Hot Dry Rock Geothermal Project Research Articles

VSP and Cross-hole seismic surveys used to determine reservoir characteristics of a hot dry rock geothermal system

Monitoring microseismic emissions during fracturing and circulation experiments at the Camborne School of Mines 2 km deep hot dry rock geothermal project has been carried out over the past 5 yr. These microseismic locations provide the only information on the position of the stimulated region within the granite rock mass. A well was drilled into the microseismic cloud on the basis that enhanced permeability would be found. However, the evidence for a relation between microseismicity and improved permeability is conflicting. Calculated shear displacements are insufficient to provide the observed permeability and it is known that only a relatively small percentage of the microseismicity that occurs is located. Vertical seismic profiles and cross-hole seismic surveys have been used to help understand the relation between the microseismic and hydraulic reservoirs. Changes in signal attenuation and velocity as raypaths cross the reservoir have been measured. These changes indicate that regions of microseismicity represent permanent increases in crack frequency and/or aperture.

水圧破砕による複数のフラクチャの成長 ＩＩ 高温岩体の水圧破砕によるフラクチャ成長のシミュレーション

At the Fenton Hill Hot Dry Rock Geothermal Project in the United States, more than ten hydraulic fracturing experiments have been conducted to stimulate a hot dry rock reservoir. Two hydraulic fracturing attempts, Exp. 2059 and Exp. 2062, successfully established a large fracture system connecting an injection well, EE-3A and a production well, EE-2. The fracture systems created by the two experiments showed different fracture extension pressures. In order to understand the combined behavior of fractures, thecomputer code FEHM was used to simulate the two layered fracture system. As in part 1 of this series of papers, an exponential relation between fracture aperture and fracture pressure was used. Parameters in the modelwere fitted to simulate specificexperiments.In this paper, the parameters were chosen to fit the data obtained from each experiment described above. The extension of each fracture was then estimated using these parameters at a constant flow rate, 0.8m3/min. Next, the fracture extension is simulated when water is pumped into the combined two-layer fracture system. From the computer simulation, the following results are obtained.1. When two fractures with different characteristics obtained by experiments 2059 and 2062 extend at the same time by pumping water at a fixed flow rate of 0.8m3/min, the higher permeability fracture (Exp. 2059: upper fracture) extends almost at the single fracture rate, while thefracture of Exp. 2062 (lower fracture) which has lower permeability extends at half the single fracture rate. The wellhead fracture extension pressure is only 0.5MPa less than that when upper fracture extends alone.2. By doubling the flow rate and keeping other parameters constant, both fractures extended faster and the lower fracture showed a marked increase in fracture extension rate. The results suggest that if we try to make multiple fractures along openhole section in an effort to create a large artificial reservoir, it is advisable to use the largest flow rate allowable under the given conditions.

Shear-wave polarizations in a hot dry rock geothermal reservoir: Anisotropic effects of fractures

Hydraulic fracturing at the Hot Dry Rock Geothermal Project in Cornwall stimulated extensive cracking in the Carnenellis granite. The acoustic emissions were monitored by a surface network of three-component seismometers which recorded acoustic events with double-couple mechanisms indicating the release of significant shear stress. Three-component shear wavetrains display splitting diagnostic of crack-induced anisotropy. Equal-area projections of the shear-wave polarizations display patterns characteristic of propagation through parallel vertical liquid-filled microcracks, where the orientations of the leading shear-wave arrivals are parallel to the strike of the vertical cracks. This strike is confirmed by the elongated distribution of the acoustic events, and indicates the stress direction at the depth of the fractures. This direction is approximately parallel to one of the sets of joints at the surface and si 20° away from the axis of compression measured about 10km northwest of the HDR site. The consistency of the shear-wave polarizations before and after water injection indicates that the intact granite rock mass to 2 km depth has anistropic symmetries which are uncharged by the hydraulic fracturing. This demonstrates the presence of stress-aligned liquid-filled microcracks (extensive-dilatancy anistropy, EDA) in the granite as in most rocks in the cool brittle upper 10–20 km of the crust. There are two main applications demonstrated by this case study. 1. (1) The orientation of hydraulic features can be predicted from the polarizations of shear waves before water injection. The most direct way to do this would be to record shear vibrators in three-component vertical seismic profiles, as suggested by Crampin [1] 2. (2) The correlation between shear wave polarizations, the existence and orientation of EDA microcracks, stress orientations, and preferred directions of flow demonstrates that three-component shear wavetrains contain important information about the internal geometry of cracked reservoir rock. Again, the most direct way to access this information would be to record shear vibrators in three-component VSPs.

Application of cross‐hole seismic measurements in site investigation surveys

Application of the cross‐hole seismic method to site investigation surveys for civil engineering developments is illustrated by case histories from the United Kingdom using the borehole sparker probe developed in McCann et al. (1975) as the seismic source. At Newborough, Anglesey, it is shown that this source generates both a compressional‐wave and a vertically polarized shear‐wave in the borehole; the measured velocity values are used to determine the dynamic elastic moduli of the saturated sand deposit at this site. At Cocking, Sussex and East Fleet, Dorset, the presence of an air‐filled void between two boreholes is shown to have a significant effect on the velocity of propagation and amplitude of the transmitted seismic signals. This work is extended at Maidstone, Kent, where the technique is used to confirm the presence of a cavity below the foundation of a house. In the Hot Dry Rock Geothermal Project at Rosemanowes, Cornwall, increased fracturing of the granitic rock mass following explosive stimulation in one of the exploration boreholes is shown to result in a significant reduction in the compressional‐wave velocity.

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.