A hydrothermal system is defined as a subterranean geothermal reservoir that transfers heat energy upward by vertical circulation of fluids driven by differences in fluid density that correspond to differences in temperature (see Figure 2). Hydrothermal systems can be classified into two types—vapor-dominated and hot water— depending on whether the fluid is steam or liquid water, respectively.
Most high-temperature geothermal resources occur where magma (molten rock) has penetrated the upper crust of the Earth. The magma heats the surrounding rock, and when the rock is permeable enough to allow the circulation of water, the resulting hot water or steam is referred to as a hydrothermal resource. Such resources are used today for the commercial production of geothermal power. They benefit from continuous recharge of energy as heat flows into the reservoir from greater depths.
Deep geothermal systems (a.k.a. enhanced geothermal systems or EGS) are defined as engineered reservoirs that have been created to extract heat from economically unproductive geothermal resources. The deep geothermal/EGS concept is to extract heat by creating a subsurface fracture system to which water can be added through injection wells. The water is heated by contact with the rock and returns to the surface through production wells, just as in naturally occurring hydrothermal systems.
Hydrofracturing and stimulation techniques are used widely in the oil and gas industry to extend production, and can be used to greatly extend and expand use of geothermal resources. Figure 3 gives a graphic idea of the domestic scope of geothermal resources at just 6 kilometers (3.7 miles), a nominal drilling depth in the oil and gas industry.
Figure 2. Illustration of a hydrothermal reservoir, showing the natural recharge, fractures, and heat source. Courtesy: Geothermal Education Office
Figure 3. Estimated Earth temperature at 6-km (3.7-mile) depth. Courtesy: Southern Methodist University Geothermal Laboratory.
Hot water from geothermal resources is used directly to provide heat for buildings, crop and lumber drying, industrial process heat needs, aquaculture, horticulture, ice melting on sidewalks, roads, and bridges, and district heating systems. In direct use applications, a well (or series of wells) brings hot water to the surface; a mechanical system— piping, heat exchanger, pumps, and controls—delivers the heat to the space or process.
Often, direct use applications use geothermal fluids not hot enough for electricity generation. To improve efficiencies, used water from geothermal power plants can be ‘cascaded’ down for lower temperature uses, such as in greenhouses or aquaculture. Flowers, vegetables, and various fish species and alligators are examples of products from greenhouse and aquaculture systems.
Geothermal heat pumps (GHPs) use the Earth’s huge energy storage capability to heat and cool buildings, and to provide hot water. GHPs use conventional vapor compression (refrigerant-based) heat pumps to extract the low-grade heat from the Earth for space heating. In summer, the process reverses and the Earth becomes a heat sink while providing space cooling (see Figure 5). GHPs are used in all 50 U.S. states today, with great potential for near-term market growth and savings.
The geopressured resource consists of deeply buried reservoirs of hot brine, under abnormally high pressure, that contain dissolved methane. Geopressured brine reservoirs with pressures approaching lithostatic load are known to occur both onshore and offshore beneath the Gulf of Mexico coast, along the Pacific west coast, in Appalachia, and in deep sedimentary basins elsewhere in the United States.
The resource contains three forms of energy: methane, heat, and hydraulic pressure. In the past, DOE conducted research on geopressured reservoirs in the northern Gulf of Mexico sedimentary basin, and operated a 1-megawatt (MW) power plant using the heat and methane from the resource.
Sometimes referred to as the ‘produced water cut’ or ‘produced water’ from oil and gas wells, co-produced geothermal fluids are hot and are often found in water/food fields in a number of U.S oil and gas production regions (See Table 1). This water is typically considered a nuisance to the oil and gas industry (and industry is accountable for proper disposal), but could be used to produce electricity for internal use or sale to the grid.
Like geopressured resources, co-produced geothermal resources can deliver near-term energy savings, diminish greenhouse gas emissions, and extend the economical use of an oil or gas field. New low-temperature electric generation technology may greatly expand the geothermal resources that can be developed economically today.
Estimated equivalent geothermal power from processed water associated with existing hydrocarbon production, using 140°C (285°F) as a nominal fluid temperature.
State |
Total Processed Water, 2004 (bbl) |
Power, MW @ 140˚C (285)˚F) |
Alabama |
203,223,404 |
47 |
Arkansas |
258,095,372 |
59 |
California |
5,080,065,058 |
1169 |
Florida |
160,412,148 |
37 |
Louisiana |
2,136,572,640 |
492 |
Mississippi |
592,517,602 |
136 |
Oklahoma |
12,423,264,300 |
2860 |
Texas |
12,097,990,120 |
2785 |
Total |
32,952,140,644 bbl |
7,585 MW |
Courtesy: Southern Methodist University
