Experimental evaluation of wellbore integrity along the cement-rock boundary

Leakage of CO(2) and brine from geologic storage reservoirs along wellbores is a major risk factor to the success of geologic carbon sequestration. We conducted multiphase [supercritical (sc)CO(2)-brine] coreflood experiments that simulate a leakage pathway along the cement/rock interface. A composite core constructed of oil-well cement and siltstone separated by a simulated damage zone (defect) containing ground cement and siltstone was flooded with brine + scCO(2) at 10 MPa and 60 °C parallel to the defect. During coinjection of scCO(2), the effective brine permeability decreased from ~200 to 90 mD due to transition to two-phase flow and then further declined to 35 mD. CO(2) injection resulted in a pH drop from 11 to 4 and carbonate-undersaturated conditions in the produced brine. Microscopy revealed leaching and erosion along the defect, a carbonation front extending 5 mm into the cement, parallel to the damage zone, and no change in the dimensions of the defect. Carbonation of cement does not appear to explain the permeability drop, which is attributed to the migration and reprecipitation of alteration products derived from cement within the defect. This study shows the potential for self-limiting flow along wellbore defects despite flow of aggressive scCO(2)-brine mixtures.