ALBERT

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: One of Sweden’s most successful geologists, Professor Ove Stephansson, passed away on February, 19 2020. During his professional life he published more than 200 articles, conference papers and books.

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: In this article, the concept of Fatigue Hydraulic Fracturing (FHF) is described, and its geothermal application is discussed. The basic idea behind fatigue fracturing is to vary the effective stress magnitudes at the fracture tip to optimize fracture initiation and growth. The optimization process can include lowering seismic radiated energy and/or generating fracture networks with various geometry and permeability. Historically, we start referring to results from mechanical laboratory core testing, discrete element simulation of fluid-induced seismicity, and application of cyclic water-fracs at the enhanced geothermal system site Groß-Schönebeck, Germany. Then, an in situ experiment at Äspö Hard Rock Laboratory is summarized to bridge the gap between laboratory core testing and wellbore-size hydraulic fracture treatments in hard rock. Three different fluid injection schemes (continuous, progressive and pulse injection) are tested underground in naturally fractured, crystalline rock mass in terms of associated induced seismicity and permeability performance. Under controlled conditions, hydraulic fractures are extended to about 20–40 m2 in size from a 28 m long, horizontal borehole drilled from a tunnel at 410 m depth. The facture process is mapped by an extensive array of acoustic emission and micro-seismic monitoring instruments. Results from three water-injection tests in Ävrö granodiorite indicate that the fracture breakdown pressure in tendency becomes lower and the number of fluid-induced seismic events becomes less when continuous, conventional fluid injection is replaced by progressive fluid-injection with several phases of depressurization simulating the fatigue treatment. One reason for this may be that in the dynamic, fatigue treatment a larger fracture process zone is generated compared to the size of the fluid pressurized zone developing during the injection phases into crystalline rock. We see mine-scale tests with hybrid sensor arrays of importance to identify and understand the actual hydraulic fracture mechanisms in hard rock. In addition, the mesoscale data obtained underground allow downscaling to laboratory core results, and upscaling to borehole reservoir stimulation results.

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: A near field network with 11 acoustic emission (AE) sensors was installed for the in situ underground experiment (Nova project 54-14-1) that took place 410 m below surface in the Äspö Hard Rock Laboratory, Sweden. The acquisition system for the piezoelectrical sensors has been improved to record signals with 1 MHz sampling rate, to detect signals produced by weaker sources and enhance the microseismic catalogue. The acquisition system was capable to operate in trigger and continuous mode. The basic idea of the experiment was to compare hydraulic fracturing growth and induced seismicity under controlled conditions for different loading scenarios as conventional versus progressive, and pulse-like water injections. In this work, we consider continuous recordings and apply recently developed automated full waveform detection and location algorithms which are based on the stacking of characteristic functions calculated from squared amplitudes. Waveform stacking and coherence techniques are adapted to detect and locate AE signals for massive datasets with extremely high sampling. We significantly increase the detection rate in comparison to trigger mode routines. Most detection concentrated during the fluid injection occurred around the fracking stages. Frequency-magnitude distribution characteristics are investigated using a relative magnitude scale estimated from the amplitude recorded at AE sensors. We demonstrate that the stacking of characteristic functions yields to a significant improvement of the detection and location also in presence of noisy records, supporting the adoption of similar techniques for other induced and natural seismic activity monitoring systems.

Description: Multistage mini hydraulic fracturing tests were performed in a borehole located in central Hungary in order to determine in-situ stress. At depth of about 500 to 560 meters, observed pressure versus time curve in metamorphic rock (mica schist) show a typical results. After each pressurization cycle, the fracture breakdown pressure in the first fracturing cycle is lower than the reopening pressures in the subsequent reopening and step-rate phases. It is assumed that the composition of the drilling mud and observed foliation of the mica schist have a significant influence on the pressure values. In order to investigate this problem, numerical modeling was performed using the discrete element code (ITASCA Particle Flow Code, PFC), which has been proven as an effective tool to investigate rock engineering problems associated with hydraulic fracturing. The code presented in this study enables simulating hydro-mechanically coupled fluid flow in crystalline rock with low porosity and pre-existing fractures (represented by the smooth joint contact model in PFC) in two dimensions. In this study, the sensitivity of the effect of foliation angle and fluid viscosity on the peak pressure is tested. The anomalous characteristics of the pressure behavior are interpreted in that way that the drilling mud penetrates the sub-horizontal foliation plane, it clogs the plane of weakness and makes the opened fracture tight. Eventually, the process prevents leak-off from the opened fracture that might explain the increased fracture reopening pressure in subsequent cycles.

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.