In Situ Direct Displacement Information on Fault Reactivation During Fluid Injection
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ORIGINAL PAPER
In Situ Direct Displacement Information on Fault Reactivation During Fluid Injection Maria Kakurina1 · Yves Guglielmi2 · Christophe Nussbaum3 · Benoît Valley1 Received: 17 January 2020 / Accepted: 23 May 2020 © The Author(s) 2020
Abstract The three dimensional (3D) displacement induced by fluid injection was measured during two fault reactivation experiments conducted in carbonate rocks at the Rustrel Low Noise Underground Laboratory (LSBB URL), France, and in shale rocks at the Mont Terri Rock laboratory, Switzerland. The faults were activated by injecting high pressure fluid and using the StepRate Injection Method for Fracture In-Situ Properties, which allows a coupled pressure-flowrate-3D displacement monitoring in boreholes. Both experiments mainly show complex aseismic deformation of preexisting fractures that depend on (1) the fluid pressure variations related to chamber pressurization and leakage into the formation and (2) irreversible shear slip and opening of the reactivated fractures. Here we detail the processing of the 3D displacement data from both experiments to isolate slip vectors from the complex displacement signal. Firstly, we explain the test protocol and describe the in situ hydromechanical behavior of the borehole/fault system. Secondly, we define the methodology of the displacement data processing to isolate slip vectors with high displacement rates, which carry information about the key orientation of fault reactivation. Finally, we discuss which slip vectors can potentially be used to solve the stress inversion problem. Keywords Geomechanics · Fracture reactivation · Displacement rate · Slip vector · Stress inversion
1 Introduction 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 * Maria Kakurina [email protected] 1
University of Neuchâtel, CHYN, Emile‑Argand 11, 2000 Neuchâtel, Switzerland
2
Energy Geoscience Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
3
Federal Office of Topography, Swisstopo, Seftigenstrasse 264, 3084 Wabern, Switzerland
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
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