Geothermal exploration

Geothermal resource exploration, development, and production draw on the techniques of both the mining and oil/gas industries. The geologic setting of geothermal resources is similar to deposits of metal ores, and geothermal systems are thought to be the modern equivalent of metal ore-forming systems. Hence, exploration draws most heavily on the techniques of the mining industry. Development of the resource and its production as hot fluid uses the techniques of the oil/gas industry with modifications because of the high temperatures and the much higher flow rates needed for economic production.

Exploration begins with selection of an appropriate area based on general knowledge of areas with above average heat flow. The best guides for more detailed investigation are the presence of thermal springs (the equivalent of oil seeps). However, to develop undiscovered resources, geologists must rely on other techniques. Because the target is a region of above average temperature, heat flow studies can indicate elevated subsurface temperatures. Among other methods being used and investigated for regional exploration are remote sensing of elevation changes, age of faulting, and geochemical techniques.

Hydrothermal systems suitable for geothermal development must have adequate temperature and sufficient flow for economic production. Geochemical techniques can be used to determine subsurface temperatures when hot springs are present, and shallow temperature-gradient holes can be used to project subsurface temperatures below the level of drilling. Geophysical tools are also used to determine the approximate size of the reservoir. Because high flow rates are needed for geothermal production, most geothermal production comes from highly fractured reservoirs. Geophysical methods that can determine fracture intensity are of great importance to the explorationist.

Geochemical studies

The interpretation of the chemistry of hot springs and fumaroles is an important tool used in geothermal exploration. The solubility of minerals strongly depends on temperature, and the kinetic rate of rock-water reactions is relatively slow. Thus, water equilibrated with rocks in a geothermal system can retain their dissolved mineral content as they move to the surface, and the composition of hot springs can be used to determine the temperature of equilibration. The geochemistry of thermal springs is the most widely used geothermal exploration tool for estimating subsurface temperatures prior to drilling wells.

The most widely used geothermometer is based on the solubility of silica. Because more than one form of silica, with different solubilities, can be present in the subsurface, caution must be used in applying the thermometer. The two most common forms of silica in geothermal systems yield the following composition-temperature relationships over the temperature range of 0 to 250°C.

....................(1)

and

....................(2)

where SiO₂ is the concentration of silica in mg SiO₂ per kg water.

The second most widely used geothermometer, Na-K-Ca, was developed by Fournier and Truesdell, and a magnesium correction was added by Fournier and Potter.

....................(3)

The concentration units are moles/kg, β = 1/3 for water equilibrated above 100°C, and β = 4/3 for water equilibrated below 100°C.

Because of the importance of geothermometers for exploration and for interpreting chemical changes in geothermal reservoirs during production, a rich literature on the geochemistry of geothermal systems is available. Four publications provide a particularly useful understanding of the chemistry of geothermal systems, how to sample thermal springs, and the application of geochemistry to understanding geothermal systems.

Geophysical techniques

Geophysical methods in geothermal exploration and field operations

Geophysical methods can help locate permeable structures with high-temperature water or steam and estimate the amount of heat that can be withdrawn from the ground in a given time period. Once a field is developed, geophysical measurements can be used to help site additional production and injection wells, to understand the details of the permeability structure, and to provide constraints on reservoir models used in the management of the geothermal field. The primary exploration targets are:

• Colocated heat
• Fluid
• Permeability

Wright et al. provide a useful review of geophysical techniques for geothermal exploration.

Geophysical interpretation in geothermal fields is complicated by two factors. First there are a great variety of rock types in which different geothermal systems might be found:

• Young sediments in the Salton Trough, California
• The Franciscan mélange at The Geysers, California
• A mixture of rocks such as tuffs, flows, mudslides, and intrusive rocks at Pacific rim, volcanic-hosted fields

Second, the geologic structures at geothermal systems are often quite complex, and structure may not determine the location or economic viability of the geothermal field. Consequently, the exploration strategy for geothermal energy differs from that for petroleum fields and is more similar to mineral exploration.

Temperature at depth can be sensed directly in boreholes or estimated by extrapolation of heat-flow measurements in both shallow and deep holes. Heat-flow measurements combine observed temperature gradients and thermal conductivity measurements to determine the vertical heat transport in areas where conduction is the primary mechanism of heat transport. If the temperature gradient changes dramatically with depth, these measurements indicate areas where heat transfer is dominated by advection. Heat-flow measurements provide evidence both of regions where geothermal systems are more likely and of the extent of localized convecting systems. Because the fluid flow patterns can be complex, the deeper zones of hot fluids are often not directly beneath the shallow high heat-flow anomalies.

Subsurface temperatures can also be inferred from physical properties of rock masses. The information needed to plan and interpret a geophysical campaign can be provided by laboratory measurements of the density, seismic, electrical and mechanical properties of rocks as a function of:

• Temperature
• Pressure
• Porosity
• Matrix material
• Alteration
• Saturation

Locating zones of sufficient permeability for economic production is difficult. Electrical self-potential (SP) provides the only direct signal from subsurface fluid flow; all other methods require the inference of permeability from:

• Causes
  • Zones of extension
  • Intersecting faults
  • State of stress
  • Seismicity
• Secondary effects
  • Temperature distribution
  • Zones of mineral alteration

Surface geophysical methods have provided important information for siting early wells at many geothermal fields. For example, the gravity anomalies caused by dense, thermally-altered sediments in the Imperial Valley, California, guided much early drilling. However, surface and borehole geophysics is much more important later in the development of a field, when wells must be sited to provide adequate production or injection capability, or to provide constraints to tune reservoir models.

Examples of specific methods as applied to geothermal

Both natural and induced seismicity reflect physical processes occurring within or beneath the geothermal system. The significance of these events, or of their absence, depends on the specific setting of the geothermal system being examined. It has been argued that, for fluids to keep moving from hot regions toward cooler regions, microseismicity must occur to keep fractures open. Consequently, passive seismic techniques for the detection of microseismicity have long been used to explore for geothermal fields. However, several fields, such as Dixie Valley, Nevada and Olkaria, Kenya have little or no detectable seismicity, and for others, such as The Geysers, California, we do not know whether there was seismicity before production began. On the other hand, seismicity can provide information about the tectonic setting in which the geothermal system occurs. For example, in the Salton Trough, the natural seismicity outlines the plate boundaries, whose oblique motion provides the extension required for the shallow injection of magma and the resulting fluid circulation. Historical and paleoseismic information may also provide valuable information about the setting of a geothermal system. For example, Caskey et al. have found that the Dixie Valley field sits in a seismic gap indicated by both 20th-century events and Holocene fault ruptures. Finally, if seismicity only occurs at shallow depths, then the brittleductile transition may shallow because of locally high heat flow.

Microearthquakes can also be useful to constrain the processes occurring during operations in a field. For example, Beall et al. and Smith et al. showed that much of the seismicity at The Geysers can be used to map the descending plume of injected fluid, and microearthquakes detected from a deep borehole seismic systems have been used to map artificial fractures (e.g., Fehler et al. at Fenton Hill, New Mexico, or Weidler et al. at Soultz, France).

Passive seismic observations are also used to generate velocity images of the crust. By simultaneously solving for the earthquake locations, time, and the velocity and attenuation structures, three-dimensional (3D) images of geothermal fields can be developed. Inferences about steam saturation and porosity can be drawn by comparisons of the P- and S-wave images or by comparing the velocity and attenuation images. Inferences about fracture orientation can be inferred if shear-wave velocity depends on polarization. These surveys can be repeated to look at the effects of production and injection.

Exploration seismology has historically not been successful in delineating economic geothermal fields, probably because of the complex structures in which they occur and the somewhat tenuous relationship between the structure and the producing fields. Recent work has focused on using the large number of first arrival times to develop a two-dimensional (2D) or potentially a 3D velocity model that can be used for migration to image steeply dipping structures. The velocity image provides valuable information about the structure as well as improving the imaging of discrete reflectors.

Many electrical methods have been applied to geothermal exploration and characterization. Passive electrical SP anomalies have been interpreted to indicate zones of strong upward flow of hot water. DC and induction methods with a broad variety of geometries, frequencies, penetration depths and resolutions have identified high-conductivity anomalies that are interpreted to be warm or heavily altered areas, or to indicate structures that might control fluid movement. Repeated electrical methods have also been used to identify zones where cool recharge is entering a geothermal system.

Potential field methods, including gravity and magnetics, are used in traditional ways to delineate faults, basin geometries and other structures, and to identify intrusions or buried eruption deposits that might provide heat or influence flow paths as demonstrated by Soengkono. The interpretation of these data depends strongly on the nature of the particular system being studied. For example, in the sedimentary section in the Salton Trough, California, the known resource areas are all marked by gravity highs caused by alteration of the sediments by high temperature circulating fluids. However, in most fields, an area of relatively high gravity would typically not be related to the geothermal system.

Although they are not traditionally thought of as geophysical techniques, geodesy and deformation measurements can provide valuable information about the processes occurring within a geothermal system.

Other than temperature-depth logging and spinner surveys to identify inflow areas, borehole logging has not been extensively used in geothermal areas. Several factors contribute to this. The high temperatures can be a problem for traditional logging tools. The tool designs and standard interpretation principles are optimized for relatively flat sedimentary sections, a situation which is unusual in geothermal environments. Finally, geothermal wells often have severe lost circulation zones that require casing to be set rapidly to save the hole. This can preclude openhole logging. High-temperature logging tools can alleviate some of these problems.

Two scientific projects have provided public access to logging data sets from drillholes in geothermal systems. The Salton Sea Scientific Drilling project collected a large suite of traditional well logs, repeated temperature logs, borehole gravity, and vertical seismic profile (VSP) measurements. At Dixie Valley, extensive borehole televiewer studies and mini-hydraulic fracture tests to determine effective stress have led to an understanding of which fractures are open and why. If interpreted as measurements of specific formation properties rather than as a means to correlate between wells, additional borehole geophysical measurements could provide valuable information in operating geothermal fields.

Integrated geophysical methods can provide valuable information about a geothermal system both during exploration and exploitation. The specific methods that are valuable, and the way disparate data sets might be combined, strongly depend on the nature of the system being examined and the questions being asked. The value of geophysical measurements is enhanced if they are interpreted in terms of a conceptual or numerical model that is also constrained by other information, whether it be geological and geochemical exploration data or knowledge gained during the operation of a field. This integration is potentially most effective during exploitation when the reservoir models calculate the geophysical effects as well as the pressure drawdowns and fluid flows. A similar approach to exploration might prove to be very valuable.

Noteworthy papers in OnePetro

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External links

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See also

Geothermal energy

Geothermal drilling and completion

PEH:Geothermal_Engineering