Hydraulic Stimulation Research Articles

Keys to successful negotiation of indigenous land use agreements and building consent for the development of natural resources, The Great Sandy Desert Project, a case study

Theia Energy Pty Ltd discovered a substantial hydrocarbon resource in the Canning Basin, NW in Western Australia, just 150-km SE of Broome. Since 2014, Theia Energy has built a robust working relationship with the traditional owners of the land that hold native title over their permits. This relationship has developed through negotiation meetings, on-country cultural heritage surveys and cultural awareness training. The relationship has culminated in the negotiation of indigenous land use agreements (ILUA) with the Karajarri People that manage the impacts of potential development of the resource including the impacts of hydraulic fracture stimulation, and provide significant economic development opportunities. The consistent exchange of information and views has developed mutual understanding and respect allowing for the effective and pragmatic treatment of risks to allow for the exploration, and if successful the development, of the onshore Canning Basin. The Great Sandy Desert Project has the potential to bring substantial benefits to regional communities and is significant for economic development of the West Kimberley, state revenue and national liquid fuel security. This study discusses important lessons learned in building trust with traditional owners and custodians of the land and how cultural values can be protected while providing social and economic opportunities for Indigenous Australians in a sustainable and culturally appropriate way. It reflects on the outcomes of the many scientific inquiries into hydraulic fracture stimulation and the perception of risks and impacts of traditional owners which must be addressed in a genuine, meaningful and effective way. It also sets out how traditional owners have, through the negotiation of the ILUA with Theia Energy and in the absence of clear policy from Government, chosen to establish their own system of regulation and economic participation in the project to address and manage risks and impacts from the development of resources on their country.

Sulphur isotopes in deep groundwater reservoirs: Evidence from post-stimulation flowback at the Pohang geothermal facility, Korea

A hydraulic stimulation was carried out on a granodiorite reservoir in an enhanced geothermal system in August 2017 in Pohang, Korea. Water injected into the 4.2 km deep PX-1 well contained c. 330−360 mg/L sulphate, with a negative δ34S. The resulting flowback water became more saline with time, with sulphate and chloride concentrations and dissolved sulphate δ34S all increasing. Compared with conservative advective-dispersive and mixing models, the flowback contained surplus sulphate with an elevated δ34S. The PX-1 reservoir fluid is saturated with respect to anhydrite at downhole temperatures and pressures. Dissolution by injected surface water of secondary anhydrite along fracture surfaces, most likely with elevated δ34S reflecting the reservoir fluid, is likely to have resulted in an excess of 34S-enriched sulphate in the flowback fluid. An alternative hypothesis involving oxidation of pyrite is also plausible but is stoichiometrically inadequate to account for the observed sulphate excess, and unlikely from a sulphur isotopic perspective. This analysis thus contributes to the evidence for water-rock reactions during stimulation of the Pohang granodiorite.

Fault slip in hydraulic stimulation of geothermal reservoirs: Governing mechanisms and process-structure interaction

Hydraulic stimulation of geothermal reservoirs in low-permeability basement and crystalline igneous rock can enhance permeability by reactivation and shear dilation of existing fractures. The process is characterized by interaction between fluid flow, deformation, and the fractured structure of the formation. The flow is highly affected by the fracture network, which in turn is deformed because of hydromechanical stress changes caused by the fluid injection. This process-structure interaction is decisive for the outcome of hydraulic stimulation, and, in analysis of governing mechanisms, physics-based modeling has potential to complement field and experimental data. Here, we show how recently developed simulation technology is a valuable tool to understand governing mechanisms of hydromechanical coupled processes and the reactivation and deformation of faults. The methodology fully couples flow in faults and matrix with poroelastic matrix deformation and a contact mechanics model for the faults, including dilation because of slip. Key elements are high aspect ratios of faults and strong nonlinearities in highly coupled governing equations. Example simulations using our open-source software illustrate direct and indirect hydraulic fault reactivation and corresponding permeability enhancement. We investigate the effect of the fault and matrix permeability and the Biot coefficient. A higher matrix permeability leads to more leakage from a permeable fault and thus suppresses reactivation and slip of the fault compared to the case with a lower matrix permeability. If a fault is a barrier to flow, increase of pressure because of the fluid injection results in stabilization of the fault; the situation is opposite if the fault is highly permeable compared to the matrix. For the given setup, lowering the Biot coefficient results in more slip than the base case. While conceptually simple, the examples illustrate the strong hydromechanical couplings and the prospects of physics-based numerical models in investigating the dynamics.

Creation of a Mixed‐Mode Fracture Network at Mesoscale Through Hydraulic Fracturing and Shear Stimulation

AbstractEnhanced Geothermal Systems could provide a substantial contribution to the global energy demand if their implementation could overcome inherent challenges. Examples are insufficient created permeability, early thermal breakthrough, and unacceptable induced seismicity. Here we report on the seismic response of a mesoscale hydraulic fracturing experiment performed at 1.5‐km depth at the Sanford Underground Research Facility. We have measured the seismic activity by utilizing a 100‐kHz, continuous seismic monitoring system deployed in six 60‐m length monitoring boreholes surrounding the experimental domain in 3‐D. The achieved location uncertainty was on the order of 1 m and limited by the signal‐to‐noise ratio of detected events. These uncertainties were corroborated by detections of fracture intersections at the monitoring boreholes. Three intervals of the dedicated injection borehole were hydraulically stimulated by water injection at pressures up to 33 MPa and flow rates up to 5 L/min. We located 1,933 seismic events during several injection periods. The recorded seismicity delineates a complex fracture network comprised of multistrand hydraulic fractures and shear‐reactivated, preexisting planes of weakness that grew unilaterally from the point of initiation. We find that heterogeneity of stress dictates the seismic outcome of hydraulic stimulations, even when relying on theoretically well‐behaved hydraulic fractures. Once hydraulic fractures intersected boreholes, the boreholes acted as a pressure relief and fracture propagation ceased. In order to create an efficient subsurface heat exchanger, production boreholes should not be drilled before the end of hydraulic stimulations.

Experimental Investigation of Hydraulic Fracturing and Stress Sensitivity of Fracture Permeability Under Changing Polyaxial Stress Conditions

AbstractUnderstanding and predicting fracture propagation and subsequent fluid flow characteristics is critical to geoenergy technologies that engineer and/or utilize favorable geological conditions to store or extract fluids from the subsurface. Fracture permeability decreases nonlinearly with increasing normal stress, but the relationship between shear displacement and fracture permeability is less well understood. We utilize the new Geo‐Reservoir Experimental Analogue Technology (GREAT cell), which can apply polyaxial stress states and realistic reservoir temperatures and pressures to cylindrical samples and has the unique capability to alter both the magnitude and orientation of the radial stress field by increments of 11.25° during an experiment. We load synthetic analogue materials and real rock samples to stress conditions representative of 500–1,000 m depth, investigate the hydraulic stimulation process, and then conduct flow experiments while changing the fluid pressure and the orientation of the intermediate and minimum principal stresses. High‐resolution circumferential strain measurements combined with fluid pressure data indicate fracture propagation can be both stable (no fluid pressure drop) and unstable (fluid pressure drop). The induced fractures exhibit both opening and shear displacements during their creation and/or during fluid flow with changing radial stress states. Flow tests during radial stress field rotation reveal that fracture normal effective stress has first‐order control on fracture permeability but increasing fracture offset can lead to elevated permeabilities at maximum shear stress. The results have implications for our conceptual understanding of fracture propagation as well as fluid flow and deformation around fractures.

Numerical investigation on the hydraulic stimulation of naturally fractured Longmaxi shale reservoirs using an extended discontinuous deformation analysis (DDA) method

The Silurian Longmaxi formation in the Sichuan Basin, which is the most important gas-producing shale reservoir in China, is characterized by relatively small effective thickness, large in-situ stress difference and highly-developed natural fracture (NF) system. Microseismic monitoring shows that complex hydraulic fracture (HF) networks with relatively small height could be stimulated in the Longmaxi shale reservoirs. Traditional two-dimensional (2D) numerical models cannot capture the three-dimensional (3D) nature of the limited-height HFs, which is not suitable for investigating the fracturing of Longmaxi shale reservoirs. This work develops a fully coupled hydro-mechanical model for HF network propagation based on the discontinuous deformation analysis (DDA) method. A new efficient method is proposed to consider the 3D effect in 2D DDA-based fracturing simulations. Hence an approximate 3D solution of fracture-induced stress and deformation could be obtained. This method is verified against previous works. Then, a series of simulations is performed to investigate the influence of reservoir thickness, in-situ stress difference and NF pattern on the hydraulic stimulation of Longmaxi shale reservoirs. Modeling results show that limited reservoir thickness is favorable to the formation of HF network. Complex and dense fracture network is easily to be formed in thin reservoirs. Large in-situ stress difference will impede the hydraulic stimulation. However, when the reservoir thickness is small, dense fracture network is still likely to be formed under large stress differences. Besides, the NF system is essential for the formation of HF network. The modeling results indicate that as a result of the limited thickness and highly-developed NF system, complex and dense fracture networks could be stimulated in the Longmaxi shale reservoirs. This work partly accounts for the initial success of gas recovery in the Longmaxi shale reservoirs. It also provides a feasible numerical method and theoretical supports for further optimization of shale gas exploitation in China.

Hydromechanical analysis of the second hydraulic stimulation in well PX-1 at the Pohang fractured geothermal reservoir, South Korea

In this study, we investigate numerically the hydro-mechanical behavior of fractured crystalline rock due to one of the five hydraulic stimulations at the Pohang Enhanced Geothermal site in South Korea. We use the commercial code FracMan (Golder Associates) that enables studying hydro-mechanical coupled processes in fractured media in three dimensions combining the finite element method with a discrete fracture network. The software is used to simulate fluid pressure perturbation at fractures during hydraulic stimulation. Our numerical simulation shows that pressure history matching can be obtained by partitioning the treatment into separate phases. This results in adjusted stress-aperture relationships. The evolution of aperture adjustment implies that the stimulation mechanism could be a combination of hydraulic fracturing and shearing. The simulated extent of the 0.01 MPa overpressure contour at the end of the treatment equals to ∼180 m around the injection point.

Modeling of fiber-optic strain responses to hydraulic fracturing

We modeled cross-well strain/strain rate responses of fiber optic sensing, including distributed strain sensing (DSS) and low-frequency distributed acoustic sensing (DAS), to hydraulic stimulation. DSS and low-frequency DAS have been used to measure strain or the strain rate to characterize hydraulic fractures. However, the current application of DSS/DAS is limited to acquisition, processing, and qualitative interpretations. The lack of geomechanical models hinders the development of the technology toward quantitative interpretation and inversion. We have developed a strategy to use the displacement discontinuity method to model the strain field around kinematically propagating fractures. For a horizontal monitoring well, modeling results were able to explain the heart-shaped extending pattern before a fracture hit, the polarity flip due to fracture interaction during stimulation, and the V-shaped pattern when a fracture does not intersect with the monitoring well. For a vertical monitoring well, modeling shows the different characters of strain rate responses when a fracture is near and far away from a vertical monitoring well. We also investigated the effects of fractures with various geometries such as elliptic and layered fractures. We compared and verified the modeling with field data from the Hydraulic Fracturing Test Site 2, a research experiment performed in the Permian Basin. Our modeling work can be used to identify patterns in field observations. The results also help to improve acquisition design and lay the groundwork for quantitative interpretation and inversion.

Hydro-mechanical modeling of the first and second hydraulic stimulations in a fractured geothermal reservoir in Pohang, South Korea

This study presents comprehensive coupled hydro-mechanical numerical modeling of the first and second hydraulic stimulations at the Pohang enhanced geothermal system (EGS) site in order to improve the understanding on the key stimulation mechanisms in the fractured reservoir. Two models for PX-2 and PX-1 wells were developed including fracture zones of different orientations and permeability. Shear dilation and frictional plastic strain-softening were implemented in a non-linear stress dependent fracture model. A history matching of the wellhead pressure curves was carried out in the early days of each stimulation. The PX-2 model with a fracture jacking mechanism achieved the reproduction of highly reversible non-linear aperture changes observed in situ. The PX-1 model successfully simulated a wellhead pressure peak and drop associated with shear slip and dilation by applying frictional plastic strain softening. Simulated critical wellhead pressure for shear slip near the PX-1 well was greater than the prediction by a simple slip potential analysis due to local stress changes by the poroelastic effect. The numerical modeling confirmed that the combination of shear dilation and jacking was adequate to capture the pressure evolution during the increasing step-rate test at PX-1. Besides, spatio-temporal changes in pressure, total stress, and permeability evaluated in the fracture zones greatly enhanced the understanding on the coupled behavior caused by the hydraulic stimulations. The possibility of zones with lower permeability away from the PX-1 well was suggested by an alternative model. The current numerical study demonstrates that the key hydro-mechanical processes of shear slip and dilation and hydraulic jacking observed in the fractured reservoir can be successfully reproduced.

New geomechanical application utilizing MWD gamma ray logs: An Eagle Ford case study

Understanding mechanical rock properties is essential for optimal well placement and hydraulic stimulation design which directly impacts hydrocarbon productivity from shale reservoirs with ultra-low permeability. Current industrial practice is to assess spatial variability of subsurface mechanical properties through analyzing core and dipole sonic logs. Seismic inversion is another commonly applicable approach. However, although numerous existing approaches have contributed to upstream operators mitigating operational risk, these individual or combined evaluations ultimately require significant cost and time for exploration. These approaches also have limitations identifying fine-scale variability within inter-well spacing or along the length of a horizontal well due to sparse pilot-well control and the intrinsic limitation of seismic resolution. In this study, we propose a new approach to capture spatial variability of Young's modulus utilizing MWD (Measurement While Drilling) gamma ray logs of horizontal wells. The MWD gamma ray log is essentially acquired for the purpose of having geometric and stratigraphic information on well position while drilling in real time. And, due to cost and time related issues, shale operators commonly acquire only MWD log rather than core or wireline log set for horizontal production drilling while the massive pad drilling has the advantage of large areal coverage. Under the circumstance, MWD gamma ray log is often only data having geological implication. In purpose of utilizing MWD gamma ray log for geomechanical approaches, we examined a relationship between gamma ray and Young's modulus acquired from pilot wells. The MWD gamma ray logs were converted into Young's modulus logs and ultimately populated for three-dimensional property modeling. To validate the property model, an integrated assessment using wireline logs, microseismic, and three-dimensional seismic data was conducted. The study results demonstrate that this new geomechanical approach utilizing MWD gamma ray logs can be supportive or even serve as an alternative to the existing approaches. The method enables identification of finer-scale spatial variability than sparse pilot-well control and conventional seismic analysis, which is attributed to the advantage of long-lateral, multi-benched, staggered, and dense production well placement. Based on the reliability and cost-effectiveness of the new approach, we expect it could help upstream operators mitigate risk and uncertainty related to geomechanical properties without excessive spending for exploration and ultimately improve hydrocarbon productivity with optimal well completion strategies.

Application of geochemical modelling for hydraulic stimulation in enhanced geothermal systems

ABSTRACT In an enhanced geothermal system (EGS), the use of fracking fluid generally causes changes in the geochemical properties of a hydrothermal reservoir. In this study, the geochemical changes were estimated through geochemical modelling using PHREEQC, and the modelling results were compared with field data and evaluated. As a result, the model showed an increase of the electrical conductivity in time which is consistent with the measurements. From the comparison between two results, it was found that the injected water was mixed with groundwater and the influence of the groundwater was significant when the hydraulic retention time of the fluid was longer than a day. Also, the chemical properties of the bedrock groundwater significantly could influence on the geochemical reactions in the geothermal reservoir. From the results, it can be concluded that the consideration of the chemical properties of groundwater or formation water is essential for the successful EGS operation.

Pseudo-probabilistic identification of fracture network in seismic clouds driven by source parameters

SUMMARYFracture networks in underground reservoirs are important pathways for fluid flow and can therefore be a deciding factor in the development of such reservoirs for geothermal energy, oil and gas production or underground storage. Yet, they are difficult to characterize since they usually cannot be directly accessed. We propose a new method to compute the likelihood of having a fracture at a given location from induced seismic events and their source parameters. The result takes the form of a so-called pseudo-probabilistic fracture network (PPFN). In addition to the hypocentres of the seismic events used to image the fracture network, their magnitudes and focal mechanisms are also taken into account, thus keeping a closer link with the geophysical properties of the rupture and therefore the geology of the reservoir. The basic principle of the PPFN is to estimate the connectivity between any spatial position in the cloud and the seismic events. This is done by applying weighting functions depending on the distance between a seismic event and any location, the minimum size of the rupture plane derived from the event magnitude, and the orientation of the rupture plane provided by the focal mechanism. The PPFN is first tested on a set of synthetic data sets to validate the approach. Then, it is applied to the seismic cloud induced by the deep hydraulic stimulation of the well GPK2 of the enhanced geothermal site of Soultz-sous-Forêts (France). The application on the synthetic data sets shows that the PPFN is able to reproduce fault planes placed in a cloud of randomly distributed events but is sensitive to the free parameters that define the shape of the weighting functions. When these parameters are chosen in accordance with the scale of investigation, that is, the typical size of the structures of interest, the PPFN is able to determine the position, size and orientation of the structure quite precisely. The application of the PPFN to the GPK2 seismic cloud reveals a large prominent fault in the deep-northern part of the seismic cloud, supporting conclusions from previous work, and a minor structure in the southern upper part, which could also be a branch of the main fault.

Impact of reservoir heterogeneity on fluid injection-induced fault slip: insights from numerical simulation

In order to meet the growing energy demand worldwide, the technology of hydraulic fracturing for shale gas extraction has been rapidly developed and applied in recent years. However, man-made fluid injection has also caused several problems of high societal concern such as induced earthquakes. The focus of this paper is on the process of induced fault activation in shale reservoirs caused by hydraulic stimulation activities. We constructed a two-dimensional numerical model representing the horizontal cross-section of a faulted reservoir and simulated important coupled hydro-mechanical processes during fluid injection. We parameterized our model with the site investigation data of an actual shale gas project including in-situ stress, rock properties and injection protocol, etc. We represented the natural fault zone as a geological composite involving a fault core and a damage zone consisting of a set of subsidiary fractures parallel to the fault strike. We coupled the solid deformation and fault displacement with Darcy-type fluid flow based on poroelasticity principles and hydro-mechanical constitutive relationships. The heterogeneous nature of the permeability of the shale formation is mimicked by a random field model governed by a log-normal probability density function and directional correlation lengths, such that we can explore the impact of reservoir heterogeneity on the fault slip behaviour. In addition, we also studied the effects of fault length, dip angle and injection point-to-fault distance on the fault slip. Specifically, we analysed the link between fault slip and fluid flow field, permitting the capture of fluid movement in the heterogeneous faulted reservoir, the temporal and spatial evolution of preferential flow channels and their consequences on induced fault slip. The results of this research have important implications for understanding the triggering processes and mechanisms of fault slip during shale gas exploitation as well as many other injection-related engineering activities.

Linkages between hydrothermal alteration, natural fractures, and permeability: Integration of borehole data for reservoir characterization at the Fallon FORGE EGS site, Nevada, USA

Between 2015 and 2018, the Fallon FORGE EGS (Engineered or Enhanced Geothermal System) site in Churchill County, Nevada, USA, underwent detailed geoscientific characterization to evaluate its suitability to operate as a field demonstration site to test and develop EGS technologies. This included the compilation, synthesis and acquisition of geophysical, geological, geochemical, hydrological, down-hole log, and mineralogy datasets. Site activities culminated in the drilling of a 2481 m-deep EGS observation well (21−31) to further verify the geology and rock properties of the proposed EGS reservoir formations, which are older than the Miocene and include felsic intrusions, felsic volcanics, and meta-sedimentary lithologies. In this paper, we integrate and evaluate borehole data from well 21−31, including drill cuttings and sidewall cores (petrographic, XRD, hyperspectral), wireline geophysical logs (e.g., gamma, neutron porosity, resistivity, sonic scanner), borehole imagers (FMI, BHTV), and pressure and temperature logs. Hydrothermal alteration and veining is found throughout the entire pre-Miocene interval of well 21−31, and includes epidote mineral assemblages that suggest past temperatures >250 °C. This is higher than the hottest measured equilibrated temperature at similar depths in other wells on the FORGE site today, and suggests that the system was higher temperature in the past. Overall lithologic variability is high, with volcanic, intrusive, metamorphic, and sedimentary rocks present in the bottom 700 m of the well. Natural fractures are abundant throughout the pre-Miocene section, but are more frequent in the felsic intrusive and felsic volcanic lithologies. The dominant orientation of these natural fractures observed in FMI logs is NNE-SSW with an average strike of 010° ± 40°, subparallel to the interpreted SHmax orientation of 009° ± 14° that is indicated by drilling-induced structures in the wellbore in the BHTV log. Therefore, natural fractures appear optimally oriented for reactivation via hydraulic stimulation. Mud loss locations and a non-equilibrated temperature log suggest that there are some zones with relatively-higher permeability and/or open fractures. However injection test results indicate an injectivity index of 0.025 L/min/kPa, which is an order of magnitude lower than a typical threshold for commercial wells. Analytical modeling of injection test results indicate that the majority of the well is low permeability (< 10−16 m²), with some discrete, localized zones of higher permeability (∼ 10 -12 m², less than 0.5 percent of the pre-Miocene (deep) section intersected by the well).

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.

Hydromechanical processes and their influence on the stimulation effected volume: observations from a decameter-scale hydraulic stimulation project

Abstract. Six hydraulic shearing experiments have been conducted in the framework of the In-situ Stimulation and Circulation experiment within a decameter-scale crystalline rock volume at the Grimsel Test Site, Switzerland. During each experiment fractures associated with one out of two shear zone types were hydraulically reactivated. The two shear zone types differ in terms of tectonic genesis and architecture. An extensive monitoring system of sensors recording seismicity, pressure and strain was spatially distributed in 11 boreholes around the injection locations. As a result of the stimulation, the near-wellbore transmissivity increased up to 3 orders in magnitude. With one exception, jacking pressures were unchanged by the stimulations. Transmissivity change, jacking pressure and seismic activity were different for the two shear zone types, suggesting that the shear zone architectures govern the seismo-hydromechanical response. The elevated fracture fluid pressures associated with the stimulations propagated mostly along the stimulated shear zones. The absence of high-pressure signals away from the injection point for most experiments (except two out of six experiments) is interpreted as channelized flow within the shear zones. The observed deformation field within 15–20 m from the injection point is characterized by variable extensional and compressive strain produced by fracture normal opening and/or slip dislocation, as well as stress redistribution related to these processes. At greater distance from the injection location, strain measurements indicate a volumetric compressive zone, in which strain magnitudes decrease with increasing distance. These compressive strain signals are interpreted as a poro-elastic far-field response to the emplacement of fluid volume around the injection interval. Our hydromechanical data reveal that the overall stimulation effected volume is significantly larger than implied by the seismicity cloud and can be subdivided into a primary stimulated and secondary effected zone.

Fracture properties estimation using distributed acoustic sensing recording of guided waves in unconventional reservoirs

Perforation shots excite guided waves that propagate in a low-velocity unconventional shale reservoir. They have a frequency content of up to 700 Hz and are dispersive. We have analyzed horizontal crosswell perforation shots recorded by a distributed acoustic sensing (DAS) array. As guided waves propagate through a previously stimulated area, we observe a dramatic influence on the guided SH waves in the form of delayed arrival times, scattering, phase incoherency, and loss of amplitude and frequency. The leaky compressional waves undergo a gradual slowdown. Using a simple geometric analysis of the spatial locations of the distortions in the direct arrivals of the guided SH waves, we can estimate the half-lengths of the induced fractures, which range from 50% to 75% of the distance between the perforated and monitoring wells. Furthermore, we find that the propagation disturbances originate from the middle of the stimulated area. Other diffracted signals, notably from frac plugs, are clearly visible in the data. We report the first large-scale use of DAS records of guided waves. Their potential for high-resolution imaging and inversion of subsurface properties before and after hydraulic stimulation opens new possibilities for the use of seismology in optimizing production from unconventional reservoirs.

Well Proximity Governing Stress Drop Variation and Seismic Attenuation Associated With Hydraulic Fracturing Induced Earthquakes

AbstractWe use broadband seismic data collected within 3 km of a hydraulic fracturing (HF) well in northeast British Columbia, Canada, to estimate the stress drop values of HF‐induced earthquakes and their spatial variation. Applying both spectral ratio and clustered single‐spectra fitting methods to 484 induced earthquakes (M‐1.0 to 3.0), we find that earthquakes close to the injection well have lower stress drop values than those at greater distance. Stress drop values are generally invariant within clusters either proximal (~0.1–1 MPa) or distal (~1–10 MPa) to the well, suggesting that dynamic ruptures in rocks with similar rheological properties tend to have relatively constant stress drop values. Clustered single spectrum fitting also suggests that the seismic quality factor (Q) is lower proximal to the well. We interpret the lower stress drop values and higher seismic attenuation proximal to the well as a result of higher fracture density and/or elevated pore pressure in the rock matrix due to hydraulic stimulation.

Characterizing a decametre-scale granitic reservoir using ground-penetrating radar and seismic methods

Abstract. Ground-penetrating radar (GPR) and seismic imaging have proven to be important tools for the characterization of rock volumes. Both methods provide information about the physical rock mass properties and geological structures away from boreholes or tunnel walls. Here, we present the results from a geophysical characterization campaign that was conducted as part of a decametre-scale hydraulic stimulation experiment in the crystalline rock volume of the Grimsel Test Site (central Switzerland). For this characterization experiment, we used tunnel-based GPR reflection imaging as well as seismic travel-time tomography to investigate the volumes between several tunnels and boreholes. The interpretation of the GPR data with respect to geological structures is based on the unmigrated and migrated images. For the tomographic analysis of the seismic first-arrival travel-time data, we inverted for an anisotropic velocity model described by the Thomsen parameters v0, ϵ and δ to account for the rock mass foliation. Subsequently, the GPR and seismic images were interpreted in combination with the geological model of the test volume and the known in situ stress states. We found that the ductile shear zones are clearly imaged by GPR and show an increase in seismic anisotropy due to a stronger foliation, while they are not visible in the p-wave (v0) velocity model. Regions of decreased seismic p-wave velocity, however, correlate with regions of high fracture density. For geophysical characterization of potential deep geothermal reservoirs, our results imply that wireline-compatible borehole GPR should be considered for shear zone characterization, and that seismic anisotropy and velocity information are desirable to acquire in order to gain information about ductile shear zones and fracture density, respectively.

Dynamic Fault Interaction during a Fluid-Injection-Induced Earthquake: The 2017 Mw 5.5 Pohang Event

ABSTRACT The 15 November 2017 Mw 5.5 Pohang, South Korea, earthquake has been linked to hydraulic stimulation and fluid injections, making it the largest induced seismic event associated with an enhanced geothermal system. To understand its source dynamics and fault interactions, we conduct the first 3D high-resolution spontaneous dynamic rupture simulations of an induced earthquake. We account for topography, off-fault plastic deformation under depth-dependent bulk cohesion, rapid velocity weakening friction, and 1D subsurface structure. A guided fault reconstruction approach that clusters spatiotemporal aftershock locations (including their uncertainties) is used to identify a main and a secondary fault plane that intersect under a shallow angle of 15°. Based on simple Mohr–Coulomb failure analysis and 180 dynamic rupture experiments in which we vary local stress loading conditions, fluid pressure, and relative fault strength, we identify a preferred two-fault-plane scenario that well reproduces observations. We find that the regional far-field tectonic stress regime promotes pure strike-slip faulting, whereas local stress conditions constrained by borehole logging generate the observed thrust-faulting component. Our preferred model is characterized by overpressurized pore fluids, nonoptimally oriented but dynamically weak faults and a close-to-critical local stress state. In our model, earthquake rupture “jumps” to the secondary fault by dynamic triggering, generating a measurable non-double-couple component. Our simulations suggest that complex dynamic fault interaction may occur during fluid-injection-induced earthquakes and that local stress perturbations dominate over regional stress conditions. Therefore, our findings have important implications for seismic hazard in active georeservoir.