Deep Geothermal Reservoirs Research Articles

Impact of a regional fault zone on the properties of a deep geothermal carbonate reservoir unit (Devonian of NRW)

Impact of a regional fault zone on the properties of a deep geothermal carbonate reservoir unit (Devonian of NRW)

Insights into geochemical characteristics of the geothermal fluids of northwestern algeria: An updated conceptual model

The vigorous tectonic activity that affected the Northwestern part of Algeria has generated a Mio-Plio-Quaternary volcanic activity and led to the emergence of several thermal resurgences. These latter consist of thermal springs, travertine deposits and some hydrothermal alteration zones. The chemistry of twenty-one hot spring water samples highlight the heterogeneity of the water mineralization process in NW Algeria. Four major types of water are described. Na+-Cl− and Na+-SO42− with a high salinity up to 3794 mg/L accompanied by higher concentrations of F−, B, Li+, Br− and Cs. These are probably derived from halite and gypsum rich evaporites from the surrounding environment. The other two types are Ca2+-HCO3− and Na+-HCO3−. Chloride rich waters are associated with local carbonate formations. The stable isotopes (δ18O and δ2 H) reveal a fossil system since a large depletion of both isotopes from present-time meteoric waters cannot only be plausibly explained in terms of "inland" and "altitude" effects, we may suppose their recharge under climatic conditions much colder than the present ones taking place probably during the glacial period. The gas mixing models of N2He*1000-Ar*100 with O2CO2He*1000 demonstrate an unarguable meteoric origin for the thermal waters, displaying air-saturated water (ASW) signatures.The collected thermal waters are ‘immature’ while the deep geothermal reservoir temperatures were estimated to be about at 133 °C. The updated geochemical conceptual model indicates two main processes in the flow path of the fossil meteoric water. 1) In the Bouhdjar geothermal zone, the infiltrated meteoric waters are conductively heated at depth because of the high heat flow (∼120 mW/m2), during their ascent, along hydrothermal conduits and towards the ground surface, they have possibly been influenced by heat advection from the underlying magma chambers and/or a percolation with intrusive bodies beneath spring vents. 2) The meteoric waters are recharged from high altitude areas of the Atlasic lands and are deeply circulated and heated at greater depths, hot waters probably mix with cooler Mg-rich waters and/or leach Mg2+ from the surrounding rocks when rising to the surface. Since the locations of the hot springs are close to populated areas, a direct use of this resource is feasible to supply some energy demands that include electricity generation using binary cycles or direct uses of heat.

3DHIP-Calculator—A New Tool to Stochastically Assess Deep Geothermal Potential Using the Heat-In-Place Method from Voxel-Based 3D Geological Models

The assessment of the deep geothermal potential is an essential task during the early phases of any geothermal project. The well-known “Heat-In-Place” volumetric method is the most widely used technique to estimate the available stored heat and the recoverable heat fraction of deep geothermal reservoirs at the regional scale. Different commercial and open-source software packages have been used to date to estimate these parameters. However, these tools are either not freely available, can only consider the entire reservoir volume or a specific part as a single-voxel model, or are restricted to certain geographical areas. The 3DHIP-Calculator tool presented in this contribution is an open-source software designed for the assessment of the deep geothermal potential at the regional scale using the volumetric method based on a stochastic approach. The tool estimates the Heat-In-Place and recoverable thermal energy using 3D geological and 3D thermal voxel models as input data. The 3DHIP-Calculator includes an easy-to-use graphical user interface (GUI) for visualizing and exporting the results to files for further postprocessing, including GIS-based map generation. The use and functionalities of the 3DHIP-Calculator are demonstrated through a case study of the Reus-Valls sedimentary basin (NE, Spain).

The use of supercritical CO2 in deep geothermal reservoirs as a working fluid: Insights from coupled THMC modeling

A coupled THMC (thermal-hydrological-mechanical-chemical) model is developed and applied to explore the potential feasibility of using scCO 2 (supercritical carbon dioxide) as a working fluid in geothermal reservoirs . This is achieved by examining the evolution of the kinetics of mineral precipitation-dissolution and its associated impact on the evolution of the rock permeability and porosity. The pH of the reservoir rapidly reduces from 7 to ∼4.5–5 due to the fast dissolution of calcite . Chemical reactions and mineral dissolution and precipitation near the injector are suppressed by the plug-flow penetration of anhydrous scCO 2 displacing the original pore fluid . A conceptual three-zone model is proposed to illustrate the kinetic process of feldspar dissolution and precipitation depending on timing. The initial high concentration of K + prompts feldspar to precipitate in the first stage by consuming K + until 1 y , Feldspar were dissolved into precipitations of illite , smectite , and siderite at 1-6 y , with albite , muscovite and kaolinite mostly precipitated in the last stage 6–10 y . The precipitations of secondary clay minerals and quartz serve to maintain the integrity of caprock sealing. Continuous scCO 2 injection under fully coupled THMC model shows a 1.4-times enhancement of fracture permeability and 1.2-times enhancement of matrix permeability dominated by chemical dissolution and thermal unloading process. The pronounced thermal drawdown is the principal factor in enhancing permeability and porosity near injection well. Furthermore, the expansive capability of CO 2 provides extra benefits in enhancing formation pressure to ensure consistent high flow rates, while achieving a higher thermal energy extraction efficiency and preventing scaling issues in wellbore . The mass concentration of scCO2 in the production well increased to 0.82 after 1.2 × 10 8 s also leads to the enhancement of fluid enthalpy up to 6.5 × 10 5 J/kg, due to the high heat capacity of scCO 2 . The injected CO 2 are sequestered at ∼2 × 10 7 kg at t = 2 × 10 8 s (6.34 y ) as the solubility trapping mechanism.

Barite Scaling Potential Modelled for Fractured-Porous Geothermal Reservoirs

Barite scalings are a common cause of permanent formation damage to deep geothermal reservoirs. Well injectivity can be impaired because the ooling of saline fluids reduces the solubility of barite, and the continuous re-injection of supersaturated fluids forces barite to precipitate in the host rock. Stimulated reservoirs in the Upper Rhine Graben often have multiple relevant flow paths in the porous matrix and fracture zones, sometimes spanning multiple stratigraphical units to achieve the economically necessary injectivity. While the influence of barite scaling on injectivity has been investigated for purely porous media, the role of fractures within reservoirs consisting of both fractured and porous sections is still not well understood. Here, we present hydro-chemical simulations of a dual-layer geothermal reservoir to study the long-term impact of barite scale formation on well injectivity. Our results show that, compared to purely porous reservoirs, fractured porous reservoirs have a significantly reduced scaling risk by up to 50%, depending on the flow rate ratio of fractures. Injectivity loss is doubled, however, if the amount of active fractures is increased by one order of magnitude, while the mean fracture aperture is decreased, provided the fractured aquifer dictates the injection rate. We conclude that fractured, and especially hydraulically stimulated, reservoirs are generally less affected by barite scaling and that large, but few, fractures are favourable. We present a scaling score for fractured-porous reservoirs, which is composed of easily derivable quantities such as the radial equilibrium length and precipitation potential. This score is suggested for use approximating the scaling potential and its impact on injectivity of a fractured-porous reservoir for geothermal exploitation.

Combining autoencoder neural network and Bayesian inversion to estimate heterogeneous permeability distributions in enhanced geothermal reservoir: Model development and verification

Determining permeability distributions in reservoirs is critical for the management of limited earth resources. While hydraulic fracturing is widely used to enhance the permeability of deep geothermal, gas and oil reservoirs, it remains challenging to infer heterogeneous distributions of permeability. Typically, a limited number of boreholes are available at which reservoir imaging and tracer testing can be conducted. The number of observations is often far fewer than the number of estimable permeability parameters, making model inversion ill-posed. To overcome this problem, the autoencoder neural network was combined with a Bayesian inversion algorithm based on Markov Chain Monte Carlo (MCMC) sampling, in order to estimate the spatial distributions of permeability in an enhanced geothermal reservoir, conditional to temperature and outflow rate observations from a single-well-injection-withdrawal test (SWIW). The autoencoder neural network was used to reduce parameter dimensionality by four orders of magnitude. MCMC sampling was used to estimate low-dimensional parameters via inversion of SWIW observations. A high-resolution permeability distribution was reconstructed from the low-dimensional parameterization through reapplication of the autoencoder neural network. Application to a synthetic enhanced geothermal system demonstrated that the methodology achieved rapid stabilization and low permeability estimation error (<10%). By combining deep-learning method with Bayesian inversion, permeability distributions in geo-energy reservoirs can be estimated from a limited set of borehole data.

Seismic characterization of a Triassic-Jurassic deep geothermal sandstone reservoir, onshore Denmark, using unsupervised machine learning techniques

The Triassic-Jurassic deep sandstone reservoirs in onshore Denmark are known geothermal targets that can be exploited for sustainable and green energy for the next several decades. The economic development of such resources requires accurate characterization of the sandstone reservoir properties, namely, volume of clay, porosity, and permeability. The classic approach to achieving such objectives has been to integrate well-log and prestack seismic data with geologic information to obtain facies and reservoir property predictions in a Bayesian framework. Using this prestack inversion approach, we can obtain superior spatial and temporal variations within the target formation. We then examined whether unsupervised facies classification in the target units can provide additional information. We evaluated several machine learning techniques and found that generative topographic mapping further subdivided intervals mapped by the Bayesian framework into additional subunits.

Geological Characteristics and Distribution of Granite Geothermal Reservoir in Southeast Coastal Areas in China

The southeast coastal areas in China have distributed lots of granite outcrops of different periods. Previous research has shown that granite geothermal reservoirs are also distributed under sedimentary basins in these areas, such as in Zhangzhou basin. Therefore, granites with fractures buried in deep can be used as a potential deep geothermal reservoir in these areas. In order to study geological conditions of the deep granite reservoir and discuss the genesis of the deep granite geothermal system, rock geochemistry and zircon U-Pb chronology from outcrop and parts of the drilling cores of granitic rocks have been analyzed, combined with the anatomy of the deep seismic data and electromagnetic detection data in selected area. Based on the results of geochemistry and zircon U-Pb chronology, most granites in this area are of Yanshanian periods. Based on the seismic data, the thickness of the overlying strata on granite in Huangshadong area of Huizhou City is up to 1.5 km. According to the regional geological survey, multi-stage joints are developed in the granite, and most of hot springs rise from intersection of fracture with different directions to the surface. The heat source in the study area mainly comes from the mantle carried up by the deep NNE-trending faults. There are a large number of thermal springs at the intersection of the surface and the NW-trending fault, and the NW-trending fault provides the drainage conditions for the upwelling of underground thermal springs. There is a huge amount of deep granite geothermal resources in the southeast coastal area. The analysis of deep granite geological conditions and genetic models can provide guidance for the evaluation of deep granite geothermal resources and the further optimization of favorable zones in these areas.

Utilizing geothermal energy from enhanced geothermal systems as a heat source for oil sands separation: A numerical evaluation

Burning natural gas to heat water for oil sands separation is and energy and carbon-intensive process. This study presents a COMSOL Multiphysics simulation model that comprehensively investigates the feasibility of utilizing geothermal energy from enhanced geothermal systems (EGSs) as a heat source for oil sands separation. The simulation model was built with discrete fractures and was verified with analytical results. The results demonstrate that a doublet EGS performs better than a triplet EGS for heat production, because a doublet system has more uniform flow distribution and longer flow paths. The heat production performance of an EGS can be improved by decreasing the injection temperature or creating more fractures and wider fracture apertures to increase the injection flow rate in a deeper geothermal reservoir. By adjusting the parameters of water, fractures, and the geothermal reservoir, the theoretical model shows that a doublet EGS can provide hot water (≥60 °C) for 30 years, thus saving 957.35 × 106 m3 of natural gas and reducing 1.96 × 106 tonnes of greenhouse gas emissions. This study provides a rational approach for evaluating potential EGS projects in future to provide hot water for oil sands separation.

Descriptive and quantitative analysis of fracture systems in a carbonate rock mass complex

The study area is located in the Franconian Alb in Southern Germany and is an Upper Jurassic, karstified and fractured, regularly bedded carbonate rock mass with thin marl interlayers. Structurally, these deposits are heterogeneous and a classification into structural types is striven for in this study. The structural classification is based on discontinuity and fault characteristics gained from field work. The study area is considered an analogue for Germanys most productive deep geothermal reservoir horizon in the South German Molasse Basin. The aim of the field study is to describe and quantitatively analyse the different structural types, and to optimize the scanline method for a specific geological setting to gain comprehensive information with regard to fracture permeability and rock mass behaviour. The structural types are described in terms of discontinuity orientation and density, rock strength, and fault characteristics. To optimize the scanline method for the field study additional discontinuity attributes were added. One of these accessory attributes, the existence of discontinuity lags, will be discussed. It occurs when vertical and sub-vertical fractures cross sedimentary layers and continue with a small horizontal displacement in the next sedimentary layer. The results of this study are used to improve derived fracture networks with respect to thermo-hydro-mechanical models in the geothermal sector. Following article discusses the first results of the above-mentioned field study.

Well placement optimization for large-scale geothermal energy exploitation considering nature hydro-thermal processes in the Gonghe Basin, China

Doublet well system composed by extraction and injection wells is widely used for heat production from deep geothermal reservoirs. The large-scale implementation of doublet well systems lead to hydro-thermal interaction with the natural hydro-thermal flow system; this affects heat production efficiency and sustainable management of geothermal energy and groundwater resources. However, the interaction between artificial operations and nature hydro-thermal status was seldom considered in the well placement optimization. In this study, the natural status of hydro-thermal flow was determined by fitting numerical modeling output to the groundwater level and temperature observations, in a realistic site of the Gonghe Basin, China. The well separation and relative position were then optimized against background water flow direction. It was found that a well separation of 1200 m with the injection well located upstream (south) to the extraction well can maximize the volume of injected water entering extraction well, without influence on the outflow temperature. Based on this finding, we further compared the effects of “extensive well placement” where the wells were located sparsely in entire geothermal field, and “intensive well placement” where the wells were located centrally in a local region. The calculation showed that the performance of intensive well placement was superior to extensive well placement with higher outflow temperature and lower interruption in groundwater levels. Further optimization of intensive well placement resulted in the outflow temperature higher than 92 °C under the total outflow rate of 30,000 m3/d for 30 years, and the groundwater level changes can be controlled below 6.0 m. The multiple-step well placement optimization approach and novel intensive heat production mode regarding nature status of hydro-thermal flow in geothermal reservoirs, provides important reference to the other geothermal field for large-scale geothermal exploitation.

Influential factors on the development of a low-enthalpy geothermal reservoir: A sensitivity study of a realistic field

A realistic deep low-enthalpy geothermal reservoir based on real data with high detail and complicated sedimentary structure is utilized to perform sensitivity analyses of the geological features influencing reservoir properties. We perform simulations using the Delft Advanced Research Terra Simulator (DARTS). Compelling numerical performance of DARTS makes it suitable for handling a large ensemble of models including efficient sensitivity and uncertainty analyses. The major finding is that shale facies, generally ignored in hydrocarbon reservoir simulations, can significantly extend the predictive lifetime of geothermal reservoirs exploited by deep well doublets. It is important to accurately account for the shale facies in the simulation, though with an additional computational overhead. The overburden layers can improve doublet performance, but the impact depends on reservoir heterogeneity. In addition, heterogeneity will also divert the flow path with even a minor shift in the well placement. The discharge rate, an essential parameter of geothermal operation strategy, inversely corresponds to the doublet lifetime but positively correlates with the energy production for studied parameter ranges. Low sensitivity of doublet lifetime to vertical-horizontal permeability ratio and permeability-porosity correlation is observed. All these systematic findings for a realistic geothermal field with characterization at unprecedented level of detail can help to provide a general guideline for forward simulation and farther improve the profitability of geothermal energy production in realistic deep geothermal reservoirs through computer-assisted modeling and optimization.

Estimation of the Deep Geothermal Reservoir Temperature of the Thermal Waters of the Active Continental Margin (Okhotsk Sea Coast, Far East of Asia)

Low-enthalpy thermal waters (30–70 °C) with nitrogen as a dominant associated gas are spread within the active continental margin of the Russian Far East (east and north of the Okhotsk Sea Coast) and traditionally are of great importance for recreation and balneology facilities. The thermal waters are chemically classified into three groups: (i) Na–HCO3(SO4) type, with low TDS (0.2 g/L) and lowest temperature (&lt;50 °C) and high pH (9.1–9.3), (ii) Na–SO4 type with TDS (~1 g/L), highest temperature (70 °C) and weak alkaline pH (8.7) and (iii) Ca–Na–Cl type with high TDS (15 g/L), moderate T (59 °C) and neutral pH (7.5). The δ18O and δD values suggest that the thermal waters originate from meteoric water, and they are not isotopically fractionated. Silica and cation geothermometers and thermodynamic equilibrium calculations using the GeoT and PHREEQC programs indicate a reservoir temperature for the Na–HCO3(SO4) type thermal waters of 103–121 °C and for Na–SO4 and Ca–Na–Cl types of 136 and 153 °C, respectively. The evaluation of the mixing degree of the thermal water with cold groundwater shows that the equilibration temperature ranges between 148 and 153 °C. Estimated circulation depths for thermal manifestations range from 2.7 to 4.3 km and may be as great as 6 km.

Three-dimensional resistivity structure in the hydrothermal system beneath Ganzi Basin, eastern margin of Tibetan Plateau

The Ganzi geothermal system located in the eastern margin of Tibetan Plateau, is found to distribute the strongly active surface manifestations, such as boiling springs, and fumaroles. The resistivity structure of high-temperature hydrothermal system beneath Ganzi Basin has been obtained from the three-dimensional (3-D) magnetotelluric inversion of 103 broadband MT stations in period range from 320 Hz to 1000s. The MT stations have an average spacing of 500 m and completely cover the Ganzi Basin. The detailed 3-D resistivity structure which the authors found has been described characteristics of the shallow and deep geothermal reservoirs as well as to correlate with the structural features present in the Ganzi Basin. The shallow geothermal reservoir is mainly composed of the sedimentary layer, with a depth of 300−800 m, while the deep geothermal reservoir mainly consists of the fault fracture zone with a depth of 1−5 km. The porosity of the geothermal reservoirs is in the range 5–10 %, estimated based on the resistivity model. The Ganzi fault provides the pathway for transport heat and the circulation of geothermal fluids. The possible heat source for the Ganzi geothermal system has been attributed to tectonic deformation heating in the crust, radiogenic heat production derived from the decay of U, Th and K, and heat from the mantle.

S-wave experiments for the exploration of a deep geothermal carbonate reservoir in the German Molasse Basin

There are many successful geothermal projects that exploit the Upper Jurassic aquifer at 2–3 km depth in the German Molasse Basin. However, up to now, only P-wave seismic exploration has been carried out. In an experiment in the Greater Munich area, we recorded S-waves that were generated by the conventional P-wave seismic survey, using 3C receivers. From this, we built a 3D volume of P- to S-converted (PS) waves using the asymptotic conversion point approach. By combining the P-volume and the resulting PS-seismic volume, we were able to derive the spatial distribution of the vp/vs ratio of both the Molasse overburden and the Upper Jurassic reservoir. We found that the vp/vs ratios for the Molasse units range from 2.0 to 2.3 with a median of 2.15, which is much higher than previously assumed. This raises the depth of hypocenters of induced earthquakes in surrounding geothermal wells. The vp/vs ratios found in the Upper Jurassic vary laterally between 1.5 and 2.2. Since no boreholes are available for verification, we test our results against an independently derived facies classification of the conventional 3D seismic volume and found it correlates well. Furthermore, we see that low vp/vs ratios correlate with high vp and vs velocities. We interpret the latter as dolomitized rocks, which are connected with enhanced permeability in the reservoir. We conclude that 3C registration of conventional P-wave surveys is worthwhile.

Relationships between P-wave velocity and mechanical properties of granite after exposure to different cyclic heating and water cooling treatments

The bedrocks of deep geothermal reservoirs are exposed to cyclic water cooling during the exploitation of deep geothermal energy. Therefore, it is important to understand the physico-mechanical parameters of geothermal reservoir rocks. This paper reports on the P-wave velocity (Vp), uniaxial compressive strength (UCS) and elastic modulus (E) of granite specimens after exposure to different cyclic heating and water cooling treatments based on laboratory tests, and the relationships between Vp, UCS and E established through regression analysis. The physico-mechanical parameters of granite specimens all decrease remarkably in the first few thermal cycles, and their rates of decrease gradually diminish with thermal cycles, which is beneficial for the long-term exploitation of deep geothermal resources. Both UCS and E show a logarithmic correlation with Vp of granite under different high temperatures. There is a transformation from a linear relation (1 and 5 cycles) to an exponential relation (from 10 to 30 cycles) between Vp, UCS and E with thermal cycles. Such a correlation can provide a good estimation and avoid the costly, time-consuming and tedious mechanical tests. SEM observation reveals the change mechanism of the deterioration of physico-mechanical parameters, which can guide the well borehole stability during the deep geothermal energy exploitation.

High-resolution 3D seismic imaging and refined velocity model building improve the image of a deep geothermal reservoir in the Upper Rhine Graben

Over the past 35 years, geothermal projects have been developed in the Upper Rhine Graben (URG) to exploit deep geothermal energy. Underneath approximately 2 km of sedimentary deposits, the deep target consists of a granitic basement, which is highly fractured and hydrothermally altered. Therefore, it has high potential as a geothermal reservoir. Despite dense 2D seismic data coverage originally acquired for oil exploration (for a target two-way traveltime between 300 and 700 ms), the faults at the top of the granitic basement (between 1400 and 4000 ms) are poorly imaged, and their locations remain uncertain. To gain a better understanding of this large-scale faulting and to ensure the viability of future geothermal projects, a 3D seismic survey was acquired in the French part of the URG during the summer of 2018. This paper describes how an integrated project, combining seismic data processing, high-end imaging, and enhanced interpretation, was conducted to improve the understanding of this complex basin for geothermal purposes. By revealing the deep granite layer and its complex associated fault network, the insight from this project can help accurately locate future production wells.

Experimental Investigation of Mechanical Properties and Failure Behavior of Fluid-Saturated Hot Dry Rocks

Thousands of cubic meters of fluid are continuously injected for a long term to create complex fracture patterns in hydraulic fracturing of hot dry rocks. However, the physics and mechanics behind the interaction of fluid–rock are not fully understood at present. To reveal the related damage mechanisms of saturated rock samples such as damage initiation and evolution at various alternative stress levels, a series of in-house laboratory tests were performed on a TAW-series triaxial rock mechanics testing system, combined with ultrasound measurement and acoustic emission (AE) monitoring. After saturation with nano-emulsion and distilled water, ultrasound velocity of longitudinal wave was increased by 40%. Saturation weakens these mechanical parameters such as the crack damage stress ratio, fracture toughness and cohesive strength under different stress conditions. Fluid-saturated rock sample has higher AE hit rate than dry rock sample. Meanwhile, many step-like jumps appeared on the curve of cumulative AE events. Failure envelop, b-value and frequency spectrum were analyzed out to compare the mechanical difference between fluid-saturated and dry rock samples. The experimental results demonstrate that the saturation increased the pore pressure in rocks and further promoted crack propagation in hydraulic fracturing. Moreover, nano-emulsion liquid is more advantageous than distilled water for enhancing fracture complexity. This investigation provides for better understanding of the mechanisms of complex fracture formation in deep geothermal reservoirs.

Transient thermo-solutal convection in a tilted porous enclosure heated from below and salted from above

The confinement of CO2 in deep geothermal reservoirs as a means of mitigation of greenhouse gas emissions is continuously motivating research on the retention capacity of these deep aquifers. An important physical containment mechanism is related with CO2 dissolution and thermo-solutal convection. In this context, numerical simulations are performed in this work to assess the effect of inclination, Rayleigh number, and buoyancy ratio on the convective transport in a rectangular porous medium. The porous enclosure is heated from below and cooled from above, whereas a solute is dissolved through the upper boundary with a constant concentration condition and no mass loss through the other boundaries. A set of governing parameters is considered in this assessment: two buoyancy ratios with dominant solute buoyant forces (10 and 100), three Rayleigh numbers (10, 50, and 80), and three inclination angles plus the horizontal case for reference (5°, 10°, and 15°). The solution to the problem is based on a Finite Volume method along with the fixed point iteration for the coupled differential equations, and a Conjugate Gradient algorithm for the algebraic system. The model is validated and tested under mesh analysis. The numerical results show that the inclination angle has a minor effect on the convective mixing properties of the porous medium in comparison with the Rayleigh number and the buoyancy ratio. Increasing the angle slightly decreases the mixing rate as a consequence of the formation of preferential flow paths associated with the inclination, these preferential flow paths make mixing less efficient and give rise to zonation of solute concentration.

Permeability Enhancement From a Hydraulic Stimulation Imaged With Ground Penetrating Radar

AbstractWe present evidence of permeability enhancement from hydraulic stimulation experiments in fractured crystalline rock. A total of 9.49 m3 was injected in two fractured intervals of a 300 m long borehole. Repeated Ground Penetrating Radar (GPR) measurements in the same borehole were carried out prior to and following the stimulation. The initial measurements revealed fractures in the vicinity of the borehole that could be traced up to distances of 50 m away. The data measured post‐stimulation were used in a difference‐imaging approach to illuminate changes in the GPR reflections caused by the stimulations. The changes delineate the enhancement of a large and complex fracture network. These changes likely correspond to changes in local aperture, thus permeability. Our results indicate that borehole GPR yields unique information on subtle changes in hydraulic properties within a relatively large volume and provides a new perspective on the characterization and monitoring of deep geothermal reservoirs.