Description: In this study, a series of hydraulic fracturing tests under different injecting conditions was performed on Pocheon granite rock to account for the evolution of hydro-mechanical behavior during the fracturing process. We investigated the effect of the fluid viscosity and pressurization rate on the fracturing process of granite. Two different type of injection fluids, water and oil, were used under different pressurization rate. Visual inspection techniques such as X-ray computed tomography and thin section imaging were employed to capture the fracture pattern together with AE monitoring. As a result, the water injection case has larger saturation zone into the formation at breakdown while the oil infiltrates only vicinity of main fracture. The AE monitoring results show that the oil injection cases have a big sudden rise in the cumulative AE hit energy during fracture propagation which is more manifest under high pressurization rate. The induced fractures are observed to be larger in aperture and less tortuous for the higher fluid viscosity and higher pressurization rate cases through thin section images. On the other hand, the sleeve testing cases yield relatively very small aperture of induced fractures.

Description: In this study, we used the Particle Flow Code 2D (PFC2D) to simulate interaction of hydraulic fractures and natural fractures in low permeable hard rock. Natural fractures are simulated using the smooth joint model of PFC2D. We modified our fluid flow algorithm to model larger fracture permeability, and we investigated interactions of hydraulic fractures and natural fractures by varying the angle of approach and viscosity of the fracturing fluid. We also investigated seismic events evolving in a complex fracture network. The results demonstrate that our modelling tool is able to capture all possible interactions of hydraulic and natural fractures: Arrest, Crossing, Slippage of hydraulic fracture, Dilation of natural fracture, Closing/Opening of natural fracture. With low angle of approach, the hydraulic fracture coalesces with the natural fractures and results in hydro-shearing and propagation of hydro-wing fractures at the tips that are mostly Mode I type. We tested the model containing multiple natural fractures with varied fluid viscosity. Hydraulic fracture generated by high viscosity fluid tends to be localized, linear and less influenced by the natural fractures. In the complex network of natural fractures, fluid columns built along the fracture network increase the local state of stress by stress shadowing. Hydro-shearing of the natural fractures that were under increased stress state can be explained as the main mechanism responsible for occurrence of larger magnitude microseismic events.

Description: The ability to control injection-induced seismicity in energy technologies like geothermal and shale gas is an important factor for assessing the safety, the seismic hazard and the life time of reservoirs. Since fracture propagation is an unavoidable process in energy extraction, we propose a new approach to optimize the seismic radiated energy with respect to the hydraulic energy during fluid injection by using cyclic and pulse pumping schemes. We use data from laboratory and mine-scale injection experiments performed at a decimeter and a decameter scale in granitic rock. We observe that the seismic radiated energy and the permeability enhancement process strongly depend on injection style and rock type. Replacing the conventional constant flow rate scheme by cyclic/pulse injection with variable flow rates (1) lowers the fracture breakdown pressure, (2) modifies the seismic event distribution, and (3) has an impact on the resulting fracture pattern. As possible explanation, we introduce the concept of fatigue hydraulic fracturing which is the result of pressure cycles and depressurization phases during which crack tip stresses are relaxed. Cyclic fluid pressure oscillations with a secondary pump allow for an efficient rock fragmentation process. During hydraulic fatigue a significant portion of the hydraulic energy is converted into damage and fracturing of rock. This finding appears to have potentially significant implications for managing the economic and physical risk posed to communities affected by fluid-injection-induced seismicity.

Description: Samples of flowback water from a 4.3 km deep geothermal borehole in granite (Pohang, South Korea) were collected following a period of hydraulic stimulation by injection of surface water. Electrical conductivity, temperature and water chemistry of the flowback water were measured. To a first approximation, the data conform closely to a simple ‘mixing tank’ model, with an exponential trend between two end members: an initial injected surface water to a more brackish ‘resident groundwater’ composition. Significant deviation from the ‘mixing tank’ trend would be an indication of significant recent water-rock interaction or other anomalous factors. Such a deviation can tentatively be seen in Na+/Cl- data, especially between 88 and 200 m3 flowback (2.8 to 8.8 hr).

Description: Flowback water from the 4215 m deep (True Vertical Depth) PX-1 borehole, following the August 2017 hydraulic stimulation of a granodiorite geothermal reservoir in Pohang, South Korea, was monitored for a suite of physicochemical, chemical and isotopic parameters. The results provide unique insights into mixing processes, fluid evolution and rapid water-rock interaction in a deep geothermal system. Injected water for stimulation was relatively fresh, oxidising surface water, with temperature 29.5 °C and pH c. 6.5. The flowback water showed an increasing content of most solutes, with the evolution conforming to an exponential ‘flushing’ model for conservative solutes such as chloride. Flowback water became progressively Na–Cl dominated, with a circumneutral pH (7.1) and negative oxidation-reduction potential (c. −180 mV). Some solutes (including, Na, K and Si) increased more rapidly than a flushing model would suggest, implying that these had been acquired by the flowback water due to mineral hydrolysis. Stable isotopes of O and H indicate that initially meteoric waters have undergone geothermal oxygen isotope exchange with minerals. Evolution of redox species in recovered water suggests progressively oxidising zonation around the injection borehole in an otherwise reducing reservoir. Rapidly increasing silica concentrations in flowback water suggests extensive quartz dissolution and indicated a reservoir temperature of up to 169 °C. This lends plausible, if equivocal support to the hypothesis that quartz dissolution by injection water may have contributed to triggering movement on the pre-stressed fault associated with the November 2017 Mw 5.5 Pohang earthquake.

Description: Faults and fractures form the largest contrast of fluid flow in the subsurface, while their permeability is highly affected by effective pressure changes. In this experimental study, fractured low-permeability Flechtingen (Rotliegend) sandstones were cyclically loaded in a MTS tri-axial compression cell. Two different loading scenarios were considered: “continuous cyclic loading” (CCL) and “progressive cyclic loading” (PCL). During continuous cyclic loading, a displaced tensile fracture was loaded hydrostatically from 2 to 60 MPa in several repeated cycles. During progressive cyclic loading, the load was increased with a step-wise function (15, 30, 45 and 60 MPa) and unloaded after every loading step. For full elasticity of rock matrix deformation each rock sample has been preconditioned up to 65 MPa. After that, an artificial tensile fracture was introduced into the sample using the Brazilian Disk test. The fractured sample was installed into the MTS triaxial cell at a given offset of 0.5 mm and hydrostatic loading was applied accordingly. The fracture permeability was measured continuously using the cubic law calculated from the hydraulic aperture. Fracture closure was measured using LVDT extensometers during the entire experiment and the resulting fracture closure and stiffness was calculated accordingly. The total deformation of the sample was corrected by the amount of elastic deformation of the rock matrix to obtain the fracture closure only. Potential changes to the fracture surface topography before and after the experiments were analysed from high-resolution surface scans obtained by a 3D profilometer using the fringe pattern projection. The scale-independent roughness exponent was calculated using power spectral density method assuming self-affinity. The fracture aperture distribution and contact-area ratio was calculated by matching the best fitting principal planes of the bottom and top surface and applying a grid search algorithm. The results showed a “stress-memory” effect of fracture stiffness during progressive loading that can be used to identify previous stress states in fractures. This effect is characterized by a transition from a non-linear to a linear (reversible to non-reversible) behaviour of specific fracture stiffness when a previous stress-maximum is exceeded. Furthermore, the evolution of fracture permeability shows less reduction during progressive cyclic loading compared to continuous cyclic loading. The data measured during the flow-through experiment under varying effective pressure are provided in the file “MTS_data.zip”. The data are provided as separate text-files as well as in Excel format with different spreadsheets, such that each figure in the paper can be recalculated and that the underlying data is comprehensive. The name of all three rock samples is given in the file name including the type of the experiment (CCL or PCL). The fracture surfaces and the fracture aperture distributions are found within the file “Surface_data.zip”. This file contains the fracture data of each of the three rock samples as point cloud data (text-files), as well the data calculated from the surfaces.

Description: Reykjavik is almost entirely heated by geothermal energy. Yet, recent growth of the city significantly increased the heat demand. Past experiences in Iceland’s capital region showed that hydraulic stimulation of existing geothermal wells is suited to improve hydraulic performance and energy supply. However, fluid injection may also trigger felt or even damaging earthquakes, which are of concern in populated areas and pose a significant risk to stimulation operations. Consequently, soft stimulation concepts have been developed to increase geothermal well performance while minimizing environmental effects such as induced seismicity. In a demonstration project of hydraulic soft stimulation in October 2019, more than 20.000 m3 of water were injected into well RV-43 in Reykjavik in multiple stages and with different injection schemes. The hydraulic performance of the well was improved without inducing felt seismicity. An a priori seismic risk assessment was conducted and for the first time the risk was continuously updated by an adaptive traffic light system supported by a sophisticated realtime microseismic monitoring. Our results confirm that it is possible to improve the performance of geothermal wells in Reykjavik and worldwide with acceptable technical, economic, and environmental risks. Here we provide an overview of the entire stimulation project including site description, stimulation design, zonal isolation, logging, seismic risk assessment and mitigation measures, realtime seismic, hydraulic and chemical monitoring, and stimulation results and challenges.

Description: 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.

Description: Earthquakes associated with fluid injection in various geo-energy settings, such as shale gas and deep geothermal energy, have shelved many projects with great potential. However, the injection-rate dependence of earthquake nucleation length, i.e., the slowly slipping (creeping) fault length in preparation for a subsequent earthquake (Kaneko & Lapusta, 2008), remains elusive. In this study, we take a step towards this issue by performing fluid injection experiments on low-permeability granite samples containing a critically stressed sawcut fault at different local injection rates (0.2 mL/min and 0.8 mL/min) and confining pressures (31 MPa and 61 MPa) (c. f., Ji & Wu, 2017; Wang et al., 2020). An array of local strain gauges and acoustic emission (AE) hypocenter locations were used to monitor the precursory slip of critically stressed faults before injection-induced stick-slip failure (c. f., Passelègue et al., 2020; Wang et al., 2020). The nucleation length was determined for each injection-induced stick-slip event, and its dependence on effective normal stress and injection rate was explored. Herein, we compile the processed data obtained from the experiments in four Excel worksheets. The full description of the methods is provided in Ji et al. (2022).

Description: Hydraulic fracturing (HF) in crystalline rock has become increasingly important in geothermal development, especially for enhanced geothermal system (EGS). However, induced or triggered earthquakes reported from EGS sites is one of the main technical hurdles encountered. New hydraulic treatments with minimal environmental impact (i.e., controlled and mitigated induced seismicity) are of great interest. The replacement of conventional HF, which employs continuous injection, by cyclic HF (CHF) that produces cycles of alternating high and low injection rates or injection pressures is suggested to assist reduction of induced seismicity. Multiscale demonstration of the cyclic hydraulic treatment was conducted within the Work Package 5 of the EU Horizon 2020 international collaboration project “Demonstration of soft stimulation treatments of geothermal reservoirs” (Acronym: DESTRESS). Proof of concept of cyclic treatment by laboratory hydraulic fracturing under X-ray CT observations was led by Korea Institute of Civil Engineering and Building Technology (KICT). We developed experimental techniques and performed a series of hydraulic fracturing equipment allowing for different sizes of rock samples and various injection schemes to be tested. Laboratory HF and CHF tests on intact granite cores containing preexisting microcracks were performed under both biaxial and true triaxial stress conditions, combined with acoustic emission (AE) monitoring. Injectivity of fractured samples were measured by injection test for evaluation of hydraulic performance. Computed tomography and thin section microscopy were applied for grainscale observations on hydraulic fractures to help further understand hydraulic fracturing mechanism. Experimental findings show that CHF systemically reduced the breakdown pressure (BP) by ~20% and the maximum amplitude of AE by ~14 dB on average, compared with conventional HF. At the grain scale, intragranular fracturing dominated regardless of the injection pattern, whereas intergranular fractures between quartz and feldspar grains were more frequently observed in CHF, which explains the reduction in BP. Cyclic injection tends to form fracturing paths of least resistance thus to mitigate maximum amplitude of AE during fracturing. In addition, CHF creates complex fractures with more branches. However, CHF increases injectivity less than conventional HF and this is likely due to the lack of single predominant fracture in CHF fractured samples. Fractures generated in conventional HF contributed greatly to the increase of fluid flow. A modified CHF consisting of combination of cyclic injection and pulse pressurization at the peak of each cycle was tested and gave an improvement in both injectivity and decreasing induced seismicity, and is suggested as a promising alternative injection scheme for cyclic hydraulic treatment.