Geothermal exploration
Encyclopedia
Geothermal Exploration is a multidisciplinary process used to explore the subsurface in search of viable active geothermal regions with the ultimate goal of building a geothermal power plant, where hot fluids drive turbines to create electricity. Exploration methods include a broad range of disciplines including geology
, geophysics
, geochemistry
and engineering
.
Geothermal regions with adequate heat flow to fuel power plants are found in rift zones, subduction zones and mantle plumes. Hot spots
are characterized by 4 geothermal elements. An active region will have:
Exploration involves not only identifying hot geothermal bodies, but also low-density, cost effective regions to drill and already constituted plumbing systems inherent within the subsurface. This information allows for higher success rates in geothermal plant production as well as lower drilling costs.
As much as 42% of all expenses associated with geothermal energy production can be attributed to exploration. These costs are mostly from drilling operations necessary to confirm or deny viable geothermal regions. Some geothermal experts have gone to say that developments in exploration techniques and technologies have the potential to bring the greatest advancements within the industry.
Thermal gradient holes (TGH), exploration wells (slim holes), and full-scale production wells (wildcats) provide the most reliable information on the subsurface. Temperature gradients, thermal pockets and other geothermal characteristics can be measured directly after drilling, providing valuable information.
Geothermal exploration wells rarely exceed 4 km in depth. Subsurface materials associated with geothermal fields range from limestone
to shale
, volcanic rocks and granite
. Most drilled geothermal exploration wells, up to the production well, are still considered to be within the exploration phase. Most consultants and engineers consider exploration to continue until one production well is completed successfully.
Generally, the first wildcat well has a success rate of 25%. Following more analysis and investigation, success rates then increase to a range from 60% to 80%. Although expenses vary significantly, drilling costs are estimated at 400$/ft. Therefore it is becoming paramount to investigate other means of exploration before drilling operations commence. To increase the chances of successfully drilling, innovations in remote sensing
technologies have developed over the last 2 decades. These less costly means of exploration are categorized into multiple fields including geology, geochemistry and geophysics.
industry and is now being adapted to geothermal exploration. Seismic waves propagate and interact with subterranean components and respond accordingly. Two sub categories exist that are relevant to the source of the seismic signal. Active seismology relies on using induced/man-made vibrations at or near the surface. Passive seismology
uses earthquakes, volcanic eruptions or other tectonic activity as sources.
Passive seismic studies use natural wave propagation through the earth. Geothermal fields are often characterized by increased levels of seismicity. Earthquakes of lesser magnitude are much more frequent than ones of larger magnitude. Therefore, these micro earthquakes
(MEQ), registering below 2.0 magnitude on the Richter scale, are used to reveal subsurface qualities relating to geothermal exploration. The high rate of MEQ in geothermal regions produce large datasets that don’t require long field deployments.
Active Seismology, which has history in the oil and gas industry, involves studying man made vibrational wave propagation. In these studies geophone
s (or other seismic sensors) are spread across the study site. The most common geophone spreads are in line, offset, in-line with center shot and Fan shooting.
Many analytical techniques can be applied to active seismology studies but generally all include Huygens Principle, Fermat’s Princeple and Snell’s law. These basic principles can be used to identify subsurface anomalies, reflective layers and other objects with high impedance contrasts
.
studies use changes in densities to characterize subsurface properties. This method is well applied when identifying dense subsurface anomalies including granite bodies, which are vital to locate in the geothermal exploration projects. Subsurface fault lines
are also identifiable with gravitational methods. These faults are often identified as prime drilling locations as their densities are much less than surrounding material. Developments in airborne gravitational studies yield large amounts of data, which can be used to model the subsurface 3 dimensionally with relatively high levels of accuracy.
Changes in groundwater
levels may also be measured and identified with gravitational methods. This recharge element is imperative in creating productive geothermal systems. Pore density and subsequent overall density are affected by fluid flow and therefore change the gravitational field
. When correlated with current weather conditions, this can be measured and modeled to estimate the rate of recharge in geothermal reservoirs.
Unfortunately, there are many other factors that must be realized before data from a gravity study can be interpreted. The average gravitational field the earth produces is 920 cm/c^2. Objects of concern produce a significantly smaller gravitational field. Therefore, instrumentation must detect variations as small as 0.00001%. Other considerations including elevation, latitude and weather conditions must be carefully observed and taken into account.
(MT) measurements allow detection of resistivity
anomalies associated with productive geothermal structures, including faults and the presence of a cap rock, and allow for estimation of geothermal reservoir temperatures at various depths. MT has successfully contributed to the successful mapping and development of geothermal resources around the world since the early 1980s, including in the U.S.A. and countries located on the Pacific Ring of Fire
such as Japan
, New Zealand
, the Philippines
, Ecuador
, and Peru
.
Geological materials are generally poor electrical conductors and have a high resistivity. Hydrothermal fluids in the pores and fractures of the earth, however, increase the conductivity of the subsurface material. This change in conductivity is used to map the subsurface geology and estimate the subsurface material composition. Resistivity measurements are made using a series of probes distributed tens to hundreds of meters apart, to detect the electrical response of the Earth to injection of electrical impulses in order to reconstruct the distribution of electrical resistance in the rocks. Since flowing geothermal waters can be detected as zones of low resistance, it is possible to map geothermal resources using such a technique. However, care must be exercised when interpreting low resistivity zones since they may also be caused by changes in rock type and temperature.
The Earth’s magnetic field varies in intensity and orientation during the day inducing detectable electrical currents in the Earth’s crust. The range of the frequency of those currents allows a multispectral analysis of the variation in the electromagnetic local field. As a result it is possible a tomographic reconstruction of geology, since the currents are determined by the underlying response of the different rocks to the changing magnetic field.
. At the curie point, materials will change from ferromagnetic to paramagentic. Locating curie temperatures for known subsurface materials provides estimates on future plant productivity. For example, titanomagnetitite, a common material in geothermal fields, has a curie temperature between 200-570 degrees Celsius. Simple geometric anomalies modeled at different depths are used to best estimate the curie depth.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, geophysics
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
, geochemistry
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...
and engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
.
Geothermal regions with adequate heat flow to fuel power plants are found in rift zones, subduction zones and mantle plumes. Hot spots
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
are characterized by 4 geothermal elements. An active region will have:
- Heat Source - Shallow magmatic body, decaying radioactive elements or ambient heat from high pressures
- Reservoir - Collection of hot rocks from which heat can be drawn
- Geothermal Fluid - Gas, vapor and water found within the reservoir
- Recharge Area - Area surrounding the reservoir that rehydrates the geothermal system.
Exploration involves not only identifying hot geothermal bodies, but also low-density, cost effective regions to drill and already constituted plumbing systems inherent within the subsurface. This information allows for higher success rates in geothermal plant production as well as lower drilling costs.
As much as 42% of all expenses associated with geothermal energy production can be attributed to exploration. These costs are mostly from drilling operations necessary to confirm or deny viable geothermal regions. Some geothermal experts have gone to say that developments in exploration techniques and technologies have the potential to bring the greatest advancements within the industry.
Drilling
Drilling provides the most accurate information in the exploration process, but is also the mostly costly exploration method.Thermal gradient holes (TGH), exploration wells (slim holes), and full-scale production wells (wildcats) provide the most reliable information on the subsurface. Temperature gradients, thermal pockets and other geothermal characteristics can be measured directly after drilling, providing valuable information.
Geothermal exploration wells rarely exceed 4 km in depth. Subsurface materials associated with geothermal fields range from limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
to shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
, volcanic rocks and granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
. Most drilled geothermal exploration wells, up to the production well, are still considered to be within the exploration phase. Most consultants and engineers consider exploration to continue until one production well is completed successfully.
Generally, the first wildcat well has a success rate of 25%. Following more analysis and investigation, success rates then increase to a range from 60% to 80%. Although expenses vary significantly, drilling costs are estimated at 400$/ft. Therefore it is becoming paramount to investigate other means of exploration before drilling operations commence. To increase the chances of successfully drilling, innovations in remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
technologies have developed over the last 2 decades. These less costly means of exploration are categorized into multiple fields including geology, geochemistry and geophysics.
Seismology
has played a significant role in the oil and gasOil and gas
Oil and gas are two commonly associated fossil fuels. The phrase may refer to:*Oil and gas field*Oil and gas law in the United States-See also:* Gasoline* Heating oil* Hydrocarbon exploration* Natural gas* Petroleum industry* Petroleum...
industry and is now being adapted to geothermal exploration. Seismic waves propagate and interact with subterranean components and respond accordingly. Two sub categories exist that are relevant to the source of the seismic signal. Active seismology relies on using induced/man-made vibrations at or near the surface. Passive seismology
Passive seismic
Passive seismic is the detection of natural low frequency earth movements, usually with the purpose of discerning geological structure and locate underground oil, gas, or other resources. Usually the data listening is done in multiple measurement points that are separated by several hundred meters,...
uses earthquakes, volcanic eruptions or other tectonic activity as sources.
Passive seismic studies use natural wave propagation through the earth. Geothermal fields are often characterized by increased levels of seismicity. Earthquakes of lesser magnitude are much more frequent than ones of larger magnitude. Therefore, these micro earthquakes
Microearthquake
A microearthquake is a very low intensity earthquake which is usually three or less on the Richter scale. In addition to having natural tectontic causes, they may also be seen as a result of underground nuclear testing or even large detonations of conventional explosives for producing excavations...
(MEQ), registering below 2.0 magnitude on the Richter scale, are used to reveal subsurface qualities relating to geothermal exploration. The high rate of MEQ in geothermal regions produce large datasets that don’t require long field deployments.
Active Seismology, which has history in the oil and gas industry, involves studying man made vibrational wave propagation. In these studies geophone
Geophone
The term geophone derives from the Greek word "geo" meaning "earth" and "phone" meaning "sound".A geophone is a device which converts ground movement into voltage, which may be recorded at a recording station...
s (or other seismic sensors) are spread across the study site. The most common geophone spreads are in line, offset, in-line with center shot and Fan shooting.
Many analytical techniques can be applied to active seismology studies but generally all include Huygens Principle, Fermat’s Princeple and Snell’s law. These basic principles can be used to identify subsurface anomalies, reflective layers and other objects with high impedance contrasts
Acoustic impedance
The acoustic impedance at a particular frequency indicates how much sound pressure is generated by a given air vibration at that frequency. The acoustic impedance Z is frequency dependent and is very useful, for example, for describing the behaviour of musical wind instruments...
.
Gravity
GravimetryGravimetry
Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of gravitational field or the properties of matter responsible for its creation are of interest...
studies use changes in densities to characterize subsurface properties. This method is well applied when identifying dense subsurface anomalies including granite bodies, which are vital to locate in the geothermal exploration projects. Subsurface fault lines
Fault Lines
Fault Lines, a documentary series produced and broadcasted by Al Jazeera English, is the channel's flagship program about the Americas.Josh Rushing, Zeina Awad and Sebastian Walker host the series, currently enjoying it's third season....
are also identifiable with gravitational methods. These faults are often identified as prime drilling locations as their densities are much less than surrounding material. Developments in airborne gravitational studies yield large amounts of data, which can be used to model the subsurface 3 dimensionally with relatively high levels of accuracy.
Changes in groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
levels may also be measured and identified with gravitational methods. This recharge element is imperative in creating productive geothermal systems. Pore density and subsequent overall density are affected by fluid flow and therefore change the gravitational field
Gravitational field
The gravitational field is a model used in physics to explain the existence of gravity. In its original concept, gravity was a force between point masses...
. When correlated with current weather conditions, this can be measured and modeled to estimate the rate of recharge in geothermal reservoirs.
Unfortunately, there are many other factors that must be realized before data from a gravity study can be interpreted. The average gravitational field the earth produces is 920 cm/c^2. Objects of concern produce a significantly smaller gravitational field. Therefore, instrumentation must detect variations as small as 0.00001%. Other considerations including elevation, latitude and weather conditions must be carefully observed and taken into account.
Resistivity and Magnetotellurics
MagnetotelluricsMagnetotellurics
Magnetotellurics is an electromagnetic geophysical method of imaging the earth's subsurface by measuring natural variations of electrical and magnetic fields at the Earth's surface. Investigation depth ranges from 300m below ground by recording higher frequencies down to 10,000m or deeper with...
(MT) measurements allow detection of resistivity
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
anomalies associated with productive geothermal structures, including faults and the presence of a cap rock, and allow for estimation of geothermal reservoir temperatures at various depths. MT has successfully contributed to the successful mapping and development of geothermal resources around the world since the early 1980s, including in the U.S.A. and countries located on the Pacific Ring of Fire
Pacific Ring of Fire
The Pacific Ring of Fire is an area where large numbers of earthquakes and volcanic eruptions occur in the basin of the Pacific Ocean. In a horseshoe shape, it is associated with a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts and/or plate movements...
such as Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
, the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
, Ecuador
Ecuador
Ecuador , officially the Republic of Ecuador is a representative democratic republic in South America, bordered by Colombia on the north, Peru on the east and south, and by the Pacific Ocean to the west. It is one of only two countries in South America, along with Chile, that do not have a border...
, and Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....
.
Geological materials are generally poor electrical conductors and have a high resistivity. Hydrothermal fluids in the pores and fractures of the earth, however, increase the conductivity of the subsurface material. This change in conductivity is used to map the subsurface geology and estimate the subsurface material composition. Resistivity measurements are made using a series of probes distributed tens to hundreds of meters apart, to detect the electrical response of the Earth to injection of electrical impulses in order to reconstruct the distribution of electrical resistance in the rocks. Since flowing geothermal waters can be detected as zones of low resistance, it is possible to map geothermal resources using such a technique. However, care must be exercised when interpreting low resistivity zones since they may also be caused by changes in rock type and temperature.
The Earth’s magnetic field varies in intensity and orientation during the day inducing detectable electrical currents in the Earth’s crust. The range of the frequency of those currents allows a multispectral analysis of the variation in the electromagnetic local field. As a result it is possible a tomographic reconstruction of geology, since the currents are determined by the underlying response of the different rocks to the changing magnetic field.
Magnetics
The most common application magnetism has in geothermal exploration involves identifying the depth of the curie point or curie temperatureCurie depth
In geophysics, the Curie depth is the depth at which rocks in a specific geographical area encounter the Curie temperature. This depth can be approximated from aeromagnetic survey data through spectral analysis or forward modeling....
. At the curie point, materials will change from ferromagnetic to paramagentic. Locating curie temperatures for known subsurface materials provides estimates on future plant productivity. For example, titanomagnetitite, a common material in geothermal fields, has a curie temperature between 200-570 degrees Celsius. Simple geometric anomalies modeled at different depths are used to best estimate the curie depth.
Geochemistry
This science is readily used in geothermal exploration. Scientists within this field relate surface fluid properties and geologic data to geothermal bodies. Temperature, isotopic ratios, elemental ratios, mercury & CO2 concentrations are all data points under close examination. Geothermometers and other instrumentation are placed around field sites to increase the fidelity of subsurface temperature estimates.US Geothermal Potential
Geothermal Energy is an underdeveloped energy resource and warrants further investigation and exploration. According to the U.S. Department of Energy, Utah's geothermal capabilities alone, if fully developed, could provide 1/3 of the state's power needs. Currently, the United States is planning to organize national geothermal databases, expand USGS resources nationally and develop geophysical projects to validate advances in exploration technologies. Below lists U.S. counties and regions that potentially can utilize geothermal power and would warrant further exploration.| U.S. State | County/Region |
|---|---|
| Arizona | Cochise, Graham, Greenlee, Maricopa, Pima, Pinal, Yauapia, Yuma |
| California | Alpine, Colusa, Contra Costa, Imperial, Inyo, Kern, Lake, Lassen, Los Angeles, Modoc, Mono, Monterey, Napa, Orange, Placer, Plumas, Riverside, San Bernardino, San Diego, San Luis Obsipo, Santa Barbara, Shasta, Sierra, Sonoma, Ventura |
| Colorado | Archuleta, Chaffee, Fremont, Garfield, Gunnison, Mineral, Ouray, Park, Routt, Saguache |
| Idaho | Ada, Adams, Bear Lake, Blaine, Boise, Bonneville, Camas, Canyon, Caribou, Cassia, Custer, Elmore, Franklin, Fremont, Gem, Lemhi, Oneida, Owyhee, Payette, Teton, Twin Falls, Valley, Washington |
| Montana | Beaverhead, Deer Lodge, Gallatin, Jefferson, Lewis and Clark, Madison, Park, Roosevelt, Rosebud, Sanders, Silver Bow, Stillwater |
| Nevada | Carson City, Churchill, Douglas, Elk, Eureka, Humboldt, Lincoln, Lyon, Nye, Pershing, Story, Washoe, White Pine |
| New Mexico | Donna Ana, Grant, Hidalgo, McKinley, Rio Arriba, San Miguel, Sandoval, Valencia |
| Oregon | Baker, Clackamas, Crook, Harney, Klamath, Lake, Lane, Linn, Malheur, Marion, Umatilla, Union, Wasco |
| Utah | Box Elder, Cahce, Davis, Iron, Juab, Millard, Salt Lake, San Pete, Sevier, Uintah, Utah, Weber, Washington, Benton, Grant, King, Lincoln, Okanogan, Skamania |
| Alaska (Not Counties) | Adak, Akutan, Baranof, Bell Island Hot Springs, Chena Hot Springs, Circle Hot Springs, Goddard, Makushin, Manley Hot Springs, Melozi Springs, Morzhovoi, Nancy, Portage, Pilgrim Springs, Serpentine Hot Springs, Sitka, Unalaska |
| Nebraska | Cheyenne, Keya Paha, Kimball, Scottsbluff |
| North Dakota | McLean |
| South Dakota | Butte, Corson, Dewey, Fall River, Haakon, Harding, Jackson, Jones, Lawrence, Meade, Mellette, Pennington, Perkins, Stanley, Todd, Tripp, Ziebach |
| Texas | Atacosa, Bell, Bexar, Brazoria, Burleson, Concho, Dallas, El Paso, Falls, Gonzale, Hardin, Hill, Karnes, Live Oak, McLennan, Milam, Navarro, Presidio, Webb |
| Wyoming | Hot Springs, Lincoln, Natrona |