Frictional behaviour and transport properties of simulated fault gouges derived from a natural CO2 reservoir

- Natural CO2 analogue fields offer an opportunity to better assess CCS-related risks.
- CO2-brine-rock interactions may affect the mechanical properties of fault gouges.
- Mechanical behaviour of quartz-rich gouge is not strongly impacted by CO2-exposure.
- Rapid CO2 leakage and degassing could lead to carbonate precipitation in a fault.
- Carbonate precipitation may impact frictional strength or seismic potential.

We investigated the effects of long-term CO2-brine-rock interactions on the frictional and transport properties of reservoir-derived fault gouges, prepared from both unexposed and CO2-exposed sandstone, and from aragonite-cemented fault rock of an active CO2-leaking conduit, obtained from a natural CO2 field (Green River, Utah). Direct shear experiments (5-90 MPa effective normal stress; lab dry or wet; 20-100 °C) showed that the sandstone-derived gouges are characterised by virtually normal stress- and temperature-independent friction coefficients (μ ≈ 0.5-0.6). The data exhibited stable, velocity-strengthening behaviour moving towards near-neutral velocity-dependent behaviour with increasing effective normal stress. The carbonate-rich fault rock gouges exhibited higher friction coefficients (μ ≈ 0.6-0.7), with a transition from velocity-strengthening behaviour at room temperature (dry) to velocity-weakening behaviour at 100 °C (dry and wet), i.e. a transition from stable sliding to potentially unstable or seismogenic slip. Cross-fault permeability decreased up to 1.5 orders with increasing displacement, showing slightly lower values for the carbonate-rich gouges. We infer that the mechanical behaviour of fault gouges derived from the sandstones studied will not be strongly influenced by long-term CO2-exposure, due to the low content of reactive minerals in the protolith. Significant changes in frictional strength or (micro)seismic potential of faults present in a CO2 storage system are only expected when there is major carbonate precipitation in the fault damage zone due to rapid CO2 leakage and degassing.