Enhanced Geothermal Systems Research Articles

Risk assessment of earthquakes induced during HDR development: A case study in the Gonghe Basin, Qinghai Province, China

Earthquakes induced during hot dry rock (HDR) geothermal development have attracted extensive attention and constrained the further exploitation of HDR resources. Seismic risk evaluation of HDR projects is essential. The first HDR demonstration project in China is located in the Gonghe Basin in Qinghai. The EGS (enhanced geothermal systems) site is located near Gonghe County and is very seismically sensitive. The seismic risk induced by hydraulic fracturing and long-term cycling during the HDR development process was evaluated using a multi-field coupling method based on analysis of the geothermal geological conditions, ground stress, and natural fracture characteristics. During hydraulic fracturing, the predicted magnitude of earthquake events is generally below 2.0, with a range of 0.7–1.3. During a 30-year development period, the predicted maximum earthquake magnitude is less than 1.28. The HDR development will not have an impact on the local geological safety. Considering the uncertainty of the geology, a further discussion of measures to reduce the risk of earthquakes is presented. An entire induced seismic risk evaluation process method is developed, which can provide a reference for related research.

Numerical Simulation of Enhanced Geothermal System With Complex Fracture Networks

Abstract Geothermal resource is one of the most promising renewable energies, which is successfully accessed by the enhanced geothermal system (EGS) characterized by a complex fracture network. Therefore, complex fracture networks with hydraulic and natural fractures of different fracture orientations and intensities are established in this study. The natural fractures are randomly distributed in the geothermal reservoir. The working fluid flowing and heat exchange in the geothermal reservoir are simulated by coupling model, considering the effects of temperature variation on the density, viscosity, thermal conductivity and specific heat capacity of working fluid, and the permeability of reservoir matrix. Effects of natural fracture orientation and intensity, position of injection and production wells, and geothermal reservoir permeability on the heat production behavior are analyzed. The simulation reveals that the low-temperature region in the geothermal reservoir could breakthrough along natural fractures. When the direction of injection and production wells is consistent with the orientation range of natural fractures, the reservoir exploitation ratio and the heat extraction rate are the largest, but the temperature stability time of the produced fluid is the shortest and the cooling rate is the fastest. Increasing the intensity of natural fractures is helpful to increase the heat extraction rate, but the temperature of production wells decreases more easily. Increasing the distance perpendicular to the hydraulic fracture can better improve the performance of production wells. High reservoir permeability is not conducive to the stability of production fluid temperature, but can greatly increase the exploitation ratio of the geothermal reservoir.

Effect of initial water saturation on the performance of fracturing fluids with and without polyallylamine under simulated EGS conditions

Objectives/scopeStimuFrac (US Patents 9,873,828 B2 and 9,447,315 B2), a CO2-reactive polymer aqueous solution [polyallylamine (PAA) 1wt% in water] combined with CO2, can potentially be used as a less water-intensive fracturing fluid for enhanced geothermal systems (EGS). Our previous results show that in hot dry rock (HDR), PAA/CO2 fracturing fluids outperformed other fluids such as water, CO2, and CO2/water in generating large fractures with less fluid consumed. The objective of this work is to investigate the effect of initial water saturation on the performance of StimuFrac by conducting hydraulic fracturing tests with ½ foot cubic rock samples held under representative EGS stress/temperature conditions and by using cyclic injection strategies (under constant injection rate). The resulting fracture hydraulic conductivities, breakdown pressures, and volumes of fluids required are compared. Methods/procedures/processTo simulate geothermal reservoir conditions, in all tests, the rock sample was held under triaxial confinement and at 200 °C, and different volumes of water were initially injected into the rock sample before any fracturing processes were initiated. For the single-cycle PAA (or water) alternating CO2 (PAG or WAG) injection fracturing experiments, one complete cycle consisted of two steps: (1) injecting a PAA slug (or water slug) followed by (2) injecting CO2 to initiate and propagate the fracture. For experiments involving multiple injection cycles, the CO2 injection pressure is increased until it peaks and begins to decline (indicating fracture initiation at this moment), and then continued being injected for another 30 s to propagate the fracture. Then, these two-step cycles [injection of PAA (or water) followed by CO2 injection (up to 2–4 mL/min)] are repeated. Applications/significance/noveltyThe results of this study suggest that water saturation significantly affects the fracturing fluid transmission into the rock pore space, thus affecting fracture initiation and propagation. In this study, fracturing tests via a single injection cycle or multiple injection cycles were performed. When the rock samples are split in half following testing, it is evident that fracture propagation is considerably restricted under high water saturation conditions (where a three-day initial water injection was conducted) in comparison to stimulation experiments carried out in hot and dry rock. The fractures propagate less than 1/3 of the distance from the wellbore to the outer rock surface, and in some cases, no fracture is generated. This may be caused by leak-off dominating the fracturing process and the fluid injection rate is insufficient to overcome leak-off, even under high injection rate conditions. Additionally, CO2 could be leaking off into the wellbore annulus and this may be making it more difficult to generate sufficiently high-pressure gradients away from the near-wellbore region. Under low water saturation conditions (dry rock or after 1-day initial water injection), PAA/CO2 consistently generated significantly larger fractures compared with the other fluids. CO2 generated large fractures only in the hot dry rock and only when using high injection rates, though data variability is high.

Poroelastic stress relaxation, slip stress transfer and friction weakening controlled post-injection seismicity at the Basel Enhanced Geothermal System

Induced seismicity is a limiting factor for the development of Enhanced Geothermal Systems (EGS). Its causal mechanisms are not fully understood, especially those of post-injection seismicity. To better understand the mechanisms that induced seismicity in the controversial case of the Basel EGS (Switzerland), we perform coupled hydro-mechanical simulation of the plastic response of a discrete pre-existing fault network built on the basis of the monitored seismicity. Simulation results show that the faults located in the vicinity of the injection well fail during injection mainly triggered by pore pressure buildup. Poroelastic stressing, which may be stabilizing or destabilizing depending on the fault orientation, reaches further than pressure diffusion, having a greater effect on distant faults. After injection stops, poroelastic stress relaxation leads to the immediate rupture of previously stabilized faults. Shear-slip stress transfer, which also contributes to post-injection reactivation of distant faults, is enhanced in faults with slip-induced friction weakening.

Lessons learned from hydrothermal to hot dry rock exploration and production

In this paper, we investigate geothermal exploration and production in 189 hydrothermal projects and 42 hot dry rock projects around the world. The hydrothermal fields for a working hydrothermal system to generate electricity should have the elements of heat source, water-saturated porous or fractured reservoir, caprock, heat transfer pathway, and good heat preservation condition and geothermal power energy intensity of 10–20 MW per km2 within at least 5 km2 area in tectonically active region. The hot water or steam flow rate in this hydrothermal system is normally larger than 40 L/s with temperature of 150 °C or above. The power generated from enhanced geothermal system (EGS) in hot dry rock projects are generally less than 2 MW because the flow rate in most cases is much less than 40 L/s even with the hydraulic fractures using the modern stimulation technology learned from the oil and gas industry. The natural fracture in the subsurface is generally beneficial to the hydraulic fracturing and heat recovery in the hot dry rock. Moreover, the hydraulic fracture parameters, injection rate and well spacing, drilling strategy should be properly designed to avoid the short-circuit between injector and producer and low heat productivity. In the future, CO2 enhanced geothermal recovery associated with CO2 sequestration in the high temperature oil, gas, and geothermal fields maybe a good choice. On the other hand, both near-real-time seismic monitoring to limit the pumping rate and the closed-loop of the Eavor-Loop style system without hydraulic fracture can contribute greatly to heat recovery of hot dry rocks and mitigate the risks of the hydraulic fracturing induced earthquake. Furthermore, the hybrid solar and geothermal system performs better than the stand-alone geothermal system.

Effect of temperature-dependent rock thermal conductivity and specific heat capacity on heat recovery in an enhanced geothermal system

The modeling of heat recovery from an enhanced geothermal system (EGS) requires rock thermal parameters as inputs such as thermal conductivity and specific heat capacity. These parameters may encounter significant variations due to the reduction of rock temperature during heat recovery. In the present study, we investigate the effect of temperature-dependent thermal conductivity and specific heat capacity on the thermal performance of EGS reservoirs. Equations describing the relationships between thermal conductivity/specific heat capacity and temperature from previous experimental studies were incorporated in a field-scale single-fracture EGS model. The modeling results indicate that the increase of thermal conductivity caused by temperature reduction accelerates thermal conduction from rock formations to fracture fluid, and thus improves thermal performance. The decrease of specific heat capacity due to temperature reduction, on the contrary, impairs the thermal performance but the impact is smaller than that of the increase of thermal conductivity. Due to the opposite effects of thermal conductivity increase and specific heat capacity decrease, the overall effect of temperature-dependent thermal parameters is relatively small. Assuming constant thermal parameters measured at room temperature appears to be able to provide acceptable predictions of EGS thermal performance.

Mapping Buried Fault Zones in a Granitic Pluton Using Aeromagnetic Data

Globally, the largest geothermal resources are linked to the crystalline basement, which could be exploited using Enhanced Geothermal Systems (EGS). The planned underground laboratory GeoLaB aims to facilitate the large-scale application of EGS technologies by performing in-situ high-flowrate experiments on fault zones. The Tromm Granite at the northwestern margin of the Upper Rhine Graben was preselected as one of two potential sites for GeoLaB. To improve the understanding of the natural fault network in the target area, a drone-based aeromagnetic survey was conducted in March 2022, covering an area of about 13.3 km2 in the center of the pluton. A selection of eight filters to enhance the signal was tested against each other using a synthetic model. Based on this comparison, the tilt derivative appears to be the most reliable method for mapping and characterizing natural fault zones. The filtered aeromagnetic dataset reveals a network of linear anomalies interpreted as altered fault zones. The relative abundance of magnetic lineaments correlates broadly with slip tendency in the current stress field. Thus, the aeromagnetic may provide insights into the reactivation potential and deformation history of fault zones, which significantly influence the hydraulic properties. However, additional geophysical surveys or exploration wells are needed to validate the results.

Comparative study on heat extraction performance of three enhanced geothermal systems

Enhanced geothermal systems (EGSs) in this study are classified as fracturing-EGS (F-EGS), pipe-EGS (P-EGS) and excavation-EGS (E-EGS) according to reservoir stimulation strategies. However, the heat extraction performances of three EGSs employing different stimulation strategies are not fully understood. Here, we define the region where the pore pressure increment calculated by a hydraulic fracturing process is higher than rock tensile strength as the stimulation region for establishing a more accurate F-EGS model, and then compare three geothermal systems to select a appropriate reservoir stimulation strategy. We find that the F-EGS model assuming an entire stimulated region significantly exaggerates the heat extraction results. The optimal conditions for P-EGS are low injection rates and short operation times, which is suiTablefor seasonal heating or multi-energy co-generation projects including a thermal recovery phase. Theoretically, E-EGS has better geothermal extraction performance than F-EGS based on existing model assumptions, but its construction feasibility and economics need further exploration. H2O is more suiTableas a heat exchange fluid in E-EGS than supercritical CO2. This study provides a reference for geothermal mining simulation and reservoir stimulation strategy selection.

In-situ flow visualization with Geo-Positron-Emission-Tomography in a granite fracture from Soultz-sous-Forêts, France

We investigate the fluid flow field in a fractured granite core sample. Sequential imaging with Positron-Emission-Tomography (PET) allows direct reconstruction of flow streamlines, thus providing a unique insight into the fluid dynamics of complex fractured crystalline materials. Pulse migration experiments using the positron-emitting radionuclide 18F− as tracer were conducted on a fractured granitic drill core, originating from a depth of 1958 m of the Enhanced Geothermal System (EGS) reference site at Soultz-sous-Forêts, France. The flow field was analyzed as a function of in- and outlet positions across the fracture, as well as applied flow rates. Different flow path characteristics were identified. Both the fracture aperture variation and the topography of the fracture surface affect the flow field with consequences on flow channeling and preferential flow paths. Furthermore, pulse migration experiments were also numerically simulated with a 2.5D model using COMSOL Multiphysics®.While the higher flow rate experiments show wider and higher dispersion of the flow path, lower velocity results in more localized flow and channeling behavior. This type of study thus yields enhanced experimental insights into the hydrodynamics of fracture flow and its relation to the rough structure of a single fracture, compared to input-output experiments. It can help to validate model simulations and experimentally determine hydrodynamic parameters needed for reactive transport modeling that are otherwise estimated with a high degree of uncertainty.

Research status and development trend of key technologies for enhanced geothermal systems

Enhanced geothermal system (EGS) is a primary method to develop geothermal resources stored in hot dry rock (HDR), but it faces several key problems, such as unreasonable hydraulic fracture networks at high reservoir temperature, unclear multi-scale and multi-field coupling regularity, low heat extraction efficiency caused by the flashing flow in geothermal wells, and low thermoelectric conversion efficiency of geothermal fluid, which restricts the large-scale commercial development of geothermal resources. To resolve these major bottleneck problems, systematically reviews and analysis of the research progress and development trend of EGS are conducted in this paper. Particular attentions are devoted to four key technologies involved in the development of HDR geothermal resource by EGS: (1) the hydraulic fracturing technology for HDR reservoirs, including reservoir reconstruction methods, hydraulic fracturing network forming mechanisms and fracture propagation prediction models is illustrated in detail; (2) the fracture characterization methods, mathematical models and solution methods are described from three aspects including pore-scale multi-field coupled models, reservoir-scale multi-field coupled models and upscaling methods; (3) the efficient extraction technology of wellbore thermal fluid, involving the mechanism of flashing flows in geothermal wells and the experimental and numerical methods for investigating the characteristics offlashing flows are discussed; (4) the HDR geothermal power generation technologies, considering the principles of geothermal power generation, the types of power generation systems and the main application markets are introduced. In conclusion, EGS is a technology-intensive system, however, due to the complex working conditions of the underground reservoirs and the instability of the ground equipment, theoretical research tends to be separated from the practice. For the purpose of promoting the applicability of EGS, intimate combination and mutual guidance with the pilot tests are necessary to develop a production-research combined mode, and to raise the awareness and break through the key points in a constant back-and-forth.

Numerical study of hydraulic fracturing treatments and geothermal energy extraction from a naturally fractured granitic formation

Enhanced geothermal system (EGS) development is an extremely complex process involving reservoir creation and heat extraction. At present, hydraulic fracturing is still the most widely used method to create geothermal reservoirs. We utilize field-measured data from the Gonghe Basin (Northwest China) geothermal field, including drilling data, temperature logging data, and data from lab tests, to perform sensitivity analysis using the 3D fracturing simulator MShale and evaluate different fracturing treatments of a 3.7 km deep granitic formation. The simulation results show that the created reservoir geometry highly depends on the mechanical rock properties, in-situ stress states, and natural fracture systems. The optimum reservoir performances are achieved by analyzing the geological system and performing adequate planning, including a sensitivity analysis of the reservoir rock parameters, natural fracture development status, and in-situ stress states. Wherein the natural fracture development level determines the hydraulic stimulation technique. For the granitic formation in the Gonghe Basin, we found that water fracturing provides the best performance for the formation with highly developed natural fractures, whereas gel-proppant fracturing is more suitable for tight and intact formations. The fracturing treatment affects the production performance of EGSs. The power generation capacity of the formation with highly developed natural fractures treated with water fracturing is 1.61–3.94 MW, whereas that of the formation with few or without natural fractures treated with gel-proppant fracturing is 0.84–2.71 MW.

Audio Magnetotelluric and Gravity Investigation of the High-Heat-Generating Granites of Midyan Terrane, Northwest Saudi Arabia

The Midyan Terrane (northwest Saudi Arabia) is characterized by the presence of a massive belt of radioactive granitic rocks and thick sedimentary cover near the coastal areas. The area is greatly influenced by the tectonic activities of the Red Sea and Gulf of Aqaba, implying its high potentiality of geothermal energy. In the present work, geophysical surveys, including audio magnetotelluric and gravity methods, were integrated to investigate the subsurface structural pattern of the study area, which identified regional deep and shallow fault systems and detected the subsurface geometry/extension of the granitic rocks as well as detecting the thickness of the sedimentary basins near the coastal area. A total number of 80 audio magnetotelluric and 246 gravity stations were recorded, analyzed, and interpreted. Two high-potential geothermal targets were indicated: high-heat-generating granites and thick anomalous sedimentary basins near the coastal areas. High-heat-generating granites are significant in terms of enhanced geothermal systems (EGSs) whereas sedimentary basins play a crucial role in the formation of conventional geothermal systems. Both areas require more exploration plans to evaluate the energy potential of geothermal reservoirs. The results also contribute to the identification of the subsurface orientation and geometry of radioactive granites, providing the necessary parameters to enhance a volumetric estimation for geothermal reserves.

Investigation on the effect of tree-like bifurcation fracture networks properties on the EGS thermal recovery performance

In order to truly describe the fracture network of the enhanced geothermal system (EGS), the tree-like fractal bifurcation theory is used to reflect the heterogeneity of the fracture network. The temperature-seepage coupling model is set up, the influences of bifurcation fracture networks and well layouts on thermal recovery performance are analyzed. The results show that the increase of bifurcation level, angle and length ratio all have positive effect on the thermal recovery performance. A higher bifurcation level produces a greater number of secondary fractures, a higher bifurcation angle and length ratio leads to a relatively spread fracture distribution, thereby improving thermal recovery performance. A higher aperture ratio is prone to premature thermal breakthrough, which limits the thermal recovery process. Under the conditions and scopes of our study, the optimal bifurcation level is 5, angle is 120°, length ratio is 0.55 and aperture ratio 0.4. This study provides some good guidance of efficient fracturing design for better thermal recovery performance.

Laboratory study of fracture initiation and propagation in Barre granite under fluid pressure at stress state close to failure

Hydraulic fracturing is frequently used to increase the permeability of rock formations in energy extraction scenarios such as unconventional oil and gas extraction and enhanced geothermal systems (EGSs). The present study addresses uncertainties in the hydraulic fracturing process pertaining to EGSs in crystalline rock such as granite. Specifically, there is debate in the literature on the mechanisms (i.e. tensile and/or shear) by which these fractures initiate, propagate, and coalesce. We present experiments on Barre granite with pre-cut flaws where the material is loaded to high far-field stresses close to shear failure, and then the fluid pressure in the flaws is increased to move the Mohr's circle to the left and observe the initiation and propagation of fractures using high-speed imaging and acoustic emissions (AEs). We find that the hydraulic fractures initiate as tensile microcracks at the flaw tips, and then propagate as a combination of tensile and shear microcracks. AE focal mechanisms also show elevated levels of tensional microfracturing near the flaw tips during pressurization and final failure. We then consider a numerical model of the experimental setup, where we find that fractures are indeed likely to initiate at flaw tips in tension even at relatively high far-field stresses of 40 MPa where shear failure is generally expected.

Effect of water-cooling shock on fracture initiation and morphology of high-temperature granite: Application of hydraulic fracturing to enhanced geothermal systems

Deep geothermal energy exploitation is primarily accomplished using an enhanced geothermal system (EGS) to extract heat from hot dry rock (HDR). The use of hydraulic fracturing technology to build large-scale fracture networks is essential for improving the EGS heat extraction efficiency. Water-cooling shock on high-temperature rock degrades the mechanical properties of the rock while increasing the fracturing effect of HDR. A thermal–hydraulic–mechanical coupling model is proposed herein to investigate the effect of water-cooling shock on fracture initiation and morphology in HDR hydraulic fracturing. The validity of the coupling model is confirmed using theoretical analytical solutions and physical experimental results, such as hydraulic fracturing experiments on high-temperature granite, as well as analytical solutions for the stress shadow effect and fracture initiation pressure. The numerical simulation is performed using a coupling model to investigate the characteristics of fracture propagation and damage evolution during the HDR hydraulic fracturing process as well as to investigate the fracturing effect of water-cooling shock on high-temperature rock. Finally, the effects of initial rock temperature, in-situ stress, and heat transfer coefficient on fracture morphology are investigated. The results show that the combined effects of tensile strength deterioration water-cooling shock, thermal stress caused by cold shrinkage, and pore water pressure are primarily responsible for the initiation and propagation of hydraulic fractures. The water-cooling shock can considerably reduce the fracture initiation pressure, induce more secondary fractures, and form more complex fracture networks. The initial temperature and heat transfer coefficient of the rock affect the thermal stress and tensile strength deterioration. The formation of an intricate fracture network can be aided by a high initial rock temperature, a large surface heat transfer coefficient, and small in-situ stress differences.

Single‐Well Pore Pressure Preconditioning for Enhanced Geothermal System Stimulation

AbstractThe stress state is an important parameter in terms of both earthquake nucleation and rupture. Here, a new stimulation technique is proposed for Enhanced Geothermal Systems (EGSs), which have previously been burdened with a number of high‐profile incidences of induced seismicity. This stimulation technique pre‐emptively alters, or preconditions, the stress state before injection. This preconditioning is achieved through fluid production, such that the zone of reduced pore pressure around the well results in a stress‐ and fracture‐energy‐ barrier to potential nucleating and/or propagating ruptures near the point of injection. Using an existing 1‐D linear slip‐weakening model, it is shown how this methodology has the potential to either suppress the nucleation of dynamic events or halt their propagation. In particular, reducing the pore pressure around the region to be stimulated such that the residual shear stress rises above the in‐situ value of shear stress results in ultimately stable nucleation regimes. These results hold for injection times which are small compared to the required time of production, but this methodology results in nucleation lengths, or, analogously, stimulated areas, which are orders of magnitudes larger than those safely achievable without preconditioning. An example of how this approach may be applied in the field as well as other possible methods to achieve a preconditioned reservoir are provided. Both laboratory‐scale and meso‐scale testing of preconditioning are recommended to further constrain the applicability of this methodology for the creation of EGSs.

Frictional stability and permeability evolution of fractures subjected to repeated cycles of heating-and-quenching: granites from the Gonghe Basin, northwest China

Fluid injection into enhanced geothermal system (EGS) reservoirs can reactivate subsurface fractures/faults and trigger earthquakes—requiring that frictional stability and permeability evolution characteristics are adequately evaluated. This behavior potentially becomes more complicated when the impacts of temperature and cycled thermal stresses, and the resulted damage accumulation on both stability and transport characteristics are getting involved. We conducted coupled shear-flow experiments on saw-cut fractures recovered from an analog surface outcrop representative of a reservoir at 2450 m in the Gonghe Basin of northwestern China. The rocks were subjected to variable numbers of repeated heating-quenching (25-180-25 °C) cycles for shear-flow experiments at an effective stress of ~ 3 MPa and with velocity stepped between 10-1-10-1-10 μm/s. The smooth fractures return frictional coefficients in the range ~ 0.69 to 0.72 and are little affected by the thermal cycling. The frictional stability parameter (a–b) decreases and the instantaneous permeability increases with an increase in the number of heating-quenching cycles, during which intergranular and intragranular microcracks were generated in fracture surface. The above results indicate that the heating-quenching cycles during hydraulic fracturing of geothermal reservoir could affect both the fracture frictional instability and permeability evolution.

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.

Hydraulic Stimulation of Enhanced Geothermal System: A Case Study at Patuha Geothermal Field, Indonesia

This paper discusses a case study on the enhanced geothermal system (EGS), which entails a hydraulic stimulation at Patuha geothermal field, West Java, Indonesia. Patuha field is a volcano-hosted vapor-dominated geothermal system. A hydraulic stimulation was designed with 6,360 m3 (=40,000 barrels) of water injection to evaluate the enhancement in steam production. The water injection was conducted in three phases: a step-up period, a plateau period with a constant rate, and a step-down period, which raised the bottom hole pressure effectively and positively influenced formation. In addition, in EGS, rapid restorations of pressure and temperature to their pretest values near the bottom hole were observed, unlike in a typical nonvolcanic system. Hall plots, i.e., the Hall integral and Hall derivative, confirmed that the geothermal reservoir was stimulated, and thus, the hydraulic stimulation increased the steam production up to 11.4% at other production wells that were located near the cold-water injector.

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.