Cryo-SEM of liquid-bearing rock

Scanning electron microscopy has been used to examine microstructure of dry soils, sedimentary rocks and other porous materials for three decades. There are many studies of sand grain surface texture, pore morphology, and clay swelling. However, pore geometry and surface topography are only part of the story of how two or more fluids flow through porous media, whether they be unconsolidated or consolidated. The other part is how the fluids distribute in the pore space. Fluid distribution in pore space is largely governed by relative wettability of pore walls. Wetting fluid tends to reside on walls as a thin film and to occupy small pores totally, whereas nonwetting fluid tends to occupy the center of larger pores. Which fluid is more strongly wetting depends on the local nature of the wall. Contact angles indicate wettability of planar, homogeneous solid surfaces, but roughness and compositional heterogeneity, which seem to be common in sedimentary rock, complicate matters.