Stress and rock mechanics issues of relevance to HDR/HWR engineered geothermal systems: review of developments during the past 15 years

This paper reports the findings of the Stress and rock mechanics working group of the Academic Review of Hot Dry Rock/Hot Wet Rock (HDR/HWR) Engineered Geothermal Systems convened in Sendai, Japan in 1997. Key developments in the fields of stress and rock mechanics that are relevant to the development of HDR/HWR systems and that have occurred since the last Academic Review in 1982 are described. Rock mechanics is here taken to include basic studies of fluid flow through fractures. Key unresolved issues that are important for HDR/HWR systems are also discussed.     

Numerical simulation of flow and heat transfer in fractured crystalline rocks: Application to the Hot Dry Rock site in Rosemanowes (U.K.)

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.     

Development of hot dry rock technology at the Hijiori test site

Since 1985, the New Energy and Industrial Technology Development Organization (NEDO) has conducted a Hot Dry Rock project at the Hijiori test site, Yamagata prefecture. The objective of this project is to develop and test technologies such as borehole logging, hydraulic fracturing, fracture mapping and reservoir evaluation, which are essential for the development of a Hot Dry Rock power generation system. In 1991, heat was successfully extracted from a shallow reservoir at a depth of 1800 m for three months using one injection well (SKG-2) and three production wells (HDR-1, HDR-2 and HDR-3). About 80% of the injected water was recovered from these production wells. The thermal output of hot water and steam reached about 8 MW. Since 1992, a deep reservoir at a depth of 2200 m has been developed. In 1995 and 1996, heat extraction tests were conducted using one injection well (HDR-1) and two production wells (HDR-2 and HDR-3). A long-term circulation test, lasting about two years, is planned to evaluate the reservoir, starting in 2000.     

Enhanced geothermal systems (EGS) using CO2 as working fluid—A novel approach for generating renewable energy with simultaneous sequestration of carbon

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.     

A three-dimensional thermo-poroelastic model for fracture response to injection/extraction in enhanced geothermal systems

Water injection in enhanced geothermal systems sets in motion coupled poro-thermo-chemo-mechanical processes that impact the reservoir dynamics and productivity. The variation of injectivity with time and the phenomenon of induced seismicity can be attributed to the interactions between these processes. In this paper, a three-dimensional transient numerical model is developed and used to simulate fluid injection into geothermal reservoirs. The approach couples fracture flow and heat transport to thermo-poroelastic deformation of the rock matrix via the displacement discontinuity (DD) method. The use of the boundary integral equations, for the pressure diffusion and heat conduction in the rock matrix, eliminates the need to discretize the infinite reservoir domain. The system of linear algebraic equations for the unknown displacement discontinuities, and fluid and heat sources are used in a finite element formulation for the fluid flow and heat transport in the fracture. This yields a system of equations which are solved to obtain the temperature, pressure, and aperture distributions within the fracture at every time step. In this way, the temporal variation of the fracture aperture and fluid pressure, caused by pressurization and thermo-poroelastic stresses, are determined. Numerical experiments using the model illustrate the feed-back between matrix dilation, shrinkage, and pressure in the fracture. It is observed that whereas the poroelastic effects dominate the early stage of injection pressure profile and the fracture aperture evolution, thermoelastic effects become dominant for large injection times.     

Monitoring production using surface deformation: the Hijiori test site and the Okuaizu geothermal field,Japan

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.     

Pressure analysis of the hydromechanical fracture behaviour in stimulated tight sedimentary geothermal reservoirs

Hydromechanical phenomena in fractured sediments are complex. They control the flow in stimulated tight sediments and are crucial for the exploitation of geothermal energy from such rocks. We present the analysis of a cyclic water injection/production (huff–puff) process, a promising method to extract geothermal energy from tight sedimentary reservoirs. It uses a single borehole, which considerably reduces investment costs. A huff–puff test was performed in a 3800-m deep sedimentary formation (borehole Horstberg Z1, Lower Saxony, Germany). The analysis presented herein explains the downhole pressure measurements by a simplified reservoir model containing a single vertical fracture. The model addresses the flow behaviour between the fracture and the rock matrix in a layered formation, and the coupling between fluid flow and the mechanical deformation of the fracture. The latter aspect is relevant to predict the efficiency of the geothermal reservoir because cooled regions resulting from a particular injection/production scheme can be identified. The analysis methods include: (1) the curve-fitting code ODA used for a determination of different flow regimes (radial or linear), (2) an analytical solution for the calculation of the injection pressure, assuming a time-dependent fracture area, and (3) the simulator ROCMAS, which numerically solves the coupling between fluid flow and fracture deformation. Whereas each single approach is insufficient to explain the complete test data, a combination of the results yields an understanding of the flow regimes taking place during the test.     

Temperature-dependent scale precipitation in the Hijiori Hot Dry Rock system,Japan

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.     

Simulation of heat extraction from crystalline rocks: The influence of coupled processes on differential reservoir cooling

Processes operating during the extraction of heat from fractured rocks influence dynamically their fluid flow and heat transport characteristics. The incorporation of pressure- and temperature-dependent rock parameters, coupled with geomechanical deformation, is particularly important for predictive modelling of geothermal reservoirs hosted in crystalline rock masses. Changes in flow and transport parameters of fractures caused by variations in local effective stress are computed using an experimentally validated geomechanical model [McDermott, C.I., Kolditz, O., 2006. Geomechanical model for fracture deformation under hydraulic, mechanical and thermal loads. Hydrogeol. J. 14, 487–498]. Local effective stress changes are linked to alterations in reservoir fluid pressures, and to in situ stress conditions, including the build-up of thermal stresses resulting from the cooling of the rock mass. These processes are simulated using a finite-element model in order to study the behaviour of the Spa Urach (southwestern Germany) potential geothermal reservoir. The model couples mechanical deformation and alteration of fracture parameters with pressure-, temperature- and salinity-dependent fluid parameter functions. The effects of potential reservoir damage on reservoir productivity are investigated to help identify optimal heat recovery schemes for the long-term economical exploitation of geothermal systems. Simulation results indicate that preferential fluid flow paths and shortcuts may develop, depending on the mechanical and thermal stress releases that occur during intense exploitation of these systems.     

Geochemical modelling of the Soultz-sous-Forêts Hot Fractured Rock system: comparison of two reservoirs at 3.8 and 5 km depth

This study was carried out within the framework of the European Hot Fractured Rock project in Soultz-sous-Forêts, France. Due to the very high salinity of the brine contained in the fractured granite and the limited kinetic data for high temperature, a new code called FRACHEM has been developed to simulate the coupled thermal-hydraulic and chemical processes of flow in the fractured system. The goal is to forecast the long term behaviour of the reservoir under exploitation. Among the minerals taken into account for this simulation (calcite, dolomite, quartz and pyrite), carbonate reactions predominate. Their reaction rates are higher than the quartz and pyrite reaction rates. The amount of quartz and pyrite precipitated is neglected. As a consequence of calcite dissolution, the porosity increases at the injection point with resulting permeability changes .     

增强型地热系统中液-岩化学作用数值模拟研究

增强型地热系统（Enhanced Geothermal System, EGS）利用深层岩石中连通的裂隙网络进行流体工质循环,从而实现地热能的持续开采。EGS运行时循环流体工质会与深层岩石产生化学反应,引起岩石中矿物的溶解/沉积,使热储中的裂隙网络形貌产生动态变化,对地下流动与传热过程造成影响。本文分析了EGS中液–岩化学作用特点,详细阐述了在多孔介质热流动模型中耦合入液–岩化学反应的方法,基于已开发成功的EGS传热传质数值模型初步建立了传热–流动–化学（Thermal-Hydraulic-Chemical, THC）多场耦合数值模型,并使用该模型对五井布局EGS的长期运行过程进行了模拟分析,模拟时仅考虑方解石在水流体中溶解和沉积。模拟结果显示,循环流体的注入温度以及注入流体中的矿物离子浓度的设定十分重要。如果二者没有达到较为合适的“平衡”,就会导致注入井附近渗透率和孔隙率的持续变化,对EGS的导流能力造成极大影响。     

Modelling of a european prototype hdr reservoir

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.     

The precipitation of solids from potential heat exchange fluids for use in a hdr geothermal system in granite

An important consideration in the development and operation of a Hot Dry Rock geothermal system is the selection of a heat transfer fluid and the chemical composition of this fluid during circulation. The chemical reaction of the circulation fluid with the reservoir rock may lead to the undesirable corrosion or scaling of the reservoir itself, or associated engineering structures. Two potential circulation fluids for use in a high temperature (200°C) HDR system in granite in SW England, are a dilute (TDS < 120mg/1) groundwater, and a modified seawater composition. The reaction of these fluids with granite has been evaluated experimentally, with particular emphasis upon the characterisation of solid precipitates. Secondary solids associated with the reaction of groundwater with granite consist of clay and Ca-zeolite. Product fluids were alkaline (pH 9.1), of low salinity (TDS < 600mg/1) and were relatively benign for heat exchange purposes. The amount of clay precipitated may be linked to the amount of Mg in the fluid, but is less than 0.5 wt percent of the initial solid starting material. Chemical analysis of precipitated clay by analytical transmission electron microscopy reveal a range of composition between illite and smectite. Secondary solids associated with seawater-granite reaction include anhydrite, magnesium hydroxide sulphate hydrate (MHSH) and clay. The precipitation of MHSH and clay is instrumental in governing the low pH (pH 3.5) of the product fluid, which would pose problems concerning the corrosion of pumps, heat exchangers, etc in a possible HDR geothermal system. The suitability of each of the potential heat exchange fluids may be linked to their initial Mg contents which govern the acidity of the reacted fluid and the amount of precipitated clay.     

Fluid circulation and heat extraction from engineered geothermal reservoirs

A large amount of fluid circulation and heat extraction (i.e., thermal power production) research and testing has been conducted on engineered geothermal reservoirs in the past 15 years. In confined reservoirs, which best represent the original Hot Dry Rock concept, the flow distribution at any given time is primarily determined by three parameters: (1) the nature of the interconnected network of pressure-stimulated joints and open fractures within the flow-accessible reservoir region, (2) the mean pressure in the reservoir, and (3) the cumulative amount of fluid circulation—and therefore reservoir cooling—that has occurred. For an initial reservoir rock temperature distribution and mean fluid outlet temperature, the rate of heat extraction (i.e., thermal power) is at first only a function of the production flow rate, since the production temperature can be expected to remain essentially constant for some time (months, or even years). However, as reservoir circulation proceeds, the production temperature will eventually start to decline, as determined by the mean effective joint spacing and the total flow-accessible (i.e., heat-transfer) volume of the reservoir. The rate of heat extraction, which depends on the production flow rate, can also vary with time as a result of continuing changes in the flow distribution arising from reservoir cooling.The thermal power of engineered reservoirs can most readily be increased by increasing the production flow rate, as long as this does not lead to premature cooldown, the development of short-circuit flow paths, or excessive water losses. Generally, an increase in flow rate can be accomplished by increasing the injection pressure within limits. This strategy increases the driving pressure drop across the reservoir and the mean reservoir pressure, which in turn reduces the reservoir flow impedance by increasing the amount of joint dilation. However, the usefulness of this strategy is limited to reservoir operating pressures below the fracture extension pressure, and may lead to excessive water losses, particularly in less-confined reservoirs. Under such conditions, a downhole production-well pump may be employed to increase productivity by recovering more of the injected fluid at lower mean reservoir operating pressures.     

Faulting mechanisms and stress regime at the European HDR site of Soultz-sous-Forêts, France

The state of stress and its implications for shear on fault planes during fluid injection are crucial issues for the HDR (Hot Dry Rock) or EGS (Enhanced or Engineered Geothermal System) concept. This is especially true for hydraulic stimulation experiments, aimed at enhancing the connectivity of a borehole to the natural fracture network, since they tend to induce the shearing of fractures, which is controlled by the local stress regime.During the 2000 and 2003 stimulation tests at Soultz-sous-Forêts, France, about 10,000 microearthquakes were located with a surface seismological network. Hundreds of double-couple (DC) focal mechanisms were automatically determined from first-motion polarities using the FPFIT program [Reasenberg, P.A., Oppenheimer, D., 1985. FPFIT, FPPLOT and FPPAGE: Fortran computer programs for calculating and displaying earthquake fault-plane solutions. US Geological Survey Open-File Report 85-739, 25 pp.]. The majority of these mechanisms indicate normal-faulting movement with a more or less pronounced strike-slip component. Some quasi-pure strike-slip events also occurred, especially in the deeper part of the stimulated rock volume, at more than 5 km depth.Although we found a double-couple solution for all events, we tried to observe and quantify the proportion of the non-double-couple (NDC) component in the seismic moment tensor for several microseisms from the 2003 data. The study shows that the NDC is higher for the events in the vicinity of the injection well than for the events far from the well.We used the method of Rivera and Cisternas [Rivera, L., Cisternas, A., 1990. Stress tensor and fault-plane solutions for a population of earthquakes. Bull. Seismol. Soc. Am. 80, 600–614.] to perform the inversion of the deviatoric part of the stress tensor from P-wave polarities. This method was applied to different datasets from the 2000 test, taken from the shallower and deeper parts of the stimulated region. The results show a stable, horizontal, NE-SW-oriented trend of the minor horizontal stress, but a rotation of the major stress from a sub-vertical direction (top of the stimulated region) to a sub-horizontal one (bottom of the stimulated region). This implies a change from a normal-faulting to a strike-slip regime, in agreement with our fault-plane solutions. Finally, we applied the stress components to the nodal planes of several events and were able to determine their fault plane and obtain a 3D image of the fracture network, based on real data.     

Multifracture response to supercritical CO2‐EGS and water‐EGS based on thermo‐hydro‐mechanical coupling method

Hydraulic‐fracturing treatments have become an essential technology for the development of deep hot dry rocks (HDRs). The deep rock formation often contains natural fractures (NFs) at micro and macroscales. In the presence of the NF, the hydraulic‐fracturing process may form a complex fracture network caused by the interaction between hydraulic fractures and NF. In this study, analysis of carbon dioxide (CO₂)‐based enhanced geothermal system (EGS) and water‐based EGS in complex fracture network was performed based on the thermo‐hydro‐mechanical (THM) coupling method, with various rock constitutive models. The complexity of the fracture geometry influences the fluid flow path and heat transfer efficiency of the thermal reservoir. Compared with CO₂‐based EGS, water‐based EGS had an earlier thermal breakthrough with a rapid decline in production temperature. CO₂ can easily gain heat rising its temperature thus reducing the effect of a premature thermal breakthrough. Both CO₂‐based EGS and water‐based EGS are affected by in‐situ stress; the increase in stress ratio improved the fracture permeability but resulted in an early cold thermal breakthrough. When the same injection rate is applied to water‐based EGS and CO₂‐based EGS, water‐based EGS displayed higher injection pressure buildup. Water‐based EGS had higher reservoir deformation area than CO₂‐based EGS, and thermoelastic constitutive model for water‐based EGS showed larger deformed area ratio than thermo‐poroelastic rock model. Furthermore, higher values of rock modulus accelerated the reservoir deformation for water‐based EGS. This study established a novel discussion investigating the performance of CO₂‐based EGS and water‐based EGS in a complex fractured reservoir. The findings from this study will help in deepening the understanding of the mechanisms involved when using CO₂ or water as a working fluid in EGS.     

Changes in fracture aperture and fluid pressure due to thermal stress and silica dissolution/precipitation induced by heat extraction from subsurface rocks

The numerical model developed by Suresh Kumar and Ghassemi [Suresh Kumar, G., Ghassemi, A., 2005. Numerical modeling of non-isothermal quartz dissolution in a coupled fracture–matrix system. Geothermics 34, 411–439] is used to study fluid pressure and permeability changes in a fracture in a rock mass by taking into account the effects of thermal stresses and silica precipitation/dissolution, which is computed using linear reaction kinetics. Fluid flow in the fracture is calculated based on the cubic law. Solute transport mechanisms by advection and dispersion are included in the model. Mass exchange between the horizontal fracture and the rock matrix is accounted for by assuming diffusion-limited solute transport. Heat transfer between the fracture and the rock matrix is modeled considering only conduction, while heat transport within the fracture includes thermal advection, conduction, and dispersion in the horizontal plane. Pressures of the circulating fluid through the fracture are allowed to vary with time, while the flow rate is assumed to remain constant.     

Assessment of hdr reservoir stimulation and performance using simple stochastic models

Experience in developing and circulating HDR reservoirs at Rosemanowes in Cornwall, UK, and more recently at Soultz-Sous-Forets in Alsace, France, suggests that the natural fracture system controls fluid flow and that artificial fractures are relatively unimportant. Two simple models, based on standard analytic solutions for the mechanical behaviour of cracks combined with a simple representation of the frictional properties of fracture surfaces, have been developed. The models describe the stimulation and circulation of HDR reservoirs in naturally fractured basement. In these models a connected fracture network is assumed, but the details of the spatial relationships between fractures within a conceptually ellipsoidal stimulated zone are ignored. On the other hand the models take close account of the size and orientation distributions of the natural fracture sets when calculating changes in fracture volume and reservoir impedance. The models provide estimates of the rock volume to fluid volume ratios required for stimulations, the injection flow rate for differing injection pressures, the fluid flux/fracture area partitioning, and hence thermal drawdown behaviour. The two models have been applied to field data from the Rosemanowes HDR research site gathered during circulation from 1985 to 1992. The first model enables the calculation of adequate stimulation pressures and fluid volumes; the second, reservoir injectivity curves and fluid flux/fracture surface area distributions within the reservoir.     

水平井地质模型-数值模型-FSD耦合分段压裂优化研究

为解决水平井分段改造和均衡层间产气差异的难题,从岩石细观力学角度出发,建立了岩石渗流-应力-损伤耦合模型(FSD模型);考虑岩石的非均质性,应用弹性有限元法作为应力分析工具,计算分析岩石的应力场和位移场,对水压裂缝的萌生、扩展过程中渗透率演化规律及其渗流-应力耦合机制和渗流与损伤耦合机制进行模拟分析。采用基于储层特征、完井方式、流体性质及油藏数值模拟等综合信息的Multi Well Simulator开放式软件平台,通过定义损伤断裂延伸准则和FSD模型方程,可以实现油藏数值模拟和FSD动态耦合分析。以D403-2H井为例,建立地质模型-数值模型-FSD模型,对射孔参数、措施层段、分段工具位置等进行优化,对合理划分措施井段和调整产气剖面进行研究。现场应用结果表明,该技术能够有效解决层间矛盾突出的水平井的层间储量动用不均衡的问题。     

Effects of heat extraction on fracture aperture: A poro–thermoelastic analysis

Poroelastic and thermoelastic effects of cold-water injection in an enhanced (or engineered) geothermal system (EGS) are investigated by considering flow in a pre-existing fracture in a hot, rock matrix that could be permeable or impermeable. Assuming plane fracture geometry, expressions are derived for changes in fracture aperture caused by cooling and fluid leak-off into the matrix. The corresponding induced pressure profile is also calculated. The problem is analytically solved for the cases pertaining to a constant fluid injection rate with a constant leak-off rate. Results show that although fluid loss from the fracture into the matrix reduces the pressure in the crack, the poroelastic stress associated with fluid leak-off tends to reduce the aperture and increase the pressure in the fracture. High rock stiffness and low fluid diffusivity cause the poroelastic contraction of the fracture opening to slowly develop in time. The maximum reduction of aperture occurs at the injection point and become negligible near the extraction point. The solution also shows that thermally induced stress increases the fracture aperture near the injection point and, as a result, the fluid pressure at this point is greatly reduced. The thermoelastic effects are particularly dominant near the inlet compared to those of poroelasticity, but are pronounced everywhere along the fracture for large times. Although poroelasticity associated with leak-off does not change the fracture aperture significantly for low permeability rocks, it can lead to pore pressure increase and cause nearby fractures to slip.