scholarly journals Meter-scale stress heterogeneities and stress redistribution drive complex fracture slip and fracture growth during a hydraulic stimulation experiment

2021 ◽  
Author(s):  
Linus Villiger ◽  
Valentin Samuel Gischig ◽  
Grzegorz Kwiatek ◽  
Hannes Krietsch ◽  
Joseph Doetsch ◽  
...  
Keyword(s):  
Shear Zone ◽  
Induced Seismicity ◽  
Damage Zone ◽  
Test Site ◽  
Crystalline Rock ◽  
Stress Redistribution ◽  
Small Scale ◽  
Hydraulic Stimulation ◽  
Source Mechanisms ◽  
Stimulation Of

Summary We investigated the induced seismicity, source mechanisms and mechanical responses of a decameter-scale hydraulic stimulation of a pre-existing shear zone in crystalline rock, at the Grimsel Test Site, Switzerland. The analysis reveals the meter-scale complexity of hydraulic stimulation, which remains hidden at the reservoir-scale. High earthquake location accuracy allowed the separation of four distinct clusters, of which three were attributed to the stimulation of fractures in the damage zone of the shear zone. The source mechanism of the larger-magnitude seismicity varied by cluster, and suggests a heterogeneous stress field already prevailing before stimulation, which is further modified during stimulation. In the course of the experiment, stress redistribution led to the aseismic initiation of a tensile-dominated fracture, which induced seismicity in the fourth of the identified seismic clusters. The streaky pattern of seismicity separated by zones without seismicity suggests fluid flow in conduits along existing fracture planes. The observed sub-meter scale complexity questions the forecasting ability of induced seismic hazard at the reservoir scale from small-scale experiments.

scholarly journals Hydro-mechanical processes and their influence on the stimulation effected volume: Observations from a decameter-scale hydraulic stimulation project

2020 ◽  
Author(s):  
Hannes Krietsch ◽  
Valentin S. Gischig ◽  
Joseph Doetsch ◽  
Keith F. Evans ◽  
Linus Villiger ◽  
...  
Keyword(s):  
Shear Zone ◽  
Mechanical Response ◽  
Compressive Strain ◽  
Shear Zones ◽  
Test Site ◽  
Crystalline Rock ◽  
Injection Point ◽  
Hydraulic Stimulation ◽  
Fracture Fluid ◽  
As Stress

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 one out of two different shear zone types were hydraulically reactivated. An extensive monitoring system of sensors recording seismicity, pressure and strain was spatially distributed in eleven boreholes around the injection locations. As a result of the stimulation, the near-wellbore transmissivity increased up to three orders in magnitude, while jacking pressures of the stimulated structures reduced during most of the experiments. Transmissivity change, jacking pressure and seismic activity were different for the two shear zone types, suggesting that the shear zone characteristics govern the seismo-hydro-mechanical 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 m–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 the strain magnitude decreases with increasing distance. This compressive strain signals are interpreted as a poro-elastic far-field response to the emplacement of fluid volume around the injection interval. The exceptional hydro-mechanical 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.

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

Solid Earth ◽  
2020 ◽  
Vol 11 (5) ◽  
pp. 1699-1729 ◽  
Author(s):  
Hannes Krietsch ◽  
Valentin S. Gischig ◽  
Joseph Doetsch ◽  
Keith F. Evans ◽  
Linus Villiger ◽  
...  
Keyword(s):  
Shear Zone ◽  
Compressive Strain ◽  
Shear Zones ◽  
Test Site ◽  
Crystalline Rock ◽  
Injection Point ◽  
Hydraulic Stimulation ◽  
Spatially Distributed ◽  
Fracture Fluid ◽  
As Stress

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.

scholarly journals The seismo-hydro-mechanical behaviour during deep geothermal reservoir stimulations: open questions tackled in a decameter-scale in-situ stimulation experiment

2017 ◽  
Author(s):  
Florian Amann ◽  
Valentin Gischig ◽  
Keith Evans ◽  
Joseph Doetsch ◽  
Reza Jalali ◽  
...  
Keyword(s):  
Rock Mass ◽  
Geothermal Energy ◽  
Numerical Models ◽  
Test Site ◽  
Crystalline Rock ◽  
Coupled Processes ◽  
Small Scale ◽  
Intermediate Scale ◽  
Grimsel Test Site

Abstract. In this contribution we present a review of scientific research results that address seismo-hydro-mechanical coupled processes relevant for the development of a sustainable heat exchanger in low permeability crystalline rock and introduce the design of the In-situ Stimulation and Circulation (ISC) experiment at the Grimsel Test Site dedicated to study such processes under controlled conditions. The review shows that research on reservoir stimulation for deep geothermal energy exploitation has been largely based on laboratory observations, large-scale projects and numerical models. Observations of full-scale reservoir stimulations have yielded important results. However, the limited access to the reservoir and limitations in the control on the experimental conditions during deep reservoir stimulations is insufficient to resolve the details of the hydro-mechanical processes that would enhance process understanding in a way that aids future stimulation design. Small scale laboratory experiments provide a fundamental insights into various processes relevant for enhanced geothermal energy, but suffer from 1) difficulties and uncertainties in upscaling the results to the field-scale and 2) relatively homogeneous material and stress conditions that lead to an over-simplistic fracture flow and/or hydraulic fracture propagation behaviour that is not representative for a heterogeneous reservoir. Thus, there is a need for intermediate-scale hydraulic stimulation experiments with high experimental control that bridge the various scales, and for which access to the target rock mass with a comprehensive monitoring system is possible. Only few intermediate-scale hydro-shearing and hydro-fracturing experiments have recently been performed in a densely instrumented rock mass. No such measurements have been performed on faults in crystalline basement rocks. The In-situ Stimulation and Circulation (ISC) experiment currently performed in a naturally fractured and faulted crystalline rock mass at the Grimsel Test Site (Switzerland) is designed to address open research questions, which could not be investigated in the required detail so far. Two hydraulic injection phases were executed to enhance the permeability of the rock mass: a hydro-shearing phase and then a hydraulic fracturing phase. During the injection phases the rock mass deformation across fractures and within intact rock, the pore pressure distribution and propagation and the micro-seismic response were monitored at a high spatial and temporal resolution.

scholarly journals Induced seismicity risk analysis of the hydraulic stimulation of a geothermal well on Geldinganes, Iceland

2020 ◽  
Vol 20 (6) ◽  
pp. 1573-1593
Author(s):  
Marco Broccardo ◽  
Arnaud Mignan ◽  
Francesco Grigoli ◽  
Dimitrios Karvounis ◽  
Antonio Pio Rinaldi ◽  
...  
Keyword(s):  
Risk Assessment ◽  
Risk Analysis ◽  
Induced Seismicity ◽  
Energy Demand ◽  
Total Duration ◽  
Individual Risk ◽  
Hydraulic Stimulation ◽  
Starting Point ◽  
Damage Risk ◽  
Stimulation Of

Abstract. The rapid increase in energy demand in the city of Reykjavik has posed the need for an additional supply of 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 be a nuisance to the population and even damage 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 risk assessment with in situ data collected during the stimulation. Therefore, it is important to stress that this a priori study represents a baseline model and starting point to be updated and refined after the start of the project.

scholarly journals Influence of reservoir geology on seismic response during decameter-scale hydraulic stimulations in crystalline rock

Solid Earth ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 627-655 ◽  
Author(s):  
Linus Villiger ◽  
Valentin Samuel Gischig ◽  
Joseph Doetsch ◽  
Hannes Krietsch ◽  
Nathan Oliver Dutler ◽  
...  
Keyword(s):  
Seismic Response ◽  
Fluid Pressure ◽  
Shear Zones ◽  
Test Site ◽  
Fracture Network ◽  
Crystalline Rock ◽  
Hydraulic Fractures ◽  
Geothermal Systems ◽  
Hydraulic Stimulation ◽  
Shear Dilation

Abstract. We performed a series of 12 hydraulic stimulation experiments in a 20m×20m×20m foliated, crystalline rock volume intersected by two distinct fault sets at the Grimsel Test Site, Switzerland. The goal of these experiments was to improve our understanding of stimulation processes associated with high-pressure fluid injection used for reservoir creation in enhanced or engineered geothermal systems. In the first six experiments, pre-existing fractures were stimulated to induce shear dilation and enhance permeability. Two types of shear zones were targeted for these hydroshearing experiments: (i) ductile ones with intense foliation and (ii) brittle–ductile ones associated with a fractured zone. The second series of six stimulations were performed in borehole intervals without natural fractures to initiate and propagate hydraulic fractures that connect the wellbore to the existing fracture network. The same injection protocol was used for all experiments within each stimulation series so that the differences observed will give insights into the effect of geology on the seismo-hydromechanical response rather than differences due to the injection protocols. Deformations and fluid pressure were monitored using a dense sensor network in boreholes surrounding the injection locations. Seismicity was recorded with sensitive in situ acoustic emission sensors both in boreholes and at the tunnel walls. We observed high variability in the seismic response in terms of seismogenic indices, b values, and spatial and temporal evolution during both hydroshearing and hydrofracturing experiments, which we attribute to local geological heterogeneities. Seismicity was most pronounced for injections into the highly conductive brittle–ductile shear zones, while the injectivity increase on these structures was only marginal. No significant differences between the seismic response of hydroshearing and hydrofracturing was identified, possibly because the hydrofractures interact with the same pre-existing fracture network that is reactivated during the hydroshearing experiments. Fault slip during the hydroshearing experiments was predominantly aseismic. The results of our hydraulic stimulations indicate that stimulation of short borehole intervals with limited fluid volumes (i.e., the concept of zonal insulation) may be an effective approach to limit induced seismic hazard if highly seismogenic structures can be avoided.

scholarly journals Induced seismicity risk analysis of the hydraulic stimulation of a geothermal well on Geldinganes, Iceland

2019 ◽  
Author(s):  
Marco Broccardo ◽  
Arnaud Mignan ◽  
Francesco Grigoli ◽  
Dimitrios Karvounis ◽  
Antonio Pio Rinaldi ◽  
...  
Keyword(s):  
Risk Assessment ◽  
Risk Analysis ◽  
Induced Seismicity ◽  
Energy Demand ◽  
Total Duration ◽  
Hot Water ◽  
Individual Risk ◽  
Hydraulic Stimulation ◽  
Damage Risk ◽  
Stimulation Of

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.

The STIMTEC-X hydraulic stimulation campaign at the URL Reiche Zeche Mine, Germany

2021 ◽  
Author(s):  
Carolin Boese ◽  
Joerg Renner ◽  
Georg Dresen ◽  
Keyword(s):  
Acoustic Emission ◽  
High Resolution ◽  
Stress Field ◽  
Stress Measurement ◽  
Damage Zone ◽  
Local Stress ◽  
Seismic Monitoring ◽  
Underground Laboratory ◽  
Small Scale ◽  
Hydraulic Stimulation

<p>In 2018-2019, the STIMTEC hydraulic stimulation experiment was conducted at the Reiche Zeche underground laboratory in Freiberg, Saxony/Germany, to investigate the role of hydro-mechanical processes for the often required enhancement of hydraulic properties in deep geothermal projects. We applied the same injection protocol to each of the ten stimulated intervals in the 63 m-long injection borehole. Yet, we observed significant small-scale variability in the seismic and hydraulic responses to stimulation and in parallel stress field heterogeneity on the meter scale. While acoustic emission (AE) activity was high in the upper part of the injection borehole, no AE events were detected in its deepest part, ending in a high-permeability damage zone.</p><p>To investigate the stress field and seismic variability throughout the experimental volume and their interrelation further, we started the follow-on experiment STIMTEC-X. The initial phase involved eleven local stress measurements performed in October 2020 in three existing boreholes, previously used for monitoring purposes, with varying orientations and lengths. This phase of the experiment was seismically monitored in real-time using an adaptive, high-resolution seismic monitoring network comprising six AE-type hydrophones, six regular AE sensors and four accelerometers. The hydrophones were installed in combination with hydraulic gauges or the double packer probe used for localized injection to make best use of the existing infrastructure. Hydrophones were optimally placed for each measurement configuration anew with at least one deployed in the direct vicinity (~3-4 m) of the injection interval. We detected low-magnitude AE activity (M<-3.5) at high resolution, spatially distributed between distinct clusters identified previously during the STIMTEC experiment. Overall, these records indicate a doubling of the seismically active volume. We also performed eight dilatometer tests to determine deformation characteristics of induced hydrofracs and pre-existing fractures. A circulation experiment between the injection borehole and two newly drilled boreholes of 23 m and 30 m depth is pending. Here, we present the seismicity associated with the STIMTEC and STIMTEC-X hydraulic stimulation campaigns and focal mechanism solutions. We focus on how they contribute to 3-D volumetric stress field characterisation between local stress measurement points.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.5069b42fb60064884501161/sdaolpUECMynit/12UGE&app=m&a=0&c=928e8094105ded0e25c2a2da4e49946e&ct=x&pn=gnp.elif&d=1" alt=""></p><p>Figure 1: Borehole layout (cyan - injection borehole, yellow: seismic monitoring boreholes, green: hydraulic monitoring borehole, red: mine-back validation boreholes) and acoustic emission (AE) events during the STIMTEC (yellow and orange circles) and STIMTEC-X (purple circle) experiments at the Reiche Zeche underground laboratory in Freiberg, Germany. Damage zones (transparent red) and hydraulically stimulated (dark blue rings) and/or hydraulically tested intervals (light blue rings) are shown. Stimulation of the intervals resulted in >11000 AE events with most events occurring during the periodic pumping sequences following the hydrofracturing. The seismic clouds extend about 5 m radially around the boreholes.</p>

scholarly journals Influence of reservoir geology on seismic response during decameter scale hydraulic stimulations in crystalline rock

2019 ◽  
Author(s):  
Linus Villiger ◽  
Valentin Samuel Gischig ◽  
Joseph Doetsch ◽  
Hannes Krietsch ◽  
Nathan Oliver Dutler ◽  
...  
Keyword(s):  
Seismic Response ◽  
Mechanical Response ◽  
Fluid Pressure ◽  
Shear Zones ◽  
Test Site ◽  
Fracture Network ◽  
Crystalline Rock ◽  
Hydraulic Fractures ◽  
Geothermal Systems ◽  
Hydraulic Stimulation

Abstract. We performed a series of 12 hydraulic stimulation experiments in a 20 × 20 × 20 m foliated, crystalline rock volume intersected by two distinct fault sets at the Grimsel Test Site, Switzerland. The goal of these experiments was to improve our understanding of stimulation processes associated with high-pressure fluid injection used for reservoir creation in enhanced or engineered geothermal systems. In the first six experiments, pre-existing fractures were stimulated to induce shear dilation and enhance permeability. Two types of shear zones were targeted for these hydroshearing experiments: i) ductile ones with intense foliation and ii) brittle-ductile ones associated with a fractured zone. The second series of six stimulations were performed in borehole intervals without natural fractures to initiate and propagate hydraulic fractures that connect the wellbore to the existing fracture network. The same injection protocol was used for all experiments within each stimulation series so that the differences observed will give insights into the effect of geology on the seismo-hydro-mechanical response rather than differences due to the injection protocols. Deformations and fluid pressure were monitored using a dense sensor network in boreholes surrounding the injection locations. Seismicity was recorded with sensitive in-situ acoustic emission sensors both in boreholes and at the tunnel walls. We observed high variability in the seismic response in terms of seismogenic indices, b-values, spatial and temporal evolution during both hydroshearing and hydrofracturing experiments, which we attribute to local geological heterogeneities. Seismicity was most pronounced for injections into the highly conductive brittle-ductile shear zones, while injectivity increase on these structures was only marginal. No significant differences between the seismic response of hydroshearing and hydrofracturing was identified, possibly because the hydrofractures interact with the same pre-existing fracture network that is reactivated during the hydroshearing experiments. Fault slip during the hydroshearing experiments was predominantly aseismic. The results of our hydraulic stimulations indicate that stimulation of short borehole intervals with limited fluid volumes (i.e., the concept of zonal insulation) may be an effective approach to limit induced seismic hazard if highly seismogenic structures can be avoided.

Sign in / Sign up

Export Citation Format

Share Document