Field characterization of elastic properties across a fault zone reactivated by fluid injection

Abstract

We studied the elastic properties of a fault zone intersecting the Opalinus Clay formation at 300 m depth in the Mont Terri Underground Research Laboratory (Switzerland). Four controlled water injection experiments were performed in borehole straddle intervals set at successive locations across the fault zone. A three‐component displacement sensor, which allowed capturing the borehole wall movements during injection, was used to estimate the elastic properties of representative locations across the fault zone, from the host rock to the damage zone to the fault core. Young's moduli were estimated by both an analytical approach and numerical finite difference modeling. Results show a decrease in Young's modulus from the host rock to the damage zone by a factor of 5 and from the damage zone to the fault core by a factor of 2. In the host rock, our results are in reasonable agreement with laboratory data showing a strong elastic anisotropy characterized by the direction of the plane of isotropy parallel to the laminar structure of the shale formation. In the fault zone, strong rotations of the direction of anisotropy can be observed. The plane of isotropy can be oriented either parallel to bedding (when few discontinuities are present), parallel to the direction of the main fracture family intersecting the zone, and possibly oriented parallel or perpendicular to the fractures critically oriented for shear reactivation (when repeated past rupture along this plane has created a zone).

Permeability Variations Associated With Fault Reactivation in a Claystone Formation Investigated by Field Experiments and Numerical Simulations

“…Stresses following the lithostatic gradients are applied on all the boundaries, and permeable flow boundaries are imposed. A strike‐slip stress regime is applied to simulate displacements having the same direction as those monitored during the first rupture (Ev.1) with σ 1 = 9.4 MPa oriented N125°, σ 2 = 6.0 MPa vertical, and σ 3 = 4.7 MPa oriented N035° (Jeanne et al, ). The paleostress regime that generated the Main Fault is not accurately known, but the strike‐slip regime adopted here appears to be relatively consistent with paleostress estimations from fault planes striations (Nussbaum et al, ).…”

“…The best fit, chosen by eye, is found with an initial permeability of 6 × 10 −17 m 2 and a Young's modulus of 1.0 GPa for the reactivated fault. A permeability of 10 −17 m 2 and a Young's modulus of 0.4 GPa and 1.4 GPa normal and parallel to the laminar structure of the claystone for the surrounding rock mass are used for the rock mass (a sensitivity analysis to these mechanical properties is shown in Jeanne et al, ). During injection, the fracture permeability evolves due to changes in effective stress.…”

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

“…characterizes the initial reversible expansion of the injection chamber, which is preferentially oriented almost perpendicular to the N50°35-to-45°dipping fault family affecting the chamber. Jeanne et al (2017) showed that this u 1 ! orientation and magnitude can be explained by the rock elastic modulus magnitude, which is 10 times lower perpendicular to existing fault planes than parallel to existing fault planes.…”

“…The relation between the compliance of the probe and the elastic response of the borehole wall has been studied extensively through laboratory tests and fully coupled hydromechanical modeling using the TOUGH-FLAC simulator (Jeanne et al, 2017). It was shown that the device is~10 times more compliant than the rock, and that the orientation and magnitude of displacements below the stimulation pressure strongly depends on the elastic anisotropy of the rock.…”

Tracking Fluid Flow in Shallow Crustal Fault Zones: 2. Insights From Cross‐Hole Forced Flow Experiments in Damage Zones

“…Drillcores from exhumed faults and deep boreholes provide some constraints on the permeability of fault zone materials, indicating that damage zones are regions of enhanced permeability where most flow, if not all, is concentrated (Brace, , ; Evans & Goddard, ). Few studies have been designed, however, to characterize the internal flow structure of damage zones due to the difficulty of accessing and hydraulically testing faults at depth, except in sedimentary basins where a growing number of underground experiments has been conducted recently (Cappa et al, ; Jeanne et al, , ). In deep crustal fault zones, much of our knowledge on fault‐controlled flow processes comes from indirect observations, such as seismicity (Jansen et al, ), which informs on fluid migration at the hectometer to km‐scale but generally lacks the spatial resolution to track flow at the scale of discrete subsidiary fracture systems.…”