Brine-saturated kaolinite mudrocks preserve capillary sealing to CO2 at high pressure and temperature

Abstract

Caprock sealing capacity is essential for carbon geological storage in saline aquifers and depleted oil and gas formations. Clay-rich caprocks and fault gouge are expected to hold buoyant CO₂ in the storage formation by capillary forces. However, all direct capillary sealing capacity measurements of clay-rich rocks to CO₂ were so far limited to pressures below ∼20 MPa and/or temperatures below 50 °C, typically lower than target storage conditions. This paper presents new results of brine absolute permeability, capillary CO₂ breakthrough pressure, and post-breakthrough CO₂ permeability for resedimented kaolinite clay plugs at fluid pressures greater than 41 MPa, temperatures of 60 °C and 80 °C, and mean effective stress of ∼6.8 MPa. The results show that breakthrough pressure (P_CO2 - P_w) is always positive and remains in the interval between ∼ 1.4 MPa and 2.8 MPa within the range of pressure and temperature explored. Moreover, average post-breakthrough CO₂ relative permeability is ∼5 %. An additional test with a clay mixture representative of a shale from the North Sea, at similar pressure-temperature conditions held a differential pressure, i.e., no breakthrough, over three months with a maximum difference P_CO2 - P_w = 5.71 MPa. Results and analysis support the water-wet properties of clays at high pressure and temperature and the resulting capillary sealing capacity to CO₂. These results support expectations that clay-rich caprocks are satisfactory seals for holding buoyant CO₂ via capillary forces. Results also suggest that if the sealing capacity is surpassed, clay-rich caprocks can limit advective flow because of their low CO₂ relative permeability and potential for resealing through snap-off.