ABSTRACT: Supercritical carbon dioxide (CO 2 ) is used as a heat transmission fluid for the geothermal development in enhanced geothermal systems (EGS), aiming at improving the efficiency of heat mining and alleviating the emission of the world greenhouse gas. Buoyancy effects contributed by the density difference between water and carbon dioxide pose a great impact on the operation of this CO 2 -based EGS. A numerical simulation tool based on the unified pipe-network method (UPM) is developed in the current study to analyze the complex mechanism of multi-phase and multi-physical coupled process in the geothermal development. The reliability of the current numerical model solved by a sequential implicit time scheme is verified against results obtained using a fully implicit time scheme. Case studies are performed for the development of CO 2 -based EGS using typical properties of geothermal reservoirs embedded with fracture networks. Higher values of the CO 2 storage volume and the net heat extraction rate are achieved when the CO 2 buoyancy effect is considered in the same operation condition. The influences of different operation strategies, including varying production pressure differentials and injection flow rates, on the performance of the heat extraction and the carbon dioxide sequestration are also analyzed.

ABSTRACT: This paper aims to experimentally investigate the heat-treated hot dry rock (HDR) samples in response to different loading rates in order to understand the mechanism of fracture initiation and propagation behavior in geothermal rock masses. Semi-circular Bend (SCB) specimens with heat treatment, which are cored from the Yanjiao Town of Sanhe City in north China, are subjected to acoustic emission (AE) test under uniaxial stress and compared with HDR specimens at room temperature. The experimental results show that the value of fracture toughness at a relative high loading rate of 0.50mm/min is 1.2 ~ 1.5 times as that at a loading rate of 0.15 mm/min. When the heat-treated temperature is more than 600°C, the artificial fracture will not initiate from the initial notch crack because of the thermal shock cracks in the heat-treated samples, and the corresponding fracture toughness is sharply decreased by 1 ~ 3 folds of the specimens without any heat treatment. The AE features at the higher temperature are obviously different from that at a lower temperature. There is a step-like increase on the AE curves for the higher heat-treated specimens, and the total AE numbers are 4 ~ 6 times more than that for the specimens at room temperature.

ABSTRACT: A series of fracture and flow tests are being performed at the Sanford Underground Research Facility (SURF) as part of the EGS Collab project. The tests involve generating a communicating fracture(s) between two boreholes, and monitoring flow through the generated fracture(s). A long-term flow test was performed at the end of October 2018 through early November of 2018 between the injection and production wells at the EGS Collab site on the 4850 Level of SURF. This paper will present an analysis of the pressure, and flow during this test. Analysis and interpretation of the variation in the efficiency of the connection as the injection conditions changed is presented. Recovery of injected fluid is between 70 and 85%. Injection conditions varied from constant pressure, to constant rate, with formation induced pressure rise at constant flow rate, and multiple flush cycles to induce pressure decrease from apparent fracture plugging.

ABSTRACT: Geothermal energy is a renewable energy resource that can be found in abundance on the earth. Effective fracturing technology is the precondition to develop it efficiently. In this paper, the tests of the P-wave velocity, uniaxial compressive strength, and permeability of the granite after high temperature treated with air cooling and liquid nitrogen cooling were conducted. The variations of the P-wave velocity, uniaxial compressive strength, and permeability of the granite after treatment were obtained. Results indicate that the p-wave, uniaxial compressive strength, and Young's modulus decreased after the treatment, while the permeability increased. The p-wave and permeability are positively correlated with the treatment temperature. The scanning electron microscope (SEM) and x-ray diffraction (XRD) were employed to observe the microstructure and mineral content of the granite, respectively. Many micro-cracks were produced after liquid nitrogen treatment. All the experimental results indicate that liquid nitrogen cooling can induce significant thermal stress and has a positive impact on rock failure. It is expected to provide some guidance for the application of liquid nitrogen fracturing in geothermal reservoirs, especially in hot dry rock reservoirs.

ABSTRACT: We investigate the uniaxial compressive strength (UCS) anisotropy and elastic constants of the Poorman formation foliated schist at the 4850’ level of Sanford Underground Research Facility (SURF) in Lead, South Dakota. As seen in rock cores retrieved from the experiment site, the foliation characteristics vary from distinct planar foliation bands to tight folding with thicknesses varying from the centimeter to meter scale. From each sample group, 1” diameter and 2” long samples were sub-cored with planar foliation oriented 0°, 30°, 45°, 60°, and 90° from the sub-core axis to the foliation plane normal (beta angle, ß). Two additional groups of seemingly “isotropic” samples were prepared from monitoring boreholes where the foliation is tightly folded and deformed at the centimeter scale. We found UCS is weakest when the normal to the foliation plane is oriented 45° to 60° from the sample axis and the resulting failure plane follows the foliation orientation. Dynamic Young's modulus was estimated using elastic modulus relationships for isotropic rocks. To compare the estimate with the true dynamic Young's modulus, we assumed the planar samples behave as transversely isotropic material and utilized the 5 samples in each group to solve for the independent stiffness constants (c 11 , c 33 , c 13 , c 44 , c 66 ). Here we present our findings on the influence of foliation orientation on UCS and static and dynamic Young's modulus measured under triaxial stress conditions similar to the in-situ stress conditions at the EGS Collab Experiment 1 field site.

ABSTRACT: Heat extraction from low permeability sedimentary rocks of the Buntsandstein formation (lower Triassic) at a depth of 3800 m and temperature of approximately 144 °C has been tested in an abandoned gas well, Horstberg, in the North German Basin. The multilayer characteristic of the Buntsandstein allows us to apply a mono-borehole concept where the circulation of cold and warm water occurs only within one single well. An induced hydraulic fracture provides the vertical hydraulic connectivity between the upper and lower layers, i.e. Solling and Detfurth, respectively. In this study, we investigate numerically the effect of stress on the aperture, storage and permeability of the induced hydraulic fracture (hydromechanical characterization) and investigate the effective heat transfer area and residence time that continually changes in response to pore pressure and stress changes within the reservoir (thermal characterization). The opening and closure of the fracture control the rate of heat transfer between fracture and adjacent rock. The simulation considers thermo-poroelastic behavior of the rock. A finite element software was used to apply Biot theory of poroelasticity, i.e. fluid flow in a deformable porous media and to apply Barton-Bandis model to characterize fracture deformation. The results were compared with the forthcoming field experiments, i.e. tracer and circulation tests.

ABSTRACT: The goal of this work is to study various mechanisms of dynamic fracture in brittle materials validated against recent and ongoing small scale experiments performed at Sandia National Laboratory. We perform our analysis using Lawrence Livermore's GEODYN hydrocode. We present model calibration and verification performed against the available data. We consider in our analysis multiple energy sources with different timing and spatial separation which could be implemented during the experimental campaign.

ABSTRACT: To understand the effect of cold water injection on the mechanical properties and brittleness of rock, a series of mechanical tests were performed on granite samples. Firstly, granite samples were slowly heated to a series of target temperatures and then rapidly cooled down using flowing water. Thereafter, optical microscopic observation and uniaxial/triaxial compression tests were performed. Based on the experimental results, the thermal effect on brittleness was further analyzed. Experimental results show that numerous thermally-induced micro cracks were obviously generated when the thermal treatment levels were higher than 300 °C. The variation of mechanical parameters under compression showed a transition from thermal hardening to thermal cracking at 300 °C. The uniaxial compressive strength and Young's modulus significantly decreased only when the thermal treatment levels were higher than 300 °C. Meanwhile, significant energy dissipation was observed prior to the appearance of peak stress. The energy-based brittleness evaluation result shows that the brittleness of granite slightly increased before 300 °C, which followed by sharply decreasing due to the generation of thermally-induced micro cracks. These experimental results may provide references for the thermal effect on the mechanical properties of granite during the exploitation of HDR.

ABSTRACT: We present a new fault injection model for induced seismicity in the context of geothermal systems. The characteristic and novelty of our model is that it is both (1) fast and thus allows a probabilistic assessment and (2) purely physic-based in the sense that none unconstrained engineering approach to mimic processes or parameters distributions are used.

ABSTRACT: The Yangbajing geothermal field occurred in Yangbajing Cenozoic basin in Lhasa-Gangdese Magmatic Belt between Yalu-Brahmaputra River Suture Zone and Bangong Nujiang Suture Zone, controlled by strike-slip and detachment fault system south the Nyainqentanglha Mountain.Formation of hydrothermal system in the geothermal field has direct relationship with secondary faults of the fault system south the Nyainqentanglha acting as a perfect geothermal channel, in trending of northeast, northwest and north northeast in chronological order respectively. The hydrothermal activities display strongly in intersection areas of the several groups of faults, and have joined in geothermal circulation since about late period of Middle Pleistocene - early period of Late Pleistocene.There are two types of geothermal reservoir in the Yangbajing geothermal field, such as shallow basement fracture and Quarternary porous reservoir, and deep-seated hyperthermia structural basement fracture reservoir.

ABSTRACT: This work presents results on grain scale cracking behaviors, inhomogeneous deformation and fracture process zone of Australian Harcourt granite under tensile conditions. Brazilian test was performed with cracking initiation and propagation being monitored by a high speed camera (2×10 5 frames/s). Digital image correlation (DIC) was utilized to obtain full-field kinetics and, the development of fracture process zone was characterized using an extended DIC algorithm. The obtained real-time information was further cooperated with the distribution of mineral grains obtained by the synchrotron micro CT and the digital photograph of crack surfaces. The initiative location and the interaction between the micro-cracking process and mineral grains were identified and analyzed. It is found that the first crack initiates on a quartz grain near a biotite particle with some deviation away from the disc geometrical center. Besides, biotite grains trend to deviate or arrest micro cracks resulting from the cleavage structure and higher ductility. The fracture process zone presents as a narrow band whereas it occurs and develops prior to visible cracks.

ABSTRACT: Near-realtime seismic monitoring of fluid injection allowed control of induced earthquakes during the stimulation of a geothermal well near Helsinki, Finland. The injection well was drilled down to 6.1 km-depth into Precambrian crystalline rocks. The well was deviated 45° from vertical and an open hole section at the bottom was divided into several injection intervals. A total of 18,160 m 3 of fresh water was pumped into crystalline basement over a period of 49 days in June and July, 2018. The stimulation was monitored in near-real time using a deep seismic borehole array and series of borehole stations. Earthquakes were processed within a few minutes and results informed a Traffic Light System (TLS). Using near-realtime information on induced-earthquake rates, locations, magnitudes, and evolution of seismic and hydraulic energy, pumping was either stopped or varied between wellhead-pressures of 60-90 MPa and flow rates of 400-800 l/min. This procedure avoided the nucleation of a project-stopping red alert at magnitude M2.1 induced earthquake, a limit set by the TLS and local authorities.

ABSTRACT: Borehole breakouts and tensile wall fractures have been considered essential input to determine horizontal principal stress orientation. However, when the wellbore experiences significant cooling, pervasive tensile failure could misleading information for conventional breakouts. Particularly in Enhanced Geothermal Systems (EGS) wells, high temperature and low permeability make the thermally induced failure become more prevalent. Incorporating temperature change, a simple compressive failure model based on Mohr-Coulomb failure criterion for inclined wellbores has been developed. Different failure profiles around a wellbore were distinguished. In applying at sub-vertical well of NWG 55-29 at Newberry EGS site in Oregon. Temperature survey after drilling and at equilibrium phase revealed cooling up to 105°C. Besides showing extensive breakouts at azimuth of E-W, borehole televiewer (BHTV) log observation shows infrequent breakouts at N-S azimuth. Yet, there is no sign of tensile wall fractures. The model depicts that the extensive breakouts are thermally induced, X-mode. Therefore, the extensive breakouts presents the direction of maximum horizontal stress (S Hmax ), instead of classical minimum horizontal stress (S hmin ) orientation. This result is consistent with observation of tensile wall fractures at a nearby well and fairly consistent with shear-wave splitting from teleseismic study on the Newberry Caldera.

ABSTRACT: Hydraulic fracturing can be recognized as an emerging method used in the mining of heat in Enhanced Geothermal Systems as well as in the extraction of oil and gas entrapped within shale formations. While there are several experimental studies focusing on the initiation and propagation of hydraulically-induced fractures under uniaxial and biaxial loading conditions, a very limited number of experimental studies investigate the effect of triaxial loading conditions on fracture initiation and propagation.

ABSTRACT: Natural fractures can have a significant influence on hydraulic stimulation. One must estimate the peak and residual geomechanical strengths, hydraulic conductivity, and in-situ stress state for the exiting natural fractures in a site in order to identify those natural fractures that are prone to hydraulic opening and/or shear. We apply a triaxial direct-shear method to measure these fracture properties in the Poorman's schist for the EGS Collab project's Experiment 1 test bed at the Sanford Underground Research Facility. In addition, we measure rock matrix density, matrix permeability, and acoustic velocity anisotropy. Using this data, we identify that south-east striking foliation-parallel fractures in this test bed are vulnerable to shear stimulation and can be hydrosheared at in-situ stress conditions. However, natural infilled fractures which are relatively easy to locate and are ubiquitous at the site were found to be too strong and high-friction for hydroshearing, with exception for those parallel to the foliation of the rock.

ABSTRACT: A series of tracer tests have been performed to characterize the stimulated fracture network in Experiment 1 of the EGS Collab project. In the present study, we use a brute-force, Monte Carlo approach to model one of the conservative tracer tests and characterize the flow patterns in the fracture network. A fracture model involving a hydraulic fracture and a natural fracture is first developed based on geophysical data and observations, and massive realizations are performed to simulate tracer transport processes in the fracture model by assuming either uniform or heterogeneous aperture fields in the fractures. The modeling results indicate that the flow field in the natural fracture might be more channelized than that in the hydraulic fracture, and the fluid/tracer may leak from the hydraulic fracture to matrix through a sink to the left of the hydraulic fracture.

ABSTRACT: Geo-mechanical properties of geothermal reservoir rocks are important in different stages of geothermal resources development, including drilling optimization, reservoir stimulation (or re-stimulation) and development of fluid-flow and multi-physical (thermo-hydro-chemo-mechanical) constitutive models. Laboratory experiments were performed on a cylindrical granodiorite rock specimen retrieved from a well at a depth of 5561.5 ft in Patua geothermal field in Northern Nevada. In this study, Autolab 1500 (a high-pressure/temperature fully servo-controlled triaxial instrument) was used to perform a cyclic and triaxial multi-stage elastic tests on the granodiorite specimen in order to characterize the hysteresis, elastic-plastic, and strength properties of the specimen under different loading conditions. Laboratory results revealed linear elastic with very small (or no) plastic deformation and pressure dependent mechanical responses of the specimen under stresses up to 50 MPa of confining and 60 MPa of differential stress. Young's modulus and Poisson's ratio of the specimen under different stress conditions are reported in the paper. Within the elastic region, the Young's modulus increased, while the Poisson's ratio decreased with the increment of the differential stress during loading stage. Moreover, the effect of the closure of the micro-cracks present in the specimen was observed in specimen's response at lower stress levels.

ABSTRACT: A fully coupled thermal-hydraulic-mechanical (THM) model was developed to investigate the heat extraction process of an artificial enhanced geothermal system (EGS). The random fracture network in the stimulated geothermal reservoir was generated by the fractal theory. The local thermal non-equilibrium (LTNE) theory was adopted to simulate the heat exchange between the rock matrix and the injection cold fluid. Temperature-dependent fluid thermodynamic properties and pressure-dependent fracture/pore permeability were also incorporated into the thermo-poroelastic coupling model by some empirical formulas. The proposed multiphysics model was validated by several analytical solutions. The evolution of temperature, effective stress and reservoir permeability during heat extraction was analyzed in detail. The sensitivity of heat production performance to heat convection coefficient, injection rate, injection temperature, fracture network morphology was discussed. Results indicate that at the early stage, the interconnected large-scale fractures around wellbore dominate the mass and heat transport. Some scattered small-scale fractures contribute to uniformly propelling the cooling of the heat rock mass. The change in effective stress associated with the thermo-poroelastic effect may induce fracture shear dilation and pore expansion, resulting in the permeability enhancement of the overall reservoir. In some regions, where the temperature drop is insignificant and the pore pressure decreases, thereby inducing compressive stress to make fractures closure. It is more practical to use the LNTE theory to analyze the heat extraction process in fractured geothermal reservoirs. The heat extraction efficiency, thermal breakthrough time and service-life of an EGS are seriously affected by the hydraulic conductivity and connectivity of fracture networks under constant injection rates and injection temperatures. Generating a complex and scattered fracture network but without preferential channels is conducive to extracting more heat from geothermal reservoirs.

ABSTRACT : The estimation of the in-situ stress state is required for the design and execution of deep engineering operations related to Enhanced Geothermal System (EGS). Borehole failures, often referred as borehole breakouts, which are controlled by local stress concentration around the wellbore, are recognized being a useful indicator to assess in-situ stress conditions. However, breakouts evolve with time and this may affect our ability to use them for quantifying the stress state. We use a unique data set from the deep geothermal well of Rittershoffen GRT-1 in order to verify the hypothesis concerning wellbore breakout geometrical evolution. In GRT-1 wellbore, imaging has been acquired 4 days, 348 days and 946 days after drilling completion. Thermal, hydraulic and chemical stimulations have been performed between the first and the second image acquisition. Using this data set, we were able to describe in-situ the breakout evolution with time. We show increase in the extension of breakouts along the well. Contrary to the common assumptions, we also show that breakout widen, but within the limit of the accuracy of our analysis they do not deepen. The consequences of the breakout evolution for stress characterization are significant and add up to other important uncertainties in such analyses like the estimation of strength parameters.

ABSTRACT: In this paper we present a discrete element modelling based fluid injection simulation in a blocky structured rock mass. The hydro-mechanical couple model is developed on the basis of a commercial software, Particle Flow Code 2D. The fluid fl ow algorithm is modified to enable simulation of primary fluid flow through the interfaces between the rock mass blocks and secondary fluid flow into the rock block matrix. The modelling results shows that several key observations in the field of geothermal reservoir stimulation and long-term wastewater disposal are well captured, which include: post shut in seismicity, large magnitude events at the fringe of the seismicity cloud, fluid pressure induced seismicity and poro-elastic stress triggered seismicity at far distance from the injection. In addition, we examined the effect of cyclic injection where there is a relaxation time between the main stimulation cycles. The results show that cyclic injection lowers the seismicity magnitudes, and mitigates the effect of poro-elastic stress triggering. The model is being applied to a few field scale studies, e.g. Enhanced Geothermal Systems and longterm waste water disposal, where a fault zone is present near to the injection and later activated with large seismic events by the fluid injection.