Texas Geothermal Energy
Though little recognized by the general public, geothermal energy ("earth heat") is the third largest source of renewable energy in the United States, behind hydropower and biomass. Hundreds of thousands of megawatts of geothermal power lie beneath the surface in the United States, practically inexhaustible and waiting for new exploration tools, better resource techniques, lower-cost drilling technologies, and improved methods of predicting the behavior of geothermal reservoirs. As new technologies are being developed, so is the interest in mining the earth's heat.
Accelerating oil and electricity prices have also encouraged the use of the earth's heat, which is generated both from drilling wells deep into the earth to produce electricity from heated water (hydrothermal heat) and from the high temperatures of the earth itself closer to the surface, which is used to heat and cool buildings and homes. This method most often uses geothermal heat pumps that work with heat exchangers to transfer heat between warm and cool spaces.
As of 2012, 146 new geothermal energy projects were under development in the U.S., which will provide an additional 5,102 MW of electric power. The Geothermal Energy Association credits the federal production tax credit with the current expansion of the geothermal industry.
Texas Geothermal Resources
Texas has thousands of wells that have high enough temperatures for the possible development of geo-pressured and geothermal resources. Geothermal energy can be generated anywhere in the state. However the biggest technical issue facing geothermal development is determining where and how deep the resources are and how companies can get to and utilize them.
In 2007 state lands were leased for the first time to research possible geothermal energy development. Ten percent of any income from energy produced on this land will go to the state's Permanent School Fund.
Using Oil & Gas Wells
Accessing geothermal power by drilling for water or steam is similar to drilling for oil and gas. Texas has an advantage in drilling for geothermal resources as the state has decades of experience with oil and gas extraction, with access to detailed subsurface analyses of heat resources, reservoirs and deep-water availability.
Oil and gas wells co-produce additional hot fluids that require disposal. This fluid could be a bonanza for the geothermal industry, while sparing the oil and gas industries the expense of disposal of these co-produced fluids. Equally advantageous is the existence of large amounts of data on existing wells.
Using tapped out oil and gas wells could greatly reduce the costs involved in exploration and drilling even though retrieving geothermal resources to generate electricity is a significantly different process from that of oil and gas drilling and would involve redesign and re-drilling.
Texas Geothermal Research
SECO has worked with the Southern Methodist University's Geothermal Laboratory and the University of Texas at Permian Basin (UTPB) to analyze existing oil and gas-well data to develop the expertise to determine how this information can be used for geothermal energy exploration. SMU's Geothermal Laboratory estimates that within the next ten years, Texas could have 2,000 to 10,000 MW in generating capacity from geothermal resources accessed through oil and gas wells.
In January 2007, SECO published a groundbreaking study, Geopowering Texas: Conversion of Deep Gas Wells and Fields into Geothermal Energy Wells, (pdf) by Dr. Richard Erdlac at UTPB. This data provides advance knowledge of the best areas to go for heat energy acquisition. The three-year exploration and resource research project focused on West Texas, identifying and assessing potential sites for converting depleted deep gas wells and fields into geothermal energy wells to generate renewable electrical power.
The study proposed that the idea behind Texas geothermal production is the reuse of the wells drilled by the oil and gas industry that are either sufficiently deep to encounter hot water, or that could be deepened into these hot zones. In Pecos County, for example, temperatures of nearly 300°F are found in wells of around 18,000 feet. One purpose of the study was to develop the needed subsurface databases and maps to enable the energy industry to expand their activities to acquiring the subsurface heat for generating renewable electrical power from sedimentary basins. Researchers estimate that electric power production potential from Texas oil and gas wells range from 400 MW to over 2,000 MW.
In 2008, SECO published a second geothermal study performed by the Southern Methodist University’s Geothermal Laboratory, Texas Geothermal Assessment for the I-35 Corridor East (pdf). The use of hot wastewater from existing hydrocarbon wells is a feasible electrical energy solution. The project goal was to develop site-specific locations for geothermal electrical power production in existing Texas hydrocarbon fields, with the ultimate goal to develop specific examples for proof-of-concept tests of geothermal electrical generation in Texas.
SECO also contracted with Southern Methodist University's Geothermal Laboratory to create a geothermal outreach and networking group, in conjunction with the U. S. Department of Energy's GeoPowering the West program, with the intent to move Texas from a state of little geothermal knowledge and consumption, to one where geothermal becomes an important component of the renewable energy industry.
Texas Geothermal Maps
The map of Texas geothermal resources below shows that a largely untapped energy resource lies underneath our feet that can be developed for electrical power generation. Hydrothermal, geopressured and hot dry rock resources areas are identified. The orange color represents the known potential for hydrothermal uses such as space heating, fish farming, desalinization and resort spas. The green color shows the known potential for geopressure uses such as heating, enhanced oil recovery and electricity. The blue areas show the known potential for hot dry rock uses such as heating and electricity.
The map also shows that there are at least five major regions within Texas that have a strong potential for geothermal electrical power production. These include East Texas, the Gulf Coast, the Delaware-Val Verde Basin region, the Trans-Pecos region, and the Aardvark Basin where it enters the Texas Panhandle. The region consisting of the Maverick Basin along the South Texas-Mexico border may represent an independent sixth area for consideration. These regions are based upon the existence of oil and gas wells with temperatures that can get above 212 degrees Fahrenheit. The locations and boundaries of the geothermal areas illustrated are approximate.
Source: Virtus Energy Research Associated, adapted by author
The following three descriptions refer to the resources identified by the map shown above.
Hydrothermal resources are composed of hot water and/or steam that is found in fractured or porous rock at shallow to moderate depths as a result of the intrusion of molten magma into the earth's crust, or from the deep circulation of water through fault or fracture systems.
A geopressured resource consists of hot brine saturated with methane and found in large, deep aquifers that are under higher pressure due to water trapped in the burial process. The northern Gulf of Mexico is the largest region discovered that contains this type of resource.
Hot dry rock
Hot dry rock is a heated geological formation formed in the manner of hydrothermal resources, but containing no water due to the absence of aquifers or fractures required to transport water. This resource is huge in comparison to hydrothermal resources.
Future of Geothermal in Texas
Future development of geothermal energy in Texas will fall along three lines: geoexchange systems (heat pumps), direct use activities, and electric power generation. Several different companies that install geoexchange systems are centered in Texas. Direct use application is less developed, and is used only locally at several places in Central Texas. Electrical power generation has not yet become commercially established.
Heat Pump (Geoexchange System)
A geothermal heat pump system consists of pipes buried in the shallow ground near the building. Fluid (mostly water) circulating in the pipes carries heat into a building in the winter and pulls heat out of the building in the summer, exchanging the heat with the cooler surroundings at either end of the loop. Like an air conditioner or furnace, a geothermal heat pump is an electrically powered heating and cooling system, but it moves heat from and to the earth to take advantage of the almost constant temperature occurring just a few feet underground, which is usually warmer than the air in winter and cooler than the air in summer.
Geoexchange systems are more expensive to install, but much more efficient and cost- effective than competing fuel technologies, and can be developed anywhere in Texas for our homes, businesses and schools. Costs can be recouped in two to 10 years through energy savings.
Direct geothermal use taps hot water or heat from below the ground, near the earth's surface, to generate electricity for industrial heating needs, fish farming, food processing facilities, pasteurizing milk, spas and hot springs, nurseries, and residential and commercial heating. The hot water can also be piped directly under roads and sidewalks to melt snow, and even to heat a network of buildings in a community. Dehydration, the drying of onions and garlic, is the largest industrial use of geothermal energy.
Direct use of geothermal energy for homes and commercial operations can achieve savings up to 80% lower than fossil fuels.
Texas Trans-Pecos region
The key to the development of direct use geothermal energy in Texas is access to hot water. While there are some hot springs that have been identified in the Trans-Pecos region, an undeveloped source of hot water comes from oil and gas wells that were drilled by prospectors in the search for oil and gas.
Geothermal Power Plant
A geothermal power plant uses geothermal steam or heat to drive turbine-generators to produce electricity. Three different types make use of the various temperature ranges of geothermal resources: dry steam flash and binary.
The typical size range for a geothermal power plant is from 10 to 250 MW. One MW (8,760,000 kWh) of electricity will meet the needs of approximately 1,000 households for one year. Meeting a 250 MW generating plant output from fossil fuels would require 1,288,994 bbls of oil, or 7,227,722 mcf of natural gas, or 280,769 tons of coal per year.
In traditional geothermal electricity production, using near-surface high temperature water or steam, three methods are used to convert thermal energy into the mechanical energy of a spinning turbine. The following three descriptions refer to these methods.
Power plants using dry steam systems were the first type of geothermal power generation plants built. Dry steam is extremely hot water that is already in the form of steam and thus ready to drive a steam turbine. In this type of well, the very hot pressurized steam is produced at high speed. A dry steam power plant brings the steam to the surface of the earth to turn a turbine and generate electricity. At the end of the process, the water is returned to the thermal reservoir through an injection well to replenish this renewable energy resource. This technology is well developed and commercially available.
Flash steam plants are the most common type of geothermal power generation plants in operation today. A flash steam power plant vaporizes water above 360° F by releasing it from the pressurized reservoir into a lower-pressure flash tank. The hot water is 'flashed' into steam through a pressure drop within the tank. The steam then drives a turbine to produce electricity. Flash steam power generators are in the size range of 10MW to 55MW, with 20MW being a standard size being used in many countries.
The use of two separate fluids for power generation gives the name 'binary' to this type of power plant. In this process, the geothermal fluid is brought to the surface and passed through a heat exchanger to heat another "working fluid" which is vaporized and used to turn the turbine/generator units. The geothermal water, and the working fluid are each confined in separate circulating systems or "closed loops" and never come in contact with each other. The secondary fluid is reused again and again as it runs through vaporization and condensing phases. The geothermal fluid is injected back into the ground in a closed-loop system. The binary cycle method has the greatest potential for expanded electricity generation because it allows producers to take advantage of lower-temperature fluids (225° F - 360° F). Additionally, this method produces no air emissions.