Complexity of Fault Rupture and Fluid Leakage in Shale: Insights From a Controlled Fault Activation Experiment

Abstract

We observed rupture growth caused by controlled fluid injections at 340‐m depth within a fault zone in the low‐permeability Opalinus Clay in the Mont Terri Underground Research Laboratory (Switzerland). The rupture mechanisms were evaluated using measurements of the three‐component borehole wall displacements and fluid pressure in two sections of the fault zone and located horizontally 3 m apart from each other. One section was set across a secondary segment of the fault and used for stepwise fluid injection intended to trigger rupture growth. The other section was set across the principal shear zone of the fault for monitoring. After stepwise pressure increase up to 5.95 MPa at injection, rupture initiated as slip activation, followed by an overall opening of the fault planes connected to the injection. After 19 s of continued injection, displacements arrived at the monitoring point on the principal shear zone. These displacements are about 2.4 times larger than in the secondary fault segment. Overall, the displacements corresponded to a normal fault activation. About 9 s after the displacement front arrived, a strong pressure increase of 4.17 MPa was measured at the monitoring point, indicating a hydraulic connection had formed along the initially very low permeability fault planes between the injection and the monitoring points. Our analyses highlight that the fault activation is consistent with the state of stress but that injection pressure must be close to the normal stress acting on the fault for permeability to be generated and for fluid leakage to occur.

Estimating perturbed stress from 3-D borehole displacements induced by fluid injection in fractured or faulted shales

“…A detailed description of the hydromechanical response of the whole test sequence is given in Guglielmi et al (2020). Here, we focus on the pressure test conducted step-by-step with an engine pump ( Fig.…”

In Situ Direct Displacement Information on Fault Reactivation During Fluid Injection

“…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).…”

“…Moreover, the slip orientation on the SE-dipping fractures is consistent with the first slip event, occurring during the pressurization of the interval located 3 m below the analyzed test. Guglielmi et al (2020) showed that this slip, occurring on a SE-dipping fracture (136°/32°), is favorable for activation under the stress state obtained by Martin and Lanyon (2003). Finally, the SW-dipping fractures are typically sealed with calcite and celestite (Nussbaum et al 2011), which adds cohesion and prevents the reactivation of these fractures.…”

“…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 SIM-FIP 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 2018Cappa et al , 2019, and in shale rocks at the Mont Terri rock laboratory, Switzerland (Guglielmi et al 2017;Jeanne et al 2018Jeanne et al , 2017Guglielmi 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).…”

In situ hydromechanical responses during well drilling recorded by fiber-optic distributed strain sensing

“…In conventional coherent optical time-domain reflectometer (COTDR) method, Rayleigh backscatter spectrum generated for each region in the longitudinal direction of the optical fiber is obtained through measurement ( Fig. 1) (Hartog, 2017). From the frequency shift between the reference Rayleigh-scattering power spectrum (RSPS) and a target RSPS using the cross-correlation method, the strain or temperature change at the point can be calculated.…”