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Abstract. The rapid increase in energy demand in the city of Reykjavik has posed the need for an additional supply of hot water from deep geothermal energy. The deep hydraulic (re-)stimulation of well RV-43 on the peninsula of Geldinganes (north of Reykjavik) is an essential component of the plan implemented by Reykjavik Energy to meet this energy target. Hydraulic stimulation is often associated with fluid-induced seismicity, most of which is not felt on the surface, but which, in rare cases, can cause nuisance to the population and even damage to the nearby building stock. This study presents a first of its kind pre-drilling probabilistic induced-seismic hazard and risk analysis for the site of interest. Specifically, we provide probabilistic estimates of peak ground acceleration, European microseismicity intensity, probability of light damage (damage risk), and individual risk. The results of the risk assessment indicate that the individual risk within a radius of 2 km around the injection point is below 0.1 micromorts, and damage risk is below 10−2, for the total duration of the project. However, these results are affected by several orders of magnitude of variability due to the deep uncertainties present at all levels of the analysis, indicating a critical need in updating this a-priory risk assessment with in-situ data collected during the stimulation.

In this article, a high-resolution acoustic emission sensor, accelerometer, and broadband seismometer array data set is made available and described in detail from in situ experiments performed at Äspö Hard Rock Laboratory in May and June 2015. The main goal of the hydraulic stimulation tests in a horizontal borehole at 410 m depth in naturally fractured granitic rock mass is to demonstrate the technical feasibility of generating multi-stage heat exchangers in a controlled way superiorly to former massive stimulations applied in enhanced geothermal projects. A set of six, sub-parallel hydraulic fractures is propagated from an injection borehole drilled parallel to minimum horizontal in situ stress and is monitored by an extensive complementary sensor array implemented in three inclined monitoring boreholes and the nearby tunnel system. Three different fluid injection protocols are tested: constant water injection, progressive cyclic injection, and cyclic injection with a hydraulic hammer operating at 5 Hz frequency to stimulate a crystalline rock volume of size 30 m × 30 m × 30 m at depth. We collected geological data from core and borehole logs, fracture inspection data from an impression packer, and acoustic emission hypocenter tracking and tilt data, as well as quantified the permeability enhancement process. The data and interpretation provided through this publication are important steps in both upscaling laboratory tests and downscaling field tests in granitic rock in the framework of enhanced geothermal system research. Data described in this paper can be accessed at GFZ Data Services under 10.5880/GFZ.2.6.2023.004. [ABSTRACT FROM AUTHOR]

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).

Fault zones often serve as the major fluid pathways in a variety of geo-energy systems, such as deep geothermal systems. However, injection-induced instability of faults can sometimes lead to large-magnitude earthquakes. Cyclic injection has thus been proposed as an alternative injection protocol to better manage and mitigate the associated seismic risks. The risks of injection-induced seismicity depend primarily on the extent and magnitude of the fluid pressure perturbation. When fluid is injected into a fault zone, the local fault permeability will be enhanced, which in turn promotes the migration of fluid along the fault. This nonlinear process is further complicated during cyclic injection via alternating the injection pressure. In this study, both numerical and analytical modelling are conducted to investigate cyclic fluid injection into a fault zone with pressure sensitive permeability, in which the local fault permeability changes as a function of the local effective stress. The match with laboratory-scale experimental and field-scale analytical results of cyclic fluid injection verifies the accuracy of the numerical model. The parametric study reveals that the injection pressure attenuation, quantified by the amplitude ratio and phase shift, is enhanced by a lower initial fault permeability, a smaller stress sensitivity coefficient and a shorter period of pressure cycle (i.e. a higher frequency). Besides, the amplitude of the pressure cycle has a negligible effect on the injection pressure attenuation. We also discuss the implications of our results for the less amenable far-field seismic hazard and post shut-in seismicity. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper, an underground experiment at the Äspö Hard Rock Laboratory (HRL) is described. Main goal is optimizing geothermal heat exchange in crystalline rock mass at depth by multistage hydraulic fracturing with minimal impact on the environment, that is, seismic events. For this, three arrays with acoustic emission, microseismicity and electromagnetic sensors are installed mapping hydraulic fracture initiation and growth. Fractures are driven by three different water injection schemes (continuous, progressive and pulse pressurization). After a brief review of hydraulic fracture operations in crystalline rock mass at mine scale, the site geology and the stress conditions at Äspö HRL are described. Then, the continuous, single-flow rate and alternative, multiple-flow rate fracture breakdown tests in a horizontal borehole at depth level 410 m are described together with the monitoring networks and sensitivity. Monitoring results include the primary catalogue of acoustic emission hypocentres obtained from four hydraulic fractures with the in situ trigger and localizing network. The continuous versus alternative water injection schemes are discussed in terms of the fracture breakdown pressure, the fracture pattern from impression packer result and the monitoring at the arrays. An example of multistage hydraulic fracturing with several phases of opening and closing of fracture walls is evaluated using data from acoustic emissions, seismic broad-band recordings and electromagnetic signal response. Based on our limited amount of in situ tests (six) and evaluation of three tests in Ävrö granodiorite, in the multiple-flow rate test with progressively increasing target pressure, the acoustic emission activity starts at a later stage in the fracturing process compared to the conventional fracturing case with continuous water injection. In tendency, also the total number and magnitude of acoustic events are found to be smaller in the progressive treatment with frequent phases of depressurization. [ABSTRACT FROM AUTHOR]

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