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Observation of a Repeated Step-wise Fracture Growth During Hydraulic Fracturing Experiment at the Grimsel Test Site
Hydraulic fracturing (HF) experiments were conducted at the Grimsel Test Site (GTS), Switzerland, with the aim to improve our understanding of the seismo-hydro-mechanical processes associated with high-pressure fluid injection in a moderately fractured crystalline rock mass. Observations from one of these HF experiments indicate simultaneous propagation of multiple fractures during continuous fluid injection. The pressure measured in one observation interval show a cyclic response indicating repeated step-wise fracture growth. This is interpreted as a stick-split mechanism propagating fractures in an episodic manner and connecting them to the natural fracture network. In addition, transient partial closure and opening of fractures on the time-scale of seconds to minutes were observed from pressure and deformation monitoring. Our data set provides unprecedented insight in the complexity of hydraulic fracture propagation.
Hydromechanical insight of fracture opening and closure during in-situ hydraulic fracturing in crystalline rock
Six hydraulic fracturing (HF) experiments were conducted in situ at the Grimsel Test Site (GTS), Switzerland, using two boreholes drilled in sparsely fractured crystalline rock. High spatial and temporal resolution monitoring of fracture fluid pressure and strain improve our understanding of fracturing dynamics during and directly following high-pressure fluid injection. In three out of the six experiments, a shear-thinning fluid with an initial static viscosity approximately 30 times higher than water was used to understand the importance of fracture leak-off better. Diagnostic analyses of the shut-in phases were used to determine the minimum principal stress magnitude for the fracture closure cycles, yielding an estimate of the effective instantaneous shut-in pressure (effective ISIP) 4.49±0.22 MPa. The jacking pressure of the hydraulic fracture was measured during the pressurecontrolled step-test. A new method was developed using the uniaxial Fibre-Bragg Grating strain signals to estimate the jacking pressure, which agrees with the traditional flow versus pressure method. The technique has the advantage of observing the behavior of natural fractures next to the injection interval. The experiments can be divided into two groups depending on the injection location (i.e., South or North to a brittle-ductile S3 shear zone). The experiments executed South of this zone have a jacking pressure above the effective ISIP. The proximity to the S3 shear zone and the complex geological structure led to near-wellbore tortuosity and heterogeneous stress effects masking the jacking pressure. In comparison, the experiments North of the S3 shear zone has a jacking pressure below the effective ISIP. This is an effect related to shear dislocation and fracture opening. Both processes can occur almost synchronously and provide new insights into the complicated mixedmode deformation processes triggered by high-pressure injection.
Seismo-hydromechanical interaction during in-situ hydraulic fracturing experiments
Hydraulic fracturing is a common technique used in a variety of fields like civil and mining engineering, oil & gas and geothermal industry. It can be used to enhance the permeability of low permeable rocks, to increase the connectivity of natural fractures, to modify the rock mass strength, or to measure the Earth’s stress field. In the context of deep geothermal energy exploitation, a heat exchanger needs to be created at depth with characteristics favorable for heat extraction i.e. sufficient permeability and heat exchanger area. The creation of the heat exchanger for geothermal heat extraction remains a critical element with high associated risks including poor reservoir performance and induced seismicity. Hence the need for a better understanding of the coupled seismic-hydromechanical processes during stimulation operations. The execution of experiments on the intermediate-scale has the advantage of a better control on the processes associated with induced seismicity and reservoir performance compared to full-scale and allow to use comprehensive real time monitoring of pore pressure, rock mass deformation and seismicity. This scale is closer to the full-scale stimulation than laboratory scale, where seismo-hydromechanical interactions are generally focused on single fractures. The decameter-scale In-situ Stimulation and Circulation (ISC) project took place between 2015 and 2018 at the Grimsel Test Site (GTS), Switzerland. The GTS is located in the Central Swiss Alps, beneath the mountains of the Grimsel Pass. Overall, the moderately crystalline fractured rock mass shows a pervasive foliation and was intersected by six major sub-vertical shear zones. For each of the two assumed stimulation endmembers, hydraulic shearing and hydraulic fracturing, six experiments were conducted. Prior to the experiments, the test volume was characterized in great detail with respect to geology, geophysics, hydrogeology and in-situ stress field. This doctoral thesis aims at better understanding tensile fracture growth. It includes study of fracture toughness and fracture process zone on laboratory scale and the investigation of the seismo-hydromechanical coupled processes during in-situ hydraulic fracturing experiments. The tested intact Grimsel Granodiorite samples indicate that the resistance against material failure is significantly higher across the foliation plane than along it. The results from Digital Image Correlation (DIC) confirm the development of a semi-elliptical fracture process zone (FPZ) with an average length to width ratio of about two for both principal directions. This agrees well with the available results in the literature. The experimental results of the length of the FPZ give supporting evidence to the fact that a nonlinear cohesion stress distribution provides an accurate cohesive model that agrees well with the experimental results. Additionally, the conformity of the ratio of the FPZ length in two principal directions with the theoretical predictions gives supporting evidence to the proportionality of the FPZ length with respect to the square of fracture toughness to tensile strength. At the decametric scale during the in-situ experiment, the hydromechanical coupled responses of the rock mass and its fractures were captured by a comprehensive monitoring system installed along the tunnels and within dedicated boreholes. At the borehole scale, these processes involved newly created tensile fractures intersecting the injection interval while at the cross-hole scale, the natural network of fractures dominated the propagation process. The six HF experiments can be divided into two groups based on their injection location (i.e., south or north to a brittle ductile shear zone), their similarity of injection pressures and their response to deformation and pressure propagation. The experiments executed north of the shear zone, show smaller injection pressures and larger backflow during bleedoff phases. In addition, we observe re-orientation of the seismic cloud as the fracture propagated away from the wellbore. The re-orientation during propagation is interpreted to be related to a strong stress heterogeneity and the intersection of natural fractures striking different from the propagating hydraulic fracture. This leads in the details to complex geometry departing from theoretical mode I fracture geometries. The seismic activity was limited to about 10 m radial distance from the injection point. In contrast, strain and pressure signals reach further into the rock mass indicating that the process zone around the injection point is larger than the zone illuminated by seismic signals. Furthermore, strain signals indicate not just single fracture openings but also the propagation of multiple fractures. Various methods to estimate the fracture opening and fracture contact pressure were applied and compared from single injection borehole observations with the strain gauge in distance from the injection point. The results show, that the fracture opening pressure was also observed at the strain gauge, associated with a strong increase of fracture transmissivity. The combination of injection pressure and strain observation allows to define an aperture-stress relationship with a general trend toward decreasing normal fracture stiffness during fracture opening. The fracture contact pressure can be estimated, but hydromechanical superposition of pump shut-in and corresponding pressure loss and interaction of the connected surrounding fractures make this task very challenging and error-prone. The pore pressure data set differentiate two distinct responses based on lag time and amplitudes. This allow to distinguish a near- and far-field response. The near-field response is due to pressure diffusion and the far-field response is due to stress perturbation. The far-field pore pressure response is consistent for all experiments, indicating the dominant failure mechanism. This change in the far-field are very sensitive and can be used as a complementary method to seismic monitoring during hydraulic stimulations. The exceptional hydromechanical dataset allow to test numerical stimulations and can help to improve injection strategies, the monitoring design and the numerical modelling.
Observations of fracture propagation during decameter-scale hydraulic fracturing experiments
Various in-situ hydraulic fracturing experiments were carried out in the naturally fractured, crystalline rock mass of the Grimsel Test Site (GTS) in Switzerland. The purpose was to study the geometry of the newly created fractures and their interaction with the pre-existing fracture network using transient pressure and rock mass deformation observations. Under controlled conditions, six hydraulic fractures with similar injection protocols were executed in two sub-vertical injection boreholes. The rock mass is intersected by two E-W striking shear zones (S3), and two biotite-rich meta-basic dykes with a densely fractured zone in between. The S3 shear-zone intersecting the rock volume of interest acts as a high-permeability connection to the tunnel for the experiments executed south of it. Strong variation in injectivity enhancement, jacking pressure, break down pressure, instantaneous shut-in pressure and fluid flow recovery among the different injection intervals indicate different stress conditions north and south of S3.
Poroelasticity Contributes to Hydraulic-Stimulation Induced Pressure Changes
High-pressure fluid injections cause transient pore pressure changes over large distances, which may induce seismicity. The zone of influence for such an injection was studied at high spatial esolutions in six decameter-scaled fluid injection experiments in crystalline rock. Pore pressure time series revealed two distinct responses based on the lag time and magnitude of pressure change, namely, a near- and far-field response. The near-field response is due to pressure diffusion. In the far-field, the fast response time and decay of pressure changes are produced by effective stress changes in the anisotropic stress field. Our xperiments confirm that fracture fluid pressure perturbations around the injection point are not limited to the near field and can extend beyond the pressurized zone.
Influence of reservoir geology on seismic response during decameter-scale hydraulic stimulations in crystalline rock
We performed a series of 12 hydraulic stimulation experiments in a 20 m×20 m×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-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.
Stress Measurements for an In Situ Stimulation Experiment in Crystalline Rock: Integration of Induced Seismicity, Stress Relief and Hydraulic Methods
An extensive campaign to characterize rock stresses on the decameter scale was carried out in three 18–24 m long boreholes drilled from a tunnel in foliated granite at the Grimsel Test Site, Switzerland. The survey combined stress relief methods with hydrofracturing (HF) tests and concomitant monitoring of induced seismicity. Hydrofracture traces at the borehole wall were visualized with impression packer tests. The microseismic clouds indicate sub-vertical south-dipping HFs. Initial inversion of the overcoring strains with an isotropic rock model yielded stress tensors that disagreed with the HF and microseismic results. The discrepancy was eliminated using a transversely isotropic rock model, parametrized by a novel method that used numerical modelling of the in situ biaxial cell data to determine the requisite five independent elastic parameters. The results show that stress is reasonably uniform in the rock volume that lies to the south of a shear zone that cuts the NNW of the study volume. Stress in this volume is considered to be unperturbed by structures, and has principal stress magnitudes of 13.1–14.4 MPa for σ1, 9.2–10.2 MPa for σ2, and 8.6–9.7 MPa for σ3 with σ1 plunging to the east at 30–40°. To the NNW of the uniform stress regime, the minimum principal stress declines and the principal axes rotate as the shear zone is approached. The stress perturbation is clearly associated with the shear zone, and may reflect the presence of more fragmented rock acting as a compliant inclusion, or remnant stresses arising from slip on the shear zone in the past.
In situ observation of helium and argon release during fluid-pressure triggered rock deformation
Temporal changes in groundwater chemistry can reveal information about the evolution of flow path connectivity during crustal deformation. Here, we report transient helium and argon concentration anomalies monitored during a series of hydraulic reservoir stimulation experiments measured with an in situ gas equilibrium membrane inlet mass spectrometer. Geodetic and seismic analyses revealed that the applied stimulation treatments led to the formation of new fractures (hydraulic fracturing) and the reactivation of natural fractures (hydraulic shearing), both of which remobilized (He, Ar)-enriched fluids trapped in the rock mass. Our results demonstrate that integrating geochemical information with geodetic and seismic data provides critical insights to understanding dynamic changes in fracture network connectivity during reservoir stimulation. The results of this study also shed light on the linkages between fluid migration, rock deformation and seismicity at the decameter scale.
Hydraulic stimulation and fluid circulation experiments in underground laboratories: Stepping up the scale towards engineered geothermal systems
The history of reservoir stimulation to extract geothermal energy from low permeability rock (i.e. so-called petrothermal or engineered geothermal systems, EGS) highlights the difficulty of creating fluid pathways between boreholes, while keeping induced seismicity at an acceptable level. The worldwide research community sees great value in addressing many of the unresolved problems in down-scaled in-situ hydraulic stimulation experiments. Here, we present the rationale, concepts and initial results of stimulation experiments in two underground laboratories in the crystalline rocks of the Swiss Alps. A first experiment series at the 10 m scale was completed in 2017 at the Grimsel Test Site, GTS. Observations of permeability enhancement and induced seismicity show great variability between stimulation experiments in a small rock mass body. Monitoring data give detailed insights into the complexity of fault stimulation induced by highly heterogeneous pressure propagation, the formation of new fractures and stress redistribution. Future experiments at the Bedretto Underground Laboratory for Geoenergies, BULG, are planned to be at the 100 m scale, closer to conditions of actual EGS projects, and a step closer towards combining fundamental process-oriented research with testing techniques proposed by industry partners. Thus, effective and safe hydraulic stimulation approaches can be developed and tested, which should ultimately lead to an improved acceptance of EGS
On the link between fracture toughness, tensile strength, and fracture process zone in anisotropic rocks
This paper presents experimental results on the anisotropy of the fracture toughness, Brazilian tensile strength, and the fracture process zone (FPZ) in granodiorite samples. The fracture toughness is measured using semi-circular bending tests, while Brazilian disk tests were conducted to measure the tensile strength indirectly. Digital image correlation (DIC) was employed to obtain full-field surface deformation associated with the fracture propagation and identify the FPZ. An averaging scheme is proposed to determine the length and width of the FPZ from the strain field. The DIC results confirm a semi-elliptical FPZ developing ahead of the crack tip, with an average length-to-width ratio of approximately two. The results also indicate that the theoretical models such as Irwin and strip-yield with uniform traction, which are based on plastic deformation near the crack tip, underestimate the extent of the inelastic zone, while the stripyield model with a linear cohesion stress distribution overestimate the length of the process zone. The anisotropy ratio of the FPZ length obtained from the models, however, agrees very well with the ratio obtained from the DIC measurements. This evidence supports the basis of the theoretical models that predict the FPZ length to be proportional to the square of fracture toughness over tensile strength.