Experimental investigation of the sealing capacity of generic clay-rich caprocks

Abstract For a set of generic clay-rich lithotypes, gas breakthrough experiments with helium (He) and carbon dioxide (CO2) were conducted at elevated pressures and temperatures. Core plugs of 38 mm diameter and approx. 15 mm length were prepared and measurements were performed at confining pressures between 15 and 29 MPa and temperatures of 25–45 °C. Intrinsic permeability coefficients of the samples ranged between 3 × 10−18 and 2 × 10−22 m2. Gas breakthrough and subsequent viscous gas flow could only be achieved for an argillaceous limestone and two mudstone samples with intrinsic permeability coefficients of 4 × 10−18 and 2 × 10−20 m2, respectively. For the limestone, capillary gas breakthrough occurred between 1.2 and 2.9 MPa (He). The gas (He and CO2) breakthrough pressure for the immature mudstone lay between 5 and 13 MPa. Gas flow rates recorded after breakthrough revealed a linear relationship between effective gas permeability and differential pressure. Effective gas permeability coefficients were normalized to single-phase water permeability coefficients to obtain relative permeabilities. The highest relative gas permeability values of approximately 0.05 and 0.10, respectively, for He and CO2 were observed at a differential pressure of 10 MPa. The mature shales with intrinsic permeabilities below 10−20 m2 represent effective barriers and showed no indications of capillary gas breakthrough up to a pressure difference of at least 20 MPa for He and 9 MPa for CO2. Gas transport through these caprocks under the conditions of this study occurs only by diffusion, with average diffusion coefficients in the order of 10−11 m2/s. Analyses of the sample plugs after exposure to CO2 revealed no recognizable mineral alterations. This is attributed to the fact that transport and reaction rates are very low and that only a small proportion of the internal surface area is exposed to the permeating fluid/CO2.