Abstract At present, along with conventional energy sources continually consumed, renewable energy sources are increasingly favored, especially the clean and inexhaustible geothermal resources have been universally valued both at home and abroad. In particular, the Enhanced Geothermal Systems (EGS), which is mainly aimed to exploit the thermal energy of Hot Dry Rock (HDR) at depths of 3 to 10 kilometers underground, has been full of interest to many countries. However, so far there hasn't been an EGS being successfully put into commercial operation because of its shortcomings such as small scale, low efficiency, etc. In this article, in response to the bottleneck of the study on the development of traditional EGS based on drilling technology (EGS-D), a conceptual model of EGS based upon excavation technology (EGS-E) is innovatively proposed and its main components of underground structure are described in this paper. As for ‘High ground stress, High ground temperature and High osmotic pressure’ initial conditions with regards to deep rock mass, the excavation experience, which is worth being learnt from extensive review of previous study as well as practical experience such as the successful excavation of ultra-deep mines in the gold field of South Africa, is summed up. The underground spatial structure that may be reasonable to the so-called EGS-E is being tried establishing. It is expected to provide with a basis for our subsequent numerical modeling. 1. Introduction Currently, seeking and developing clean new energy is the basic energy exploitation strategy, and the clean and inexhaustible geothermal resources have been universally valued both at home and abroad. Geothermal energy is the heat energy mainly generated by the transmutation of radioactive elements in rocks, which is 2.0934×10 18 kJ annually. And the geothermal energy stored at depths of less than 10 kilometers underground was estimated to be 170 million times the amount of heat released from all the coals stored in the earth by Pollack and Chapman in 1977 (Wang Ruifeng, 2002). It can be seen that the reserves of geothermal energy are very considerable. In spite of its advantages of stability, continuity and high utilization coefficient, the scale of the geothermal energy with temperature less than 150 °C at depths of less than 3 kilometers underground is usually too small to maintain the demand for long-term stable electricity production which is mainly hydrothermal and only accounts for 10% of all the geothermal energy stored in the earth (Guo Jian et al., 2014). Therefore, the enhanced geothermal system (EGS) which aims at exploiting the geothermal energy from hot dry rock (HDR) at depths of 3 to 10 kilometers has gradually attracted people's attention.

Abstract The geothermal energy extraction using the fracture-type reservoir in deep crust more than 350–400 °C is suggested. When using the fracture-type reservoir, there is a possibility of aseismic slip rather than seismic slip. However, characteristic and influence on permeability of the aseismic slip is unknown. Therefore, in this study, to clarify the occurrence condition, characteristics and influence on permeability of aseismic slip, injection-induced slip experiment using cylindrical specimen with a 45° tilted tensile fracture was conducted under the condition 200–500 °C. As a result, the followings were clarified. 1) there was a difference in characteristics between the slip start and the subsequent slip, 2) the slip velocity at the beginning of slip was affected by the surface shape of the fracture, 3) and the slip velocity of the subsequent steady slip tended to decrease as the temperature increased. Under 350–500 °C, the pore pressure at the beginning of slip decreased as temperature increased. Therefore, it is suggested that slower slip with a smaller pore pressure, namely, a more stable slip, may occur as the temperature increases. The permeability change before and after the slip experiment was increased at 200, 250 and 300 °C, didn't change at 350 °C and decreased by half at 500 °C. But since it is not a large decrease of more than one order, it is considered that a sufficient permeability can be maintained in the real geothermal reservoir. 1. Introduction New concept of engineered geothermal development where reservoirs are created in ductile basement is proposed (Asanuma et al., 2012). This potentially has a number of advantages. Suppression of felt earthquakes from/around the reservoirs is one of them (Muraoka et al., 2013). When using this type reservoir, there is a possibility of aseismic slip rather than seismic slip. However, characteristic and influence on permeability of the aseismic slip is unknown. Therefore, in this study, to clarify the occurrence condition, characteristics and influence on permeability of aseismic slip, injection-induced slip experiment using cylindrical specimen with a 45° tilted tensile fracture was conducted under the condition 200–500 °C.

Abstract Laboratory experiments on hydraulic fracturing were carried out to investigate the damage evolution during cyclic injection using acoustic emission monitoring. Cylindrical granite specimens with 50 mm in diameter and 100 mm in height were tested under a uniaxial vertical load of 25 MPa. A central borehole of 8 mm in diameter along the specimen long axis was drilled, and water was pumped in at a rate of 50 mm 3 /s. A total of 25 specimens were tested, 5 under continuous injection, and 20 under cyclic injection with different levels of maximum pressure starting from 76.9% to 101.2% of the average breakdown pressure under continuous injection. The results indicate that the level of maximum pressure during cyclic injection, is inverse to the number of cycles that lead to failure. Moreover, there was an average reduction of 25.9 dB of the maximum AE amplitudes recorded during cyclic injection. AE activity was detected from the early stages of cyclic injection, and only right before failure during continuous injection. The crack mode classification shows similar ratios of tensile to shear cracks for continuous and cyclic injection, and the 3D location of these events indicates that the 3.5% of shear cracks during cyclic injection are well distributed among the AE cloud. Finally, the hydraulic energy was higher for cyclic injection, especially for those cases with lower levels of maximum pressure. The radiated seismic energy ranges from 2.3 × 10 −14 J to 6.3 × 10 −11 J even for cases that failed at low number of cycles, however with increasing cycles the values are persistently low. 1. Introduction The stimulation of Enhanced Geothermal Systems through hydraulic fracturing, as a way to increase its permeability, has been associated with undesired levels of induced seismicity (Majer et al., 2007). An alternative cyclic hydraulic fracturing scheme was suggested with the aim to reduce induced seismicity. This concept is based on a hydro-mechanical model, and numerical simulation results show a reduction in breakdown pressure and seismicity (Zang et al., 2013). Later, this concept was implement in laboratory scale experiments that also resulted in a reduction of breakdown pressure and AE activity. Zhuang et al. (2016) reported a reduction of nearly 20% of breakdown pressure during cyclic hydraulic fracturing in granite specimens. Another study conducted with Tennessee sandstone, reported a 16% breakdown pressure reduction (Patel et al., 2017). However, no experimental study has covered the effect of maximum pressure during cyclic injection, fixed as a fraction of the average breakdown pressure under continuous injection. In this study, laboratory experiments were carried out on cylindrical granite specimens to investigate damage evolution during cyclic hydraulic fracturing using acoustic emission (AE) monitoring for fracture detection and propagation. The effect of maximum allowed pressure during cyclic injection on the number of cycles to failure and the maximum AE amplitude was investigated. Also, the AE crack type as well as the hydraulic and seismic energies were analyzed and discussed. The results were compared with a group of tests carried out under continuous injection, and some conclusion were drafted.

Abstract A large number of laboratory experiments about the influence of heating or heating-cooling cycles on the mechanical properties of various granites are reviewed. Both scanning electron microscopy (SEM) and particle-based discrete element modeling (DEM) are employed to quantitatively elucidate the mechanisms responsible for temperature-dependent mechanical properties of granites, from a perspective of microcracking. Both SEM observations and DEM simulations give consistent results and show that there exists a temperature threshold beyond which the thermally-induced microcracks increase drastically. Both intergranular and intragranular microcracks are observed in the granites after thermal treatment, and intergranular ones are dominant. A continuous increase in temperature can generally weaken granites, mainly by inducing significant thermal stress and generating tensile microcracks. The weakening of granites after a heating-cooling cycle is due only to the thermally induced microcracks. With increasing grain size the magnitude of Brazilian tensile strength reduction of granites due to thermal treatments becomes small, whereas with increasing heterogeneity in grain size distribution, the magnitude of Brazilian tensile strength reduction of granites due to thermal treatments becomes great. This is because the two competing mechanisms, i.e., the length and number of the thermally induced microcracks in granites. 1. Introduction Since the first enhanced geothermal system (EGS) was conceived at the Fenton Hill project, the United States, in the 1970s, EGS projects have been pursued around the world (McClure and Horne, 2014). EGS projects involve finding vast blocks with high temperature (> ~200 °C) and connected fracture networks. Working fluid (e.g., water or supercritical CO 2 ) is first injected and circulated through the fracture networks in geothermal reservoirs and eventually pumped back to the surface as steam. In the world EGS projects are commonly located in granite rocks with various mineralogical properties (Zhao et al. 2018). The mechanical response of "hot granites" to cooling becomes an important question to geologists and engineers.

Abstract Despite attempts to engineer viable deep reservoirs for the recovery of thermal energy at high enthalpy and mass flow rates - dating back to the 1970s - this goal has been surprising elusive. The record is replete with failed attempts, examples on life support and some successes. The key difficulties are in (i) accessing the reservoir inexpensively and reliably at depth, (ii) in penetrating sufficiently far through the reservoir, and (iii) in stimulating the reservoir in a controlled manner to transform permeability from microDarcy to higher than milliDarcy levels with broad and uniform fluid sweep and (iv) to create and retain adequate fluid throughput and heat transfer area throughout the project lifetime. We discuss key controls on permeability evolution in such complex systems where thermo-hydro-mechanical-chemical and potentially biological (THMC-B) effects and feedbacks are particularly strong. At short-timescales of relevance, permeability is driven principally by deformations - in turn resulting from changes in total stresses, fluid pressure or thermal and chemical effects. We explain features of reservoir evolution with respect to both stable and unstable deformation, the potential for injection-induced seismicity and its impact on both reservoir performance and in interrogating the evolving state of the reservoir. 1. Introduction The estimated thermal resource in the upper 5 km of crust below the US is of the order of 10 7 EJ. This compares favorably both with the hydrothermal resource at a mere 10 4 EJ and to the annual energy budget for the US, at ∼100 EJ/year. Recovering even a fraction of this baseload resource would contribute significantly to a new low carbon energy economy. The intrinsic goal of recovering thermal energy from the shallow crust (∼5 km for Engineered Geothermal Systems) requires that high-fluid-throughput and thermally-long-lived geothermal reservoirs may be universally engineered and developed, at will, and at any geographic location. High-fluid-throughput in traditional basement rocks requires that reservoir permeabilities at depth (∼5 km) must be elevated from the microDarcy to the milliDarcy range - this avoids untenable pumping costs and avoids inadvertently fracturing the reservoir by extreme fluid overpressuring of the heat-exchange fluid. Although fracturing would appear desirable in developing conduits with high-fluid-throughput, it typically violates the second tenet of a desired long thermal life, which requires that high heat-transfer area is maintained concurrent with high flow rates. This is only feasible if fluid circulation in the reservoir has a broad and even sweep through media with a short thermal diffusion length (small fracture spacing) thus avoiding short-circuiting and damaging feedbacks of thermal permeability enhancement.

Abstract Enhanced geothermal system (EGS) has the potential to offer a large amount of clean energy by extracting stored thermal energy from the subsurface. The effectiveness of heat extraction is dependent not only on the permeability of fractured rocks but also on the stability of preexisting and induced fractures. A better understanding of fracture slip in granite during fluid injection is critical to optimize the strategies of hydraulic stimulation. We experimentally investigated the shear behaviors of a sawcut fracture, a gouge-filled fracture, and a natural fracture in Bukit Timah granite in response to fluid injection under a constant normal stress and a constant critical shear stress, respectively. In the most cases, the pore pressure at the injection-induced failure exceeds that predicted by the Mohr-Coulomb failure criterion. This is attributed to the nonuniform distribution of fluid over the fracture plane, which is associated with lower permeability of fracture and host rocks and higher injection rate. The shear behaviors of sawcut and natural fractures show a complex combination of creep and stick before injection failure, which is presumably dependent on the state of asperity contacts. The gouge-filled fracture always creeps preceding the injection-induced failure, because the stiffness of testing system overweighs the critical rheologic stiffness of fracture. For these three fractures, the slip rate at injection failure increases with higher injection rate, releasing more strain energy. The slip rate induced by fluid injection in this study falls within the slip rate range of slow slip events observed in natural faults. 1. Introduction Harvesting heat trapped in igneous rocks offers us an affordable and sustainable solution to reduce our dependence on fossil fuels. Because of the extremely low permeability of igneous rocks, fluid injection has been used to create and/or activate fractures, enhancing the permeability of the host rocks. Besides the permeability evolution of rock fractures, the effectiveness of heat extraction is also dependent on the frictional stability of preexisting and induced fractures, because frictional instabilities of fractures result in seismic events. For example, fluid injection-induced seismicity in an enhanced geothermal system (EGS) in Basel, Switzerland led to the closure of the project (Majer et al. 2007), so did the California project in the United States also aiming at extracting underground geothermal energy (Elsworth et al. 2016). Therefore, a better understanding of the frictional stability evolution of fractures in igneous rocks is of critical importance to optimize the hydraulic stimulation strategy for EGS. The fracture is induced to slip when the shear stress acting on the fracture exceeds the shear strength of it according to the Mohr-Coulomb failure criterion (Zoback 2010) and the effective stress law (Terzaghi 1923). When a fracture starts to slip, the slip can either be seismic or aseismic, depending on the friction rate parameter and relative magnitude of critical rheologic stiffness of the fracture and the stiffness of elastic surroundings (Marone 1998). Specifically, when the friction rate parameter is positive, the fracture is intrinsically rate strengthening and can always slip stably (aseismic), while a fracture with a negative friction rate parameter is conditionally rate weakening, tending to slip unstably (seismic) if the critical rheologic stiffness of the fracture is larger than the stiffness of elastic surroundings.

Abstract An understanding of thermal effects on mechanical behavior of crystalline rocks is important for many underground projects, such as enhanced geothermal systems, nuclear waste repositories, CO 2 sequestration and underground coal gasification, where the temperature can be up to 300 °C or even approach the melting point of rocks. A large number of laboratory tests have been conducted to study the thermal effects on mechanical properties of crystalline rocks, like granites, which showed that elastic modulus, compressive and tensile strengths generally decrease with increasing temperatures. It is known that the rock mechanical behavior is dependent on the formation, growth and eventual interaction of micro-cracks, but the responsibility and mechanism of micro-cracking caused by thermal loading for the macroscopic mechanical properties of granites are still not clear. In this study, particle mechanics method is used to simulate the thermal cracking processes in granite at a scale of laboratory test, in order to elucidate the changing macroscopic mechanical properties with increasing temperature from the point of view of micro-cracking. The main conclusions are: a monotonous increasing temperature (heating) can generally reduce the elastic modulus, compressive and tensile strengths of granite, mainly by increasing thermal stresses and secondarily by generating tensile micro-cracks. Compared with heating, the heating-cooling cycles can have a less significant influence on the rock mechanical properties, which is solely due to the increase density of thermal-induced micro-cracks.

Abstract We describe a monitoring of the elastic wave propagation in the methane hydrate-bearing sediment artificially formed in a giant pressure-cell. The giant pressure-cell consists of a cylindrical shaped cell with internal radius 50 cm and a vertical well system to simulate the depressurization method. The 6 cm thickness of iron wall of the cell shades the interior from the X-ray CT or the other electric non-destructive inspections. Therefore, the acoustic tomography survey with explosion-proofed piezoelectric sensor was proposed. As a first step, we conducted a preliminary minimal experiment by one source and three receivers to investigate the sufficiency of the system for the measurement. The sediment base was made of the Toyoura standard sand. In the final stage of the generation of methane hydrate, the P-wave velocity estimated from the travel time picked on the averaged waveform was about 3000 m/s which was consistent with previous research. In the dissociation stage, the rapid decay of the signal was observed after the initiation of the dissociation. The travel time delay was not observed before the signal disappeared.

Abstract Massive hydraulic stimulation has been the key operation to improve the deep heat exchanger permeability in Enhanced Geothermal System (EGS). In flavor of heat extraction, the EGS deep well is preferable to hold a long uncased open section and it can be completed with varying well trajectory. This feature significantly complicates the analysis of hydraulic fracturing process. This paper develops a generic model to predict hydraulic fracture initiation in EGS hydraulic stimulation to consider the long well open section with varying trajectory. The model is based on the linear elastic solution of stresses on arbitrarily-oriented wellbore wall, therefore it is referred to as the first order estimation. A simulator was developed to estimate fracture initiation pressure and the associated fracturing location in the well open section for a given in situ stress condition and well trajectory. Furthermore, application examples were carried out and the dependency of hydraulic fracture initiation in EGS well open section on well trajectory and in situ stress conditions was demonstrated. It is shown that the well trajectory optimization design is necessary as the optimum trajectory allows fracture initiation to shift from casing shoe to well toe section with a lower breakdown pressure. The proposed model can be applied in terms of the determination of optimum well trajectory at the design stage and helpful for the interpretation of hydraulic stimulation behavior after the operation.

Abstract Abandoned wells impose enduring liabilities to petroleum companies and/or governments. However, the depth and abundance of abandoned petroleum wells make them an economically attractive source of geothermal energy. Geothermal energy can be harvested from oil/gas wells and used to generate electricity, used directly for heating, incorporated into water desalination processes, or used by heat pumps for heating/cooling applications. The present research work examines the possibility of extracting geothermal energy from abandoned oil/gas wells by studying the heat transfer in underground geothermal heat exchangers installed in these wells. A double-pipe design configuration is chosen for the geothermal heat exchanger units embedded inside a petroleum borehole. Using in-situ gathered information, the effects of key parameters such as geothermal gradient, ground temperature values, and the flow inside of the tubes are evaluated. In order to provide a constant power production the inlet temperature it is proposed to adjust the temperature of the inlet fluid, so that that the difference between outlet and inlet temperatures is kept at a desirable value. Effect of adding insulation jacket on the inner pipe of the geothermal heat ground exchanger is studied. It is found that the sustainability of long term geothermal heat extraction depends on the balance between the rate at which geothermal energy is extracted and the rate at which the ground formation can replace its geothermal heat content. The results suggest that abandoned petroleum wells can be economically reused for the purpose of sustainable geothermal energy production.

Abstract Geomechanical behaviors of methane-hydrate-bearing sub-seabed layers have not been sufficiently clarified, although they are essential to ensure sustainable exploration of methane hydrate in marine sediments. In particular, the viscoelastic properties of methane-hydrate-bearing layers are thought to have great significance in the long-term prediction of the geomechanical behaviors. It has been clarified that methane-hydrate-bearing sand has significantly strong time dependence for a geomaterial. Thus a constitutive equation used for the long-term prediction of the geomechanical behaviors is required to consider the viscoelasticity (time dependence) of methane-hydrate-bearing sand. To date, however, there has not been any viscoelastic constitutive equation modeled based on experimental results. This study aims to discuss the viscoelastic properties of methane-hydrate-bearing sand specimens obtained from drained triaxial compression tests and propose a viscoelastic constitutive equation. The results of three types of triaxial compression tests, e.g., constant-strain-rate test, constant-stress-rate test and creep test, on artificial methane-hydrate-bearing sand specimens are reviewed, mainly focusing on the viscoelastic behaviors. Then, a nonlinear viscoelastic constitutive equation is proposed for methane-hydrate-bearing sand, considering the strain-rate and stress-rate dependences of mechanical properties and creep behaviors. The model prediction fits well with the test results of stress-strain relationships in constant-strain-rate and constant-stress-rate tests before the peak failure point and primary creep behavior.

Abstract It has become more important for communities to utilize locally available renewable energy sources. The use of renewable energy can reduce environmental impacts and potentially provide long-term cost savings for communities. In this paper, the authors propose a system that could cool buildings in summer and melt snow on the pedestrian sidewalks in winter using an abandoned underground mine and a hot spring in Idaho Springs, Colorado. In the proposed system, an underground mine would be used as cold thermal energy storage, and the heat of geothermal hot fluid transported from the hot spring would be re-used to melt snow in the historic downtown. To assess the feasibility of the proposed system, we conducted a series of temperature measurements at the Edgar Mine (Colorado School of Mines' Experimental Mine) and heat transfer analyses of the geothermal hot fluid flowing through pipes that will be buried in the ground under the city. The temperature measurements proved that the temperature of the underground mine was low so that we could store cold groundwater for use in summer. Furthermore, the temperature profiles of two different tunnels in the Edgar Mine were discussed to determine the most appropriate place to store cold groundwater for summer use. In the heat transfer analyses, the heat loss of the geothermal hot fluid during its transportation was calculated, and then the heat requirement for snow melting and heat supply from the geothermal hot fluid were compared. It was concluded that the heat supply in the present situation was not sufficient enough to melt snow in the whole area of the historic downtown. However, the result indicated that the proposed snow melting system could be realizable if the snow melting area is limited or additional geothermal wells are drilled. We hope this case study will serve as an example of the concept of "local consumption of locally available energy". Many communities in the world do not fully utilize thermal resources in the ground. If such communities start utilizing them in a socially and environmentally responsible manner, it will contribute more to globally sustainable development for mankind.

Abstract A new concept of Enhanced Geothermal System (EGS), in which geothermal fluids are produced from a fractured reservoir created artificially within an originally semi-brittle or ductile basement, has been proposed. This new geothermal system potentially has a number of advantages including: simpler design and control of the reservoir, nearly full recovery of injected water, sustainable production, lower cost when developed in relatively shallower zones in compression tectonic settings, large potential quantities of energy extraction from widely distributed semi-brittle or ductile zones, the establishment of a universal design/development methodology, and suppression of felt earthquakes from/around the reservoirs. To assess the potential of the new geothermal system, the "Japan Beyond-Brittle Project (JBBP)" has also been recently initiated, and the authors have conducted fundamental investigations on mechanical and hydraulic characteristics of the new type of reservoir, in which the rock first experiences hydraulically and/or thermally induced brittle failure, and then subjected to the temperature and pressure conditions where the rock exhibits semi-brittle or ductile stress-strain behavior at the original condition. Gypsum aggregate specimens have been used in the present study, because brittle, semi-brittle and ductile stress-stain behaviors of the specimen can be controlled only with confining stress level at the room temperature. At confining stresses up to 40 MPa, tri-axial compression and fluid flow experiments have been conducted on the specimens without and with fracture. It has been indicated that stress-strain behavior is independent of existence of fracture. Moreover, permeability of the specimen with fracture is much higher than that without fracture at conditions of brittle, semi-brittle, and ductile stress-strain behaviors. On the other hand, permeability of the specimen with fracture is not so different from that without fracture at conditions of a transitional behavior between semi-brittle and ductile stress-strain behaviors.

Abstract: The creation of an excavation damaged zone (EDS) is expected around all man-made openings in civil engineering, in underground mining, and in petroleum engineering. The EDS may vary the physical, mechanical, and hydraulic properties of rock mass, and in turn dominate the evolution of EDS under coupled thermal, hydraulic and mechanical (THEM) environments. Understanding the development of EDS under coupled THEM conditions is of great importance for evaluating the engineering stability and safety, and for optimizing the supporting parameters. The work begins an introduction to the coupled THEM model for the rock damage, which is proposed when the damage variable is incorporated into the classic thermo hydro elastic model according to elastic damage theory. Next, the model is numerically implemented with finite element method by employing a numerical package called CONSOLE Multi physics (CM), and is also validated against some existing experimental observations. Finally, the model is used to simulate the formation and development of EDS under coupled THEM environments, and the effect of rock mass heterogeneity, potential THEM boundary conditions on the coupled THEM responses of rock mass is examined. 1. INTRODUCTION In enhanced geothermal systems, as in reservoirs for the sequestration of CO2, radioactive waste repositories, petroleum reservoirs, and other subsurface engineered facilities, the excavation damaged zone around the underground opening is influenced in both the short- and long-term by thermal-hydro-mechanical (THEM) behavior of fractured rock mass. The coupled THEM numerical models used in rock mechanics can be traced back to early 1980s, when many models were proposed based on the extension of Boot's theory of consolidations. Beginning from 1990s, under the form work of international cooperative project entitled DECO VALE, a number of benchmark tests (BIT) and test cases (TC) have been carried out in order to support development of computer simulators for THEM processes in geological systems [1–2]. Up to now, most numerical codes are still based on the assumption of elasticity and plasticity of rocks, or based on discrete approach where only a limited number of fractures are included to represent the fractured rock mass, thus the propagation of existing fractures, as well as the initiation of new fractures in rock mass, are usually ignored. However, both the hydraulic and thermal processes are sensitive to fracture initiation and propagation. Under the coupling of complex THEM processes the existing fractures may propagate and some new fractures may initiate, which in turn alters the thermal and hydraulic processes. Therefore, it's quite significant to incorporate the damage processes of rock in the numerical models in order to characterize the coupled THEM response of fractured rocks, especially during the formation and development of excavation damaged zone (EDS). In view of this, the authors have recently developed a damage-based THEM model to study the coupled THEM process during rock failure [3], which makes it a competitive candidate for characterizing the THEM response of the EDS around underground openings. To this end, it is to numerically study the development of EDS under coupled THEM condition that defines the objective of this work.

ABSTRACT Exploration in trachyandesitic Mount Sabalan, near the town of Meshkin-Shahr in north western Iran, indicated that this area hosts geothermal reservoir. In this study, we determine the direction of principal stresses using structural geology features exposed on the surface. These features include slickensides and fault steps. Analysis of the recorded slickenside traces in six districts was carried out using multiple inverse method. Results of the analyses indicated that the direction of the three principal stresses in every district is independent of the others. However, in all districts, maximum principal stress is vertical, minimum and intermediate principal stresses are horizontal. One of the applications of stress direction is in the design of hydraulic fracturing. Since the permeability of this reservoir is rather low, this stimulation technique could be a solution for enhancing productivity. Direction of the minimum stress dictates the geometry of created fracture. In this case the fracture would propagate vertically. 1. Introduction 1.1. History of Project In 1974, for assessment of geothermal resources potential in Iran some primarily studies including aeromagnetic and gravimetric operation have been done in area more than 8300 km2. After a long standby, in 1990 previous documents were reviewed and resistivity operations were conducted. According to the resistivity anomalies and geochemical alteration Meskin- Shahr (Northwestern Sabalan) introduced as the first geothermal potential in Iran. So, three exploration (production) wells up to 3200, 3176 and 2260 m and two injection wells up to 650 m have been drilled. Completion investigations such as logging and flow tests indicated that temperature of reservoir is about 240°C and permeability of reservoir is medium to low. Permeability is one of the important factors in using captured heat. Base on some calculations and estimations, it is essential to increase permeability by artificial methods. To increase permeability, we suggest hydraulic fracturing. During the hydraulic fracturing, artificial fractures are created in reservoir. Created fractures extend perpendicular to minimum horizontal stress. So, by knowledge of principal stress directions, we can determine artificially fractured geometry and drill the injection wells in the best direction. In this study, we try to determine direction of the in-situ stresses in Meskin-Shahr geothermal field using multiple inverse method. 1.2. Regional Geology Study area is located around the Sabalan Mountain in Ardabil province, northwestern Iran (Fig. 1). Geological setting of this area such as lithology and tectonics is controlled by Sabalan volcano activities. Volcanic structure is spot and stratovolcan like Stromboli volcano in Italy. Central volcano erupted on the main conjugated fracture over the paleohorst with E-W trend. According to published studies, volcanic activity in Sabalan started Eocene and resumed in Pliocene by eruption of trachyandesitic to andesitic lava flow through the main caldera. After caldera collapsing in Early Pleistocene, central caldera partially filled by trachyandesit–trachydacite 1394 lava flows [1]. The study area is located in complex compressional zone between Iran, Arabia and Eurasia plates in Alborz- Azerbaijan structural zone.