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.