Towards a methodology for top seal efficacy assessment for underground CO2 storage

Abstract

This chapter highlights that the quantitative assessment of leakage risk and leakage rates from planned underground CO2 storage sites is a primary requirement for public acceptance, formal site approval, and credit for stored CO2 quantities under CO2 emission schedules. Leakage through the top seal can basically occur by three processes: diffusion through the pore system, capillary transport through the pore system of the seal, and multiphase migration through a fracture network; or by a combination of any of these. Diffusion results in very low leakage rates; maximum rates typically attained after several 100 000 years, being in the ppm range. Multiphase capillary migration is characterized by two main parameters: capillary breakthrough pressure and effective permeability to the non-wetting phase. The dependence of effective permeability to CO2 on capillary pressure, which in turn is a function of CO2 column height, is hysteretic in character with generally higher effective permeability during pressure decrease than during increase, at the same capillary pressure. Leakage is likely to stop at approximately 20 to 50% of the breakthrough pressure as suggested by the snap-off theory. Capillary breakthrough pressure and effective permeability is very difficult to measure for low-permeable rocks.