Engineered Geothermal Systems Research Articles

Numerical study on the heat extraction performance of enhanced geothermal systems with a well-fracture-reservoir combined model

Understanding the fluid flow and heat transfer process in the well-fracture-reservoir combined system is crucial for the development of Enhanced or Engineered Geothermal Systems (EGS). In this paper, a well-fracture-reservoir integrated numerical model is developed to study the heat extraction performance considering the interactions among the well boreholes, the fractures in the reservoir and the base rock reservoir itself. The wells and major fractures are precisely described by the direct modelling while the sub-fractures in the base rock are described by a permeability distribution. A novel method to improve the heat extraction performance of EGS by enhancing the permeability in the near-well region is proposed. Simulation results show that the fluid flow path is predominantly determined by the well layout and fracture network. The fluid pressure is non-linearly distributed between the injection and production wells in the fractures and reservoir. Further, an optimized scheme for heat extraction of an assumed geothermal site in Gonghe Basin is proposed based on the near-well enhancement approach. The overall heat extraction performance of the near-well enhanced case is improved by more than 15.0%. The present work provides a feasible improvement scheme for EGS to achieve a better heat extraction performance.

Multi-disciplinary characterizations of the BedrettoLab – a new underground geoscience research facility

Abstract. The increased interest in subsurface development (e.g., unconventional hydrocarbon, engineered geothermal systems (EGSs), waste disposal) and the associated (triggered or induced) seismicity calls for a better understanding of the hydro-seismo-mechanical coupling in fractured rock masses. Being able to bridge the knowledge gap between laboratory and reservoir scales, controllable meso-scale in situ experiments are deemed indispensable. In an effort to access and instrument rock masses of hectometer size, the Bedretto Underground Laboratory for Geosciences and Geoenergies (“BedrettoLab”) was established in 2018 in the existing Bedretto Tunnel (Ticino, Switzerland), with an average overburden of 1000 m. In this paper, we introduce the BedrettoLab, its general setting and current status. Combined geological, geomechanical and geophysical methods were employed in a hectometer-scale rock mass explored by several boreholes to characterize the in situ conditions and internal structures of the rock volume. The rock volume features three distinct units, with the middle fault zone sandwiched by two relatively intact units. The middle fault zone unit appears to be a representative feature of the site, as similar structures repeat every several hundreds of meters along the tunnel. The lithological variations across the characterization boreholes manifest the complexity and heterogeneity of the rock volume and are accompanied by compartmentalized hydrostructures and significant stress rotations. With this complexity, the characterized rock volume is considered characteristic of the heterogeneity that is typically encountered in subsurface exploration and development. The BedrettoLab can adequately serve as a test-bed that allows for in-depth study of the hydro-seismo-mechanical response of fractured crystalline rock masses.

Influence of water-rock interaction on permeability and heat conductivity of granite under high temperature and pressure conditions

Water-rock interactions exist between the geothermal host rock (GHR) and the cold injected water for Enhanced (or Engineered) Geothermal Systems (EGS), changing permeability and thermal conductivity of GHR, influencing the heat extraction of EGS. Therefore, a water-rock interaction experiment is performed under high temperature and pressure conditions for heat-treated granite. The variation in permeability and thermal conductivity, mineral composition, and solution composition of the rock matrix is consequently analyzed, and their influence on the heat extraction of EGS is discussed. The thermodynamic analysis of the chemical processes of the experiment is presented using batch chemical simulators, PHREEQC 3.0, reproducing the chemical time series of the water-rock reactions. Water-rock reaction changes the composition of the rock and the circulating fluid. For primary minerals, albite, anorthite, and k-feldspar dissolve, biotite tends to be stable, and quartz gradually increases. Secondary minerals, muscovite, calcite, kaolinite, and chlorite, are formed in our simulation results, and their source and generation mechanisms are discussed. However, the kaolinite and chlorite were not detected in the experiment due to their low quantity. Both the dissolution of primary mineral and the formation of secondary mineral change the permeability and the thermal conductivity of the granite. The permeability exhibits a transition from increasing to decreasing while the thermal conductivity shows an opposite variation. Both approach constant values but only at 60% of the initial value. However, from the long-term perspective, the change in permeability and thermal conductivity of the granitic host rock around the fracture will not exert a strong impact on the heat extraction of EGS. The test results in this context could facilitate our understanding of permeability characteristics and the evolution of thermal conductivity of a granitic host rock during the water-rock interaction, and their influence on the heat extraction of EGS during the operation process.

Extrapolation of populations of small earthquakes to predict consequences of low- probability high impact events: The Pohang case study revisited

The magnitude (MW) 5.5 Pohang, Korea, earthquake on 15 November 2017, induced by the Pohang Engineered Geothermal Systems (EGS) project, caused one fatality and ∼US$300 million of economic consequences. The Commission, appointed by the Korean Government to investigate this earthquake, has made public a release of data including magnitudes of the smaller earthquakes associated with the well stimulations. On the basis of this earthquake population, it has been proposed that a significant probability of such losses was predictable beforehand, and that the project should have been suspended, implying that its developer was remiss for not doing so. This argument depends on the low b-value estimated, ∼0.61. However, three factors are shown to contribute, individually or in combination, to inaccuracy of these magnitude determinations: the low recording bandwidth of the permanent seismograph stations in the area; miscalibration of the formula for determining local magnitudes in Korea; and the relation used to estimate magnitudes of smaller events from larger events by template matching. These factors all cause underestimation of magnitudes of the smallest events documented, resulting in underestimation of b-values. The true b-values are higher, being 1.12 for the earthquakes associated with the August 2017 stimulation of well PX-1; similar values are estimated for the other well stimulations. A consequence of this analysis is that the probability of any earthquake as large as MW = 5.5, predicted ahead of its occurrence by extrapolation using b-values, was much lower than has been claimed. This analysis highlights the need for agreed workflow specifications for reporting datasets like this, where the data might influence prosecution of EGS developers, as well as agreed specifications for acceptable economic risk arising from EGS projects.

Introduction to this special section: Geothermal energy

Geothermal energy is a global renewable resource that has the potential to provide a significant portion of baseload energy in many regions. In the United States, it has the potential to provide 8.5% of the electric generation capacity by the middle of the century. In general, geothermal systems require heat, permeability, and water to be viable for energy generation. However, with current technologies, only heat is strictly necessary in a native system. Engineered geothermal systems (EGS) introduce water into the subsurface at elevated pressures and reduced temperatures and enhance permeability through hydraulic and/or shear fracturing. Additionally, although moderate- to high-temperature resources currently dominate geothermal energy production, low-temperature resources have been utilized for direct-use cases. When well balanced and maintained, geothermal resources can produce significant amounts of heat and achieve long-term sustainability on the order of an estimated tens to hundreds of years.

Energy potential of a single-fracture, robust, engineered geothermal system

Engineered Geothermal Systems (EGS) are promising, but high-risk targets for baseline energy generation. Technology breakthrough is needed to lower the high risks from largely unknown geologic variables and the limited control of the coolant flow field underground. A new, robust EGS (REGS) arrangement and creation technology is reviewed featuring innovative fracture opening, stabilization and permeability control. The geometry, aperture support technique and coolant fluid flow isolation system are all robustly planned and created according the inventive concept. The new elements of the REGS technology are (1) the step-by-step creation, control, and tests of hydraulic fracturing to achieve a planar wing fracture or fractures osculating along the well trajectory; (2) fracture stabilization by hardening grouting injection to create a central support island in each planar wing fracture for zonal isolation; and (3) well-fracture-well fluid circulation for geothermal energy extraction from single, or clustered planar fractures as geothermal heat exchangers. The paper reviews ongoing tests to prove the key components of the REGS geologic heat exchanger with stabilized, large fracture aperture and controlled flow zones for minimized opening pressure loss, seismicity and maximized energy extraction. Flow fields and heat transport are reviewed around zonal isolation of fracture flow by a grouted, blocking island for heat exchange. The robustness is reviewed for the step-by-step construction of a REGS. The paper reports new results from using numerical, coolant flow and heat exchange models to demonstrate the geothermal energy potential of a single REGS well drilled in hot, dry rock.

The EGS Collab Project: An intermediate-scale field test to address enhanced geothermal system challenges

Three components are typically needed to extract geothermal energy from the subsurface: 1. hot rock, 2. a heat transfer fluid, and 3. flow pathways contacting the fluid and the rock. These naturally occur in many locations resulting in hydrothermal systems, however there are enormous regions containing hot rock that do not naturally have adequate fluid, and/or appropriate fluid permeability to allow hot fluid extraction. Some type of engineering or enhancement of these systems would be required to extract the energy. These enormous regions provide the possibility of long-term extraction of significant quantities of energy. Enhanced (or engineered) Geothermal Systems (EGS) are engineered reservoirs created to extract economical amounts of heat from low permeability and/or porosity geothermal resources. There are technological challenges that must be addressed in order to extract the heat. These include proper stimulation, effective monitoring, reservoir control, and reservoir sustainability. The US DOE Geothermal Technologies Office and geothermal agencies from other countries have supported field tests over a range of scales and conditions. A current US field project, the EGS Collab Project, is working nearly a mile deep in crystalline rock at the Sanford Underground Research Facility (SURF) to study rock stimulation under EGS stress conditions. We are creating intermediate-scale (tens of meters) test beds via hydraulic stimulation and are circulating chilled water to model the injection of cooler water into a hot rock which would occur in an EGS, gathering high resolution data to constrain and validate thermal-hydrological-mechanical-chemical (THMC) modeling approaches. These validated approaches would then be used in the DOE’s flagship EGS field laboratory, Frontier Observatory for Research in Geothermal Energy (FORGE) underway in Milford, Utah and in commercial EGS. In the EGS Collab project, numerous stimulations have been performed, characterized, and simulated and long-term flow tests have been completed.

Characterization of laboratory-scale hydraulic fracturing for EGS

To investigate the impact of reservoir stimulation by hydraulic fracturing, a series of advanced lab-scale block tests were conducted with multiple measurement methods to characterize the fracture and to assess the operation and the performance of an enhanced or engineered geothermal system (EGS). The stimulation test samples were 330-mm cubic blocks of Sierra White granite with one injection well and four producers drilled into it. Acoustic emissions generated during the fracture initiation and propagation were monitored with twenty AE sensors mounted on all sides of the block to help characterize the fracture initiation and propagation process. The recorded AE signals were analyzed to obtain the event location and the failure mechanism. The localized AE events shows that due to the low viscosity of the fracturing fluid (water), the fracture propagated rapidly and took only about 1.4 s to reach near the block surface. During the fracturing phase, about 47 percent of the events were determined to be shear/mixed mode failure events, 28 percent tensile failure events, and 25 percent compressional failure events. Just after the simulation, a step-rate (constant flowrate) injection test was carried out to investigate the reservoir injectivity enhancement. During this phase additional compressional failure events were observed and the percentages of tensile failure and shear/mixed mode failure decreased. The dramatic increase of the injection rate with small increments of the injection pressure demonstrates the nonlinear response of the fracture to effective normal stress variation with much higher fracture opening increase during the low effective normal stress interval. Hydraulic fracturing tests under different stress levels show that more AE hits/events are recorded when the rock block is under higher applied stress. The reconstructed 3D fracture geometry shows good agreement with the AE monitoring results. After fracturing, a tracer test was carried out and the fracture aperture was estimated to be around 139 μm. The tracer recovery curve shows two linear portions of the tracer tail on a semi-log plot similar to that observed in some field data and predicted by numerical modeling. Laser scanning of the fracture surfaces indicates that the induced fracture has a rough surface with a joint roughness coefficient (JRC) of 12.98 and tortuosity of 1.10, both of which are slightly larger than that from the Brazilian test fracture on the same rock. The thin section and SEM observations show that the fracture follows the weak boundaries between the Quartz and Albite grains or the weak beddings in the grains.

Influence of the Effective Vertical Stresses on Hydraulic Fracture Initiation Pressures in Shale and Engineered Geothermal Systems Explorations

Influence of the Effective Vertical Stresses on Hydraulic Fracture Initiation Pressures in Shale and Engineered Geothermal Systems Explorations

Evolution of permeability in a single granite fracture at high temperature

Thermal energy recovered from Hot Dry Rock (HDR) or Engineered Geothermal Systems (EGS) is clean and renewable and may fulfill current energy demands. The long term flow characteristic of fractures is a key issue related to both the rate of geothermal production and the service life of the geothermal reservoir. We explore the long term fluid transport characteristics of a long (50 mm wide and 300 mm long) artificial fracture in granite at temperatures to 200 °C. Permeability is measured under steady flow conditions in three stages: (1) stepped increments of temperature from 25 °C to 200 °C; (2) constant temperature of 200 °C applied for 27 h; (3) gradually decreased temperature from 200 °C to 25 °C. The evolution of hydraulic aperture and permeability of the fracture is evaluated over these three regimes. Hydraulic aperture and permeability decrease with the increase in temperature. Uniform high temperatures result in a continued but gradual reduction in both hydraulic aperture and permeability. The reduction in hydraulic aperture and permeability of the fracture is permanent with marginal recovery as the temperature is reset to the original 25 °C. Permeability and heat transfer efficiency increase during heating (stage 1), but decrease during high temperature production and then cooling (stages 2 and 3).

Assessment of the resource base for engineered geothermal systems in Great Britain

An assessment of the engineered geothermal system (EGS) resource base that might be available for the generation of electricity for Great Britain has been undertaken by adopting a globally self-consistent protocol that if universally adopted, would allow estimates of EGS made for different countries and regions to be comparable. Maximum estimated temperatures at depths of 5 and 7 km are greater than 200 and 300 °C respectively, a considerable increase over previous estimates. The total heat in place in the basement, to a depth of 9.5 km that is theoretically available for EGS is 357,197 EJ. If it were possible to develop just 2% of this resource, this would be equivalent to 1242 times the final UK energy consumption in 2015. The theoretical and technical potential power has been calculated from the available heat in place. The total technical potential power, to a depth of 6.5 km, is 222,393 MWe and represents just 0.4% of the theoretical potential power. Current EGS exploitation is more likely to be restricted to a depths of around 4.5 km and reservoir temperatures greater than 175 °C. In which case technical potential power is mainly restricted to regions of high heat producing granites and represents a total technical potential power of 2280 MWe. However, improvements in drilling technology are expected to enable economic drilling to depths of 7 km or greater that will enable EGS exploitation in all regions of Great Britain.

Illite dissolution kinetics from 100 to 280 °C and pH 3 to 9

Some geothermal reservoirs may require permeability enhancements (either by reactivating existing fracture networks or creating new ones) to allow sufficient fluid and heat transfer throughout the system. In these engineered geothermal systems (EGS), fluid/mineral reactivity is of interest because mineral reaction can affect fracture apertures, wall rock stability, and potential downstream precipitation. In this study we focus on the reactivity and dissolution of illite, an aluminosilicate sheet mineral common in geothermal systems, measured in flow-through experiments carried out at temperatures of 100–280°C and pH 2.7–9.7. Under these conditions, illite dissolves readily according to:Rillite=1×10-2·e-58/RT·aH+0.55+2×10-5·e-54/RT+1.5·e77/RT·aOH-0.35·f(ΔGr),where Rillite represents the surface area-normalized dissolution rate. Illite dissolves congruently under alkaline pH conditions at temperatures of 100°C and above, but as pH decreases below 5 illite dissolution is accompanied by precipitation of an aluminum oxy(hydroxide) phase identified as boehmite by X-ray diffraction of bulk samples. Precipitation of AlO(OH)(s) appears to proceed rapidly and is limited by the rate of illite dissolution in our experiments.

On the Role of Thermal Stresses during Hydraulic Stimulation of Geothermal Reservoirs

Massive quantities of fluid are injected into the subsurface during the creation of an engineered geothermal system (EGS) to induce shear fracture for enhanced reservoir permeability. In this numerical thermoelasticity study, we analyze the effect of cold fluid injection on the reservoir and the resulting thermal stress change on potential shear failure in the reservoir. We developed an efficient methodology for the coupled simulation of fluid flow, heat transport, and thermoelastic stress changes in a fractured reservoir. We performed a series of numerical experiments to investigate the effects of fracture and matrix permeability and fracture orientation on thermal stress changes and failure potential. Finally, we analyzed thermal stress propagation in a hypothetical reservoir for the spatial and temporal evolution of possible thermohydraulic induced shear failure. We observe a strong influence of the hydraulic reservoir properties on thermal stress propagation. Further, we find that thermal stress change can lead to induced shear failure on nonoptimally oriented fractures. Our results suggest that thermal stress changes should be taken into account in all models for long-term fluid injections in fractured reservoirs.

Thermal drawdown-induced flow channeling in a single fracture in EGS

The evolution of flow pattern along a single fracture and its effects on heat production is a fundamental problem in the assessments of engineered geothermal systems (EGS). The channelized flow pattern associated with ubiquitous heterogeneity in fracture aperture distribution causes non-uniform temperature decrease in the rock body, which makes the flow increasingly concentrated into some preferential paths through the action of thermal stress. This mechanism may cause rapid heat production deterioration of EGS reservoirs. In this study, we investigated the effects of aperture heterogeneity on flow pattern evolution in a single fracture in a low-permeability crystalline formation. We developed a numerical model on the platform of GEOS to simulate the coupled thermo-hydro-mechanical processes in a penny-shaped fracture accessed via an injection well and a production well. We find that aperture heterogeneity generally exacerbates flow channeling and reservoir performance generally decreases with longer correlation length of aperture field. The expected production life is highly variable (5 years to beyond 30 years) when the aperture correlation length is longer than 1/5 of the well distance, whereas a heterogeneous fracture behaves similar to a homogeneous one when the correlation length is much shorter than the well distance. Besides, the mean production life decreases with greater aperture standard deviation only when the correlation length is relatively long. Although flow channeling is inevitable, initial aperture fields and well locations that enable tortuous preferential paths tend to deliver long heat production lives.

수직 횡등방성 전기적 이방성을 고려한 자기지전류탐사 모델링

Magnetotelluric (MT) survey investigates electrical structure of subsurface by measuring natural electromagnetic fields on the earth surface. For the accurate interpretation of MT data, the precise three-dimensional (3-D) modeling algorithm is prerequisite. Since MT responses are affected by electrical anisotropy of medium, the modeling algorithm has to incorporate the electrical anisotropy especially when analyzing time-lapse MT data sets, for monitoring engineered geothermal system (EGS) reservoir, because changes in different-vintage MT-data sets are small. This study developed a MT modeling algorithm for the simulation MT responses in the presence of electrical anisotropy by improving a pre-existing staggered-grid finite-difference MT modeling algorithm. After verifying the developed algorithm, we analyzed the effect of vertical transversely isotropic (VTI) anisotropy on MT responses. In addition, we are planning to extend the applicability of the developed algorithm which can simulate not only the horizontal transversely isotropic (HTI) anisotropy, but also the tiled transversely isotropic (TTI) anisotropy.

Results from Newberry Volcano EGS Demonstration, 2010–2014

The Newberry Volcano EGS Demonstration is a 5 year field project designed to demonstrate recent technological advances for engineered geothermal systems (EGS) development. Advances in reservoir stimulation, diverter, and monitoring are being tested in a hot (>300°C), dry well (NWG 55-29) drilled in 2008. These technologies could significantly reduce the cost of electrical power generation from geothermal resources. The project, funded in part by the Department of Energy, began in 2010 with two years of permitting, technical planning, and development of a project-specific Induced Seismicity Mitigation Plan (ISMP). Well stimulation carried out in 2012 indicated that casing repairs were needed; confirmed by further wellbore logging and analysis in 2013. Repairs were completed in August 2014, and the well was re-stimulated in the fall. 9500m3 (2.5 million gallons) of groundwater were injected at a maximum wellhead pressure of 195bar (2850 psi) over 4 weeks of hydraulic stimulation. Injectivity changes, thermal profiles and seismicity indicate that fracture permeability in well NWG 55-29 was enhanced. The fifteen-station microseismic array (MSA) located 398 events in 2014, ranging in magnitude from M 0 to M 2.26. Temperature logs run after injection of thermally-degradable zonal isolation material (TZIM) showed that at least two flow zones were blocked and one or two new zones opened because of the injected TZIM. Break-down products of TZIM were detected in flow-back fluids, indicating that the material degraded as predicted. This work demonstrates the viability of large-volume low-pressure stimulation coupled with non-mechanical diverter technology and microseismic monitoring for reservoir mapping.

How hot are the Cairngorms?

Heat flow measured over the East Grampians batholith in the 1980s was found to be unexpectedly low and at odds with high radiogenic heat production within the outcropping granites and a very large volume of granite predicted from an interpretation of gravity data. Past climate variations perturb temperature gradients in the shallow subsurface leading to erroneous estimates of heat flow. A reconstruction of the surface temperature history during the last glacial cycle has enabled a rigorous palaeoclimate correction to be applied to the heat flow that shows an increase of 25% over previously reported values; revised to 86 ± 7 mW m−2. An interpretation of recent mapping reveals that the surface exposures of the East Grampians granites are the roof zones of a highly evolved magma system. Rock composition, therefore, is likely to become more mafic with depth and the heat production will decrease with depth. This petrological model can be reconciled with the gravity data if the shape of the batholith is tabular with deep-seated feeder conduits. The increased heat flow value leads to revised predictions of subsurface temperatures of 129°C at 5 km depth and 176°C at 7 km depth, increases of 40% and 49%, respectively, compared to previous estimates. These temperatures are at the lower end of those currently required for power generation with Engineered Geothermal Systems, but could potentially be exploited as a direct heat use resource in the Cairngorm region by targeting permeable fractures with deep boreholes.

Maximum potential for geothermal power in Germany based on engineered geothermal systems

We estimate the maximum geothermal potential in Germany available for exploitation by operated engineered geothermal systems (EGS). To this end, we assume that (a) capabilities for creating sufficient permeability in engineered deep heat exchange systems will become available in the future and (b) it will become possible to implement multiple wells in the reservoir for extending the rock volume accessible by water circulation for increasing the heat yield. While these assumptions may be challenged as far too optimistic, they allow for testing the potential of EGS, given the required properties, in countries lacking natural steam reservoirs. With this aim, we model numerically the thermal and electric energies which may be delivered by such systems by solving coupled partial differential equations governing fluid flow and heat transport in a porous medium. Thus, our model does not represent the engineered fractures in their proper physical dimension but rather distributes their flow volume in a small region of enhanced permeability around them. By varying parameters in the subsurface, such as flow rates and well separations, we analyze the long-term performance of this engineered reservoir. For estimating the maximum achievable potential for EGS in Germany, we assume the most optimistic conditions, realizing that these are unlikely to prevail. Considering the available crystalline landmass and accounting for the competing land uses, we evaluate the overall EGS potential and compare it with that of other renewables used in Germany. Under most optimistic assumptions, the land surface available for emplacing EGS would support a maximum of 13,450 EGS plants each comprising 18 wells and delivering an average electric power of 35.3 MW e . When operated at full capacity, these systems collectively may supply 4155 TWh of electric energy in 1 year which would be roughly seven times the electric energy produced in Germany in the year 2011. Thus, our study suggests that major scientific, engineering, and financial efforts are justified for developing the drilling and stimulation technologies required for creating the permeabilities required for successful EGS. Then, EGS will have great potential for contributing towards national power production in a future powered by sustainable, decentralized energy systems.

Conceptual model and numerical analysis of the Desert Peak EGS project: Reservoir response to the shallow medium flow-rate hydraulic stimulation phase

A series of stimulation treatments were performed as part of the Engineered Geothermal System (EGS) experiment in the shallow open-hole section of Desert Peak well 27-15 (September 2010–November 2012). These injections at variable wellhead pressures, both below and above the magnitude of the least horizontal principal stress (Shmin), produced injectivity gains consistent with hydraulically induced mechanical shear and tensile failure in the surrounding rock. A conceptual framework for the overall Desert Peak EGS experiment is developed and tested based on a synthesis of available structural and geological data. These data include down-hole fracture attributes, in situ stress conditions, pressure interference tests, geochemical tracer studies, and observed induced seismicity. Induced seismicity plays a key role in identifying the geometry of large-scale geological structures that could potentially serve as preferential flow paths during some of the stimulation phases. The numerical code FLAC3D is implemented to simulate the reservoir response to hydraulic stimulation and to investigate in situ conditions conducive to both tensile and shear failure. Results from the numerical analysis show that conditions for shear failure could have occurred along fractures associated with a large northeast-trending normal fault structure located ∼400m below the injection interval which coincides with the locations of most of the observed micro-seismicity. This structure may also provide a hydrologic connection between EGS well 27-15 and injection/production wells further to the south–southwest.

Analysis of Hydraulic Fracturing and Reservoir Performance in Enhanced Geothermal Systems

Analyses of fracturing and thermal performance of fractured reservoirs in engineered geothermal system (EGS) are extended from a depth of 5 km to 10 km, and models for flow and heat transfer in EGS are improved. Effects of the geofluid flow direction choice, distance between fractures, fracture width, permeability, radius, and number of fractures, on reservoir heat drawdown time are computed. The number of fractures and fracture radius for desired reservoir thermal drawdown rates are recommended. A simplified model for reservoir hydraulic fracturing energy consumption is developed, indicating it to be 51.8–99.6 MJ per m3 fracture for depths of 5–10 km.