure for a large-scale subsurface experimental facility to investigate coupled thermal-hydrological-mechanical-chemical-biological 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. Many natural and engineered earth systems involve coupling of multiple processes in rocks that vary across a wide range of scales. The most pervasive process in the Earth?s crust that gives rise to strongly coupled phenomena is the flow of fluids (water, CO2, hydrocarbons, magmas) through fractured heated rock under stress. Understanding changes in the reactivity, deformability, life-supporting and transport properties of rocks that fluids infiltrate is important in a broad range of geological engineering and geological science endeavors. Despite this fundamental importance, the interactions remain poorly understood.
The project will: (1) Determine properties of Homestake rocks: geological, geochemical, mechanical, thermal, isotopic, and reactivity. (2) Upscale these data to elucidate transport mechanisms (conductive versus convective), natural reaction rates in fractures, and microbial community evolution. (3) Evaluate monitoring strategies, in-situ probes and sampling methods, and necessary measurements. (4) Select a candidate site for the evaluating coupled processes. (5) Develop a work-breakdown-structure. (6) Develop a coupled numerical model to evaluate potential effects on the rock mass and optimal heater configuration, power, and monitoring borehole orientations.
The models and insight from these experiments will have broad applicability to engineered systems, e.g., enhanced geothermal systems, CO2 sequestration and subsurface contaminant transport. Educational outreach will involve facility tours and a traveling benchscale ?mock-up? demonstration experiment.
The goal of this project was to better undertand coupled thermal, hydrological, mechanical, chemical, and biological (THMCB) processes in the Earth's crust by constructing mathematical models of hydrothemal circulation in a controlled experimental setting at the Homestake Mine. This mine is the site a large scale deep undergroung science and engineering lab (DUSEL) being developed for a range of interdisciplinary studies. The particular goal of this project was to determine whether using a heated vertical borehole or borehole array could generater the appropriiate fluid flow, and thermal conditions to provide a suitable framework for setting up a long-term THMCB experiiment at the DUSEL site. WE found that if the experiment were to reach steady state within 10 years, the permeability of the rock adjacent tot he borehole or borehole array would have to equal or exceed a value of approximately 10^-10 m^2. This is a relatively large peremability, but premilimiary data from the mine suggested that the highly fractured rodk formations could be nearly that high. If the permeaility was approximately 10^-11 m^2, the experimental set up might still work of addtional permeability could be created by thermal fracturing or some other means. Alternatively, one could consider a non-steady state circulation system. The main conclusion is that the proposed experimental set up was worth for exploration and feasibility study. The broader impacts of this work conern the need for better understanding of coupled THMCB processes which are prevalent in the Earth's crust. These processes are important for the formation of ore deposits and for the development of enhanced (or engineered) geothermal energy resources. THMCB processes can also shed light on life in extreme environments and mahy play a role in crustal seismicity.
