Frictional Instabilities and Carbonation of Basalts Triggered by Injection of Pressurized H2O‐ and CO2‐ Rich Fluids

The safe application of geological carbon storage depends also on the seismic hazard associated with fluid injection. In this regard, we performed friction experiments using a rotary shear apparatus on precut basalts with variable degree of hydrothermal alteration by injecting distilled H2O, pure CO2, and H2O + CO2 fluid mixtures under temperature, fluid pressure, and stress conditions relevant for large‐scale subsurface CO2 storage reservoirs. In all experiments, seismic slip was preceded by short‐lived slip bursts. Seismic slip occurred at equivalent fluid pressures and normal stresses regardless of the fluid injected and degree of alteration of basalts. Injection of fluids caused also carbonation reactions and crystallization of new dolomite grains in the basalt‐hosted faults sheared in H2O + CO2 fluid mixtures. Fast mineral carbonation in the experiments might be explained by shear heating during seismic slip, evidencing the high chemical reactivity of basalts to H2O + CO2 mixtures. Plain Language Summary The injection of H2O+CO2 mixtures in basalts has been proposed for CO2 storage as an appealing option to the injection of CO2 fluids. In fact, H2O+CO2 fluids should react with basalts and induce the precipitation of carbonate minerals. The huge advantage of this storage technique is that, by turning CO2 into rock (New York Times 9/2/2015), it prevents the risk of CO2 leakage driven by buoyancy forces in the storage site. However, though it is well known that fluid injection may induce seismicity (e.g., Ellsworth, Science, 2013), the frictional behavior of faults in basalts injected by H2O+CO2 fluids is still poorly known. To provide insights on the induced‐seismicity potential of this CO2 storage technique, we reproduced the ambient conditions of typical H2O+CO2 storage sites by exploiting the rotary shear apparatus Slow to High Velocity Apparatus (INGV‐Rome, Italy). Our experimental results show that fluid composition and degree of alteration of basalts had a negligible role in controlling the maximum fluid pressure that the experimental fault can sustain before failure. Evidences for mineral carbonation after the experiments sheared with CO2‐rich water attest the high chemical reactivity of basalts in acidic environment. Key Points We injected pressurized H2O‐, CO2‐, and H2O + CO2‐rich fluids in unaltered and altered basalt‐hosted preloaded faults Frictional instabilities occurred at similar fluid pressures and normal stresses independently of the d