Project: Friction in faulted Opalinus Clay at high temperature and pressure
Funding: Mont Terri
Period: 01.01.2015 – 31.12.2021
Understanding of the mechanical and physical properties of shales is of major importance in many fields such as faults hydro-mechanical behavior, cap-rock, unconventional reservoir studies or nuclear waste disposal. In particular, relationships between fluid transport properties, applied stress and textural anisotropy are critical both in intact and fractured shales. Therefore, these relations need to be investigated in the laboratory in order to have a better understanding on in-situ mechanisms. In this project, we will focus on the effect of pore fluid pressure and micro-structural texture on frictional properties of Opalinus Clay. This formation is the host rock for a high-level radioactive waste repository in Switzerland. Indeed, with a hydraulic conductivity of 2.10-13 m/s, the rock is practically impermeable. The objective of this project is to provide better understanding of the various couplings between hydraulic and mechanical interactions in shale reservoir. In particular, this project provides a detailed study of how the friction properties control the transport properties of reactivated fractures in Opalinus clay. Most previous studies do not address some complex phenomenon accompanying frictional slip such dilatancy [e.g., Marone and Kilgore, 1993; Samuelson et al., 2009], which can, in return, produce different frictional responses. However, shear induced dilatancy of fluid saturated fault is critical for reservoir safety. Here, we will experimentally study the evolution of both the fluid transport properties (porosity, and permeability) and friction properties during slip evolution at pressure up to 50 MPa and at ambient temperature. These data will provide new constraints on the permeability evolution during the creation and aging of nuclear waste repository. This study will also provide the opportunity to correlate specific fault zone structures with fluid flow and mechanical properties, in particular the non-linear relationship between hydraulic aperture change and mechanical fracture opening. For the first time, fault zone structures observed in the Mont Terri laboratory could be related to seismic behavior. This project would also help interpreting the FS experiments (In-situ clay faults slip hydro-mechanical characterization) planned for September at the Mont Terri laboratory.
Critical questions we address on the hydro-mechanical coupling issues in shale reservoir are:
- the frictional properties of cohesive and non-cohesive rock samples under reservoir conditions (i.e pre-post peak friction, coehesion and healing)
- the role of pore pressure, and fluid type (brine and Co2) on frictional behavior and fault stability
- the elastic properties, permeability and porosity variation during deformation and healing
- Poro-elastic properties of fault zones such as Skempton’s A and B and Biot’s coefficient?