Experimental measurement of, and controls on, permeability and permeability anisotropy of caprocks from the CO2 storage project at the Krechba Field, Algeria

The long-term success of the geological storage of CO2 is dependent on the integrity of the sealing horizons, yet there is a paucity of data on permeability, permeability anisotropy, and factors that affect them. Using samples from an ongoing field trial for CO2 sequestration, this paper presents measured vertical (k(v)) and horizontal (k(h)) permeabilities across a range of effective pressures. Petrological and petrophysical analyses highlight what are the dominant controls on permeability. The Krechba field in Algeria is one of the largest CO2 storage projects currently running with over 3M tonnes of CO2 injected since 2004. Experimental samples of the caprock and underlying storage domain were recovered from the base of the succession. Caprock permeability ranges from 10(-23) to 10(-19) m(2). Permeability decreases with decreasing porosity and pore throat radius and increasing clay mineral content. Primary depositional heterogeneous distribution of clay minerals produced contrasting layers of relatively low and high permeability resulting in extreme k(h)/k(v) ratios of up to 50,000. Samples with the same porosity, mean pore throat size and clay mineral content can have k(h)/k(v) differing by >4 orders of magnitude. The data was used to model permeability using the Yang-Aplin model. Accuracy of the predicted permeabilites was found to reflect the measured permeability anisotropy. The results highlight that lateral migration of CO2 will be significant and that the caprock succession at Krechba should provide a good seal, even with decreasing effective pressure during injection, in the absence of significant modification by deformation and/or reaction with the CO2-rich fluids.