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

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 degrees C) showed that the sandstone-derived gouges are characterised by virtually normal stress- and temperature-independent friction coefficients (mu approximate to 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 (mu approximate to 0.6-0.7), with a transition from velocity-strengthening behaviour at room temperature (dry) to velocity-weakening behaviour at 100 degrees 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 longterm 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. (C) 2016 Elsevier Ltd. All rights reserved.