5-year in situ caprock fault leakage experiment at Mont Terri: A mechanics-based analysis

The potential for caprock fault reactivation and CO 2 leakage remains a primary risk for Geological Carbon Storage. The long-term geomechanical processes governing fault leakage and sealing behaviour remain poorly constrained in situ, despite extensive laboratory and numerical studies. This study presents a five-year, meso-scale in situ injection experiment conducted at the Mont Terri Underground Rock Laboratory (Switzerland). CO 2 -rich brine was injected directly into a fault zone within a caprock analogue (Opalinus Claystone) under sustained low-flow rate and high-pressure conditions. The mechanics-based analysis integrates pressure, flow rate, and distributed strain sensing (DSS) data from six monitoring boreholes with detailed geological characterisation of the fault zone structure. Long-term pressure-controlled injection was conducted at 4.5 MPa, which remained within the linear hydraulic regime below the Fault Opening Pressure (FOP > 4.5 MPa). Two mini-stimulations exceeded this threshold via stepwise increase in flow and pressure-controlled injection to characterise fault reactivation and CO 2 -rich brine leakage potential. During mini-stimulations, mechanical fault reactivation was observed at injection pressure of ≈ 6 . 1 MPa . DSS measured strain ( ∼ 200 μ ϵ ) predominantly localised within the upper fault core and lenses of scaly clay with high-microfracture density. While mini-stimulations caused temporary transmissivity increase, the fault zone demonstrated self-sealing despite constant pressure injection. The large portion of permanent strain observed suggests that aseismic visco-plastic creep is the dominant long-term sealing mechanism. These findings support caprock-fault self sealing capacity, suggesting that even following high-pressure reactivation, structural heterogeneity forces highly tortuous fluid migration pathways that restrict flow and limit the spatial and volumetric leakage of CO 2 -rich brine.