Research: Geothermal Exploration and Resource Development

Department of Energy Geothermal Technologies Program (DOE)

We are working to develop new and improve existing geochemical and isotopic techniques that will significantly reduce the uncertainty in locating and defining the extent and potential of new geothermal fields.

Chemical and isotopic compositions of liquid and vapor phases reflect temperature- and pressure-dependent equilibration between the fluid and the rock matrix. They provide the most—often the only—information on the existence and conditions of a geothermal reservoir. Heat flow studies, tectonic models, and other geophysical regional analyses can sometimes be used to limit an explored area. However, chemical and isotopic methods are the only technologies that:

confirm the existence of a geothermal reservoir
quantify reservoir temperatures constrained by fluid-rock equilibration
identify the source of geothermal fluids and heat
trace fluid flow paths that define zones of fluid upwelling and the geometry and extent of a resource
estimate fluid ages that constrain flow rates through the system
quantify fluid circulation depths, the resource’s extent and its potential

Furthermore, we can obtain information on both proximal and distal reservoir conditions from determining varying rates of chemical and isotopic equilibration between fluid and rock matrix.

The Basin and Range Province in the western United States is characterized by an exceptionally high surface heat flow and therefore represents an area of vast geothermal potential. Geothermal exploration in the Basin and Range is severely hampered by:

a lack of surface features (springs, fumaroles, or wells) that can be sampled for chemical and isotopic indicators of the presence of deeper, higher temperature fluid reservoirs
Chemical and isotopic compositions of accessible features may obscure the evidence of higher temperature reservoirs due to mixing and/or chemical and isotopic re-equilibration in shallow, cooler reservoirs

In essence, the deep geothermal reservoirs are “hidden.” We are now evaluating existing, and pursuing new, geochemical and isotopic techniques, so we can assess these deeper, hidden geothermal systems.

Currently geochemical and noble gas isotopic data are collected from surface springs, fumaroles, and accessible wells throughout the Basin and Range and surrounding areas. The focus of our investigation is the relationship between the location of known geothermal resources, and the presence of fault-hosted, deep, permeable fluid-flow pathways, which are identified by anomalous helium isotopic compositions in surface fluids.

Although surface emanations and shallow well fluids are strongly affected by cold recharge, thus concealing deep, higher temperature reservoirs, early work suggests that helium isotope studies may provide the best, and perhaps only, indication of deep permeability, which indicates potential for higher fluid temperatures. We are developing this technique as a regional survey tool to identify areas of high potential for more detailed evaluation. Helium isotopes are proving useful for identifying the main fluid flow paths and upwelling zones: information that we can use to direct initial drilling targets.

Enhanced Geothermal Systems (EGS)

The Department of Energy Geothermal Technologies Program (DOE) has placed a high priority on the Enhanced (Engineered) Geothermal System (EGS) concept. The Earth’s natural thermal gradient represents an extensive geothermal resource, limited only by our inability to tap the gradient by engineering productive and sustainable geothermal reservoirs. It has been estimated that exploitation of the geothermal gradient in the western United States alone could add as much as 100,000 MW of base load electrical power.

The extent to which this vast, non-polluting resource can contribute to meeting our society’s increasing demand for energy will depend on conceptual and technical advances in exploration, reservoir definition, and development of sustainable reservoirs. A fundamental roadblock to the development of EGS is our inability to manipulate fluid flow in subsurface fractures in such a way as to accurately predict where, and how, heat, fluids and permeability co-exist, or can be engineered to co-exist, in commercially viable and sustainable quantities.

To meet these challenges, we are developing chemical and isotopic techniques to:

probe the thermal and chemical evolution of natural and induced fractures
quantify fluid flow rates through fractures
determine the heat extraction efficiency of circulating fluids.

This is part of a coherent, integrated research effort examining all aspects of fractures and fracture-networks.

Currently we are working on two projects:

Monitor changes in chemical and isotopic compositions of geothermal gases induced by injection of cold water into The Geysers Geothermal Field in northern California. Relate these changes to reservoir processes, and determine if they are coupled to microseismicity induced by fluid injection.
Develop and test isotopic techniques to estimate spacing of permeable fractures. Combining fracture spacing and reservoir geometry, we hope to estimate the integrated surface area of fluid-rock interaction and calculate the heat extraction efficiency of a fluid circulation loop.

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