Capillary trapping in sandstones and carbonates: dependence on pore structure

Residual non-wetting phase saturation and wetting-phase permeability were measured in three limestones and four sandstones ranging in porosity from 0.13 to 0.28 and in absolute permeability from 2 × 10-15 to 3 × 10-12 m2. This paper focuses on the residual state established by waterflooding at low capillary number from minimum water saturation achieved using the porous plate technique, which yields the maximum residual under strongly water-wet conditions. The pore coordination number and pore body-throat aspect ratio of each rock were estimated using pore networks extracted from X-ray microtomography images of the rocks. Residual saturation decreases with increasing porosity, with no apparent difference in magnitude between the limestones and sandstones at a given porosity. Thus intraparticle/intra-aggregate microporosity does not significantly alter the efficiency of capillary trapping in the rocks considered presently. Residual saturation broadly decreases as conditions become less favorable for snap-off, i.e., with decreasing pore aspect ratio and increasing coordination number. The measured residual saturations imply that capillary trapping may be an effective mechanism for storing carbon dioxide in both sandstones and carbonates provided that the systems are strongly water-wet.