ALBERT

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: 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.

Description: The selection of earthquake focal mechanisms (FMs) for stress tensor inversion (STI) is commonly done on a spatial basis, that is, hypocentres. However, this selection approach may include data that are undesired, for example, by mixing events that are caused by different stress tensors when for the STI a single stress tensor is assumed. Due to the significant increase of FM data in the past decades, objective data-driven data selection is feasible, allowing more refined FM catalogues that avoid these issues and provide data weights for the STI routines. We present the application of angular classification with expectation-maximization (ACE) as a tool for data selection. ACE identifies clusters of FM without a priori information. The identified clusters can be used for the classification of the style-of-faulting and as weights of the FM data. We demonstrate that ACE effectively selects data that can be associated with a single stress tensor. Two application examples are given for weighted STI from South America. We use the resulting clusters and weights as a priori information for an STI for these regions and show that uncertainties of the stress tensor estimates are reduced significantly.

Description: Hydraulic fracturing tests were performed in the laboratory on cubic granite specimens with a side length of 100 mm, under true triaxial stress conditions combined with acoustic emission (AE) monitoring. Six different injection schemes were applied for investigating influence of the injection scheme on hydraulic performance and induced seismicity during fracturing. Three of them are injection rate controlled: constant rate continuous injection (CC), stepwise rate continuous injection (SC), cyclic progressive injection (CP); and the other three are pressurization rate controlled: stepwise pressurization (SP), stepwise pulse pressurization (SPP), cyclic pulse pressurization (CPP). Test results show that the SPP scheme achieves the highest increase in injectivity among the six injection schemes. The CP scheme has the lowest induced seismicity while improvement in injectivity is the least. The CPP scheme achieved a reasonable improvement in both increasing injectivity and decreasing induced seismicity, and is suggested as a promising alternative injection scheme. Microscopic observation on thin sections of fractured specimens was employed. Eighteen hydraulic fractures in ten specimens fractured by different injection schemes were measured using the ImageJ software for a quantitative evaluation at mineral scale. Intragranular fractures splitting microcline, orthoclase and quartz grains dominated (〉70%) in all cases irrespective of injection schemes. The SPP scheme creates the largest fracture length which could explain the highest injectivity among all the testing schemes. The testing cases with relatively low magnitudes of the maximum AE amplitude correspond to short fracture lengths and small portions of intragranular fractures in microcline grains. Quartz grains are more fractured than microcline and orthoclase grains. Quartz chips are frequently observed adjacent to hydraulic fractures, and this is highly related to the preexisting microcracks abundant in quartz grains, which degrade the grain strength and interact with hydraulic fractures during the fracturing process.