Hot Dry Rock Resources Research Articles

Geothermal energy in Australia

Although Australia is not usually associated with geothermal energy, it possesses significant amounts of both conventional (wet) geothermal and hot dry rock (HDR) geothermal resources. The country's conventional geothermal resources are extensive, but are low temperature and are located in areas of low population density with the result that they are not used extensively for either electricity generation or for direct heating. Australia's HDR resources, on the other hand, represent a world-class resource but have not been used to date because the technology for converting HDR resources into electricity is not yet fully commercial. The effort being directed towards exploring economically useful HDR resources and in the development of technology to convert this energy resource into electricity could make HDR resources an important part of Australia's energy mix in the near future, but there are major barriers.

Thermal modelling of the hot dry rock geothermal resource beneath GEL99 in the Cooper Basin, South Australia

Radiogenic granites beneath the Cooper Basin are purported to be the most prospective hot dry rock resource in the world, but the remote location means that any economic development of the resource must factor in a significant cost of connecting to the national electricity grid. This cost cannot be easily reduced. The other major cost of development is drilling, so it is imperative to keep this cost to a minimum. As the energy potential of the rocks is directly proportional to their temperature, it is vital to target drilling at locations likely to yield the highest temperatures at the shallowest depths. The South Australian government has granted three Geothermal Exploration Leases for commercial ventures to explore in the Cooper Basin. GEL99 is one of those leases and is currently operated by Scopenergy Limited. A series of one-dimensional conductive heat flow models were developed from thermal conductivity profiles and bottom hole temperature data extracted from numerous petroleum exploration and appraisal wells in the GEL99 area. These heat flow estimates were then extrapolated vertically into the underlying crystalline basement to predict the temperature versus depth distribution at specific points in the region. The conductive heat flow at the top of the Warburton Basin sequence was found to be about 103±8 mWm 2 beneath most of GEL99. The 220°C isotherm lies at a mean depth of around 4200–4300 m beneath most of the wells studied. Heat flow appears to be closely related to the nature of the basement rocks.

Geothermal energy from hot dry rock: A renew able energy technology moving towards practical implementation

The technology to extract useful amounts of energy from the large, ubiquitous, hot dry rock (HDR) resource has been under development for more than twenty years. Initial work during the 1970's and 1980's showed that it is possible to access and extract HDR energy using techniques originally conceived and tested by the Los Alamos National Laboratory. The process entails drilling a well deep enough to reach hot rock, injecting water at high enough pressure to open natural joints in the rock, and returning the water, heated by the rock, to the surface through one or more additional wellbores. After extraction of its thermal energy, the water is recirculated through the hot rock to mine more heat. In this closed-loop process, nothing is released to the environment except heat, and no long-term wastes accumulate. The practical potential of HDR technology was demonstrated in a series of flow tests conducted from 1992 to 1995 at the HDR pilot facility at Fenton Hill, NM. These tests showed that energy can be extracted from HDR in significant amounts over extended time periods. Extensive analyses and reservoir characterization studies carried out as part of the testing program provided a large amount of information about the dynamics of the heat extraction process and confirmed that HDR energy production facilities can be operated with minimal environmental effects. Operating strategies evaluated during the test period included the production of energy at steady-state levels for extended periods and on a variable-output schedule particularly suited to load-following electricity generation.

HDR geothermal exploration

Hot dry rock (HDR) is defined as that part of a geothermal anomaly where the fluids needed for production of steam or hot water are lacking. Most of the world's geothermal resource is not present in the form of natural hydrothermal systems but as HDR. Development of this resource through the use of manmade geothermal systems is in progress in several countries. The largest of these experiments, the Fenton Hill HDR geothermal project, is funded by the U.S. Department of Energy and the governments of West Germany and Japan. This project is located a short distance west of the rim of the Valles Caldera in the Jemez Mountains of New Mexico. As the Fenton Hill experiments progressed, it became evident that the location and extent of the HDR geothermal resource in other areas should be evaluated and that potential HDR drilling sites be located as part of a comprehensive program needed to encourage its development. Because the HDR resource lacks the sharp physical and chemical contrasts produced by natural fluids, it presents different exploration problems from those of conventional hydrothermal exploration. The purpose of a workshop, held in Los Alamos, New Mexico, June 21–23, 1982, was to review geological, geochemical, and geophysical exploration methods currently used for HDR recognition and resource evaluation and to evaluate new ideas for HDR exploration.

Candidate sites for future hot dry rock development in the United States

Generalized geologic and other data are tabulated for 24 potential hot dry rock (HDR) sites in the contiguous United States. The data show that HDR resources occur in many geologic and tectonic settings. Potential reservoir rocks at each prospect are described and each system is categorized according to inferred heat sources. The Fenton Hill area in New Mexico is discussed in detail because this region may be considered ideal for HDR development. Three other prospectively valuable localities are described: The Geysers-Clear Lake region in California, The Roosevelt Hot Springs area in Utah, and the White Mountains region in New Hampshire. These areas are singled out to illustrate the roles of significantly different geology and geophysics, reservoir rocks, and reservoir heat contents in possible HDR developments.

A regional strategy for geothermal exploration with emphasis on gravity and magnetotellurics

As part of the resource evaluation and exploration program conducted by Los Alamos Scientific Laboratory for the national Hot Dry Rock (HDR) Geothermal Program, a regional magnetotelluric (MT) survey of New Mexico and Arizona is being performed. The MT lines are being located in areas where the results of analysis of residual gravity anomaly maps of Arizona and New Mexico, integrated with other geologic and geophysical studies indicate the greatest potential for HDR resources. The residual gravity anomalies are derived by applying the concept of predicting gravity anomalies from topography. This can be accomplished by employing reductions similar to those used in some isostatic investigations, in which a regional topographic surface is used as the Bouguer reduction datum. The datum is derived by comparison of various harmonics of Bouguer anomalies and elevations of stations. Topography can be used to predict Bouguer anomalies because of isostatic compensation; the resultant anomalies can be considered high frequency residual anomalies or isostatic anomalies corrected for regional compensation. Such maps have been produced for Arizona, New Mexico, west Texas, and Chihuahua, Mexico. The main objective of the MT project is to produce a regional geoelectric contour map of the pervasive deep electrical conductor within the crust and/or upper mantle beneath the Colorado Plateau and the adjacent Basin and Range Province and Rio Grande Rift. The MT survey consists of 200 sites along several long profiles with site spacing of 15–20 km. Pre-existing available MT data are being integrated with the new data. After the data are processed, a one-dimensional inversion is applied to the sounding curve and used as a starting point for 2-D modeling. Such a project and ultimate map will be of major value in studying the regional geophysics and tectonics of the southwest United States as they now apply to HDR resources in particular and geothermal resources in general. Electrical conductivity anomalies of large areal extent are of particular interest in geothermal exploration. Correlation analysis of large conductive anomalies with other geophysical, geological, and geotectonic data is being performed. Preliminary analysis of the data has suggested several major regions of anomalously shallow high electrical conductivity. Among these is the Aquarius area of northwest Arizona which is the site of a longwavelength residual anomaly low, which when modeled and correlated with other geophysical data can be shown to be possibly related to low density and high temperature in the crust at depths of 20 km or less. Preliminary analysis of MT data indicates the possible existence of a mid-crustal high electrical conductivity anomaly in this same region.