X-ray computed microtomography (XMT) was used to establish why pore-filling Cr(III)-acetate-HPAM gels reduced permeability to water much more than to oil. Our results suggest that permeability to water was reduced to low values because water must flow through gel itself, whereas oil pressing on the gel in Berea sandstone or porous polyethylene forced pathways by dehydration—leading to relatively high permeability to oil. In very permeable sandpacks, data from other researchers support ripping or extrusion mechanisms for creating oil pathways.
Our XMT studies provide interesting insights into imbibition and drainage processes in water-wet and oil-wet porous media even before gel placement. Many of our observations were consistent with conventional wisdom. However, some were unexpected. Residual wetting-phase (water) saturations in Berea were surprisingly low-valued in small pores. We attribute this to surface roughness caused by clay coating on Berea's pore walls, which allowed efficient water drainage from small pores during oil injection.
Gels have often been injected into production wells in an effort to reduce water production without seriously damaging hydrocarbon productivity. To be effective, the gels must significantly reduce permeability to water while causing minimal reduction in permeability to oil. Many gels exhibit this disproportionate permeability reduction, but the property has not been as predictable as we would like. If the mechanism for this property were understood, gel treatments could be applied with greater reliability. To determine this mechanism for a Cr(III)-acetate-HPAM gel, we used XMT. As an important prelude to investigating gel behavior in porous media, XMT was used to characterize imbibition and drainage processes in water-wet and oil-wet porous media before gel placement.