There are two requirements for use of a geothermal resource, temperature and a fluid to transport the heat in the resource to the surface. For Ground-Source or Geoexchange Heat Pumps, the fluid is usually pumped from the surface into the ground (see Heat Pumps), although ground water may be used as the heat exchange fluid. For Direct Use or Electricity Generation, ground water either rises to the surface naturally or is pumped to the surface to transport the heat to the surface. However, in most areas, temperatures within a few miles of the surface are sufficient for direct use or electricity generation, but there is insufficient ground water to transport the heat to the surface. In these areas heat transport may be increased by the creation of an Engineered or Enhanced Geothermal System (EGS).
In areas where there is insufficient groundwater to transport heat to the surface, the problem is typically not a lack of water, but a lack of interconnected pathways, or permeability, through which the water may flow. There are two approaches to solving this problem: 1) increase the permeability of the rocks; and 2) place a heat exchanger in the ground in a borehole with the heat-transporting fluid circulating inside the heat exchanger. The second option is the most common method of heat exchange used with heat pumps, but is also used in some direct use applications. It has the advantage that fresh water may be used in the heat exchanger and the same fresh water used in the direct-use application, such as aquaculture or bathing. The first option is EGS and is the most common option tested to date for electricity generation. A number of attempts have been made around the world to establish flow between two boreholes through a series of artificially induced fractures. The ability to create a circulation system with both high productivity and constant high temperatures has not yet been demonstrated.
Diagram of the EGS Concept (from DOE, 2008, Figure 2)
Los Alamos Hot Dry Rock Experiment
The first enhanced geothermal system experiment (formerly known as Hot Dry Rock) was carried out under the leadership of the Los Alamos National Laboratory in northern New Mexico about 30 km (19 miles) west of the Laboratory. Their model was that crystalline basement rock was impermeable to water: if fractures could be created in this rock then a closed loop flow of water could be produced in this rock (where the rock was hot) to extract heat from the Earth.
During Phase I of the project two holes were drilled to a depth of about 3 km (6,560 feet) and a temperature of about 200°C (392°F). Fractures were generated at the bottom of the two holes by hydraulic fracturing. This process is a standard practice in oil and gas wells to increase productivity. The process involves pumping water down the wells, with only a short section of the well open to rock, until the pressure causes natural fractures in the rock to open or new fractures to be generated. The initial hydraulic fracturing by Los Alamos created a poor connection between the two wells. After additional experimentation, redrilling of one of the wells, and additional hydraulic fracturing, a fracture area of approximately 50,000 square meters (540,000 square feet) was produced with relatively good connection between the wells. Water was circulated between the wells through the fracture system in a closed, recirculating, pressurized water loop and heat extracted from the subsurface at rates of up to five MW thermal.
Phase II of the project attempted to establish connection between two wells at a depth of about 4.5 km (14,765 feet) at a temperature about 300°C (570°F) using the experience gained in Phase I of the project. A fracture system was created again by hydraulic fracturing with an estimated area of 1,500,000 square meters (16,145,000 square feet). As with Phase I, the initial connection between the two Phase II wells we poor, but despite many attempts to improve the connection, including chemical leaching of the fractures to decrease their resistance to flow, a high-flow connection between the two Phase II was never achieved.
The Los Alamos experiment has become the model for most subsequent, enhanced-geothermal-system projects. Typically a hole is drilled down to crystalline basement rock to a depth where the temperature is hot enough for heat extraction. Fractures are created, generally by hydraulic fracturing. A second hole is drilled to intersect the fractures and create a loop through which water may be circulated. These projects in England, Japan, Germany, France, Australia, and other states in the US have resulted in significant improvements in the understanding and technology of fracturing crystalline rock at relatively high temperatures. The systems are far from the economic generation of electricity, however. Each project has included one or more of the following problems: high costs; difficulty in establishing connection through fractures; uneconomically low flow, relatively rapid decrease in temperature; significant (>10%) loss of water in circulation. More . . .
Colorado's EGS Ranking
Colorado has been ranked as having the highest EGS geothermal resource of all 50 states in the 3 to 4 km (9,850 to 13,125 ft) depth range (Tester et al., 2006). The technology is not ready to extract this heat. A modification of the crystalline rock, EGS technique may be closer to production. Colorado has a number of deep sedimentary basins and the temperatures in the deep portions of some of these basins exceed 150°C (300°F). Oil and gas companies have decades of experience in hydraulic fracturing of sedimentary rocks and at working at these temperatures. Electricity can be generated at these temperatures (see Electricity Generation). During the next few years The Raton Basin is likely to be drilled to a depth of about 3 km (9,850 ft) with the goal of developing a sedimentary rock enhanced geothermal system. This project does not require any experimental technology. The only unknown is how well heat can be extracted from these formations. If this project is successful, large areas of the sedimentary basins of Colorado will be prospective sedimentary EGS sites.
DOE, 2008, An Evaluation of Enhanced Geothermal Systems Technology, U. S. Department of Energy, http://www1.eere.energy.gov/geothermal/pdfs/evaluation_egs_tech_2008.pdf
Tester et al., 2006, The Future of Geothermal Energy: Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century, Massachusetts Institute of technology,