Pore-lining phases and capillary breakthrough pressure of mudstone caprocks: Sealing efficiency of geologic CO<inf>2</inf> storage sites

Subsurface containment of CO2 requires an effective caprock seal. Many previous studies on seals have relied on macroscopic measurements of capillary breakthrough pressure and other petrophysical properties without direct examination of solid phases that line pore networks and contact fluids. However, pore-lining phases strongly contribute to sealing behavior through interfacial interactions among CO2, brine, and the mineral or non-mineral phases. We examined continental and marine mudstones using high resolution (i.e., sub-micron), direct observations of pore-lining phases and X-ray diffraction (XRD). Our results indicate that sealing efficiency (i.e., breakthrough pressure) is governed by pore shapes and pore-lining phases that are not identifiable except through direct characterization of pores. Bulk XRD data do not indicate which phases line the pores and may be especially lacking for mudstones with organic material. Organics can line pores and may represent once-mobile organics that modified wettability of an originally clay-lined pore network. For shallow formations (i.e., <800m depth), interfacial tension and contact angles result in breakthrough pressures that may be as high as those needed to fracture the rock-thus, in the absence of fractures, high capillary sealing efficiency is indicated. Deeper seals have poorer capillary sealing if some reduction in water-wetting occurs, with an increase in pressure and temperature, on minerals and/or pore-lining organic phases. © 2012 Elsevier Ltd.