Enhanced Geothermal System Project Research Articles

Assessment of two recent hot dry rock thermal energy production projects

In this paper, we present analyses of laboratory and field data indicating that the current two-well, injection-production system, connected with multiple hydraulic fractures, is a very promising method for extracting heat from hot dry rock (HDR) systems to generate electricity. The current two-well system could be expanded to a three-well system consisting of one injection well and two symmetric producing wells connected via the central hydraulic fracture emanating from the injection well. To improve a uniform distribution of the injected fluid among all hydraulic fracture stages in the injection well, we advocate for field implementation of a newly designed well stimulation technique, the GeoThermOPTIMAL.We first present an analysis of the post-fracturing flow data obtained from an HDR geothermal injection well at the Utah FORGE Enhanced Geothermal System (EGS) research field site. The site is adjacent to the Roosevelt hydrothermal (HT) field. The objective of the study is to assess the effectiveness of well stimulation in extracting heat from the low-permeability, hot dry granitoid rock in the Utah FORGE research site. The study includes interpreting pressure falloff data obtained during the well stimulation process and employing laboratory-measured core data as a major input in the interpretation of the field falloff data. As a confirmation of the robustness of our analysis in Utah FORGE, we reviewed and analyzed the flow test results published for an injection-production doublet at the Blue Mountain EGS (Project Red) commercial site in Nevada. From the analyses of these two field tests, we have concluded that the interpretation and findings of the Blue Mountain EGS pilot test are consistent with the interpretation and findings from the Utah FORGE field research project test results.In summary, our engineering assessments began with laboratory experiments conducted on various core samples, including those from a granite outcrop and the Utah FORGE geothermal reservoir. These experiments aimed to measure key parameters such as matrix and fracture permeabilities, and porosities (km≈10−18m2, kf,eff≈10−15m2,ϕm≈10−1,andϕf≈10−4). These data served as guides and inputs for analytical and numerical solutions used to match the field pressure response of the geothermal wells. While we did not have core samples from the Blue Mountain EGS wells, we successfully applied the Utah FORGE analysis approach to the Blue Mountain site with cautious optimism.

Numerical investigation and optimization on dynamic power generation performance of enhanced geothermal system

The Enhanced geothermal system (EGS) is a crucial technology for the development and utilization of hot dry rock (HDR). However, current research typically overlooks the strong coupling characteristics of fracture seepage, wellbore transport, and thermoelectric conversion, leading to a lack of precision in describing the dynamic operational performance of EGS throughout its entire lifecycle. A dynamic numerical model of evaporator and condenser is established by the moving boundary (MB) method. Subsequently, the dynamic power generation process is integrated with a coupled thermal-hydraulic-mechanical (THM) model of the fractured reservoir, proposing a methodology for calculating the dynamic performance of EGS throughout its lifecycle.Traditional EGS models exhibit significant deviations between calculated results and actual performance due to the neglect of instantaneous changes in injection temperature and the failure to consider the nonlinear characteristics of heat exchangers. The production pressure difference is a primary factor influencing the net output power of EGS. Therefore, selecting an appropriate injection pressure is crucial to ensuring continuous power generation throughout the EGS lifecycle. By optimizing the operating parameters of the actual EGS project, the optimal injection flow rate, fracture aperture, initial fracture permeability and well spacing are 50 kg/s, 0.03 m, 1.5 × 10−10 m2 and 350 m respectively.

Reservoir and Fracture Characterization for Enhanced Geothermal Systems: A Case Study Using Multifractured Wells at the Utah Frontier Observatory for Research in Geothermal Energy Site

Summary For enhanced geothermal systems (EGS), multistage hydraulic fracturing along a deviated or horizontal well is a key technology used to create a high-conductivity fracture network between injection and production wells in deep, low-permeability geothermal reservoirs. The purpose of the created fracture network is to allow for the efficient transfer of fluid, heated by the geothermal reservoir, from the injection to the production well; therefore, well spacing (between injection and production wells) and hydraulic fracturing must be designed not only to promote connectivity between well pairs but also to mitigate thermal short-circuiting and thermal breakthrough. Analysis of post-fracture pressure decay (PFPD) data measured after each stage of a hydraulic fracturing treatment can be used to provide critical reservoir and fracture parameters required for well and hydraulic fracturing design optimization; this method provides a low-cost alternative and complementary approach to in-situ observation techniques, such as core-through experiments, fiber optics, or image logs in offset wells. Until now, PFPD has primarily been applied to multifractured horizontal wells (MFHWs) completed in low-permeability hydrocarbon reservoirs. The goal of this study is therefore to develop a methodology to estimate fracture and reservoir parameters using stage-by-stage PFPD data associated with EGS projects. An analytical model is proposed herein to estimate fracturing fluid efficiency, fracture length, average fracture aperture, average fracture conductivity, and reservoir permeability for different possible fracture geometries in EGS reservoirs. PFPD data collected for three hydraulic fracture stages in the injection well at the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) site were analyzed to demonstrate the practical application of the proposed method. The results of this study indicate that, due to the presence of natural fractures in the target (granitic) reservoir, the hydraulic fracturing treatment (using slickwater) in the openhole section resulted in low fracturing fluid efficiency and small hydraulic fractures. In contrast, hydraulic fracturing treatments conducted in the perforated casedhole wellbore resulted in higher fracturing fluid efficiency and created larger hydraulic fractures even with smaller injected volumes. The results of the PFPD analysis were confirmed using a Formation MicroScanner image log and microseismic data collected during each stage of hydraulic fracturing.

Improving enhanced geothermal systems performance using adaptive fracture conductivity

Maximizing heat extraction from enhanced geothermal systems (EGS) depends heavily on an efficient heat sweeping or basically an effective circulation of water through fracture networks. Fracture hydraulic conductivity can play a critical role in controlling potential thermal breakthroughs; however, the exact geometry and properties of these fractures cannot be determined in practice. Thermal breakthrough remains a main challenge for maintaining efficient heat extraction throughout the lifespan of an EGS project. In this work, we explore the concept of fracture conductivity tuning using temperature-sensitive proppants to extract more heat and delay thermal breakthrough from the particle scale to the reservoir scale. We derive an empirical equation to describe fracture conductivity as a function of proppant properties and in-situ conditions including closure stress and temperature. Sensitivity analysis are performed to determine which parameters are the most crucial in determining the temperature-sensitive proppant permeability. Temperature and thermal expansion coefficient are the main contributing factors to the change in proppant permeability. Poisson’s ratio is found to have negligible impact on the proppant permeability. The derived equation is used to quantify the improvement in heat extraction from EGS at the field scale. The outcome shows that using temperature-sensitive proppant can increase the heat extracted by 47.8% compared to using typical proppant. This work assesses the feasibility of using temperature-sensitive proppants to effectively delay thermal breakthrough and sweep larger amounts of heat from EGS. In addition, this paper provides a quick and robust method to determine the required properties of such temperature-sensitive proppants to achieve uniform thermal gradient along different flow paths in the subsurface autonomously.

Monitoring induced microseismicity in an urban context using very small seismic arrays: The case study of the Vendenheim EGS project

Monitoring the seismicity induced by fluid injections in deep geothermal reservoirs is often limited by the significant anthropogenic ambient noise level inherent to most enhanced geothermal system (EGS) projects in urban contexts. We report on the performance of a monitoring network made of three small aperture (72 m) seismic arrays composed of 21 3D nodes. We test the setup for four months (from 12 December 2020 to 8 April 2021) of the Strasbourg sequence of induced earthquakes (2019–2022) related to the EGS Georhin project (Vendenheim, France). The deployment starts a few days after the MLv = 3.6 induced earthquake of 4 December 2020 and covers the early shut-in period of the wells. We use a beamforming technique to characterize the main noise sources, which consist of slow apparent velocities of surface waves emitted from mobile anthropogenic sources (motorway and railway traffic). We detect events with a phase-weighted stacking method, which is efficient when wavefronts illuminate the arrays with a high apparent velocity. Earthquakes associated with these detections are located using a matched field processing technique. The obtained catalog includes 216 seismic events, which represent four times more events than the reference catalog from Le Bureau Central Sismologique Français–Renass (the national academic agency in charge of seismicity monitoring in France), and a reduction of the completeness magnitude from 0.3 to −0.5. The clustering of the seismicity is analyzed using waveform correlation. The enriched catalog reveals intermittent seismic activity during the slow and continuous decrease in fluid pressure after shut-in.

Autonomous Fracture Conductivity Using Expandable Proppants in Enhanced Geothermal Systems

Summary Early thermal breakthrough in enhanced geothermal systems (EGS) due to the presence of preferential flow channels is a major challenge that endangers efficient and economic heat extraction in such systems. Previous studies mainly focused on adjusting circulation rates of the working fluid, which still leaves significant amounts of untapped heat behind. Currently, there is a lack of technologies for altering flow distribution within the fracture network to achieve uniform heat sweeping in the reservoir. This work presents a novel concept for making proppants to autonomously control fracture conductivity based on the surrounding temperature. Here, proppants with negative thermal expansion coefficients have demonstrated the capability for appropriate fracture conductivity adjustment as a function of temperature to achieve uniform flow across the fracture network. Particle-particle interactions governing such functions are explicitly modeled, and then the Lattice Boltzmann methods (LBM) is used to determine the potential impact of closure stress and temperature changes on the permeability of the proposed proppant packs. Microscale analyses are further used to determine the required material properties to achieve a certain improvement in the permeability of the proppant pack. Our analyses show an enhancement in permeability and the associated fracture conductivity by half of their initial values. Field-scale analysis further confirms the effectiveness of the proposed concept as 31.4% more heat can be extracted from EGS over 50 years of production when the proposed proppants are used. Such novel proppants may effectively delay thermal breakthrough, sweep heat from larger rock volumes, and elongate the life span of the EGS project.

Expertise in Complex-Well Construction Leveraged for Geothermal Wells

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 204097, “Constructing Deep Closed-Loop Geothermal Wells for Globally Scalable Energy Production by Leveraging Oil and Gas Extended-Reach Drilling and High-Pressure/High-Temperature Well-Construction Expertise,” by Eric van Oort, SPE, Dongmei Chen, SPE, and Pradeepkumar Ashok, SPE, The University of Texas at Austin, et al. The paper has not been peer reviewed. _ In the complete paper, deep closed-loop geothermal systems (DCLGS) are introduced as an alternative to traditional enhanced geothermal systems (EGS) for green energy production that is globally scalable and dispatchable. The authors demonstrate that DCLGS wells can generate power on a scale comparable to that of EGS. They also highlight technology gaps and needs that still exist for economically drilling DCLGS wells, writing that it is possible to extend oil and gas technology, expertise, and experience in extended-reach drilling (ERD) and high-pressure/high-temperature (HP/HT) drilling to construct complex DCLGS wells. Introduction CLGS is considered a subset of EGS, but the authors write that it is a distinct entity. EGS mostly involves well designs that rely on fractures for heat extraction. Such systems are different from CLGS wells in that the latter use closed conduits for thermal fluid circulation and heating. CLGS relies on fluids pumped through a closed loop. The authors treat CLGS systems as being different from EGS systems, with the understanding that drilling technologies discussed in the paper as enablers for CLGS wells apply equally to EGS wells. In the geothermal (GT) domain, the majority of attention and funding currently is assigned to EGS projects. A case is made in the complete paper to continue to develop DCLGS technology because of its favorable risk profile compared with EGS. Part I of the complete paper introduces a hydraulic model coupled with a thermal model suitable for calculating the power generation of DCLGS wells. This synopsis concentrates instead on Part II of the complete paper, in which technology gaps and needs of DCLGS drilling and well construction are highlighted and opportunities identified where oil and gas experience and technology can be directly applied and leveraged. Similarities and Differences of Deep GT and Oil and Gas HP/HT Wells - GT wells generally use larger production hole sizes than typical land wells. - Casing-cement annuli typically are cemented back to surface. - GT wells can be drilled in more-forgiving pore-pressure fracture gradient (PPFG) environments with wider drilling margins than geopressured HP/HT wells in hydrocarbon systems. - Severe lost circulation appears to be a universal problem in deep GT wells. - Drilling costs can account for 50% or more of the total capital costs for a GT energy project. - Data sets on GT wells are much smaller than those for oil and gas wells.

A Review of Enhanced Geothermal Systems with Emphasis on the Use of Backfilling Materials

Abstract: Enhanced geothermal systems (EGS) are a promising renewable energy technology that uses the heat stored in the Earth’s crust to generate electricity. In EGS, water is injected into a well drilled into a hot rock to create a network of fractures through which water can circulate and pick up heat. The main challenge of EGS is to develop and maintain these fractures to allow the efficient circulation of water. This review paper examines the use of backfilling materials in EGS to improve water circulation and increase energy extraction. Various backfilling materials are discussed, including proppants, particulate materials, and chemical additives. The effects of backfilling on the mechanical and thermal properties of the reservoir rock are also examined. Finally, case studies of EGS projects that have used backfilling materials are reviewed to evaluate their effectiveness

Comparative analysis of water and carbon dioxide injection for the thermohydraulics of an EGS project in Dikili Geothermal Field, Türkiye

A comparative numerical analysis of the thermohydraulics of an enhanced geothermal system (EGS) project in Türkiye in Dikili area is presented. The fractured granodiorite is modelled as porous media, utilizing the numerically suggested data of other authors for the corresponding hydraulic characteristics. As the heat transmission fluid, two different mediums are alternatively considered. These are the more classical medium, water and the supercritical Carbon Dioxide (sCO2). Transient calculations are performed for a time period of twenty years, comparing the temporally developing results obtained for water and sCO2 with each other. Based on modeling parameters and assumptions, higher production temperatures are observed with sCO2, in comparison to water, implying an advantage for sCO2 usage as a working fluid in EGS. This is accompanied by the further advantage of a lower pressure drop for sCO2. On the other hand, the temperature advantage is relativized by the lower specific heat capacity of sCO2 causing a decrease in the production thermal power. In general, the present re found to be encouraging for a further and more detailed analysis of the employment of sCO2 as working fluid in EGS.

Geothermal Characteristics and Productivity Potential of a Super-Thick Shallow Granite-Type Enhanced Geothermal System: A Case Study in Wendeng Geothermal Field, China

Super-thick shallow granites without a cap layer are widely distributed in the Wendeng geothermal field. To evaluate the field’s productivity potential for an enhanced geothermal system (EGS), we carried out field tests, laboratory tests and numerical simulations in succession. The geothermal characteristics and deep rock mechanical properties were identified based on real geological and core data from the borehole ZK1 in Wendeng geothermal field. Then, a numerical model of reservoir hydraulic fracturing based on a discrete fracture network was established. Thermal extraction simulations were then conducted to assess the long-term productivity of an EGS project based on the fracturing results. Possible well layout patterns and operational parameters were considered. Results indicated that, for naturally fractured formations, large well spacings should be used and reservoirs with overdeveloped natural fractures should not be selected. For the same reservoir, created by stimulation, the production performances of five-spot and triplet-well modes were different. The pressure indicator was more sensitive to the choice of well layout mode than the temperature indicator. The power generation of the five-spot well mode was slightly improved above that of the triplet-well mode. When selecting the target reservoir, the formations with high temperatures, moderate natural fractures, and high in-situ stress shielding are preferable. On this basis, a large volume of fracturing fluids should be injected to stimulate the reservoir, making the reservoir length and width as large as possible. If the desired large-scale reservoir is created, the five-point well mode should be selected.

Geothermal Potential of Hot Dry Rock in South-East Baltic Basin Countries—A Review

The beginning of 2022 was a time of major changes in the perception of energy availability and security in European countries. The aggression of Russia against Ukraine destabilizing the European energy economy, combined with the withdrawal from fossil fuels which has been going on for a dozen years, has strengthened activities to introduce new energy technologies based on renewable energy sources. One of the most promising and stable sources of renewable energy is geothermal energy, in particular enhanced geothermal systems (EGS) in hot dry rocks (HDR). These deposits occur at a great depth in almost every place on Earth, but due to their low permeability, they require hydraulic fracturing, which results in high investment costs. This technology has been developed for several decades. The current situation in Europe seems to confirm that its rapid development to a level that guarantees stable and profitable operation is crucial. This is of particular importance in the case of former member states of the economic zone of the Council for Mutual Economic Assistance, which until recently were heavily dependent on Russian energy. This review, based on the latest available data, covers potential HDR prospective areas in the countries of the south-eastern Baltic basin, including Lithuania, Latvia, Estonia and Poland. It is specific to this region that the original heat flux density is lower as a result of the paleoclimatic effect associated with the youngest ice age; however, thermal conditions do not deviate too much compared to western Europe, especially Rhine Graben, and significantly exceed the conditions of Finland, where an EGS project is currently being operated. In Lithuania, the most prospective area is the ZNI intrusion (south of Klaipeda), characterized by a geothermal gradient of up to 40 °C/1000 m. In addition, the Precambrian batholith south of Liepāja (Latvia) and the Rapakivi granites in the north and center of Estonia are promising EGS sites. Poland has relatively the most explored EGS potential, in both volcanic, crystalline and sedimentary rocks, especially in the area of the Szczecin Trough, Gorzów Block, Moglino-Łódź Trough and Karkonosze Mountains. Unfortunately, local tectonic conditions, in particular the development of faults and natural fracture zones that affect the directions of fracture propagation during hydraulic fracturing, have not been sufficiently recognized, which is one of the main barriers to the expansion of EGS pilot projects in these countries. These issues present challenges for the researchers, especially in terms of petrophysical analyses of rocks in target zones and local stress conditions, which have a key impact on fracturing operations and profitability of the systems. Despite high investment costs on the one hand and a significant slowdown in the global economy in 2022 on the other, it remains hopeful that the authorities of individual countries will decide to accelerate research work, leading to the implementation of pilot projects of EGS installations, and that this technology will be further improved to ensure a stable clean energy supply.

Hydraulic Stimulation of Geothermal Reservoirs: Numerical Simulation of Induced Seismicity and Thermal Decline

Enhanced Geothermal Systems (EGS) can boost sustainable development by providing a green energy supply, although they usually require the hydraulic stimulation of the reservoir to increase fluid flow and energy efficiency due to the low rock permeability at the required depths. The injection of fluids for hydraulic stimulation implies several risks, for instance, induced seismicity. In this work, we perform numerical simulations to evaluate the seismic risk in terms of fault reactivation, earthquake magnitude, and rupture propagation. The computational model includes the fully coupled thermo-hydro-mechanical equations and simulates faults as frictional contacts governed by rate-and-state friction laws. We apply our methodology to the Basel EGS project as a continuation of our previous work, employing the same parameters and conditions. Our results demonstrate that permeability stimulation is not only related to induced seismicity but also can induce a thermal decline of the reservoir over the years and during the energy production. The proposed methodology can be a useful tool to simulate induced earthquakes and the long-term operation of EGS.

Hindcasting injection-induced aseismic slip and microseismicity at the Cooper Basin Enhanced Geothermal Systems Project

There is a growing recognition that subsurface fluid injection can produce not only earthquakes, but also aseismic slip on faults. A major challenge in understanding interactions between injection-related aseismic and seismic slip on faults is identifying aseismic slip on the field scale, given that most monitored fields are only equipped with seismic arrays. We present a modeling workflow for evaluating the possibility of aseismic slip, given observational constraints on the spatial-temporal distribution of microseismicity, injection rate, and wellhead pressure. Our numerical model simultaneously simulates discrete off-fault microseismic events and aseismic slip on a main fault during fluid injection. We apply the workflow to the 2012 Enhanced Geothermal System injection episode at Cooper Basin, Australia, which aimed to stimulate a water-saturated granitic reservoir containing a highly permeable (k = 10^{-13} - 10^{-12}hbox {m}{^2}) fault zone. We find that aseismic slip likely contributed to half of the total moment release. In addition, fault weakening from pore pressure changes, not elastic stress transfer from aseismic slip, induces the majority of observed microseismic events, given the inferred stress state. We derive a theoretical model to better estimate the time-dependent spatial extent of seismicity triggered by increases in pore pressure. To our knowledge, this is the first time injection-induced aseismic slip in a granitic reservoir has been inferred, suggesting that aseismic slip could be widespread across a range of lithologies.

Beneath your feet and in your place: Multi-scalar imaginaries of energy, place, and local geothermal development

Research on responses to renewable energy has investigated both sociotechnical imaginaries and place imaginaries; however, interactions between competing visions of energy and place have received limited attention. This study investigates how multi-scalar imaginaries of energy and place interact within local context to shape negotiations of acceptability. In a case study of a campus Enhanced Geothermal Systems (EGS) project, document analysis of media coverage, PR materials, and policy documents are used 1. To identify imaginaries of place and energy present in public discourse across scales; and 2. To determine how multi-scalar imaginaries interact, align, and compete. Rhetoric links the EGS project with a national narrative of geothermal energy as limitless in potential, close at hand, and capable of ensuring national energy security. This geothermal imaginary is reinterpreted at different scales: regional decarbonization of heating in the Northeast, and campus climate leadership through scientific discovery. However, a local counter-narrative draws on place identity as a progressive college town and on local history with energy and extraction. A local vision of an urgent transition using a mix of tested technologies conflicts with the priorities of scientific exploration and intellectual leadership in the campus vision, which motivated the choice of a less mature technology. Multi-scalar imaginaries are embedded throughout public discourse on the EGS project, with misalignment between scales driving local perspectives toward the technology.

Transient evolution of the relative size distribution of earthquakes as a risk indicator for induced seismicity

Induced earthquakes pose a substantial challenge to many geo-energy applications, and in particular to Enhanced Geothermal Systems. We demonstrate that the key factor controlling the seismic hazard is the relative size distribution of earthquakes, the b-value, because it is closely coupled to the stress conditions in the underground. By comparing high resolution observations from an Enhanced Geothermal System project in Basel with a loosely coupled hydro-mechanical-stochastic model, we establish a highly systematic behaviour of the b-value and resulting hazard through the injection cycle. This time evolution is controlled not only by the specific site conditions and the proximity of nearby faults but also by the injection strategy followed. Our results open up new approaches to assess and mitigate seismic hazard and risk through careful site selection and adequate injection strategy, coupled to real-time monitoring and modelling during reservoir stimulation.

A regenerative Enhanced Geothermal System for heat and electricity production as well as energy storage

Enhanced Geothermal Systems (EGSs) evolved from hot dry rock (HDR) can play a crucial role in fulfilling the energy demands while boosting the transition toward carbon neutrality. This study proposed an innovative conception of EGS to integrate heat and electricity production and the storage of surplus renewable energy. Employing engineering data from the GeneSys project, Germany, a geothermal facility is numerically simulated by adopting two horizontal wells. In order to investigate the evolution of fluid and heat transmissivity during fracturing, production, and storage procedures through multiple hydraulic fractures while incorporating stress superposition effects, we performed coupled thermo-hydro-mechanical (THM) modeling with improved simulators, namely FLAC3Dplus and TOUGH2MP-TMVOC. By critically optimizing the thermal production, the electric potential caused by reservoir temperature decreases from 7.17 MW to 5.08 MW during 30 years, and the Levelized cost of electricity (LCOE) of proposed EGS is estimated at 5.46 c$/kWh; thus indicating remarkable development feasibility from an economic perspective. Finally, we have inspected several schemes for energy storage on the thermal-depleted reservoir to extend the life of geothermal power plant and make surplus energy useable efficiently. The results revealed that the EGS project could indeed be regenerative as formation temperature rises with energy storage/recovery cycles.

Heat mining from super-hot horizons of the Larderello geothermal field, Italy

Super-hot rock geothermal is an emerging source of renewable and carbon-free energy. This paper is the first attempt to explore fluid and heat flow dynamics in the reservoir-wellbore coupled system, to assess the power generation performance of a super-hot (>450 °C) enhanced geothermal system (EGS). We developed a high-performance code and built a 3-D wellbore-reservoir coupled model based on data from a recently completed deep-drilling project at Larderello, Italy. The general pattern of the super-hot EGS is characterized by a significant temperature plummet (>60 °C), after which the production fluid evolves from steam to a two-phase mixture till the end of the operation period. Reservoir pressure emerges as a key parameter to determine the temperature of the two-phase mixture. By realistically capturing phase transitions driven by coupled thermo-hydraulic processes during operations, our numerical model predicts a lower power generation efficiency compared to previous attempts based on ultra-simplified models. Although finalized at assessing the thermodynamic viability of a specific system, this modeling approach provides general information on fundamental thermo-hydraulic processes in the Earth crust that might be applied for the design of similar EGS projects elsewhere.

An environmental life cycle cost assessment of the costs of deep enhanced geothermal systems – The case studies of Reykjanes, Iceland and Vendenheim, France

Environmental life cycle costing (ELCC) is a tool which aggregates five categories of monetary costs across a project's life cycle: investment, operation, maintenance, end-of-life, and externalities. This paper summarises the results from the first two ELCC studies involving deep enhanced geothermal systems (EGS). The ReCiPe method was used to transform life cycle impact factors into economic costs for externalities. The two case studies are the pilot EGS project in Reykjanes, Iceland and the Vendenheim co-generation plant in France. The ELCC of the Reykjanes project is estimated in the range 14.47–15.78 million euros, with investment and well drilling projected to constitute 83% of these amounts. An ELCC in the range 91.90–113.97 million euros is estimated for Vendenheim, with the production plant, well drilling, and operations and maintenance costs accounting for the majority. The levelized costs of energy associated with Vendenheim (mean Є45.0/MWh/year) and Reykjanes (mean Є16.5/MWh/year) are at the lower end of the range normally reported for geothermal power projects. Although the case studies cannot be directly compared since Reykjanes involves the drilling of a single well and Vendenheim a co-generation plant and two wells, the outcomes suggest that deep EGS projects may involve cost-savings compared to conventional geothermal power ventures.

An extended methodology for multi-criteria decision-making process focused on enhanced geothermal systems

When considering geothermal project development, the complex assignment of aggregating and quantifying the influencing factors and their interactions is an inevitable task that enables comprehensive assessment of the geothermal energy utilization from many different aspects. The main purpose of the scientific work is a revised set of criteria for comprehensive evaluation of geothermal project focusing on Enhanced Geothermal Systems (EGS) and considering the geological settings, technology, economic and financial aspects of project development, and societal and environmental parameters. The newly presented work is conceived as an addition to the previous work, defining and thoroughly describing twenty-eight influencing factors for evaluation of deep geothermal energy. The mentioned factors consist of 18 newly introduced criteria, 6 replaced criteria, and 4 modified criteria which are based on the extensive literature review and expertise integrating all aspects of EGS project development. One of the key findings of the presented methodology is that it provides broad assessment of an EGS project, applicable to different end-uses including only electricity generation, only heat production and combined heat and power production. The proposed methodology could serve as a preparatory guideline for more detailed analysis and calculations for a future project development, either greenfield or brownfield, encompassing technological, geological, economic, and environmental aspects. The methodology was tested in case study where two different geothermal sites were assessed for heat production and electricity production scenarios. Both sites, with their main geological parameters, resulted in production of electricity, from 263.51 GWh up to 927.69 GWh, and heat production from 334.25 GWh up to 5,207.68 GWh. Such increase of renewable energy production from geothermal source derived the avoidance of CO2 emissions ranging from 79,686 tonnes of CO2-eq up to 1,241,542 tonnes of CO2-eq. The final grades of each criterion for the heat and electricity production scenarios resulted in favor of Site 2, where better reservoir and economic performance values derived the higher final grade of site’s overall performance. The summarized results lead to a conclusion that the proposed methodology is applicable for assessment of different end-usage options for one geothermal site and for comparison of projects at different geothermal sites.

A discrete fracture hybrid model for forecasting diffusion-induced seismicity and power generation in enhanced geothermal systems

SUMMARY We describe a 3-D discrete fracture hybrid model (DFHM) that returns forecasts of both induced seismicity and of power generation in an Enhanced Geothermal System (EGS). Our model considers pore-pressure increase as the mechanism driving induced seismicity, similarly to other hybrid models, but it uses discrete fracture modelling for flow and heat that allows accurate and realistic transient solutions of pore pressure and temperature in fractured reservoirs. Earthquakes and flow are thus considered as closely coupled processes. In the DFHM model, the creation phase of an EGS is described as a Markovian process with a transitional probability that encapsulates the irreducible uncertainty with regards to induced seismicity. We conditioned this transitional probability on field observations from the 2006 EGS project in Basel, achieving a good match with observations of seismicity evolution. Specifically, our model effectively reproduces and explains the observed long-term exponential decay of seismicity after the well was shut in, suggesting that pore pressure diffusion in a critically stressed fractured reservoir is sufficient to explain long-lasting post-injection seismic activity as observed in Basel. We then investigate alternative injection scenarios, using Monte Carlo simulations to capture the uncertainties in fault locations and stressing conditions. We show that the number of induced events depends not only on the total injected volume but also on the injection strategy. We demonstrate that multistage injection schemes are superior to single-stage ones, since the former are associated with less seismic risk and can generate at least the same revenue in the long term.