Multi-physics Characterization of Time-dependent Brittle Deformation in Basalt Undergoing Carbonation

Permanent CO₂ storage is needed to mitigate climate change. Basaltic rocks are potential reservoirs due to their abundance and high concentrations of divalent cations that allow for CO₂ mineralization. To ensure safe implementation of this storage technology, deformation behavior of basalt in the presence of reactive fluids needs to be studied in detail.

In this project, we conducted brittle creep deformation experiments on basalts from the CarbFix site in Iceland. The rocks were deformed by constant load stepping under several pore fluid conditions (dry, H₂O-saturated and H₂O+CO₂-saturated) at temperature, T=80°C and effective pressure, P_eff = 50 MPa to investigate how carbonation reactions affect the creep behavior. Our results confirm that the presence of fluid weakens the rocks as all fluid-saturated samples fractured at differential stress lower than 70 MPa while the dry samples failed at stress above 120 MPa at identical effective pressures. Furthermore, the rock-fluid interaction accelerates the creep rate and increases the stress dependence of the creep rate. The creep rate shows little to no stress dependence in dry experiment while a similar stress exponent of ~1.2 is observed in all fluid saturated experiments regardless of the fluid composition. Volumetric strain data show dilation in H₂O-fluid saturated samples occurred at differential stress below 60 MPa and in H₂O+CO₂-saturated sample below 50 MPa, while dry rocks dilate at stress above 90 MPa. Monitoring of passive acoustic emissions (AEs) shows that in saturated experiments, the AEs are randomly spatially distributed with higher b-values compared to dry experiments where AEs coalesce along a fault plane and show lower b-values. Analysis of the evolution of ultra-sound P and S intensities and velocities indicates that sample compaction competes with micro-crack activities and such competition continues during long-term creep deformation.

Our study confirms that fluids weaken the rock strength in the brittle creep regime. The strongest effect is seen in H₂O+CO₂-saturated sample suggesting that crack growth and coalescence are accelerated by a corrosive fluid. Our preliminary results suggest that the creep rates and their stress dependence are strongly affected by the presence of a fluid but are less affected by the exact fluid composition.