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The abundant geothermal energy in hot dry rock (HDR) formations is an attractive renewable energy resource with great potential. China will develop its first HDR geothermal formation in the Gonghe Basin. HDR is a hard and low-permeability granite containing very few fluids. Development requires fluids to cyclically flow between injection and production wells to extract geothermal energy in the artificial heat transfer zone. Hydraulic fracturing is the main technology for creating flow paths. But few studies have investigated fractures in HDR geothermal formations. This paper investigated fractures as flow paths in HDR geothermal formations during hydraulic fracturing. Hydraulic fractures were simulated using a custom true-triaxial hydraulic fracturing test system in a realistic formation environment, in which a scaled wellbore was used that was built in outcrop granite rock from the Gonghe Basin. Fracture creation in granite was investigated via experiments, as well as influence factors, and what experience could be achieved. This study can be used to design and evaluate hydraulic fracturing projects in potential HDR geothermal formations.

Hot dry rock geothermal energy is a type of renewable energy with great development prospects in deep strata. However, it is quite difficult to construct an artificial reservoir. In this paper, deep large-dip-angle fault zone in the Yangbajing geothermal field in Tibet of China is used as a natural artificial reservoir, and a set of hot dry rock heat extraction schemes for such fault modes is proposed. A three-dimensional thermo-hydro-mechanical coupling model is established for the scheme to study the distributions of temperature, stress and seepage during the process of mining fault-mode hot dry rock geothermal energy. The temperature of #1 production well remains 445 °C after 22-year operation and it decreases from 445 °C to less than 200 °C after 15-year operation in #2 production well. The initial vertical stress near the injection well is 199 MPa, which decreases to 193 MPa after 1 year and remains unchanged. The specific water flow in the fault zone between the wells increases negatively and exponentially with the extraction time. The seepage resistance of the fault rock mass gradually decreases. The total effective heat production is 13130 MWa after 22-year operation.

Existing hot dry rock geothermal projects are commonly confronted with some technical issues, such as corrosion and scaling, and water loss. To resolve these issues, the present work proposes a novel system for mining hot dry rock geothermal energy, in which a reservoir is combined with a heat pipe system. The new system encompasses a heat pipe placed in a single-well to extract hot dry rock geothermal energy, while an artificial reservoir is built around the main endothermic region of the well, which is permeable and saturated with carbon dioxide (CO2). This wellbore structure design may stimulate a stronger natural convection in the reservoir, resulting in a higher thermal power production. To evaluate the proposed system, an extensive numerical investigation was conducted. The comparison of the proposed system with the conventional downhole heat exchanger (DHE) system in terms of heat extraction performance indicates clear superiority of the proposed system primarily due to the associated thermosyphon effect of CO2 fluid in the reservoir. To better understand how operating and design variables affect the heat extraction performance of the system, a detailed sensitivity analysis was conducted taking into consideration a wide range of possible configurations and working conditions. The eventually obtained knowledge will guide the design of the system in practice.

Hot dry rock geothermal energy is almost inexhaustible green energy. However, it makes slow progress in practice due to slow theory development. In this study, a three dimension thermo-hydro-mechanical coupled model of fractured media was established to simulate the extraction of HDR (Hot dry rock) geothermal energy based on the geological characteristics (geothermal gradient of 50 K/km, buried depth of 6250–6750 m) of Tengchong geothermal field in China. The simulation results show the variation in both field of temperature, stress, seepage and fracture aperture during heat extraction. The temperature in fracture face increased exponentially from injection well towards production well while extracting heat. The initial rock mass temperature of 573 K decreases to 423 K after 9-year running. The initial water pressure gradient in the fracture reached 0.17 MPa/m near the injection well and then decreased to 0.052 MPa/m after 1 year. The fracture aperture was triple of the initial value and the permeability coefficient increased by nine times over the 9-year operation period. That seepage resistance of artificial storage reservoir gradually decreased could improve extracting geothermal energy more efficiently. The amount of extracted heat declined exponentially with running time. The total extracted geothermal energy over 9 years was 5977 MWa and rock mass temperature decreased to 423 K.

Heat extraction mode, e.g. well layout or arrangement of wells, of enhanced or engineered geothermal system (EGS) is crucial to its performance and directly affects its commercial viability. Assuming the subsurface target hot dry rock (HDR) has been well-fractured and the created heat reservoir can be treated as a homogeneous porous medium, we numerically simulate the long-term heat extraction process of EGSs of various well layouts, including the standard doublet well layout, two triplet well layouts, and a quintuplet well layout. The simulation results enable a detailed analysis on the effects of well layout on EGS heat extraction performance. We find simply deploying more production wells does not necessarily improve the EGS heat extraction performance; an EGS with triplet well layout can perform better than an EGS with a quintuplet well layout or worse than an EGS with the standard doublet well layout. One more finding is an EGS with the injection well positioned close to the edge of the reservoir gets more thermal compensation from the un-fractured rocks surrounding the reservoir during heat extraction. Further, we deduce an optimized EGS well layout must ensure enough long major flow path and less preferential flow in the reservoir, and the injection well is located close to the edge of the reservoir. We then design a quartuplet well layout accordingly. Results from an additional simulation with respect to the quartuplet well EGS indicate its enhanced heat extraction performance, corroborating the success of design. Last, we discuss about the hot dry rock (HDR) heat recovery factor based on numerous simulated cases and estimate the amount of HDR geothermal resource that can be converted into electricity by EGS.

The development of Hot-Dry Rock (HDR) geothermal energy in Australia with drillings to some kilometres depth yields an impetus for deep stress logging. For the Olympic Dam HDR-project, borehole Blanche-1 was drilled to almost 2 km depth and provided the possibility to estimate the in situ stresses within the granitic borehole section by the analysis of borehole breakouts and core discing, as well as by hydraulic fracturing combined with acoustic borehole televiewer logging for fracture orientation determination. Although the stress magnitudes derived by the different methods deviate significantly, they clearly indicate for the depth range between 800 and 1,740 m a compressional stress regime of S v ≤ S h

It is a well-known fact that an injection of the borehole fluids into surrounding porous rocks often results in fault reactivation,such as during hydrocarbon production from a reservoir, fluid injection for enhanced oil recovery, hot dry rock geothermal energy extraction, and waste disposal or carbon dioxide sequestration. However, no rigorously derived method for the description of spatial and temporal distribution of seismic events and for the estimation of the critical value of pore pressure of a porous rock sufficient for the generation of an microseismic has ever been developed. There a model developed within the context of Biot’s theory of poroelasticity is used to obtain the distribution of pore pressure, then the pore pressure is substituted into a Mohr–Coulomb failure criterion to predict the fault stability and the spatio-temporal cluster of microseismic events in a reservoir. in this model, the Biot system of equations is formulated for the radial symmetry case and is supplemented by the relevant boundary conditions, Then the solution is constructed analytically. A key advantage and the novelty of the proposed approach is it allows one to monitor an influence of pressure applied at the borehole within and surrounding reservoir and to predict the lower and upper bounds for the critical state of a natural rock, and it can be used in different reservoirs.

The hot dry rock geothermal energy is one kind of the clean energies which are contained in the China National Energy Strategy. This strategy is to encourage the development of the green and clean energy instead of the fossil energy. In the development of hot dry rock (HDR), the artificial geothermal reservoir is built through drilling and hydraulic fracturing. Two wells are drilled and connected by hydraulic fractures. Then, the geothermal energy can be extracted by the water circulation. The cold water is injected into one of the two wells and flowed along with the hydraulic fractures. The water becomes gas when it contacts the formation rock because of the high temperature. The phase change since the temperature and pressure are reduced at the wellhead of the another well can provide power to generate electricity. Therefore, the key technology to build artificial geothermal reservoir is drilling and hydraulic fracturing. However, there is few experiences in drilling and hydraulic fracturing in the HDR in China. This paper summarized and analyzed the research and field cases around the world to investigate the challenges and key points during drilling and hydraulic fracturing in the HDR. The investigation was based on geology condition, rock physics and chemistry, and field operation during the HDR development. The summary provided the common problems and experience of the drilling and hydraulic fracturing. In addition, the further research directions of related technologies were expounded. Finally, this paper could be referenced for the drilling and hydraulic fracturing in the development of HDR in China for effective utilization.

Geothermal energy source, one of the viable renewable energy sources, has encouraging potential to generate full base-load electricity, which has not been explored so far in Pakistan. Though the country can be benefited by harnessing the hydro-geothermal options of energy generation in areas where sources exist, but most of these sources lie in extreme remote and inaccessible rugged mountainous ranges away from the urban-industrial centers. On the other hand, the present study shows that the HDR geothermal option is one of the most viable renewable sources considering the tectonic setup of Pakistan. Results of the study highlight the HDR geothermal energy prospects at relatively deeper depths than hydro-geothermal resources in water-free condition. The basement tectonic analyses reveal that the HDR prospects could be found even just below the urban-industrial centers of Pakistan where there are no hot springs and/or geysers like southern Indus basin in Sindh province or the Kharan trough in the western Balochistan province. Presence of high earth-skin temperature gradient trends derived from satellite temperature data and the high geothermal gradient anomalous zone derived from scanty data of bottom-hole temperatures of some of the oil and gas exploratory wells, indicates encouraging prospects for HDR energy sources in southern Indus and Thar Desert regions inclusive of Karachi synclinorium area. These high geothermal gradients have been inferred to be the result of the deep-seated southern Indus and the Thar fossil-rift structures. Moreover, the prospects of the HDR geothermal energy sources have also been inferred in the Chagai Arc region and the Kharan–Panjgur tectonic depression in the western part of Pakistan based on the analysis of integrated geophysical data. If HDR prospects are developed, they can offer the sustainable, CO2-free and independent of time, of day, of weather or season, and the base-load energy-generation resource.

Responding to the need to reduce atmospheric emissions of carbon dioxide, Brown [Brown, D., 2000. A Hot Dry Rock geothermal energy concept utilizing supercritical CO2 instead of water. In: Proceedings of the Twenty-Fifth Workshop on Geothermal Reservoir Engineering, Stanford University, pp. 233–238] proposed a novel enhanced geothermal systems (EGS) concept that would use carbon dioxide (CO2) instead of water as heat transmission fluid, and would achieve geologic sequestration of CO2 as an ancillary benefit. Following up on his suggestion, we have evaluated thermophysical properties and performed numerical simulations to explore the fluid dynamics and heat transfer issues in an engineered geothermal reservoir that would be operated with CO2. We find that CO2 is superior to water in its ability to mine heat from hot fractured rock. Carbon dioxide also offers certain advantages with respect to wellbore hydraulics, in that its larger compressibility and expansivity as compared to water would increase buoyancy forces and would reduce the parasitic power consumption of the fluid circulation system. While the thermal and hydraulic aspects of a CO2-EGS system look promising, major uncertainties remain with regard to chemical interactions between fluids and rocks. An EGS system running on CO2 has sufficiently attractive features to warrant further investigation.

Rock fracture mechanics has been widely applied to blasting, hydraulic fracturing, mechanical fragmentation, rock slope analysis, geophysics, earthquake mechanics, hot dry rock geothermal energy extraction and many other practical problems. But a standard method to accurately determine fracture toughness of rocks, one of the most important parameters in fracture mechanics as an intrinsic property of rock, has not been yet well established. To obtain rock fracture toughness, disc-type specimens were used in this study. Rock fracture toughness under mixed-mode conditions was measured by using the straight-through crack assumption (STCA) applied to the cracked chevron-notched Brazilian disc (CCNBD) specimen and the semicircular bend (SCB) specimen. Size effects, in terms of specimen thickness, diameter and notch length on fracture toughness, were investigated. From the mixed-mode test results, fracture envelopes were obtained by applying various regression curves. The mixed-mode test results were also compared with the three well-known mixed-mode failure criteria.

Microseismicity accompanying hydraulic injection experiments at the Hijiori hot dry rock site was monitored by a network of ten borehole seismic stations deployed at an average distance of 2 km from the injection well. While expanding hydraulic fractures are almost aseismic, they can induce microseismic events. These events are probably caused by shear failures induced by high pore fluid pressures occurring on planes of weakness in the rock surrounding the main hydraulic fracture. Thus we can use these induced events to locate the hydraulic fracture and follow its growth. Microseismic events induced during the 1988 hydraulic fracturing experiment with a high injection pressure were located near the injection point in the early stage of the experiment and clearly migrated towards the east and distributed along a vertical plane. The strike of seismicity is nearly parallel to the direction of the maximum principal stress. The vertical orientation and east–west strike of the seismic events are essentially coplanar with the caldera ring-fault structure in the southern portion of the Hijiori Caldera. This indicates that a preexisting fracture zone was being re-opened and developed in the direction of the maximum principal stress, although microseismic events were caused by shear failures. The space–time distribution of the microseismic events shows that the events migrated at a rate proportional to time to the power 2/3. Assuming that migration of events is attributed to fracture propagation, the propagation of the hydraulically stimulated fracture can be explained by one of two petroleum industry models tested. Seismicity accompanying the 1989 circulation test with a low injection pressure was diffuse and showed a seismic cloud. The permeability was estimated from the hypocenter migration as 10−16 m2, which is intermediate between the permeability of core samples of granodiorite taken from the production well and the permeability of fractured rocks obtained by an injection test between the injection well and the production well. It is therefore concluded that the seismic cloud accompanying the circulation test is due to the permeation of water into joints which slip when the effective stress is reduced by the increased pore fluid pressure accompanying the hydraulic injection. Microseismic events occur and migrate along those joints.

This paper presents a general view of the recent progress in the studies on in situ rock stress measurements. Knowledge of rock stress is of fundamental importance in a wide range of rock engineering activities, including the safe and economic excavation design in civil and mining engineering, stress-related instability problems of underground openings, dams, slopes, stopes, tunnels and deep wellbores, the interpretation for hot dry rock geothermal energy projects and radioactive waste disposal, and the prediction research of earthquakes. Methods and techniques for in situ rock stress measurements are focussed on, and the recent researches are reviewed to discuss the subject for a future study and the matter to be solved at present.

Hot dry rock geothermal energy extraction experiments in artificial reservoirs created by hydraulic stimulation in naturally fractured crystalline rocks have been undertaken in several countries over the last 20 years. The experiments have had mixed results in terms of fluid recovery, system impedance, and heat extraction. Numerical models have not yet delivered a generally agreed understanding of the processes and sensitivities involved in reservoir creation and circulation. In this paper a two-dimensional fracture network model is described, which attempts to address the problems of both reservoir creation and circulation using rock mass characterization and in situ stress data as the primary inputs with a view to encapsulating our present understanding of how such systems work. The model is a prototype for a three-dimensional version currently under development and is intended mainly for engineering sensitivity studies. The basis of the model lies in approximations of fracture mechanical behavior drawn from the rock mechanics literature, a very simplified analysis of the operative physical processes, and mapping of the connectivity of fracture networks to a fine resolution regular grid. Taken together, these permit the approximate resolution of what is normally a supercomputer problem on a personal computer. The model is applied to field data gathered at Fenton Hill, New Mexico; Hijiori, Yamagata Prefecture, Japan; and Rosemanowes, Cornwall, England.

The propagation of a one-dimensional, fluid-filled crack in a hot dry rock geothermal energy reservoir (HDRGER) is discussed. In previous studies a number of different relationships between the normal stress on the crack, the fluid pressure, and the crack height (so-called crack laws) have been used, as have different “flow laws” to determine the relationship between flow rate and crack geometry. Here it is shown that the choice of submodel may have profound implications for the mathematical structure of the problem. In particular, two crack laws (a linear law and a hyperbolic law) are considered as well as two flow laws (a cubic law and a linear law). The model contains a dimensionless parameter that measures the relative importance of stresses due to local deformation of asperities and the long-range deformation of the crack surface. The case is considered where the former is the dominant mechanism. A perturbation analysis is performed, and it is found that for some combinations of laws a strained-coord...

The European Hot Dry Rock Geothermal Energy Project, located in the Rhine graben at Soultz-sous-Forets, Alsace, France, is entering a new phase in its development. Over the next few years the existing HDR system will be developed to form an operational Scientific Prototype HDR System. This paper provides an introduction to the collaborative reservoir modelling studies undertaken as part of the European Programme. In particular the paper addresses the general methodology adopted in the reservoir design process and focuses on one of the preliminary objectives of the study, assessment of the minimum HDR doublet separation required to meet the thermal performance objectives during circulation. Two “preliminary reservoir designs” are adopted as starting points for the study, the first based on exploitation of large scale planar fractures, the second on the development of a modular (multi-cell) system based on 3 cells supporting 51/s production each. Estimates were obtained using models based on both idealised geometry and empirical observations of reservoir circulation at the Camborne School of Mines (CSM) HDR project. The results indicate that a wellbore separation of around 400m would be required for the multi-cell system to achieve the required thermal performance of 10% thermal drawdown, or less, during 10 years circulation at 151/s production. Whereas, the wellbore separation required for the single fracture design would be in excess of 650m.

Models of two-dimensional and three-dimensional subsurface fracture networks are proposed on the basis of “fractal geometry” to describe geothermal water flow. Using this fracture network model, as well as the single fracture model and the permeable zone model, a parameter study of the performance of Hot Dry Rock geothermal energy extraction systems, such as the changes in output water temperature and thermal output with time, is performed. The results show a strong dependence of the system performance on geothermal reservoir characteristics, such as the fracture density of the rock mass. The fracture network model proposed here can describe the performance of reservoirs which vary between the single fracture model and the permeable zone model, and is expected to bridge the gap between the two models. It is confirmed that the performance of geothermal energy extraction systems can be predicted as a function of the fracture density of the rock mass by using the fracture network model.

The concept of hot dry rock (HDR) geothermal energy originated at the Los Alamos National Laboratory. In 1970 it was proposed as a method for exploiting the heat contained in those vast regions of the earth’s crust that contain no fluids in place—by far more widespread than natural geothermal resources (HDR represents over 99% of the total U. S. geothermal resource). Although often confused with the small, already mostly commercialized hydrothermal resource, HDR geothermal energy is completely different from hydrothermal energy.

As detailed in preceding chapters, the technical feasibility of HDR geothermal energy was clearly demonstrated at Fenton Hill, with the testing of two separate confined reservoirs. The major task now in view is moving this revolutionary new technology to its appropriate place in the world’s energy supply mix. Obviously, technical feasibility is not enough: HDR must also be capable of supplying useful amounts of energy economically. For that requirement to be met, several issues—which have yet to be adequately addressed—will need to be resolved.

How far back in our past did humans begin to use hot water and steam coming from vents in the earth’s surface to improve their lives? Did they make stops at such sites while moving from place to place, bathing in the warm pools or using the waters for cooking, and eventually construct villages near hot springs? We can only imagine in what ways man first availed himself of the earth’s heat; but we can assume that human populations in various areas sooner or later encountered hot waters that were bubbling up to the surface after having been raised to high temperatures by circulation through deep, hot rock—and that they made use of the heated water.

Predicting hot dry rock geothermal energy extraction requires knowledge of the coupled hydro-thermo-mechanical properties of rock and rock fractures. A description is presented of experiments using a geothermal rock mechanics testing system capable of testing medium-sized cylindrical rock specimens at axial loads to 2.67 MN, confining pressures to 140 MPa, pore pressures to 105 MPa and sample temperatures to 400°C. Tests were carried out to determine mechanical and thermal properties of intact granite and the mechanical deformations, fluid mass and heat transport in fractured rock. Descriptions of the tests and typical results are presented and discussed to illustrate the multi-functional capability of the testing system to address required research in the development of a geothermal energy system. Results obtained through tests on the Carnmenellis granite using the geothermal rock testing system show that: the strength of the rock can be reduced by increasing temperature, the permeability of the fracture is controlled by the effective normal stress, and the heat convection rate is a function of flow velocity in the fracture. The results also show that the granite has a thermal conductivity of about 3.5 W/mK.

Although still relatively uncommon, microseismic detection is accepted as a valuable technique in monitoring large-scale hydraulic injection or extraction from rock at depth. Even through microseismicity may potentially provide information on rock mass behaviour that is unavailable from other remote techniques, the interpretation of microsiesmic activity is often restricted to the analysis of their locations. This restricted usage is primarily due to our current limited understanding of fundamental mechanisms of microseismic energy release and their relations to the mechanics of rock deformation. In this paper, microseismic data will be used to investigate the mechanism of reservoir growth during viscous fluid injection into a jointed granitic rock mass at the Fjallbacka Hot Dry Rock geothermal energy site in Sweden. It will be demonstrated that using simple seismological and rock mechanics interpretation techniques it is possible to obtain a valuable insight into the behaviour of a jointed rock mass during fluid injection. It will be shown through the combined interpretation of microseismic fault plane solutions and in situ stress data, using the simple rock mechanics concept of peak shear strength, that reservoir growth took place through the shear failure of shallowly dipping natural joints due to sufficiently elevated pore fluid pressures.

SUMMARY A magnetotelluric survey across the Carnmenellis granite, part of the Cornubian batholith of SW England, is described. The granite is being investigated with reference to the extraction of Hot Dry Rock geothermal energy. The sounding data are found to be influenced by near-surface static distortion which can be minimized by the construction of a granite-average sounding tensor. The anisotropy in the average tensor can be attributed to the spatial form of the granite. This is verified by 2-D modelling and the model results are used as control in the assessment of the vertical resistivity profile. The resistivity values obtained by the survey are found to correspond to a saturated ‘wet’ granite down to at least 10km. Despite the high geothermal gradient the anticipated decrease of resistivity with increasing temperature does not take place. Below a depth of just over 1 km, resistivity values are found to increase to a depth of about 6km. The results are compared with the laboratory-observed response of granite to increasing applied pressure and stress. The resistivity/depth section appears consistent with the completion of joint and crack closure by a depth of 7 km and a transfer to a pore-dominated resistivity mechanism below this depth. An electrical base to the granite is resolved at a depth of 14 km in accord with the depth to base adopted by regional gravity models.

The geothermal energy in hot dry rock (HDR) is a vast resource found almost everywhere in the world. Research and development work conducted by the Los Alamos National Laboratory at the Fenton Hill test site has taken the technology to extract HDR energy from the conceptual stage through a concrete demonstration such that useful amounts of energy can be routinely produced. Flow testing during 1992-1995 has shown that HDR systems are reliable and resilient. HDR technology has a number of unique characteristics that make it particularly suited to the developing energy needs of the world. These include operational flexibility that can provide energy as needed to rapidly meet peaking or unanticipated power demands and the potential for cogeneration of both energy and clean water. HDR technology has been shown to be practical and versatile. The next logical step in making HDR a viable energy resource is to develop an HDR system that will generate and market power, thereby providing the practical operating and economic data needed to promote the rapid commercial application of this unique energy technology.

A comparison study was undertaken to evaluate different analytical methods for measuring boron concentrations in the geothermal fluid from the Los Alamos Hot Dry Rock Geothermal Energy Project and from natural waters collected in the vicinity of the project. Three colorimetric techniques, the carmine method and two azomethine H methods, were compared with two plasma emission spectroscopy methods and with thermal neutron capture-gamma spectroscopy. In general, the analytical results from all the methods were comparable and acceptable; however, each method had its own distinct advantages and disadvantages.

Present and potential geophysical applications of Superconducting Quantum Interference Devices (SQUIDs) include remote reference magnetotellurics, controlled-source electromagnetic sounding, airborne gradiometry, gravity gradiometers, rock magnetism, paleomagnetism, piezomagnetism, tectonomagnetism, the location of hydrofractures for hot dry rock geothermal energy and enhanced oil and gas recovery, the detection of internal ocean waves, and underwater magnetotellurics.

At the hot dry rock geothermal energy project in Cornwall, England, run by Camborne School of Mines, a number of hydraulic injections have been completed with the aim of creating zones of enhanced permeability. The fluids used in the Phase 2 system, which is located at a depth of more than 2 km in granite, have been water, and high and low viscosity gels. A large volume, low viscosity gel injection, which relies upon controlled shear stimulation appears the most promising in the context of significant horizontal in situ stress anisotropy which applies at the Cornwall site. The injection design has evolved with the aid of microseismic tracking for in situ locations and for the interpretation of stimulation mechanisms. Numerical modelling has been developed for the specific rock conditions encountered. Hydraulic and microseismic results are reasonably consistent with those suggested by the modelling.

Still in its early stages of development, hot dry rock (HDR) geothermal technology may provide the means to tap the enormous stock of energy stored as heat in the earth's crust. By drilling a well deep into impermeable granite and creating a thin fracture over which water is passed and heated, geothermal power can be generated where natural water deposits do not occur. The first such well is in operation at the Los Alamos Scientific Laboratory. The research in progress there should shed light on the many technical uncertainties surrounding the process. Given current assumptions on costs and physical characteristics of HDR reservoirs with thermal stress cracking, the potential economic feasibility of the process for generation of electric power is investigated. Preliminary results reported here indicate that under these optimistic physical assumptions, the resource may generate electricity at competitive prices at those sites where the geothermal gradient (the increase in temperature to depth) exceeds 30°C/km - a condition which characterizes much of the Western three-fourths of the U.S.

High-frequency (8–15 kHz) seismic waves that travel between two boreholes are used to probe the detailed structure of rock in the Los Alamos National Laboratory Fenton Hill hot dry rock geothermal energy reservoir. We study travel times of direct P waves and spectral characteristics of waveforms in an effort to determine whether or not large fluid-filled fractures are present in the rock between the two boreholes. Data from two experiments, separated in time by 1 year, are studied. We find dramatic differences between the two experiments in P wave velocity and in spectral characteristics of waveforms that traversed the same region. We also find variations in P wave velocity and spectra as a function of location during one experiment. Loss of high-frequency content of direct-arriving P waves during the second dual-well seismic experiment is compared with Q, found by using the coda wave technique (Aki and Chouet, 1975), and also with theoretical results on frequency dependence of transmission through a water-filled fracture to determine the minimum number of large fractures in the system. Two to four fractures with thickness about 4 mm and vertical extent of as much as 200 m are required to explain observations during the second experiment. Efficient transmission of high frequencies observed during the first experiment is interpreted as meaning that fracture thickness was less than 1 mm at that time. Increased fracture thickness during the second experiment is probably due to a 75-day period of heat extraction during which in situ temperature dropped by as much as 100° below the virgin temperature. Since the coefficient of linear thermal expansion of granite is approximately 10−5/°C, a decrease in temperature of 100°C in a region on the order of 1 m around the fracture is required to explain the increased fracture thickness.

As the value of microseismic monitoring during development of HDR reservoirs is already proven, the discussion group concentrated on assessing what further improvements might be made. They suggested improvements both to the receivers and to the methods of analysis, with particular reference to the derivation of different source mechanisms from the signals received.

Microearthquakes with magnitudes between −6 and −2 have been observed in three successive massive injections of water at the hot dry rock geothermal energy development project's demonstration site at Fenton Hill, NM. The injections were part of a program to increase the heat transfer area of hydraulic fractures and to decrease the flow-through impedance between wells in the energy extraction system under construction. The microearthquakes were used in mapping the location of the extended hydraulic fractures. The large shear- to compressional-wave-amplitude ratio observed in the signals suggests that the microearthquakes result from shear failure, probably on preexisting planes of weakness that intersect or make up the main hydraulic system. Failure resulting from increased pore pressure is a likely cause for the microearthquakes. Knowledge of the failure criterion for the reservoir rocks permits calculation of the pore pressure increases necessary for failure. At Fenton Hill, it appears that pressures 2 Pa (0.00029 psi) above hydrostatic are necessary. For this hypothesis of microearthquake occurrence to hold, the effective reservoir permeability must be four orders of magnitude above that for the bulk rock, as would be the case if there were permeable joints or fractures in the reservoir before hydraulic stimulation.

Hydrofracture stress measurments were conducted in the Carnmenellis granite at the Camborne School of Mines Hot Dry Rock Geothermal Energy Research site at Rosemanowes Quarry near Penryn, Cornwall, U.K. Tests were conducted at depths of up to 2000 m in borehole RH 12, which was drilled with nominal diameters of 311 mm and 216 mm and at an inclination of up to 32° from vertical. The tests were successful in establishing reliable values for the minimum horizontal stress, and estimates of the maximum horizontal stress. Stress anisotropy was very significant throughout the tested depth, with maximum and minimum horizontal stresses of about 70 and 30 MPa respectively at a vertical depth of 2000 m. The overburden stress at this depth was estimated as 52 MPa. The bearing of the maximum horizontal stress was established in a separate programme of overcoring stress measurement at approximately 130° relative to true north. The stress magnitudes determined by the overcoring tests were close to those measured by the hydrofracture tests at the same depth. The effects of hole inclination, test procedure and premature cracking of the borehole wall due to high compressive stresses are discussed in relation to the in-situ stress interpretation.

This is a short history, so far as it can now be assembled, of early speculations and observations concerning the existence and origin of natural heat in the earth's crust; of some of the many methods proposed to extract and use it; and of recent investigations designed to develop and demonstrate such methods. It is probably only the preface to a much longer history—still to be written—of mankind's beneficial use of this, one of earth's oldest, most plentiful, most widespread and secure energy sources.

The Phase I Hot Dry Rock Geothermal Energy reservoirs at the Fenton Hill field site grew continuously during Run Segments 2 through 5 (January 1978 to December 1980). Reservoir growth was caused not only by pressurization and hydraulic fracturing, but also by heat-extraction and thermal-contraction effects. Reservoir heat-transfer area grew from 8000 to 50,000 m2 and reservoir fracture volume grew from 11 to 266 m3. Despite this reservoir growth, the water loss rate increased only 30%, under similar pressure environments. For comparable temperature and pressure conditions, the flow impedance ( a measure of the resistance to circulation of water through the reservoir) remained essentially unchanged, and if reproduced in the Phase II reservoir under development, could result in “self pumping”. Geochemical and seismic hazards have been nonexistent in the Phase I reservoirs. The produced water is relatively low in total dissolved solids and shows little tendency for corrosion or scaling. The largest microearthquake associated with heat extraction measures less than minus one on the extrapolated Richter scale.

Large scale hydraulic injections into rock at significant depth are common in the exploitation of hydrocarbon formations, geothermal energy systems and for liquid waste disposal. One example is the hot dry rock geothermal energy project operated by the Camborne School of Mines in Cornwall, U.K., where a cumulative injected volume of over 300,000 m3 was pumped into 2 km deep boreholes in granite. During the course of these injections the location of pressurized water was followed by means of microseismic detection. It was concluded that the continuing downward growth of the stimulated region was associated with a shearing source mechanism. This result is explained in terms of the anisotropic in situ stress field interacting with critically aligned joints. The downward growth is closely related to the ratio of the maximum to minimum principal effective stress. The existence of the downward growth implies an eventual curvature in the maximum stress envelope if shear failures are not to be incipient under hydrostatic pore pressure conditions. This curvature implies that upward shear growth would occur during injections at greater depths. These results are potentially of general application in strong jointed rock with a markedly anisotropic in situ stress field.

Large, young calderas possess immense geothermal potential due to the size of shallow magma bodies that underlie them. Through the example of the Valles and Toledo calderas, New Mexico, and older, more deeply eroded and exposed calderas, it is possible to reconstruct a general view of geothermal environments associated with such magmatic systems. Although a zone of anomalous heat flow extends well beyond caldera margins, high- to moderate-temperature hydrothermal systems appear to be restricted to zones along intracaldera faults and may account for only about five percent of the thermal energy available from the magmatic system. Much of the remaining thermal energy resides within caldera fill and “basement” rocks under and along the flanks of the caldera and can only be extracted by manmade geothermal systems. Depending upon the age and cooling history, considerable thermal energy may still reside in crystallizing magma.

Summary A research hot dry rock (HDR) geothermal energy reservoir is currently under development in the Carnmenellis granite in Cornwall, U.K., at a depth of about 6,600 ft [2000 m]. This paper details the performance, analysis, and interpretation of hydraulic tests using low to medium pressures and flow rates. These tests allowed an evaluation pressures and flow rates. These tests allowed an evaluation of the in-situ hydraulic parameters relevant to initial water storage and long-term leakage. Tests were analyzed using both diffusion theory and an explicit, Coupled hydraulic/mechanical computer code. At low fluid overpressures, the permeabilities, in terms of uniform diffusion, were in the range of 1 to 10d, rising to about 60 d at overpressures of about 725 psi (5 MPa). These figures imply that long-term leakage psi (5 MPa). These figures imply that long-term leakage from an HDR reservoir is minor. The computer model was able to simulate several cycles of injections and shut-ins at progressively higher pressures and flow rates with a single set of variables. Introduction An HDR geothermal energy research project is being conducted by the Camborne School of Mines (CSM) at their test facility at Rosemanowes quarry in Cornwall (Fig. 1). The facility includes two deviated wells, RH11 and RH12, each drilled to a vertical depth of about 6,600 ft [2000 m] (Fig. 2). The holes are located within the Carnmenellis granite throughout their lengths, and are inclined at 30 from vertical in their lower sections to intersect the predominantly vertical natural jointing. The primary objective of the project is to link the two wells through the natural jointing by using a combination of explosive pretreatment and hydraulic stimulation to create a heat transfer zone (or reservoir) for the circulating water. However, it was first necessary to characterize the hydraulic behavior of the rock mass with modest flow rates and fluid overpressures before the natural joint structure was modified by major hydraulic injections. The resulting information then could give reference conditions for comparing the results of the high energy tests. The information is relevant for determining the leakage losses at the limits of the stimulated reservoir. It is also of interest to compare the results with those obtained at depths of 1,000 ft [300 m] in previous experiments at the same site to obtain a depth perspective for the hydraulic behavior. Since the test depth is unique for crystalline rocks in the U.K. and is unusual elsewhere, the results are of interest for other projects involving facilities at comparable depths (e.g., liquid waste disposal and deep-mined hydroelectric pumped storage). Geological Conditions The Carnmenellis granite pluton was emplaced about 285 million years ago, and subsequently has undergone a complex tectonic history. The pluton is exposed at the ground surface over an approximately circular area of about 50 sq miles [130 km]. The Rosemanowes quarry test site is near the center of the exposure (Fig. 1). Topography is gently undulating with a maximum relief of about 330 ft [100 m]. There is a network of dominant, near orthogonal, vertical joints throughout the granite, with typical spacings of 3 to 10 ft [1 to 3 m] near the ground surface, 10 to 30 ft [3 to 10 m] at 2,500-ft [750-m] depth, and similar to wider spacing at 6,600 ft [2000 m]. Nearly horizontal joints also occur but less frequently. All joints are typically tight, away from zones of stress relief. In-situ stresses have been measured in some detail. At a depth of 6,600 ft [2000 m], the vertical, maximum, and minimum horizontal stresses are approximately 7,500 psi [52 MPa], 8,700 to 10,200 psi (60 to 70 MPa) and 4,400 psi [30 MPa], respectively. The horizontal stress directions and major near-vertical joint set strike orientations are shown in Fig. 3. The fabric of the Carnmenellis granite consists of megacrysts of alkali feldspar set in a coarse ground mass of plagioclase, alkali feldspar, micas, and trace minerals. Grain size is variable, with megacrysts up to 0.75 in. [20 mm] long and crystals in the ground mass typically 0.08 to 0.2 in. [2 to 5 mm] in diameter. The permeability of the fabric is extremely low, with values permeability of the fabric is extremely low, with values of 10(-3) d obtained by laboratory water diffusion tests. The mechanical properties may be characterized by a uniaxial compressive strength of about 19,600 psi [135 MPa], with dimensionless triaxial strength parameters m and s of 21 and 1.0 per Hoek and Brown, parameters m and s of 21 and 1.0 per Hoek and Brown, Young's modulus of 9.43 × 10(6) psi [65 GPa], and Poisson's ratio of 0.2. Poisson's ratio of 0.2. JPT P. 1982

SYNOPSIS There exists within the crust of the earth a vast thermal resource of rock which is at elevated temperature but which lacks significant fluid flow to be exploited naturally. Measurements of heat flow made throughout the United Kingdom have identified the Cornubian batholith as such a structure, with an abnormally high geothermal gradient. Shallow depth field research has been conducted into the mechanics of linking wells together to engineer the required heat transfer area. This has shown that a combination fracturing technique, involving explosive stimulation and hydraulic pressurisation, has considerable advantages over the conventional hydraulic fracturing technique. The experimental design of a second phase of research is presented: the results of which are expected to provide definite guidelines for the engineering requirements of a system capable of producing super-heated steam for the generation of electrical power.

P wave arrival times have been picked on seismograms recorded during two dual well seismic experiments, separated in time by one year, that were carried out in the Los Alamos Scientific Laboratory Hot Dry Rock Geothermal Energy Reservoir. Average in situ P wave velocity is computed from arrival times measured along segments of the borehole. Velocity decreases as temperature drops owing to heat extraction from the reservoir. The velocity changes linearly with the change in temperature at a rate of 1.07 (km/s)/(100°C). The linear decrease in velocity leads to the important conclusion that P wave velocity can be used as an effective measure of change in in situ temperature during operation of the geothermal system. The change in velocity is attributed to an increase in the microcrack porosity which is a result of the heat extraction from the rock.

For the last ten years, MITI and NEDO have been supporting field work in Japan aimed at developing techniques of HDR reservoir creation. Early work was performed at 300 m depth at Yakedake and, more recently, two existing 1800 m boreholes have been acquired at Hijiori; the bottom hole temperature on this site exceeds 250°C. Plans are in hand for hydraulic fracturing aimed at reservoir creation.

With completion of the repairs of EE-2 wellbore in FY88, the major effort in FY89 will be directed toward the remaining preparations for the Long-Term Flow Test (LTFT) now scheduled to begin in September 1990. Procurement of components and equipment and installation of the surface system will continue through this and the next fiscal year to be ready for the LTFT in September 1990. Reservoir experiments will consist of pressurization and flow tests to establish further characteristics of the Phase II reservoir, particularly operating water losses. Laboratory studies will continue work on fracture healing and complete adsorption tests for chemically-reactive tracer work. Cleaning and relining of the EE-I pond to meet New Mexico State requirements, started in FY88, will be completed. Reanalysis of seismic data and automation of seismic data techniques will continue. Monitoring for environmental information, routine operations, security, health and safety training, and reporting activities will continue. The program's objectives are: (1) To develop HDR drilling, hardware and instrumentation technology for energy extraction and conduct a successful energy extraction field experiment of the Phase II reservoir. (2) To verify that the environmental consequences of HDR development are acceptable. (3) To improve HDR technology to the point where electricity more » could be produced commercially from a substantial number of known hot dry rock resource sites in a cost range of 5 to 8 cents/kWh. (4) To evaluate the performance of the Fenton Hill Phase II reservoir by 1992. That performance consists of system operating characteristics, including thermal drawdown, energy output, reservoir impedance, and water consumption. « less

Hot Dry Rock (HDR) geothermal energy, which utilizes the natural heat contained in the earth's crust, can provide a widely available source of nonpolluting energy. It can help mitigate the continued warming of the earth through the ''greenhouse effect,'' and the accelerating destruction of forests and crops by acid rain, two of the major environmental consequences of our ever-increasing use of fossil fuels for heating and power generation. In addition, HDR, as a readily available source of indigenous energy, can reduce our nation's dependence on imported oil, enhancing national security and reducing our trade deficit. The earth's heat represents an almost unlimited source of energy that can begin to be exploited within the next decade through the HDR heat-mining concept being actively developed in the United States and in several other countries. On a national scale, we can begin to develop this new energy source, using it directly for geothermal power or indirectly in hybrid geothermal/fossil-fueled systems, in diverse applications such as: baseload power generation, direct heat use, feedwater heating in conventional power plants, and pumped storage/load leveling power generation. This report describes the nature of the HDR resource and the technology required to implement the heat-mining concept in severalmore » applications. An assessment of the requirements for establishing HDR feasibility is presented in the context of providing a commercially competitive energy source. 37 refs., 6 figs.« less

The complete list of HDR objectives is provided in Reference 10, and is tabulated below in Tables 1 and 2 for the reader's convenience. The primary, level 1, objective for HDR is ''to improve the technology to the point where electricity could be produced commercially from a substantial number of known HDR resource sites in a cost range of 5 to 8 cents/kWh by 1997''. A critically important milestone in attaining this cost target is the level II objective: ''Evaluate the performance of the Fenton Hill Phase II reservoir''. To appreciate the significance of this objective, a brief background is helpful. During the past 14 years the US DOE has invested $123 million to develop the technology required to make Hot Dry Rock geothermal energy commercially useful. The Governments of Japan and the Federal Republic of Germany have contributed an additional $32 million to the US program. The initial objectives of the program were met by the successful development and long-term operation of a heat-extraction loop in hydraulically-fractured hot dry rock. This Phase I reservoir produced pressurized hot water at temperatures and flow rates suitable for many commercial uses such as space heating and food processing. It operated for moremore » than a year with no major problems or detectable environmental effect. With this accomplished and the technical feasibility of HDR energy systems demonstrated, the program undertook the more difficult task of developing a larger, deeper, hotter reservoir, called ''Phase II'', capable of supporting pilot-plant-scale operation of a commercial electricity-generating power plant. As described earlier in ''History of Research'', such a system was created and operated successfully in a preliminary 30-day flow test. However, to justify capital investment in HDR geothermal technology, industry now requires assurance that the reservoir can be operated for a long time without major problems or a significant decrease in the rate and quality of energy production. Industrial advisors to the HDR Program have concluded that, while a longer testing period would certainly be desirable, a successful and well-documented flow test of this high-temperature, Phase II reservoir lasting at least one year should convince industry that HDR geothermal energy merits their investment in its commercial development. This test is called the Long Term Flow Test (LTFT), and its completion will be a major milestone in attaining the Level 1 objective. However, before the LTFT could be initiated, well EE-2 had to be repaired, as also briefly described in the ''History of Research''. During this repair operation, superb progress was made toward satisfying the next most critically important Level II objective: Improve the Performance of HDR Drilling and Completion Technology. During the repair of EE-2, Los Alamos sidetracked by drilling out of the damaged well at 2.96 km (9700 ft), and then completed drilling a new-wellbore (EE-2A) to a total depth of 3.78 km (12,360 ft). As a consequence of this drilling experience, Los Alamos believes that if the original wells were redrilled today their combined cost would be only $8 million rather than the $18.8 million actually spent (a 60% cost saving). Further details, particularly of the completion of the well, can be found in the major section, ACCOMPLISHMENTS, but it can be seen that the second, Level II objective is already nearing attainment.« less