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Hydraulic fracturing is one of the key technologies in geothermal development of hot-dry-rock (HDR) geothermal energy. How to reduce the fracturing pressure and enhance the fractures connectivity of HDR has become a hot issue. In this paper, we propose to use a down-the-hole (DTH) hammer to impact granite, which assist to create new artificial fractures similar to natural fractures or widen the existing fractures near the wellbore, consequently enhancing the connectivity of fractures during hydraulic fracturing. A series of experiments were conducted to verify the feasibility of this stimulation technique, including the observation and quantification of impact fractures, and the determination of fracturing pressure. In addition, numerical simulation of impact stress wave was carried out to explain the distribution of fractures. The results showed that the granite rock mass damage caused by percussive rotary drilling was about 5–7 times larger than that caused by diamond rotary drilling. Meanwhile, the distribution of fractures was characterized by regionalization, which was mainly attributed to the attenuation of impact stress wave, and the numerical results were consistent with the experimental observations. Laboratory experiment results of hydraulic fracturing indicated that the fracturing pressure of granite samples near the wellbore after percussive drilling had been greatly reduced, which was nearly 50% lower than that of conventional drilling. It indicates that the proposed multi-fractured stimulation technique of hydraulic fracturing assisted by the impact fractures induced by a DTH hammer can be a potential solution to solve the problems of hydraulic fracturing.

Hot dry rock (HDR) has substantial potential as a thermal energy source. It can be exploited by hydraulic fracturing to extract heat and generate electricity. Because of its great depth (2–6 km) and consequent high temperature (150–650°C) and high pressure, the stimulation of HDR is difficult. The purpose of this study is to study the feasibility of cryogenic fracturing on HDR. We conducted a series of laboratory experiments on rock samples under triaxial-confining stresses. Two schemes were utilized: gas fracturing with and without liquid nitrogen (LN) (−196°C) treatment. For the LN treatment, high-temperature granitic specimens were immersed in LN for several hours before the fracturing tests. Three critical factors were considered, namely, rock specimen temperature (100–600°C), axial stress (5–20 MPa), and lateral stress (5–20 MPa). Subsequently, the fracture patterns were identified and correlations for both LN rapid-cryogenic and natural-cryogenic stimulations were developed to predict the breakdown pressure based on the experimental data. Finally, the field applications for LN cryogenic stimulation in HDR reservoir were discussed and a thermal-hydraulic coupled transient numerical model was established to provide suggestions of LN injection schedule for an effective cryogenic stimulation. The results show that the LN cryogenic stimulation could reduce the breakdown pressure level by 9–51% more than that of the untreated specimens. Particularly, LN provides superior fracturing efficiency on rock specimens above 200°C. This could be attributed to a sharp thermal gradient induced by LN, which can cause the strong local tensile stress to expand the micro-pore structures inside the high-temperature rock samples. Furthermore, with the assistance of LN, larger fracture apertures and better-connected fracture networks inside the rocks can be generated. Additionally, LN treatment, or rapid cryogenic fracturing, plays an increasingly important role when high-temperature rocks are subjected to higher anisotropic stresses. The key findings of this study could provide an efficient approach to explore HDR resources for geothermal development.

Liquid nitrogen jet fracturing is a novel stimulation technology, which is expected to be suitable for hot dry rock (HDR) reservoirs. Due to the large temperature difference between hot rock and cryogenic fluid, a great number of thermal cracks would be created during fracturing process, which is conductive to improve the penetration capacity of formation. In this study, a set of experiments were conducted to investigate the characteristics of thermal cracks. In these experiments, granite specimens with temperatures ranging from 200 ℃ to 300 ℃ were impacted by the low-pressure liquid nitrogen jet. The complexity and connectivity of cracks were quantitatively analyzed by a fractal method. The permeability and ultrasonic velocity of the granite specimens were tested in order to evaluate the damage conditions caused by thermal stress. Additionally, scanning electron microscope was adopted to analyze the microscopic characteristics of the thermal cracks. The results show that the heating process has a slight effect on thermal-crack generation compared with the liquid-nitrogen impact. The cracks mainly concentrate in the region near the impingement surface, due to the large temperature gradient there. The impacted rock breaks as the effects of tensile stress and shear stress. With an increase of initial rock temperature, the number of thermal cracks increases, and a more complex crack-network is formed in each specimen. Transient pulse evaluation and ultrasonic velocity measurements indicate that the impact of liquid nitrogen jet can improve the permeability and cause the damage of hot rock noticeably. This study demonstrates the important effect of thermal stress on crack generation during liquid nitrogen jet fracturing for HDR reservoirs, and the results shed light on the exploitation of HDR energy.

Hot dry rock (HDR) is a renewable and clean energy resource, its efficient exploration has attracted widespread attention from scholars. However, the rock encountered during drilling is characterized by poor drillability, high abrasiveness and great hardness, which aggravates the wear of bits and increases drilling costs. To address such an issue, much research has been conducted to investigate the failure mechanism of high-temperature rocks. But most of them focused on the rock cutting process by PDC cutters and the penetration of rocks was ignored. In the present study, a series of experiments were performed to determine the penetration efficiency of PDC cutters under different temperatures and back rake angles. The penetration force and 3D topography of penetration pits were measured to analyze rock failure characteristics. Besides, uniaxial compressive experiments were carried out to study the effects of temperature on rock properties. The results indicate that as the rock is heated from 25 to 240 ℃, its properties change slightly. Analogous rock failure characteristics can be found in rock indentation process, where chipping and crushing of rocks, on following another, are presented, respectively. Moreover, the occurrence of chipping is the major contributor to the increase in penetration efficiency. The further increase in rock temperature (greater than 360 ℃) results in a sharp decrease in the strength of rocks, which is accompanied by the enhancement of rock plasticity. At his moment, the plastic failure dominates rock indentation process under different back rake angles. The induced horizontal force cannot promote brittle failure and the penetration efficiency is hard to be improved, significantly affecting the performance of conventional PDC bits in high-temperature formations. Based on the results, the low penetration efficiency at high temperatures is a major obstacle that restricts the exploration of HDR. Increasing the cutting depth should be an important target in optimizing drilling parameters and the arrangement of PDC bits.

Exploring and exploiting a geothermal resource has become one of the most prolific tasks for contributing to the global sustainable development. Despite this fact, several countries, such as Spain, are still far from achieving a generalized use of these renewable systems. The reason for this underuse often derives from the lack of information and characterization of the geothermal energy source. Considering this, the present research aims to provide relevant data about the geothermal potential of the Spanish region of Avila. The geological context of this province lays the foundations for considering the region as a promising site for different geothermal uses. In order to estimate the geothermal energy potential of the region, the existing geological information has been complemented with thermal surveys carried out in the study area. The experimental measuring has consisted of the register of the underground temperature in piezometers of variable depth and natural springs distributed throughout the province. The processing of these records has allowed knowing the thermal evolution of the subsoil at the different levels evaluated in the research. Results show that there are two main potential areas in the province that could be successfully used for heating purposes (maybe as part of district heating systems) and for future deeper evaluations in the sense of Hot Dry Rock (HDR) techniques. Final conclusions have also been included in an interactive and open-source tool that allows visualizing the thermal findings with the aim of planning future geothermal uses in the region.

The Gonghe-Guide area, which is located at the northeastern edge of the Qinghai-Tibetan Plateau, has the greatest hot dry rock (HDR) geothermal resources exploration and development potential in China. Separated by the Waligong tectonomagmatic belt, the study area includes the Gonghe and Guide basins, which are characterized by distinctly different thermal state and surface thermal manifestations. Although a number of geophysical explorations have been undertaken, studies on fundamental geothermal theory remain scarce, including a lack of high-quality heat flow determinations and thermal structure studies. In this study, we obtained continuous steady-state temperature logs from three deep boreholes (GR2 and DR3 in the Gonghe basin and ZR1 in the Guide basin). The thermal conductivities and radiogenic heat production rates of the outcrop samples gathered from the two basins were measured. The heat-flow values were determined to be 116.3–122.3 mW m−2 for the Gonghe basin, yielding a mean of 119.3 mW m−2. Two existed heat flow sites (R2 and R3 boreholes) gave an average heat flow of 75 mW m−2 for the Guide basin, which is approximately that of the regional background heat flow level of the northeastern Tibetan Plateau. The 1-D theoretical crustal thermal structures of the Gonghe and Guide basins were constructed by solving steady-state heat conduction equation. The results show that the Moho temperature and mantle heat flow in the Guide basin were 1016 °C and 27 mW m−2, respectively. However, the temperature field in the Gonghe basin indicated partial melting in the crust. The high temperatures and thus heat flow in the Gonghe basin may suggest the existence of anomalous heat source body in crust. Successful inversions of vertical and horizontal temperature variations of boreholes in the Gonghe basin were obtained by numerically simulating a cooling rectangular magma chamber with a buried depth of 5–6 km and horizontal dimensions of 12–14 km. The study emphasizes that the anomalous heat flow in the Gonghe basin may be the joint thermal effect of: (1) the high regional background heat flow level of the northeastern Tibetan Plateau, which is mainly due to the radiogenic heat of thickened crust; (2) the cooling of a shallow magma chamber, which may be caused by the uplift and denudation of the plateau.

As one of the key technologies for hot dry rock (HDR) geothermal exploitation, artificial reservoir is giant hydraulic fracturing at its core and takes the theory of hydraulic fracturing of rocks under thermo-mechanical coupling as its key scientific problem. With large samples of Shandong grey granite being the experimental subject, this paper conducted hydraulic fracturing experiment under triaxial stress at 20 °C, 100 °C, 200 °C, 300 °C and 400 °C, analyzed the characteristic of water pressure loading curves in the experiment, and found that crack initiation pressure decreased remarkably after 300 °C. In order to verify the factors controlling the influence of temperature on crack initiation, this paper established a strong transient thermal stress model, carried out numerical computation on this model and compared it with the experimental results. The results showed that the cooling effect of fracturing fluid for high temperature borehole can lead to thermal shock phenomenon and cause tensile stress near the borehole surface; the area near the borehole experienced two impact shocks, namely, elastic wave and thermal wave; the mechanism for the effect of temperature on the hydraulic fracturing of granite is not changes of rock mechanical parameters, but the thermal shock generated by the action of fracturing fluid of rock at high temperature.

The objective of this work is to study the performance of 1wt% StimuFrac fluid [polyallylamine (PAA) in water] in ½-foot size rock samples under thermal and fluid phase conditions representative of enhanced geothermal systems (EGS) and reveal the mechanisms governing the fracturing process. Four fracturing fluids including water, CO2, CO2 with water, and CO2 with aqueous PAA were used. For all the water “only” fracturing tests, the conductivity of the rock fractured is quite low (less than 2 μ m 3 based on radial flow assumption), regardless of the constant flow rate or discrete pressure increments injection approach. For all three CO2-based fracturing fluids, granite was fractured at higher breakdown pressures, higher transient flow rates, and generated higher-conductivity fractures as compared to water. When partially saturating the rock sample with 1wt% PAA aqueous solution followed by fracturing with CO2, the volume expansion and viscosity increase triggered by CO2-induced cross-linking PAA leads to a faster pressure increase than CO2 in water saturated or initially dry. This faster pressurization rate is caused by a decrease in relative permeability of CO2 in the presence of the high-viscosity cross-linked fluid compared to the un-crosslinked CO2/water. It was also found that CO2 as a fracturing fluid can generate high fracture conductivity only when injected at very high flow rates in the presence of water or in hot dry rock (HDR). However, the conductivity of CO2 fracturing in HDR is highly variable. CO2 injected in the presence of 1wt% aqueous PAA generates fractures with the highest conductivity independently of injection flow rates and using 1/6 of the mass of CO2 as compared to CO2 fracturing in HDR. The results of this study suggest CO2/PAA is the best performing stimulation fluid under the studied P/T conditions. CO2/PAA offers the following three additional advantages over waterless CO2, and CO2/water fracturing fluids: i) it requires significantly lower volumes of CO2 due to the reduced leak off; ii) large fractures can be generated reproducibly at both low and high CO2 injection flow rate, and iii) the reversible (previously reported) viscosity increase could be beneficial to transport proppants when they become available for EGS.

The new discoveries of the high-temperature geothermal resources in the Gonghe-Guide area mark the significant breakthrough in the exploration of geothermal resources in the northeastern Tibetan Plateau. In this paper, we systematically reviewed all the available data presented in the geological, geophysical, geochemical and fundamental geothermal works that have been undertaken, in order to fully understand the characteristics of the geothermal resources in this area. The main features of the geothermal resources in this area can be summarized as follows: two distinct types of high-temperature geothermal resources have been identified, i.e., the hot dry rock (HDR) and hydrothermal geothermal resources in the Qiabuqia and Zhacang geothermal areas, respectively. The HDR reservoir in the Qiabuqia geothermal area is mainly composed of the Indosinian granitic rocks with the temperature gradient of 39 to 45.2 °C/km, thermal conductivity of 2.07 to 3.10 W/(m•K), and radiogenic heat production rate (RHPR) of 1.17 to 5.81 μW/m3. The geothermal water in the Zhacang geothermal area is meteoric origin with the relative enrichment of δ18O caused by the fluid-rock interaction under the high temperature condition. The chemical type of the geothermal water is SO4•Cl-Na. The local crustal partial melting or magma chamber at the 15 ∼ 35 km beneath the Gonghe basin, which is characterized with a relatively high geothermal background induced by the continental collision between the Indian and Eurasian plates, is the main cause of the HDR resources, whereas the hydrothermal resources in the Zhacang geothermal area are related to the deep circulation of the meteoric water in the damage zone of the west-east trending conductive Zhacang fault and upward transporting at the intersection of the Zhacang and the NNW-trending impermeable Reguang faults. Such a detailed review of the both geothermal resources can provide a glimpse of the geothermal resources of the northeastern Tibetan plateau as well as provide the direction for the subsequent searches for high-temperature resources elsewhere in China.

The hot dry rock (HDR) development is a hot research field in recent years. Numerical simulation based on thermal-hydraulic-mechanical coupling model is a key technique for the HDR development evaluation. The calculation of geomechanics is an important part of the coupling thermal-hydraulic-mechanical simulation, and it is usually implemented by finite element method. However, meshing methods of geological modeling software are designed to serve seepage simulation, and grid quality cannot meet the requirements of finite element method. In this paper, finite volume method has been proposed to calculate the distribution of underground stress field, in order to solve the problem of discontinuous grid vertexes and calculation failure caused by finite element method. In addition, the mesh of geological modeling software is for finite difference method or finite volume method, which cannot be used directly by finite element method. In this paper, spatial discretization pattern of quasi static stress balance equation based on the finite volume method is obtained, the local grid refinement algorithm is proposed, the solution workflow of thermal-hydraulic-mechanical (THM) coupling model is presented, and the solution of displacement discretization equations and THM one way coupling model are implemented by C++ programming language. Finally, the feasibility and accuracy of the finite volume method of stress field calculation are verified through a series of numerical test examples compared with commercial finite element software. Several THM coupling simulation cases of enhanced geothermal system are presented by finite volume method. Numerical test results show that the calculation error between finite volume method and finite element method is less than 1%. Moreover, the variation of principal stress, induced by pore pressure and thermal stress, can reach 8 to 20 MPa. The finite volume method provides a unified technical framework for geomechanics and thermal-hydraulic calculation. Moreover, the method can be directly combined with geological modeling software without using two sets of grids. The results prove that the finite volume method would be a promising approach for thermal-hydraulic-mechanical simulation.

Mechanical cutting assisted by high-pressure jet is a common and efficient drilling method in the exploration of hot dry rock (HDR). The drilling efficiency can be further improved with an in-depth understanding in the rock-breaking mechanism, which is a complex process involving the coupling of thermal stress, jet impact pressure and cutting force. To address such a multi-physics coupling issue, a loosely coupling method was adopted in this work to accomplish the exchange of data, including temperature, heat flux, nodes deformation and impinging pressure of jet in every time increment. In the fluid subdomain, the liquid nitrogen (LN2) jet and water jet were modeled and compared to demonstrate the potential of the novel jet in improving the drilling rate. In the solid subdomain, cutting force monitored in the experiment was loaded, and the concrete damage plastic model was employed to evaluate damage behaviors of the rock. Relative experiments were conducted to validate the reliability of simulation results. Results indicate that a great temperature difference develops on the coupling surface under the impingement of LN2 jet due to its cryogenic feature and lower heat transfer efficiency. Thereby, irreversible damage of the rock can be induced even at room temperature (298 K). As for the water jet, tensile damage is not observed until the rock temperature increases to 473 K. The assistance of jet impact on rock breaking is mainly reflected in two aspects: (1) the occurrence of tensile damage on the coupling surface facilitates the subsequent rock cutting; (2) the enhancement of tensile damage around the cutter promotes the initiation of cracks. Higher rock temperatures significantly improve the heat transfer rate and enhance the advantage of jet assistance. The present study can provide a new sight for the exploration of HDR.

Simulation of thermal hydraulic-mechanical (THM) coupling process plays an important role in exploiting the hot dry rock (HDR) resources by enhanced geothermal systems (EGS). At present, the simulations are mostly performed by using the traditional finite element method (FEM) with very dense grids. Because of the considerable size differences between the fractures and EGS, a large number of unstructured grids should be adopted, leading to a significant computational burden. To address this challenge, the embedded discrete fracture model (EDFM) and the extended finite element method (XFEM) are combined in this paper. By doing this, the grid number can be dramatically reduced, which improves the computational efficiency while maintaining relatively high accuracy. The proposed model is verified by comparing it with two typical analytic solutions and COMSOL. Moreover, the new model is applied to analyze the pressure, temperature and displacement evolutions in a hypothetical EGS. The influences of some key variables on thermal extraction performance are also investigated.

Geothermal energy attracts extensive attention for its excellent features like large capacity and lack of dependency on weather conditions. This paper presents a development of numerical model to study the hydraulic fracturing and heat extraction from geothermal reservoir, especially from brittle hot dry rock (HDR) reservoir. In this numerical model, an anisotropic damage model has been introduced based on Thermal-Hydraulic-Mechanical (THM) coupling code THOUGH2MP-FLAC3D and this damage-permeability model has also been integrated to demonstrate the enhancement in permeability of stimulated fracture network. Furthermore, numerical studies on planed Dikili enhanced geothermal system (EGS) were carried out by using newly developed model. As the simulated results show, a roughly circular fracture network is created, wherein simulated reservoir volume and area reaches to 49 million m3 and 1.41 million m2, respectively. The maximum permeability increases from initial permeability of 4 × 10−18 m2 to 7.52 × 10−14 m2 in z-direction. Based on these results, heat extraction calculations are performed for 10-years at different injection rates. The accumulated amount of produced net energy increases with the rising injection rate, while growth rate of the average thermal capacity over 10-year slows down. The production results suggest that an injection rate of 100 L/s can recover approximately 1.00 × 1016 J of thermal energy of over a 10-year period, with an average thermal capacity of 31.75 MWth.

Multiple-well Enhanced Geothermal Systems (EGS) can enable economic recovery of energy from underutilized hot dry rock (HDR) reservoirs. Hydraulic fracturing is a promising stimulation method for improving fluid flow and heat extraction in EGS. Laboratory simulations of EGS with hydraulic fracture stimulation have recently been completed in two large 300 × 300 × 300 mm3 granite block specimens to better understand this complex process of geothermal energy recovery. The first experiment implemented a binary well layout with an injector and producer. The second experiment used a triplet well layout with one injector and two producers. Selection of production well trajectory so as to intersect the hydraulic fractures was guided by acoustic emission (AE) events collected during stimulation. Both model reservoirs were subjected to heating and true-triaxial stress confinement throughout a series of drilling, stimulation, and flow and heat circulation tests. Stimulated thermal reservoir flow was characterized by a series of constant pressure, constant flow rate, stepped constant pressure and stepped constant flow rate injection tests. Tested blocks were cross-sectioned to characterize final locations and 3D geometries of the induced fractures. Insights and lessons learned from these experiments are presented with focus on application to field-scale EGS.

The Hijiori hot dry rock (HDR) system, Japan, consists of a shallow and a deep reservoir, both in fractured granitic rocks. During a long-term circulation test (LTCT) lasting approximately 18 months and which tested different fluid production scenarios, large changes were observed in output fluid temperatures, pressures, and flow rates. A multi-reservoir, numerical model of the Hijiori HDR system was developed using the finite element heat and mass (FEHM) transfer code and applied to simulations of the LTCT. The model reproduced the pressure, temperature, and flow data observed during the test. Based on the modeling study, it was concluded that most of the produced fluid came from the shallow reservoir, that the permeability of the deep reservoir changed during the initial part of the LTCT, and that the redistribution of injected water between the two reservoirs had little impact on the relative amounts of deep and shallow fluid production. After validating the model on the LTCT, it was used to optimize injection rates in both reservoirs. The model was also used in simulations of reservoir performance where an additional heat transfer surface area has been created in the subsurface through hydraulic fracturing.

Mineral scales were formed in wells and surface installations during a 2-year circulation test performed at the Hijiori Hot Dry Rock (HDR) site, Yamagata, Japan. Anhydrite deposited in the deeper parts of the production wells, while silica and calcium carbonate precipitated at the surface, downstream of the wellhead, in amounts that depended on the temperature and chemical composition of the produced fluid. In well HDR-2a, closer to the injector, most of the scale was calcium carbonate; in HDR-3, further away from the injection well, there was slight precipitation of amorphous silica. As fluid circulation progressed and temperature decreased, scaling in the flow line of well HDR-2a changed from amorphous silica to calcium carbonate.

Production in geothermal reservoirs often leads to observable surface displacement. As shown in this paper, there is a direct relationship between such displacement and reservoir dynamics. This relationship is exploited in order to image fluid flow at two geothermal field sites. At the first locality, the Hijiori Hot Dry Rock (HDR) test site, 17 tilt meters record deformation associated with a 2.2 km deep injection experiment. Images of fluid migration along a ring fracture system of the collapsed Hijiori caldera are obtained. At the Okuaizu geothermal field, leveling and tilt meter data provide constraints on long- and short-term fluid movement within the reservoir. A set of 119 leveling data suggest that the north-to-northeast trending Takiyagawa fault acts as a barrier to flow. The northwesterly oriented Chinoikezawa and Sarukurazawa faults appear to channel fluid from the southeast. The tilt data from Okuaizu indicate that a fault paralleling the Takiyagawa fault zone acts as a conduit to transient flow, on a time scale of several weeks. The volume strain in a region adjacent to the injection wells reaches a maximum and then decreases with time. The transient propagation of fluid along the fault may be due to pressure build-up, resulting from the re-initiation of injection.

Hot Dry Rock (HDR) technology started from an idea to help fulfill future energy needs as the availability of cheap fossil and other known fuels slowly reduces. The HDR concept itself is very simple but the development of the associated technology has taken significantly longer than anticipated. Anyone with experience of natural materials such as rocks knows that there are always imponderables that have not been really understood and indeed cannot at present be dealt with in a fully satisfactory manner. Furthermore, geology always has a habit of presenting us with new problems. Under these auspices and considering the limited funds that have been made available, it is encouraging to note that at long last light is visible at the end of the long tunnel of uncertainty. The results of the 1997 circulation test at Soultz-sous-Forets (France) certainly seem to show that we may have come up with a type of concept and an appropriate set of background site conditions to advance the technology. The concept of an HDR reservoir has evolved from that of a single penny-shaped fracture borrowed from the oil industry to the present graben or HWR (Hot Wet Rock) concept. International co-operation has been a key issue so far, and the expensive nature of this research demands that co-operation of this type continues to break new ground in the future. The necessary supporting technology has also evolved and the time appears to be ripe for taking advantage of this new and exciting development. Not all the answers are known, but at least we know now which questions to ask. It is worth remembering that there is still no commercial HDR plant in existence to provide real data on building, operating and maintenance costs for planning a new unit. This should not be regarded as an insuperable problem. If such were the case, then we would not have any aircraft, steel, shipping, telecommunication or nuclear industries. Many people feel that HDR technology will be needed sooner or later and the important question now is how quickly it can be put into practice when the need does arise! Recent moves to form a consortium for this next step at the Soultz site are very encouraging and show the promise and confidence that commercial and industrial interests have in its future.

Seismic signals produced while drilling a Hot Dry Rock (HDR) geothermal well in Soultz-sous-Foret (France) were detected using a 4-component seismic detector and two single-component detectors in the granite basement. The recorded signals had a signal-to-noise ratio of around 8 dB with respect to the background noise and show relatively high spectral whiteness. Results from three-dimensional hodogram analysis were successfully interpreted using a model in which the drill bit generates predominantly continuous shear waves. Reflected waves were detected using both the spatial and temporal cross-correlation of hodograms. The distribution of reflectivity from the tri-axial drill-bit vertical seismic profiling (TAD–VSP) suggests that the bottom of the artificial geothermal reservoir, created by hydraulic stimulation, has a high reflection coefficient. The pre-existing permeable zone inside the reservoir was also detected by the TAD–VSP. The reflection image of the TAD–VSP agrees well with that found by the AE (Acoustic Emission) reflection method. The TAD–VSP was successfully used to detect the internal structure of the deep basement.

This study examines heat transfer during forced water circulation through fractured crystalline rock using a fully 3-D finite-element model. We propose an alternative to strongly simplified single or multiple parallel fracture models or porous media equivalents on the one hand, and to structurally complex stochastic fracture network models on the other hand. The applicability of this “deterministic fracture network approach” is demonstrated in an analysis of the 900-day circulation test for the Hot Dry Rock (HDR) site at Rosemanowes (U.K.). The model design with respect to structure, hydraulic and thermal behavior is strictly conditioned by measured data such as fracture network geometry, hydraulic and thermal boundary and initial conditions, hydraulic reservoir impedance, and thermal drawdown. Another novel feature of this model is that flow and heat transport in the fractured medium are simulated in a truly 3-D system of fully coupled discrete fractures and porous matrix. While an optimum model fit is not the prime target of this study, this approach permits one to make realistic long-term predictions of the thermal performance of HDR systems.

The coupled hydro-thermo-mechanical response of a fractured media to forced fluid flow in a simple Hot-Dry-Rock (HDR) system is investigated. The geometry, a single fracture in a 21) matrix, was chosen in order to better understand the relevant processes for the long term behaviour of HDR reservoir. Linear elastic effects of temperature and pore pressure perturbations on stress in the solid matrix are assumed as well as a non-linear joint closure law. Thermal transport by both conduction and advection is included in the fracture and the matrix. The results of these models highlight the importance of the coupled hydraulic thermal and mechanical processes on long term system behaviour.

This work examines 3-D advective-conductive heat transfer during forced water circulation through fractured crystalline rocks. The subject of the modelling investigation is the Hot-Dry-Rock (HDR) site at Soultz-sous-Forěts, Alsace, France. Fracture data from borehole logging provides an insight to the complex structure of crystalline rocks. The proposed model is based on the general idea, that flow and advective transport in stimulated HDR reservoirs occur mainly in fractures, whose orientation is linked by the tectonic stress field. Therefore, a deterministic fracture network model is established which represents the mean geometrical features of a stimulated HDR system. A physical and mathematical model of a fractured porous medium is developed for describing flow and transport within the interacting system of fractures and rock matrix. The model is then translated into a discrete finite-element approach (ROCKFLOW). The numerical results are tested against analytical solutions for parallel fracture arrays. Fluid flow and heat transfer are simulated and illustrated. The 3-D model that is presented permits an effective calculation of the long-term performance of HDR systems. The thermal drawdown is computed with respect to the important reservoir parameters such as the borehole separation, the fracture spacing and the position of boreholes within the fracture network. A long-term prediction of the thermal performance of an HDR system is most important for an economical assessment of this type of geothermal energy.

This paper presents the results of heat extraction modelling for a prototype Hot Dry Rock (HDR) system at the Soultz-sous-Forets site, France. The significant data from the 1993 experimental HDR programme at the Soultz site are summarised. Geological field evidence indicates that two major fault structures are superimposed on the four natural fracture sets used in the fracture network model. The generation of independent fracture networks allows the creation of a picture of the intersection of fractures within the borehole which is reasonably realistic. Numerical modelling of the hydraulic tests carried out in the deepest well (GPK1) is extrapolated to predict the hydraulic behaviour in a two-well system, incorporating the present deep well, and a second deep well (GPK2) which is to be drilled in 1994/95. This allows the thermal performance of the planned system to be modelled, and indicates excessive thermal drawdown in the first six months of circulation, possibly as a result of preferential flow paths developed along a main fault. Confidence in the model will only be improved when experimental data from a circulating system are available to test the predictions, and a better understanding of the hydrogeologic system has been obtained. At this stage, the extrapolations to a two-well system should be considered only as demonstrative guidelines for further investigations.

The experimental Hot Dry Rock (HDR) geothermal reservoir created in 1985 at the Rosemanowes test site in Cornwall is one of a very small number of HDR reservoirs of substantial dimensions whose key performance parameters have been measured. The reservoir linked an injection well (RH12) with a production well (RH15). The minimum separation of the openholes was 133 m. The initial production temperature (at the casing shoe) was 80°C. The average injection temperature during the early part of the circulation test was about 25°C. Ten per cent thermal drawdown occurred after about 300 days' circulation at an average production flow rate of 8.5 kg/s. The mass of fluid produced in this time was 220 million kg. At the lowest injection pressure of about 4 MPa, the production flow rate was 4 kg/s, the impedance was 1.0 MPa/(kg/s), and the water loss was 1 kg/s (i.e. 20% of the injection flow rate). At higher injection pressures, impedance was reduced but water loss was increased. A comprehensive programme of reservoir characterisation was undertaken to measure as many possible of the fundamental reservoir parameters that are considered to have given rise to the observed thermal and hydraulic performance. This paper reports the main results of this reservoir characterisation programme. It is concluded that many of the parameters measured are not true estimates of the actual parameters being sought, but apparent parameters, whose values are highly dependent upon the simplifying assumptions used to obtain them. The performance of the RH12/RH15 reservoir is compared with that which would be required of an individual cell of a commercial-scale HDR reservoir consisting of many stimulated cells in parallel. By these criteria, the performance of the RH12/RH15 reservoir falls short in terms of thermal output and water loss, but meets the required impedance. The performance of the RH12/RH15 reservoir was severly limited by the unfavourable trajectory of the injection well (RH12), especially in terms of thermal output. The results from this experimental reservoir are therefore equivocal as far as validation of the HDR doublet concept is concerned.

Early attempts to hydraulically fracture and connect two wells drilled at the Hot Dry Rock (HDR) site at Fenton Hill in New Mexico produced a large volume of fractured rock, but no connection. Microearthquakes triggered by fracturing indicated that the stimulated rock zones grew in unexpected directions. Consequently one of the wells was sidetracked at a depth of 2.9 km. It was redrilled into the zones of most intense microseismic activity, and flow connections were achieved. Hydraulic communication was improved by supplemental stimulation using recently developed high temperature and high pressure open hole packers. Preliminary testing indicates a reservoir with heat production capability which greatly surpasses that attained in the earlier Phase I reservoir. Longer term testing in 1987 and 1988 will provide more complete information on reservoir behaviour.

A transient mass balance model is developed to account for the dynamic behavior of an artificially stimulated hot dry rock (HDR) geothermal reservoir system in fractured granitic rock. Fluid mixing between fractured zones, hydrodynamic dispersion within zones, pore fluid displacement, and mineral dissolution effects are incorporated into the model. A two-zone system is sufficient to account for the major observed results from field testing of the Fenton Hill HDR system.

The chemistry of fluids circulated through an artificially-stimulated, hot dry rock (HDR) fractured geothermal reservoir system in granitic rock is described in terms of mixing phenomena, geothermometry, and approach to saturation with reservoir rock minerals. Based on the similar dynamic behavior of Na + , K + , Li + , CI − , and B species and other isotopic evidence, the presence of a concentrated in-situ pore fluid was identified. Mixing and displacement of this in-situ fluid with meteoric make-up water is responsible for the observed behavior of the major dissolved species in the circulated fluid of this HDR system.

Man-made, Hot Dry Rock (HDR) geothermal energy reservoirs have been investigated for over ten years. As early as 1977 a research-sized reservoir was created at a depth of 2.9 km near the Valles Caldera, a dormant volcanic complex in New Mexico, by connecting two wells with hydraulic fractures. Thermal power was generated at rates of up to 5 MW(t) and the reservoir was operated for nearly a year with a thermal drawdown less than 10/sup 0/C. A small 60kW(e) electrical generation unit using a binary cycle (hot geothermal water and a low boiling point organic fluid, R-114) was operated. Interest is now worldwide with field research being conducted at sites near Le Mayet de Montagne, France; Falkenberg and Urach, Federal Republic of Germany; Yakedake, Japan; and Rosemanowes quarry in Cornwall, United Kingdom. To assess the commercial viability of future HDR electrical generating stations, an economic modeling study was conducted for a conceptual 75 MW(e) generating station operating at conditions similar to those prevailing at the New Mexico HDR site. The reservoir required for 75 MW(e), equivalent to 550 MW of thermal energy, uses at least 9 wells drilled to 4.3 km and the temperature of the water produced should averagemore » 230/sup 0/C. Thermodynamic considerations indicate that a binary cycle should result in optimum electricity generation and the best organic fluids are refrigerants R-22, R-32, R-115 or R-600a (Isobutane). The break-even bus bar cost of HDR electricity was computed by the levelized life-cycle method, and found to be competitive with most alternative electric power stations in the US.« less