Chemo‐mechanical behavior of clay‐rich fault gouges affected by CO₂‐brine‐rock interactions

The impact of long-term CO₂-brine-rock interactions on the frictional properties of faults is one of the main concerns when ensuring safe geological CO₂ storage. Mineralogical changes may alter the frictional strength and seismogenic potential of pre-existing faults bounding a storage complex. However, most of these reactions are too slow to be reproduced on laboratory timescales and can only be assessed using geochemical modeling. We combined modeling of CO₂-charged formation water and fault gouges (1–1000 years residence time, i.e. 10–10⁶ pore volume flushes) with friction experiments on simulated fault gouges (T = 22–150°C; σ_n^eff = 50 MPa; P_f = 25 MPa; V = 0.2-100 μm/s), having mineralogical compositions as predicted by the models. As an analogue for clay-rich caprocks overlying potential CO₂ storage sites in Europe, we used the Opalinus claystone. Our experiments showed that, although significant mineralogical changes occurred, they did not significantly change the frictional behavior of faults. Instead, initial fault-gouge mineralogy imposed a stronger control on clay-rich fault behavior than the extent of CO₂-brine-rock interactions, even under chemical conditions allowing for significant reaction. We demonstrated that the impact of mineralogical changes due to CO₂-brine-rock interactions on the frictional behavior and seismogenic potential of faults could be assessed using our combination of geochemical modeling and friction experiments. Note that a complete understanding requires evaluation of additional effects, such as that of shear velocity, effective normal stress, and other fault characteristics (maturity, shear strain).