Hydraulic Stimulation Research Articles

Separating Hydraulic Fracturing Microseismicity From Induced Seismicity by Bayesian Inference of Non‐Linear Pressure Diffusivity

AbstractMicroseismic data acquired during hydraulic stimulations is routinely used to characterize hydraulic fracture (HF) propagation away from a wellbore. When the data include induced seismicity (IS) related to induced fault slip, separating HF‐related and IS‐related events is essential for HF treatment optimization and IS mitigation. Linear and non‐linear analytical diffusivity models can be used to interpret microseismic data and quantify the propagation of HFs, but their accuracy reduces when significant induced microseismicity is present. A Bayesian quantile regression is used to extend these existing diffusivity models to data contaminated with IS. A plausible ellipsoid filters events that are clearly anomalous prior to the quantile regression. The regression effectively separates HF‐related microseismic events from induced events in a case study for all stages without interpretative bias. This reveals faults that are directed connected to HFs, as well as those solely related with induced seismicity.

A Hybrid Approach to Rock Pre-conditioning Using Non-explosive Demolition Agents and Hydraulic Stimulation

This study presents a novel approach to rock pre-conditioning to promote the sustainability of low-grade ore mining applications such as in-situ recovery and cave mining. The proposed method involves a two-stage hybrid approach, utilizing soundless cracking demolition agents (SCDAs) to initiate radial fractures in a predrilled host rock, followed by hydraulic stimulation to extend the fractures. SCDA injection in the first stage creates multiple radial fractures around the injection well. However, the extent of fractures is limited to the near vicinity of the injection well. To overcome this limitation, the second stage involves the application of hydraulic stimulation to extend the initiated fractures, which produces a greater fracture density compared to pure hydraulic stimulation. The concept was assessed using a fully coupled hydro-mechanical discrete element model that simulated the hybrid fracturing method on crystalline rock at the grain scale. The results indicate that the proposed method can create a high density of fractures around the injection well. Additionally, we identify and evaluate the key factors affecting the performance of the proposed method, including rock mass heterogeneity, stress anisotropy, and pre-existing defects, providing valuable insights for further experimental design and execution. Overall, the study offers promising results for a potential solution to enhance the efficiency of low-grade ore mining through the hybrid rock pre-conditioning method.

A deep-learning based model for fracture network characterization constrained by induced micro-seismicity and tracer test data in enhanced geothermal system

Fracture network developed in hot dry rock (HDR) primarily controls the flow and heat transport in enhanced geothermal system (EGS). Characterization of fracture network in deep geothermal reservoir is among the most challenging tasks. An interpretation framework for explicit delineating complex fracture network is proposed based on induced micro-seismic events, hydraulic stimulation and tracer test data, which are often available in EGS sites. It is processed by (1) the spatial distribution of fracture intensity, size and hydraulic aperture constrained by the hydraulic diffusivity derived from the spatio-temporal distribution of micro-seismic events, (2) numerical models and long short-term memory (LSTM) neural network models for the hydraulic stimulation and tracer test, and (3) parameter inversion model based on the multi-objective Harris hawks optimization (MOHHO) algorithm searching for optimal fracture network parameters minimizing the deviations between model predictions and field observations. This method has been applied to the Habanero EGS site, Australia. The prediction accuracy of the injection wellhead pressure and produced tracer concentration of the inversed micro-seismicity mapping DFN model is improved by 61.16% and 67.35%, respectively, compared with the stochastic DFN model. These results emphasize the importance to integrate induced micro-seismicity monitoring data for the characterization of hydraulically stimulated fracture network.

Laboratory simulation of fault reactivation by fluid injection and implications for induced seismicity at the BedrettoLab, Swiss Alps

Fluid induced fault reactivation experiments will take place as part of the “Fault Activation and Earthquake Rupture” project (FEAR) at the BedrettoLab, an underground laboratory for geosciences and geo-energy excavated within the Rotondo massif (Swiss Alps). The aim of this publication is to characterize frictional properties and permeability of the main segment of the fault zone selected for limited fluid-induced fault reactivation experiments. Firstly, we characterized fault zone microstructures in the field and in thin sections. Secondly, we assessed fault gouge mineralogy by X-ray powder diffraction analysis, yielding a composition in agreement with similar fault gouges in the same area. Finally, we performed a detailed frictional and permeability characterization in laboratory, using BRAVA (Brittle Rock deformAtion Versatile Apparatus). We performed five frictional experiments, run at the actual in-situ conditions: four experiments for frictional properties characterization; and one further experiment where we stimulated the experimental fault by fluid pressurization applying a similar injection protocol designed for the in-situ hydraulic stimulation experiment. Additionally, we performed microstructural analysis on experimental samples to link frictional and permeability properties with fault fabric evolution. The integration of experimental results with field investigations suggests that the selected fault is potentially seismogenic and can be dynamically reactivated and controlled with hydraulic stimulation. This study highlights the importance of bridging the gap between laboratory and in-situ fault characterization, where experimental results become instrumental for the correct design of injection protocols such as those of FEAR project.

Performance Evaluation of AE Sensors Installed Like Hydrophones in Adaptive Monitoring Networks During a Decametre-Scale Hydraulic Stimulation Experiment

AbstractIn the framework of the STIMTEC and STIMTEC-X hydraulic stimulation experiments at the Reiche Zeche mine, Freiberg (Germany), we installed acoustic emission (AE) sensors for the recording of picoseismicity both conventionally using pneumatic coupling and experimentally like a hydrophone, i.e. the sensors were placed in the borehole without a further coupling system or cementing. We investigate performance measures of the hydrophone-like acoustic emission (HAE) sensors such as frequency bandwidth, sensitivity, first motion polarity, coupling and placement quality to assess the sensor’s applicability in adaptive monitoring networks. HAE sensors can be paired with hydraulic equipment, especially with the double packer probe used for stimulation at the decametre scale because the monitored frequency content differs from injection-related noise. This offers a unique opportunity to improve the network geometry and consequently the quality of a seismic catalogue. We analyse the sensor characteristics using active ultrasonic transmission measurements from boreholes with different orientations in the rock volume, noise measurements preceding active centre punch hits in the access galleries and passive recordings of induced acoustic emission events. HAE sensors placed in water-filled boreholes show good sensitivity performance even without optimal coupling to the crystalline rock for recording distances up to 17 m. The HAE sensors record the wavefield adequately for first-arrival identification, polarity picking and amplitude characteristics but are less suitable for detecting S-waves. Due to the borehole geometry HAE sensors record waves with incidence angles from the side, resulting in opposite polarity compared to side-view AE sensors as observed in the field and lab. We discuss the advantages of adaptive monitoring networks with HAE sensors being optimally placed for each stimulation interval configuration anew to improve seismic event detection and quality of event hypocentre locations during hydraulic stimulations. We show that we are able to significantly reduce the azimuthal gap, halve the location uncertainties and improve the network coverage for the purpose of focal mechanism estimations.

Evidence of frictional melting in fault rock drill cuttings from the enhanced geothermal system site in Pohang, South Korea

The 5.5 magnitude (Mw) earthquake in Pohang, South Korea in 2017 was one of the largest triggered earthquakes at an enhanced geothermal system (EGS) site. Faults that ruptured in Pohang were not identified by preliminary geological investigations or geophysical surveys, and the subsequent study of the fault rocks at the Pohang EGS site was limited to depths of 3790–3816 m. In this study, we present new observations of fault rocks from drill cuttings retrieved from the Pohang EGS. The drill cuttings obtained from 3256 to 3911 m contained “mud balls,” which showed a clay matrix with foliation and a cataclastic texture, indicating a typical fault gouge or breccia. Furthermore, the mud ball samples retrieved from depths of 3256 m and 3260 m contained black fragments. Scanning and transmission electron microscopy revealed that the black fragments consisted of glass-like material, which is indicative of frictional melting during coseismic slip. The presence of these black fragments suggests that at least one seismic event had occurred at the Pohang EGS site prior to the hydraulic stimulation test.

Hydraulic conductivity of the crystalline crust: Insights from hydraulic stimulation and induced seismicity of an enhanced geothermal system pilot reservoir at 6 km depth, Espoo, southern Finland

We analyze hydraulic stimulation data from the St1 Deep Heat Project, which comprised drilling wells to >6 km depth in crystalline rock in southern Finland with an aim to study the conditions for an Enhanced Geothermal System. Combining the stimulation and borehole logging data with induced seismicity results allows for a comprehensive analysis of hydraulic conductivity at rarely tested depths of 5 – 6 km. Hydraulic conductivity was observed to be pressure-dependent, heterogeneously distributed and affected by pre-existing fractures and lithological variation. The pre-stimulation formation fracture frequency is in the range of 0 – 8 m−1 with an average of 1.9 m−1. Only a small fraction of the fractures were open and conductive implying a hydraulically heterogeneous medium. The average natural hydraulic conductivity derived from leak-off tests and well tests before stimulation and from cross-hole pressure data is of the order of 5 0·10−10 - 5 0·10−9 m/s (permeability 1· 10−17 – 1·10−16 m2), which is in agreement with permeability models for the brittle crystalline crust at this depth. Hydraulic stimulation increased conductivity to 10−8 – 10−7 m/s, but it gradually decreased back to the natural level after pressure release. Stimulation generated five micro-earthquake clusters at 4.8 – 6.3 km TVD depth. Hydraulic connections between clusters were apparently not attained. The project generated extensive experience and data sets regarding deep drilling, hydrogeological properties, and seismic response to stimulation of crystalline rock in the upper continental crust. Hydraulic conductivity turned out to be the most challenging issue for the St1 EGS development and the EGS project is not continued at the moment.

Laboratory investigation of hydraulic fracturing in granitic rocks using active and passive seismic monitoring

SUMMARY Knowledge of the fracturing processes can be important for the optimization of pressurized fluid injection operations in the deep underground rock mass. Active and passive seismic monitoring techniques have been used in the field for tracking or mapping the propagating hydraulic fracture. Although both these monitoring techniques provide valuable information about the generated fracture network, it is difficult for either technique to comprehensibly identify the different processes associated with hydraulic fracturing. The combined active and passive monitoring has the potential for better characterization of the complex hydraulic fracturing phenomena. In this study, laboratory hydraulic fracturing experiments with combined active and passive seismic monitoring were conducted on true triaxially loaded Barre granite cubes with different fluid injection rates. The seismic inelastic fracturing was detected by 16 passive acoustic emission sensors, where 3678 and 2370 seismic source events were detected for the high and low injection rate experiments, respectively. For active monitoring, strong variations in the attributes of signals were observed which were transmitted through four source–receiver pairs, placed both perpendicular and parallel to the generated hydraulic fracture. Positive velocity changes were observed for active sensor pairs with ray paths passing through the generated hydraulic fracture indicating fluid permeation, whereas isolated dry deformation was characterized by a slight but permanent velocity decrease. Compared to velocity, the energy of the active signals was 1–2 orders of magnitude more sensitive to different hydraulic fracturing processes. However, the sensitivity and signatures of the active signal attributes were found to be dependent on the frequency range and direction of ray path with respect to the location of the generated fracture network. Using the coupled evaluation of the active and passive signals we were able to systematically identify various hydraulic fracturing processes including: (1) aseismic deformation, (2) fracture initiation and fluid permeation, (3) pressure build-up, (4) fracture propagation and (5) pressure release and leak-off. The results of this study showed that combining the respective advantages of active and passive seismic techniques and using both of them to monitor the failure processes can facilitate a more comprehensive understanding and better control of the hydraulic stimulations in subsurface operations.

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.

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.

Three‐Dimensional Thermoporoelastic Modeling of Hydrofracturing and Fluid Circulation in Hot Dry Rock

AbstractGeothermal energy, featured as a renewable low‐carbon energy resource, exhibits great potential in mitigating global warming. However, efficient mining of geothermal energy from hot dry rock remains challenging due to the lack of a thermoporoelastic modeling approach that allows for integrated simulation of hydrofracturing and fluid circulation, and poor understanding of the mechanisms of hydraulic stimulations. To conquer these challenges, we propose an advanced modeling approach, which enables us to deal with the interplay between fluid transport, heat transfer, rock deformation, and three‐dimensional fractures' growth and interaction. Based on the thermoporoelastic model, a refined investigation is performed to determine the hydrofracturing and fluid circulation processes, using data collected from the EGS Collab Experiment 1. This meso‐scale testbed involves creating major hydraulic fractures, accompanied by numerous secondary hydroshearing fractures, hydraulically connecting two horizontal wellbores to build a fluid circulation system. All wellbores in the testbed were heavily instrumented with monitoring devices, offering high‐quality data to properly constrain the geomechanical model. Our study provides insights into: (a) how the EGS Collab testbed responds to propagating fractures with strong stress shadowing and long‐term injection of chilled water; (b) why the parallel pattern of two hydraulic fractures with a distance much smaller than the fracture dimension forms during multi‐staged hydraulic stimulations; and (c) what the mechanisms are that determine hydrofracturing performance. Overall, this work not only contributes to a novel modeling approach for integrated simulation of hydrofracturing and fluid circulation, but advances our understanding of creating enhanced geothermal systems with interconnected fracture networks.

Discrete element modeling for the multistage hydraulic stimulation of a horizontal well in hot dry rock

Hydraulic fracturing aimed at building a network of fractures in hot dry rock (HDR) often induces strong seismic risks. However, few studies have focused on seismic risks, and methods for increasing the stimulated reservoir volume (SRV) while reducing seismic risks have not been adequately studied. Therefore, based on the full hydro-mechanical coupling, a seismic-fracturing model with a discrete fracture network (DFN) was developed, and the methods for increasing the SRV while reducing the seismic risk of horizontal wells at the FORGE site in Utah were discussed. The results showed that the fluid channeling effect between each segment is the main reason for the uneven fracture growth in the horizontal well. Additionally, the spatiotemporal evolution of fracturing-induced seismicity shows that the two ends (L1 and L3) of the horizontal well tend to form high stress concentrations and induce larger seismic events than the middle (L2). To solve these problems, a novel hydraulic fracturing method with a differentiated-stage, variable-injection volume suitable for the multi-fracturing of horizontal wells in HDR was proposed. With the application of the method, the SRV of the horizontal well was improved effectively, and the average magnitude induced by hydraulic fracturing was also decreased by 15.4%.

The role of natural fracture activation in hydraulic fracturing for deep unconventional geo-energy reservoir stimulation

The presence of sealed or semi-sealed, multiscale natural fracture systems appears to be crucial for the successful stimulation of deep reservoirs. To explore the reaction of such systems to reservoir stimulation, a new numerical simulation approach for hydraulic stimulation has been developed, trying to establish a realistic model of the physics involved. Our new model successfully reproduces dynamic fracture activation, network generation, and overall reservoir permeability enhancement. Its outputs indicate that natural fractures facilitate stimulation far beyond the near-wellbore area, and can significantly improve the hydraulic conductivity of unconventional geo-energy reservoirs. According to our model, the fracture activation patterns are jointly determined by the occurrence of natural fractures and the in situ stress. High-density natural fractures, high-fluid pressure, and low effective stress environments promote the formation of complex fracture networks during stimulation. Multistage or multicluster fracturing treatments with an appropriate spacing also increase the stimulated reservoir area (SRA). The simulation scheme demonstrated in this work offers the possibility to elucidate the complex multiphysical couplings seen in the field through detailed site-specific modeling.

Process and optimum pH for permeability enhancement of fractured granite through selective mineral dissolution by chelating agent flooding

Chemical stimulation of geothermal reservoirs via selective mineral dissolution with eco-friendly chelating agents has been recently proposed as a novel method complementary to hydraulic stimulation. In our previous study, we demonstrated rapid and significant permeability enhancement accompanied by the creation of voids due to the selective dissolution of biotite in fractured granite at 200 °C under confining stress. However, the process of permeability enhancement and its optimum pH have not been investigated yet. In this study, we present the results of the stimulation experiments conducted on fractured granite at 200 °C under confining stress using a 20 wt% aqueous solution of the sodium salt of the chelating agent, N, N-bis(carboxymethyl)-l-glutamic acid (GLDA), at pH 1–8. The permeability enhancement was the highest at pH 4, at which preferential flow paths connecting the voids caused by biotite dissolution were observed. With decreasing and increasing pH, silica precipitation and suppressed selective dissolution of biotite, respectively, became more significant, accompanied by a decrease in permeability enhancement. Our findings suggested that the optimum pH was a combination of two pH values suitable for the creation of stress-resistant preferential flow paths by selective mineral dissolution of biotite and for the accelerated dissolution of quartz, which could not be achieved by the chelating agent. The successive use of first pH 4 and then pH 8 resulted in a more than 2-fold permeability enhancement in 4 h. Therefore, our study has strengthened the possibility and effectiveness of the new chemical stimulation method to facilitate the extensive use of enhanced geothermal systems worldwide in future.

Metre-scale damage zone characterization usingS-coda waves from active ultrasonic transmission measurements in the STIMTEC project, URL Reiche Zeche, Germany

SUMMARYStudies of controlled hydraulic stimulation experiments with active and passive seismic monitoring conducted in Underground Research Laboratories (URLs) benefit from specific knowledge of hydraulic parameters, close by microseismic monitoring revealing structural details of the rock mass, and detailed evolution of seismicity in response to injection operations. Microseismic monitoring is commonly used to characterize a stimulated reservoir volume, for example, in terms of damage evolution of the rock mass. Since seismic attenuation is affected by damage of the rock volume, active seismic sources covering sizes from the centimetre to decimetre scale may help us to investigate space–time varying attenuation properties in a reservoir. This may allow us to monitor damage evolution of the stimulated rock volume in more detail, also since active seismic sources produce stronger signals leading to a broader frequency range that can be analysed compared to passive seismic signals. Within the STIMTEC project in the URL Reiche Zeche (URL-RZ) in Freiberg (Germany), more than 300 active Ultrasonic Transmission (UT) measurements were performed before and after hydraulic stimulations in two boreholes in the targeted rock volume, an anisotropic metamorphic gneiss. The signal-frequency content ranges between 1 and 60 kHz. Assuming scattering attenuation to dominate over intrinsic attenuation, we here apply the single isotropic scattering model. S-coda waves of 88 spatially representative UT measurements are used to estimate the coda quality factor (QC). We obtain stable QC estimates for centre frequencies of octave-width frequency bands between 3 and 21 kHz. We group neighbouring UT measurements to stabilize the observations and form eight UT groups in total, covering different depth intervals in three boreholes and four different time periods to investigate scattering attenuation changes in a spatiotemporal manner. Our final mean QC ($\overline {{Q}_C} $) estimates show characteristic frequency-dependence as observed at the field scale in geological reservoirs. We find temporal variations of QC are strongly connected to hydraulic stimulation, and these variations are more significant than those resolved from velocity changes. $\overline {{Q}_C} $ estimates at frequencies above 15 kHz indicate healing of injection-induced small-scale fractures during a two-months post-stimulation phase. Larger fractures, mostly sampled by lower frequencies (<15 kHz), seem to be more persistent with time (over 15 months). We observe spatial differences of $\overline {{Q}_C} $ values near the mine galleries (driftway and vein drift) and relate these observations to different extents and characteristics of the galleries’ excavation damage zones. Our results further support previous assumptions based on borehole televiewer logs and mapped structures of an existing fault with larger damage zone that crosses the stimulated rock volume NW-SE between the galleries. We conclude that the coda analysis of active UT measurements complements established imaging methods used during experiments in URLs. In particular, coda analysis is a powerful tool for the detection of damage zones and for monitoring local fracture networks with immediate application for imaging georeservoirs considered for exploitation or underground storage of gases and liquids.

Potential of fixed-plane-perforation fracturing technique in enhanced geothermal system

Fixed-plane-perforation fracturing (PFFP) has been employed to construct complex fracture networks to stimulate unconventional reservoirs. Few studies have been reported on its application potential in geothermal energy extraction. Here we first introduced the PFFP to the enhanced geothermal system (EGS) and numerically examined its heat extraction performance by comparing to those of differing stimulation strategies. We found that much longer fractures and significantly larger damage area are induced by the PFFP than the single-perforation fracturing (SPF), and the heat extraction of the reservoir stimulated by the FPPF is approximately 60% higher than that by the SPF, suggesting the great potential of the PFFP in deep geothermal energy harvest. We further improved our previously-proposed sensitivity indicator to assess the influence degree of critical factors on the PFFP where fracture networks were characterized by the pore pressure of hot dry rock. We found that a high in-situ stress difference reduces the spacing between hydraulic fractures and increases the stress shadowing, causing an increase in heat extraction. The injection rate affects strongest on fracture network area whereas the initial fracture angle is the least. When the injection rate increases from 0.0005 m 3 /s to 0.0020 m 3 /s, the internal fracture length grows by 61.3%, the fracture network area expands by 74.7% of the total area, and the heat extraction increases by 54 GJ. Our findings benefit hydraulic stimulation strategy selection and heat extraction optimization over EGS construction. • We assess the potential of fixed-plane-perforation fracturing (FPPF) in EGS. • Geothermal mining models are optimized by considering pore pressure distribution. • PFFP creates a favorable fracture network and benefits geothermal mining. • Injection rate affects strongest on fracture network area and heat extraction.

Fluid flow of various configurations in a rock-proppant-system

A high rock mass permeability is essential for developing an enhanced geothermal system and commonly achieved through the fracture conductivity. Hydraulic stimulation of fractures in combination with the use of proppants can significantly increase and maintain rock mass permeability. To investigate and quantify the effect of proppants, which are so far not commonly used to enhance deep geothermal reservoirs, on the rock mass hydraulic conductivity we performed various sets of laboratory experiments. We performed long-term creep deformation tests on a stack of rock samples with simultaneous a) fluid flow and b) acidification and fluid flow, in both cases with and without a proppant-filled fracture between the stacked rock samples. A central borehole served as fluid inlet into the lower sample of the stack. Two more boreholes in the lower sample allowed fluid flow. Sample stacks were placed in a Hoek cell at elevated confining pressures to prevent fluid flow along the sidewalls of the cylindrical sample setup. We compared the axial deformation and injection pressures between the experimental setups. The laboratory tests are part of the ZoKrateS Project, which aims at showing the feasibility of enhancing fractured carbonate rock mass by proppant placement for geothermal applications.

Simulation-based evaluation of the effectiveness of fiber-optic sensing in monitoring and optimizing water circulation in next-generation enhanced geothermal systems

The success of multi-stage hydraulic fracturing technology in unconventional oil and gas reservoir development has attracted much attention from the geothermal community. Some pilot field experiments have been conducted to investigate the multi-stage hydraulic stimulation along horizontal or high-deviated wellbores in geothermal reservoirs (denoted as next-generation enhanced geothermal systems). One common output of such stimulation is that the created hydraulic fractures show various conductive capabilities, which could potentially lead to undesirable flow localization, early thermal breakthrough, and low cold water sweep efficiency. Monitoring the water circulation between horizontal wellbores connected through hydraulic fractures becomes important in avoiding early severe producing temperature reduction in practical operations. In this study, we investigate the effectiveness of distributed temperature sensing (DTS) and distributed strain sensing (DSS) in identifying dominant flow paths in geothermal reservoirs with multi-stage hydraulic stimulation and horizontal well completion designs. A coupled thermo-poro-elastic model for general-purpose geothermal reservoir simulation is developed and solved using a combined finite volume/finite element method, together with a fully implicit sequential coupling scheme. The temperature and strain responses along horizontal production wells are simulated and analyzed during water circulation. Reservoir temperature reduces in the area that is close to the water flow paths, which induces thermal contraction. These thermoelastic characteristics can be monitored by fiber-optic sensing, forming distinct signatures for flow-localization identification. DTS data may be dominantly influenced by the fractures with higher conductivity, ‘masking’ the signals related to fractures in the downstream, but DSS data can provide additional information for accurate identification of high conductive fractures. Successful mitigation strategies, for example plugging the dominant fractures illustrated in this study, could substantially increase the producing fluid temperature. The analyses in this study show that fiber-optic sensing is an effective technology to monitor and optimize water circulation in next-generation enhanced geothermal systems.

Dynamic development of geochemical reaction fronts during hydraulic stimulation of shale

Injection of acidic hydraulic fracture fluid (HFF) into shale formations for unconventional oil/gas production results in chemical reactions in the shale matrix that can alter fluid transport. Here, we report the results of set of experiments designed to evaluate the impact of calcite dissolution as a function of carbonate mineral content on matrix chemical reactivity and pore-space modification concomitant with imbibition. We tracked acidic HFF transport in four samples of Wolfcamp shale with calcite contents varying from 4% to 59% by monitoring the rate and spatial extent of bromide tracer transport using synchrotron-based X-ray fluorescence microprobe (XFM) imaging. Concurrently, we also carried out XFM imaging of the spatial distribution of Ca in the Wolfcamp shale cores (as a proxy of calcite distribution). Our approach thus yields a direct record of time-resolved selective ion transport resulting from the penetration of acidic HFF and the associated mineral transformations in the shale cores.We show that the variability in calcite content of Wolfcamp shale samples can directly affect the rate and spatial extent of imbibition. Although reaction of the acidic HFF with carbonates in shales enhances calcite dissolution and increases porosity, the spatial extent of calcite dissolution in the shale matrix is limited due to a rapid neutralization of pH. The relative abundance and spatial distribution of calcite control the chemical saturation state of the HFF progressing into the matrix. As a result, calcite has a major impact on the spatial extent and rate of matrix alteration and thus on HFF transport during subsurface reservoir stimulation. Consequently, increased calcite content in the shale matrix inhibits the spatial extent of the pore-volume increase and, by extension, the spatial extent and rate of imbibition.Our results thus show that the overall rates of calcite dissolution approach the rates of acidic HFF transport (i.e., Damköhler number ∼1), which could contribute to the efficiency of subsurface reservoir stimulation. A better understanding of HFF-calcite reaction rates is crucial for improving the prediction and optimization of fluid transport across HFF-shale interfaces during hydraulic fracturing.

Two-level simulation of injection-induced fracture slip and wing-crack propagation in poroelastic media

In fractured poroelastic media under high differential stress, the shearing of pre-existing fractures and faults and propagation of wing cracks can be induced by fluid injection. This paper presents a two-dimensional mathematical model and a numerical solution approach for coupling fluid flow with fracture shearing and propagation under hydraulic stimulation by fluid injection. Numerical challenges are related to the strong coupling between hydraulic and mechanical processes, the material discontinuity the fractures represent in the medium, and the strong effect that fracture deformation and propagation have on the physical processes. The solution approach is based on a two-level strategy that is classified into the coarse and fine levels. In the coarse level, flow in and poroelastic deformation of the matrix are coupled with the flow in the fractures and fracture contact mechanics, allowing fractures to frictionally slide. Fracture propagation is handled at the fine level, where the maximum tangential stress criterion triggers the propagation of fractures, and Paris' law governs the fracture growth processes. Simulations show how the shearing of a fracture due to fluid injection is linked to fracture propagation, including cases with hydraulically and mechanically interacting fractures. • Wing-crack propagation caused by injection-induced slip of fractures and faults. • Modeling of fracture slip, deformation and propagation in poroelastic media. • Two-scale solution strategy with local adaptive remeshing on fine-level and coarse-level grids. • Open-source 2D implementation combining finite volume and finite element methods. • Simulation results on hydromechanical coupling in fracture slip and propagation.