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Engineering Research Opportunities in the Subsurface: Geo-hydrology and Geo-mechanics
Executive Summary
The potential development of a national underground science and engineering laboratory offers significant opportunities to improve our understanding of the processes governing the mechanics of, and the transport of fluids in, fractured rocks. Importantly, these processes govern our ability to recover petroleum, mineral and geothermal resources, to restore contaminated sites to pristine conditions, to construct safe structures in rock, and to provide for the safe entombment of wastes.
Despite significant advances over the past three decades, our understanding of the processes
governing the transmission of stress and the motion of fluids in fractured rocks, the agents of complex thermal-hydraulic-mechanical-chemical-biological interactions, and our ability to both characterize material properties and to project system response remains limited. Lacking is access to a centralized underground laboratory, where the critical issue of scale effects may be rigorously examined with unusual spatial access to a large block of rock. The absence of rational procedures for the design of full-scale engineering structures in rock is a critical limitation compared to other branches of engineering. The potential NeSS site at the Homestake Mine represents but one opportunity for the constrained study of these process-interactions at relevant spatial scales of meters to hundreds of meters, at a broad range of stresses and temperatures, and at temporal scales of days to years to decades.
Important advances in geo-engineering and geo-hydrology are possible in at least four areas: 1) complex coupled-process interactions, 2) rock deformation and the state of stress with application to construction in rock, 3) the profound effect that fractures may exert in conditioning behavior, and 4) the resulting flow and transport of fluids.
Complex coupled-process interactions control the flow and transport of fluids, and of energy and nutrient fluxes in the fractured subsurface - these are strongly stress, temperature, and scale dependent. Experiments conducted at an underground laboratory will contribute to our understanding of key process-interactions over the short- and long-term, catalyze interactions between scientists representing a broad array of disciplines, and spur the development, deployment, and testing of new sensors and sensing techniques.
4 Similar experiments will contribute to our understanding of rock deformation and the state of stress, accommodating the strong controls of fractures, spatial scale, and process-interactions on the performance of structures constructed in rock. An underground laboratory will allow the testing of rational methods of design and new construction systems that both reduce reliance on empirical methods, and minimize over-support. Uniquely, detailed measurements of system durability may be made over extended durations, and compared with predictions of process-interactions.
Fractures control the mechanical behavior, and the flow and transport of fluids in fractured rock. Observations and tests in an underground laboratory will advance understanding in the modes of fracture formation, and enable the constrained development of geophysical methods of fracture-detection, and fracture-characterization.
Experiments at an underground science and engineering laboratory encompass those that examine the existing conditions of the access drifts and chambers, those that predict mechanical and hydraulic response as new structures are excavated, and those that seek pristine conditions in remote portions of the laboratory for the conduct of in situ tests, or the examination of the performance of structures in rock. The potential for the forensic examination of structures, including the confirmatory exhumation of test blocks, is an important and unique attribute of an underground laboratory.
Parallel opportunities exist in education from elementary to graduate level. These include an important contribution to the training of a new generation of Earth scientists and engineers through the provision of field school activities, participation in important research projects, and through economies of scale with other science and engineering communities in the U.S. Specific opportunities exist with current or proposed seismic (Earthscope-USArray), and hydrologic (Consortium of Universities for the Advancement of Hydrologic Science) initiatives.
1. Overview
The engineering accomplishments and advances in scientific understanding of outer space by the United States over the last half century are eloquent testimony to what can be achieved when the national government makes a commitment to a particular goal. The Neutrino and Subterranean Science (NeSS) project is an exciting development whereby the study of elementary particles from outer space requires a link to the science and technology of subterranean or inner space. An underground site dedicated to geoscientific investigations on various scales, such as envisaged in the NeSS project, could result in major advances in subterranean science. It was for this reason that the National Science Foundations asked the American Rock Mechanics Association Foundation to convene a workshop in September 2002 to address the rock mechanics and rock engineering research opportunities for a proposed national underground science and engineering laboratory. This is the report that emerged from that workshop; it was extensively reviewed by the U.S. and international rock mechanics communities.
The proposed development of an underground science and engineering laboratory for neutrino
experiments offers significant opportunities to improve our understanding of the processes governing the mechanics of, and the transport of fluids in, fractured media. Access to an extensive network of drifts to depths of 2500 meters, the attendant large range of in situ stresses and temperatures, and the potential long-duration of the experiments offer unique opportunities for geo-hydrological and geo-mechanical research. Importantly, the unusual spatial access to a regional-scale geologic block enables, through its dissection, the corroboration of processes not feasible by mere access through boreholes or geophysical methods. Specifically, proposed tests will examine the role of complex process interactions of temperature, stress, reactive chemistry and biology on the hydraulic and mechanical behavior of fractured rock masses, at spatial scales of meters to hundreds of meters, and at temporal scales of days to years. In addition, the facility offers important opportunities for the development and validation of new mechanical, hydraulic, geochemical tracer, and engineering geophysical methods for the characterization of mechanical and transport properties. An underground facility would allow the development and testing of new sensors and improved mathematical models to be applied to fractured rock masses, aquifers and reservoirs. These scientific needs are addressed in a series of proposed experiments, identified following.
2. Scientific Rationale
The proposed underground research laboratory addresses a number of contemporary needs in geohydrology and geo-mechanics. The processes governing the transmission of momentum, fluid, mass, and energy fluxes, particularly in fractured media, remain inadequately understood. These processes govern our ability to recover petroleum, mineral and geothermal resources, to restore contaminated sites to pristine conditions, to construct safe structures in rock, and to provide for the safe entombment of wastes. Engineering applications of particular societal significance, and key technical uncertainties involved in their development are included in Table
These include: Resource Recovery etroleum and Natural Gas Recovery from Conventional/Unconventional Reservoirs
In Situ Mining
Hot Dry Rock/ Enhanced Geothermal Systems (HDR/EGS)
Potable Water Supply
Mining Hydrology
Waste Containment/Disposal
Deep Waste Injection
Nuclear Waste Disposal
CO2 Sequestration
Cryogenic Storage/Petroleum/Gas
Site Restoration
Acid-Rock Drainage
Aquifer Remediation
Underground Construction
Civil Infrastructure
Underground Space
Secure Structures
Lacking is our ability to understand processes governing the transmission of stress and the
motion of fluids (viz., fracture geometry, connectivity, and transmission characteristics),
7 processes governing their interaction with their environment (viz., coupled THMC(B)1 feedbacks, involved in developing conduits and in modifying their properties), in effectively characterizing their mechanical and transport characteristics (viz., mechanical, hydraulic, tracer, and geophysical techniques), and in effectively projecting system response (viz. sensing and monitoring, data fusion and modeling).
Although significant advances have been made in understanding these interactions over the
past three decades, important questions remain. These relate both to the understanding of fundamental process interactions that control the response of the natural system, and how these systems may be harnessed for the recovery of minerals and energy, utilized for civil infrastructure and the safe disposal and containment of wastes, and with minimized impact on the natural environment.
2.1 Societal Needs
Important societal benefits will accrue from improved techniques and technologies to recover
minerals and energy, to provide safe disposal and containment of wastes, to afford the effective restoration of contaminated sites, and to contribute to the safe use of the subsurface for civil infrastructure. Resource Recovery: The ready supply of fuels, energy, and minerals powers modern society. The availability of a secure, extensive, and distributed supply of potable water is a societal imperative.
Petroleum products supply modern society with inexpensive and convenient transportation fuels and an endless array of plastics and petroleum products. Natural gas is an abundant fuel that generates the least CO2 when burned compared to petroleum or coal, and provides one likely fuel-stock to power the widely touted hydrogen economy2. Effective recovery of these fuels requires their initial
1Thermal-Hydraulic-Mechanical-Chemical-and-(Biological) processes influence the transport of fluids in fractured rocks. These processes may act against us, for example, limiting the delivery of amendments for bioremediation by pore or fracture clogging; or may act for us, as in improving recovery from petroleum reservoirs by hydraulic fracturing.
2 Reliable, Affordable, and Environmentally Sound Energy for America.s Future. Report of the National Energy Policy Development Group, to the President of the United States. discovery, typically through surface geophysics and drilling, and their subsequent removal from reservoirs, typically 2 to 5 km deep, via an array of vertical or horizontal branching boreholes. Endemic uncertainties relate to the hydraulic connections that may be developed by routine completion methods. Such methods include hydraulic fracturing of the wells, where the roles of structural features such as faults and fractures at a variety of length-scales control the depletion of the reserve. Ambiguity remains between features usefully observed from geophysical imaging, and what these features mean in developing the reservoir. A depleted reservoir may retain 80% of the original oil. Improved methods and understanding of the motion of fluids may increase this yield with subsequent improvement in the reserve base and in energy security. An underground laboratory may address these issues, albeit in a non-reservoir rock, through an improved understanding of the crucial role of fractures and faults on the displacement of fluids, on the integrity of wells, and in controlled appraisal of geophysical methods in defining hydraulic performance. Mined minerals are a fundamental need of modern society, for example, copper recovered for use in power electronics, and gold and other precious metals used in electronic components and devices.
As high-grade deposits are depleted, the lower grade and deeper deposits may only be recovered if mining methods are economically viable. In situ mining provides a potential solution where the desired mineral is recovered directly by a solvent that targets that mineral in particular. The solvent is injected in situ, into the ore, through boreholes drilled either from the surface or from the deep mine.
In situ mining methods offer the environmental advantage of reducing the amount of waste rock produced per ton of recovered mineral, and thereby reducing production costs. However, it also poses significant challenges in the development of controlled fracturing in the remote ores, in adequately characterizing the transport characteristics of these pathways, and in predicting the recovery of minerals from the application of tailored solvents. More directly than in the case of petroleum recovery, an underground laboratory provides a unique opportunity to explore these recovery techniques under closely controlled conditions. This evaluation may include the effective exhumation of the test cell to corroborate estimates of transport behavior from time-lapse geophysics, and predictions of reactive transport.
The recovery of geothermal energy from hot dry rock (HDR) reservoirs offers the potential to drastically reduce the emissions of greenhouse gases associated with the recovery and utilization of fossil fuels. HDR reservoirs have the advantage that they are present beneath major population centers in the United States, and the world, with an estimated reserve base one hundred times larger3 than that for fossil fuels. The further development of HDR geothermal reservoirs suffers from the disadvantage that access is limited, both for want of an inexpensive methods of drilling, and by our understanding of processes for development and production of the reservoir. A deep underground laboratory offers the opportunity of testing and observing the effectiveness of drilling methods, at depth, and in developing geophysical and tracer methods to follow the evolution of the reservoir with time, with unusual access to the reservoir level.
The availability of a dependable, secure, and uninterrupted supply of potable water is a societal imperative. As population pressure reduces the excess of availability over demand, any and all potential sources of potable water will play an increasingly important role. Ground water will become an increasingly important resource that offers significant advantages over surface supply in its potential for protection against surface-borne pathogens, maliciously introduced agents of bioterrorism, and of routine evaporative losses to the atmosphere. Non-traditional aquifers, including those that are fracture-dominated, may become increasingly important. In the Northwest, for example, deep fractured basaltic aquifers may become important, if the recharging annual supply from the snow pack is reduced, as projections of global warming indicate. Groundwater resources may become an important secondary source of supply.
Waste Containment and Disposal: Modern society produces a vast array of wastes ranging from the massive and benign unregulated discharge of CO2 from the burning of fossil fuels, to the scrupulously controlled inventory of long-lived fission products from nuclear power generation. For many of these products, deep geologic isolation is a potentially effective method of disposal, although many questions remain of its effectiveness.
Deep injection offers a convenient, environmentally safe, and economical method for the disposal of liquid and solid wastes in deep saline reservoirs, or in depleted petroleum reservoirs. An injection well is completed to depth, and the waste is pumped either as a liquid, a slurry, or as a grout that will solidify in place. Disposal is relatively inexpensive and ostensibly secure . formations that once trapped hydrocarbons over geologic time may also provide adequate long-term containment of the injected fluids. Despite the relative surety of this logic, few means exist to track the migration of these Reserve base based on a drawdown in crustal temperature by 120°C from a crustal depth of 3 km to 6 km. A 1°C drop in temperature over the same interval yields 200, 000 Quads (Quadrillion BTU), comparable to the total fossil fuel reserve of 360,000 Quads [Armstead and Tester, Heat Mining, Spon, p 57., 1987]. 10 fluids, and ensure their immobilization. Similarly, deep sequestration of CO2 is one potential method to stem the release of anthropogenic greenhouse gases to the atmosphere. Again, saline aquifers may be used, or the CO2 may be utilized as a stimulant to improve the recovery from otherwise depleted petroleum reservoirs. Current estimates of $100/ton-disposed must be reduced to $10/ton if geologic sequestration is to be economically viable. Significant unknowns remain in characterizing reservoir capacities, in certifying the integrity of caprocks, and in assuring containment over many decades. The development of an underground laboratory offers the potential to observe, in a controlled environment, the factors that influence the development of injection processes, albeit in an unlikely candidate rock, and confirm the efficacy of containment. Deep geologic isolation is the preferred method for the interment of spent nuclear fuel for all 30 developed nations who face this issue. Despite the large amount of money spent on site investigation and process characterization studies at the proposed repository for civilian and defense high-level nuclear waste at Yucca Mountain, significant uncertainty remains about the role of hydrologic processes controlled by the effects of the heated canisters. Importantly, the hot repository will alter the current hydrological regime, which in turn may modify the transport characteristics of the fractured rocks surrounding the repository. Migration pathways may seal or gape, with the coupling of subtle and of strong chemical, biological, and mechanical feedbacks alike, only marginally understood. The evelopment of an underground laboratory offers the potential to observe such interactions at a variety of length and time scales of relevance, where importantly, subsequent exhumation will not compromise the integrity of the containment structure.
Site Restoration: The mining of sulfide deposits has left a significant legacy of acid rock drainage from the resulting waste rock piles and tailings sediments. When removed from depth by mining, pulverized by the processing of the ores, and finally given free access to oxygen, water, and catalytic bacteria, sulfide waste products readily generate sulfuric acid that may be lethal to the ecosystems they encounter. The resulting problems are epitomized by the consequences of the bankruptcy at Summitville,4 where restoration costs are estimated at $120 million and in the reclamation of the
Berkeley pit5 in Butte, Montana, where the recovery plan will cost $87 million. Absent are effective and inexpensive methods to reduce or eliminate acid discharges, by either active or passive means.
The provision of a facility where tailings have been pre-disposed underground, and of the extensive
The Summitville Mine and its Downstream Effects. USGS Open File Report (update) 95-23 by Bigelow, R.C., and Plumlee, G.S. geology.cr.usgs.gov/pub/open-file-reports/ofr-95-0023/summit.htm
See the Department of Justice settlement of March 25, 2002.11 surface waste facilities that must be immobilized, provide an important natural laboratory for prototype testing of passive treatment, encapsulation, and immobilization technologies, and techniques for the value-added recovery of remnant ores. Underground Construction: Increasing urbanization and the desire to maintain environmental quality in the face of increased demands for surface space are focusing more attention on the possibility of using the underground space beneath cities. The traditional underground road, rail, fresh water supply and sewage systems are being augmented by a variety of uses. In Stockholm, sewage treatment plants are underground; in Chicago, the Tunnel and Reservoir Plan (TARP) now under construction, is intended to capture combined storm and sewer water overflows during high flow periods and to store this contaminated water in an underground reservoir until it can be processed by a sewage treatment plant. This avoids the discharge of sewage into local waterways and, in some cases, into basements.
The ability of a solid rock cover--be it a few meters, tens or hundreds of meters thick--to provide a robust isolation of an activity from the surface and the open atmosphere, is a valuable attribute that offers numerous opportunities for the geologic isolation of high level nuclear waste and for the basing of underground reactor facilities and hardened structures. Atmospheric contamination is a particularly severe hazard because of the rapidity with which it can spread and the great difficulty of controlling the movement of air in the open atmosphere. A number of industrial operations pose significant hazard when located on the surface. In 1984, for example, an explosion at the Union Carbide chemical plant at Bhopal, India, released toxic gases into the air, resulting in almost 4,000 deaths and leaving 11,000 with disabilities. In April 1986, the Chernobyl (Ukraine) nuclear explosion projected a plume of dangerous concentrations of radionuclides into the atmosphere where winds carried it rapidly well beyond the boundaries of Russia and around the
world. Nobel Laureate Andrei Sakharov recommended that nuclear plants be located underground.
Several designs had, in fact, been proposed prior to Chernobyl.6 The generator plant, 100 m~150 m underground, would be linked to the surface by a number of tunnels containing filters to trap any radionuclides released in the event of an explosion. Several underground nuclear power plants were constructed and operated in Siberia.
Watson, M.B, W.A.Kammer et al. (1972) Underground Nuclear Power Plant Siting, EQL Report No. 6 Environmental Quality Laboratory, California Institute of Technology, Pasadena, Calif. 91109, September, 150p:
Bernell L. and T. Lindbo (1965) Tests of Air Leakage in Rock for Underground Reactor Containment,
Nuclear Safety Vol.6 No.3 Spring, p.267.