In Situ Direct Displacement Information on Fault Reactivation During Fluid Injection

Fluid injection into the rock mass may lead to either new fracture development or activation of preexisting fractures by reducing the effective normal stress and driving the plane to shear failure associated with slip (Hubbert and Rubey 1959). According to the Wallace–Bott hypothesis, the slip direction induced on fractures is a function of relative magnitudes and orientations of the principal stresses, i.e. it is parallel to the resolved shear stress (Wallace 1951; Bott 1959). Therefore, the orientation of the triggered slipped vector together with the orientation of the pre-existing fracture carries important information about the stress and can potentially be used for stress inversion. This work is the first part of the research devoted to the estimation of the stress state using the slip data from a fault reactivation experiment (Kakurina 2020). Here, we discuss the information carried by the three-dimensional (3D) displacement data during the fluid injection and present a protocol to isolate a slip vector to be used for further stress inversion.

Slip induced by fluid injection is often associated with seismicity in the surrounding rock mass (Zoback and Harjes 1997; Shapiro et al. 1999; Guglielmi et al. 2008; Shapiro and Dinske 2009; Evans et al. 2012; Ellsworth 2013; McGarr 2014). The seismic slip is known to be used for stress inversion by applying the focal mechanism technique (Reasenberg 1985), which provides constraints on two potential compression-extension zones bounded by potential fault planes and defined by considering the polarities of the P-wave first arrivals. Inversion of slip vectors deduced from the focal mechanism can provide with the orientations of the principal stresses and the stress ratio using the methods described by Gephart and Forsyth (1984); Lund and Slunga (1999); Vavryčuk (2014).

Recent studies showed direct field evidence that fluid injection can cause significant aseismic deformation, typically defined as deformation with a slip velocity that is too slow to generate detectable seismic waves (Cornet 2012, 2016; Zoback et al. 2012; Avouac 2015; Guglielmi et al. 2015a; Duboeuf et al. 2017; Duboeuf 2018; Cappa et al. 2018). Aseismic motions constitute an important part of the surrounding mass response during fluid injections. They were observed at large-scale experiments at geothermal sites in Soultz-sous-Forêts, France (e.g., Cornet et al. 1997; Evans et al. 2005), at meter-scale experiments at shallow depths (~ 300 m) at the Mont Terri Rock Laboratory in Switzerland, Tournemire and Rustrel Low Noise Underground Laboratory (LSBB URL) in France (Guglielmi et al. 2015b; De Barros et al. 2016; Duboeuf et al. 2017), and at laboratory scale (Zoback et al. 2012; Goodfellow et al. 2015).

Both seismic and aseismic motions are controlled by coupled frictional and fluid flow behavior (Scholz 1998; Cornet 2012, 2016; Guglielmi et al. 2015a) changes in mechanical and hydrologic properties (Bird 1984; Saffer and Marone 2003) and evolving effective stress perturbations during fluid injection (Duboeuf 2018). In terms of rate-and-state friction, aseismic motion is considered to be stable (Segall 1991) and occurs when a fault strengthens during slip, demonstrating velocity strengthening behavior and rate-decreasing friction law. Seismic motion corresponds to an unstable slip, which represents a weakening behavior during slip.

The aseismic motion may occur prior to and work as a trigger for seismicity (Zoback et al. 2012; Guglielmi et al. 2015a; De Barros et al. 2016; Duboeuf et al. 2017; Duboeuf 2018; Cappa et al. 2019). The transition from aseismic to seismic behavior was explained using a slip rate dependency of fault friction (Ruina 1983; Avouac 2015; Guglielmi et al. 2015a). Guglielmi et al. (2015a) observed a slow slip rate (4 m/s) with a high dilatancy associated with a 20-fold increase of permeability, which was followed by faster slip (∼ 10 m/s) with reduced dilatancy and micro-earthquakes.

Since the seismic and aseismic motions are controlled by the same coupled processes, can we use the data of aseismic motion to invert stresses? It has always been a challenge to measure an aseismic slip during fluid injection (Cornet 2016), however, nowadays it is possible by using the Step-Rate Injection Method for Fracture In-Situ Properties (SIMFIP, Guglielmi et al. 2013), which allows a coupled 3D-displacement–pressure-flowrate continuous monitoring during the fluid injection. In this paper, we use the SIMFIP data obtained during two fault reactivation experiments conducted in carbonate rocks at the Rustrel LSBB URL in France (Derode et al. 2015; Guglielmi et al. 2015a; Duboeuf 2018; Duboeuf et al. 2017; Cappa et al. 2018, 2019), and in shale rocks at the Mont Terri rock laboratory, Switzerland (Guglielmi et al. 2017; Jeanne et al. 2018, 2017; Guglielmi 2016; Guglielmi et al. 2020). Originally the fault reactivation experiments were designed for estimating the fault permeability variation during the injection (Guglielmi et al. 2017; Jeanne et al. 2018). Both experiments consisted of fluid injections into a fault damage zone to reactivate the fault planes and to measure the slip during the injection.

The 3D displacement data obtained during the fluid injections contain both reversible and irreversible responses of the surrounded rock mass. The irreversible response may be associated to either seismic and aseismic motions. Seismic motions were observed during the injection at the LSBB URL and have been already used to invert stresses using focal mechanisms (Duboeuf 2018). However, the data for both sites at LSBB and at Mont Terri mostly show aseismic motion, which relates to a complex formation response to fluid injection (Guglielmi et al. 2015a; Duboeuf et al. 2017; Duboeuf 2018; Guglielmi 2016).

Here we show how to isolate slip on preexisting fractures from the irreversible response of the rock mass, using the continuous displacements of borehole wall monitored during the injection tests. We show that most of the reversible response is related to poroelastic effects associated with the injected fluid diffusion into the formation.

Irreversible response and eventually slip on preexisting fractures are evidenced by a non-zero net displacement at the end of the test and by periods of increased displacement rate during the injection. Thus, we discuss the possibility to isolate the irreversible response of the rock mass by first applying a threshold on the displacement rate. Second, we correlate the orientation of the high displacement-rate vectors (that can be related to aseismic or seismic events) with the orientation of the tested fractures. Finally, we discuss the potential use of these vectors for a stress tensor inversion.

The experiment protocol for both experimental sites followed the step-rate injection method using the SIMFIP probe developed by Guglielmi et al. (2013). A 2.4 m test interval is isolated in a borehole between two inflatable packers (Fig. 1). In comparison to traditional hydraulic methods, the SIMFIP probe enables simultaneous monitoring of the 3D dislocation between to point anchored to the borehole wall and fluid pressure within the pressurized interval. The sampling rate is up to 1 kHz. In the experiments described in this paper, the native data were sampled at 500 Hz. We downsample the data to reduce the noise and by trial and error we determined that 10 Hz is an optimal compromise between sampling rate and noise reduction. The pressure sensors range of the SIMFIP probe used in this research was 10 MPa allowing for high-pressure Step-Rate Testing (SRT). A downhole valve operated with N₂ gives control of the interval pressure and shut-in. Fluid pressure is monitored with pressure sensors below and above the packers, and in the test interval with the 0.001 MPa accuracy. Packers sealing the interval are inflated with water. The 3D displacement of the injection chamber wall is monitored by a three-component extensometer centered along the axis straddling the two packers in the injection chamber (Fig. 1a). This extensometer is fixed to the borehole wall by hydraulically operated anchors. Six small-diameter and deformable steel tubes connect two rings with varying orientations—making a cylindrical cage linking the upper and lower rings. Extension between the rings is resolved from the inversion of the deformations of the six tubes. The cage used in both experiments was the same. It is 0.49 m long and 0.1 m diameter (Fig. 1b). Tube deformations are captured with six fiber optic Bragg gratings (FBG) that are attached to each tube and distributed along with one single continuous fiber that brings the sensor signals to the surface-mounted data acquisition system. An inversion algorithm is used to calculate the relative 3D displacements (x, y, and z, respectively, oriented north, west, and vertical (positive up) for shales, and oriented north, vertical (positive up), east for carbonates) of the center of the upper ring towards the center of the lower ring from the tube deformations that are continuously monitored during the test. Figure 1b shows an example of measuring a relative displacement (\(\overrightarrow {{U_{\text{r}} }}\)), which represents normal slip movement along a fracture located between the anchors. The relative displacement (\(\overrightarrow {{U_{\text{r}} }}\)) is the resulting sum of the displacement of the upper anchor and lower anchor which is arbitrarily considered to be fixed. We used standard geological conception for displacement vector with a positive dip angle when the vector points below the horizontal and negative dip angle when the vector points above the horizontal. In addition, there is a reference FBG subject to ambient pressure and temperature which remains unstressed. It is used to correct the strain measurements from temperature and pressure variations in the chamber. The 6 + 1 FBG is distributed along with one single continuous fiber that brings the sensor signals to the surface-mounted data acquisition system which is a MicronOptics optical interrogator (http://​www.​micronoptics.​com). Attached to the straddle packer system is a magnetic orientation tool used to measure the initial azimuthal positioning of the tool with a 0.1° accuracy.

At the Mont Terri site, the experiment is located at 37.2 m depth below the laboratory gallery floor, which corresponds to a depth below ground surface of about 300 m, in the Opalinus Clay formation at the Mont Terri Rock laboratory, Switzerland (Fig. 2a). The laboratory is focused on studying the coupled hydrogeological, geochemical, and geomechanical processes in the Opalinus Clay formation. The exposure of Opalinus Clay in the laboratory galleries gives an excellent opportunity to investigate an unweathered clay fault system (Nussbaum et al. 2011; Jaeggi et al. 2017). The study of this formation is important because it is a potential host rock for deep nuclear waste disposal and a caprock analog for a geological carbon dioxide (CO₂) storage in deep geological reservoirs (Goodman et al. 2017).

The most deformed zone and the main tectonic structure of the Opalinus Clay, the Main Fault, crosses the galleries of the laboratory. The Main Fault consists of a complex thrust zone, about 0.8–3 m width, bounded by two major fault planes with dip directions varying from 140° to 165° and dip angles from 40° to 65°. The internal structure of the Main Fault is characterized by three main tectonic families of faults: (1) sub-parallel to the bedding planes faults with dip directions varying from 145° to 155° and dip angles from 50° to 55°, respectively, (2) SW-dipping faults with dip angles ranging from 10° to 40° and sub-horizontal faults (3) E-dipping faults with dipping angles from 20° to 60° and steep W-dipping faults (Nussbaum et al. 2011). The variability of fault density and intersections observed at the gallery walls and on the boreholes cores make the fault zone very complex and heterogeneous. The fracture network intersects the bedding, which mainly dips towards SSE with a dip angle evolving from 30° to 50°. The stress state which is currently considered in Mont Terri was synthesized by Martin and Lanyon (2003) from different stress testing methods: \(\sigma_{1}\) = 6–7 MPa with 210° trend and 70° plunge to sub-vertical, \(\sigma_{2}\) = 4–5 MPa with 320° trend and 7° plunge, and \(\sigma_{3}\) = 0.6–2 MPa with 052° trend and sub-horizontal to 18° plunge.

The Fault Slip experiment, conducted in the Mont Terri Rock Laboratory, consisted of injecting fluid into sealed intervals in upper, middle, and lower parts of the Main Fault to eventually trigger a local fault reactivation (Guglielmi 2016; Guglielmi et al. 2017). Here we focus on one injection test conducted between 35.85 and 38.33 m depth below the gallery floor (Fig. 2b). The depth of 37.2 m corresponds to the center of the displacement cage in the interval. In this interval, the injection was performed 6.5 m above the Main Fault in its damage zone (Guglielmi 2016; Kakurina 2020). The borehole was initially dry (no flow was observed after drilling). After deployment of the probe and sealing of the injection interval by packer inflation, the injection chamber was saturated with a fluid with equivalent chemistry to the formation fluid and to an initial pressure of 0.6 MPa to be in equilibrium with the local formation pressure (Guglielmi 2016; Guglielmi et al. 2020).

The injection interval intersects a set of fractures sub-parallel to the bedding planes with the dip directions and dip angles varying from 123° to 154° and 36° to 74°, respectively, three SW-dipping fractures with an average orientation of 240°/23° and one S-dipping fault oriented 174°/43° (Fig. 2b, c). The zone located between the two anchors of the displacement cage consists of two SW-dipping fractures (233°/25° and 247°/20°) with a 2 cm thick layer of scaly clays (Nussbaum et al. 2011; Laurich et al. 2015) located in the upper part of the displacement cage, and five SE-dipping fractures (130°/55°, 130°/46°, 131°/65°, 130°/37°, 130°/45°) in the lower part of the cage.

In carbonates, the experiment is located at 280 m depth in the galleries of the Rustrel Low Noise Underground Laboratory (LSBB URL, http://​lsbbnew.​prod.​lamp.​cnrs.​fr) in the southeast of France sedimentary basin (Fig. 2d). The laboratory is focused on the investigation of coupled Thermo-Hydro-Mechanical-Chemical (THMC) processes in porous fractured platform carbonates. The laboratory galleries give excellent access to a particular environment of unaltered fractured Cretaceous limestone (Urgonian facies), which are regarded as: (1) an exceptional access to the unsaturated zone of a large karstic aquifer, (2) an analog to the Middle East carbonate oil and gas reservoirs, (3) the area of major seismogenic faults in Provence, southeastern France.

The experiment was conducted in a 40 m × 20 m × 20 m volume in the fractured zone, which mainly includes bedding planes with dip directions from 200° to 225° and dip angles from 20° to 35°, E- and W-dipping steep fractures and S-dipping fractures with dip angles from 20° to 50° fractures (Cochard 2018; Duboeuf 2018). The experimental area has a strong heterogeneity in the rock quality and hydraulic properties (Jeanne et al. 2012).

The state of stress was estimated from hydraulic tests on preexisting fractures (HTPF) during a previous experiment using a straddle-packer probe and taking into account topographic stresses by Guglielmi et al. (2015a) and from microseismic events using focal mechanisms inversion by Duboeuf (2018). The regional stress state was estimated to be: \(\sigma_{1}\) = 6 ± 0.4 MPa is sub-vertical, \(\sigma_{2}\) = 5 ± 0.5 MPa is sub-horizontal and trending 20° ± 20°, and \(\sigma_{3}\) = 3 ± 1 MPa is sub-horizontal and trending 110° ± 20° (Guglielmi et al. 2015a).

The injection test was conducted between 12.65 and 15.13 m depth below the gallery floor, ~ 20 away from the Main Damage Zone (Fig. 2e, Cochard 2018; Duboeuf 2018; Jeanne et al. 2012). The center of the deformation unit is at the depth of 14 m across a bedding plane oriented 235°/26°, which separates two different rock facies in the interval. The facies located above the bedding plane is a low fractured grainstone with a porosity of 12–16%. The facies below the bedding place is a highly fractured packstone with low porosity (< 3%). In addition, the interval is clearly intersected by a sub-vertical E-dipping fracture (Cochard 2018, Fig. 2f).

The stress state using focal mechanisms from test 9 showed differences from the regional stress state with \(\sigma_{1}\) trending 225° ± 20° and plunging 40° ± 10°, \(\sigma_{2}\) sub-horizontal trending 330° ± 40° and \(\sigma_{3}\) trending 210° ± 10° and plunging 80° ± 5° (Duboeuf 2018). The stress computed using the focal mechanisms is different from the one computed by Guglielmi et al. (2015a). Duboeuf (2018) propose that the reason for the discrepancy is stress change induced by aseismic slip prior to seismic events.

This paper focuses on pressure step-up injection tests that were part of the hydraulic stimulation experiments conducted with an engine pump at Mont Terri (Guglielmi 2016; Kakurina 2020) and at LSBB (Duboeuf 2018; Duboeuf et al. 2017).

For both engine pump experiments pressure is increased stepwise in the intervals until a significant increase in the injected flow rate is measured. This pressure is a proxy to a fracture opening pressure.

The displacement response for both shale rocks at Mont Terri and carbonate rocks at LSBB intervals is complex (Figs. 3c–e and 4c–e). This complexity of the data comes from the sum of the responses of the complex system including (1) the probe borehole interaction, (2) the deformation associated with stress concentration at the borehole wall and background stress, (3) fluid diffusion from the injection interval into the formation. All these processes are influenced by the rock properties including the fractures. To simplify the displacement signal complexity and to define the key orientations that could be used for further stress inversion analyses, the following protocol has been applied. The goal of the protocol was thus not to fully explain the complex rock mass response to injection, but to extract deformation events that could clearly be associated to fracture slip.

The protocol consists of several steps, which are explained in detail below. Firstly, we isolate the time windows of constant pressure steps to avoid the influence of pump oscillation signal. Secondly, we defined limits of linear displacement-vs-pressure response to later estimate the threshold between displacement rates during reversible and irreversible deformation. Thirdly, we apply the threshold to the displacement rates measured during the constant pressure steps to identify the steps, which include higher displacement rates and may indicate a fracture slip.

Figure 7 shows the stereonets with raw data of the displacements (a), displacement after processing (b), and the isolated slip vectors (c). Each point on a raw data stereonet corresponds to a vector calculated between two positions of the SIMFIP sensor at a 0.1 s interval. Displacements after the processing corresponding to the displacements with a high rate from the first step representing irreversible response defined in Fig. 6 (Step 3 for Mont Terri and Step 4 for the LSBB URL). Such a high rate displacement vector will be called a ‘displacement event’. The displacement event is regarded as ‘slip-dominant’ displacement and not precisely the displacement of pure slip motion. The displacement event still contains complex displacement components originated from other than pure slip, although as shown on the stereonet of Fig. 7b some vectors align on fracture plane and can potentially represent almost purely slip motion.

The evolution of flow rate, pressure, and 3D displacement of the borehole wall during fluid injection from two fault reactivation experiments conducted at Mont Terri and the LSBB URL shows a complex response due to the superposition of multiple effects. We were interested in isolating fracture slip directions that we can interpret for stress estimation using the Wallace–Bott hypothesis. We developed a new protocol to isolate these slip vectors despite the complexity of the signal. The protocol is based on the extraction of displacement events that display high displacement rate during constant pressure periods. This approach is justified by the conceptualization of the main processes happening in the chamber during fluid injection. We compared the response to the fluid injection of fractured shales and carbonates and discussed which slip vectors can be driven by the current stress state according to the Wallace-Bott hypothesis.

Multiple and superposing processes involved during a fluid injection are related to the stress redistribution around the borehole, such as fluid diffusion within the rock mass, the elastic response of the rock and fractures, and the displacement occurring possibly on one or more fractures. We studied complex borehole 3D displacement signals during fault reactivation experiments and focused on isolating slip vectors that can be used for stress inversion. We identified two main phases in the rock mass response to injection:

During this second phase we identified that displacement rate can be considered a reasonable proxy to isolate potential rupture events. Comparing tests in shales at Mont Terri and in carbonates in LSBB URL using the same protocol allowed us to discuss the validity of the first aseismic high rate event as the most representative of the virgin state of stress around the borehole. We observed that the following ones may depend on more complex interactions between stress, fluid diffusion and different geological heterogeneities. Thus, to simplify the interpretation of the coupled displacement-flowrate-pressure response after a fracture activation and to capture its slip vector, we developed a protocol that helps to distinguish slip vectors based on displacement rate. At the beginning of the test the low displacement rate is used to define a threshold. Any higher displacement rates are considered a potential rupture event. The orientation of the events is compared with the known fractures in the interval, enabling us to identify which fracture(s) was activated and to characterize the slip orientation. After identifying all the slip events within the interval, we assume that the first slip vectors would best represent the background stress (i.e., unperturbed stress state). The first slip we identify in both rock types occurs before induced seismicity if any. This observation raises concerns that stress inversions based on seismic data analyses may not be representative of in situ static stress but of dynamically perturbed stress. The counterpoint is that the static stress deduced from the first slip may still be in the borehole stress perturbation zone. Applying the protocol to more simple geologies and in deeper stress contexts with higher deviatoric stress is a promising perspective to calibrate this approach. In any case, adding the continuous borehole displacement monitoring to a fluid stimulation test appears to be a new method to track stress perturbation related to geological heterogeneity along with a deep borehole profile.