Use of X-ray computed microtomography to understand why gels reduce relative permeability to water more than that to oil

R.S. Seright, J. Liang, W. Brent Lindquist, John H. Dunsmuir – 2003

Abstract

X-ray computed microtomography (XMT) in Berea sandstone reveals that a Cr(III)-acetate-HPAM gel creates DPR by trapping substantial volumes of oil that remain immobile during subsequent water flooding. Residual oil saturation increased from 43.5% before gel placement to 78.7% after — nearly doubling the trapped oil. With this high trapped oil saturation, water is forced to flow through narrow films, the smallest pores, and through the gel itself. In contrast, during oil flooding, oil pathways remain relatively free from constriction by the gel. This mechanism — oil trapping by gel leading to water being forced into the most restricted pathways — provides the first pore-scale mechanistic explanation for DPR consistent with direct XMT observation.

Key Takeaways

  • The primary DPR mechanism in water-wet Berea sandstone is oil trapping: gel placement nearly doubles residual oil saturation (from 43.5% to 78.7%), forcing subsequent water flow into narrow films, the smallest pores, and through gel itself.
  • Oil flooding after gel placement is much more efficient at creating flow paths than water flooding — oil pressure dehydrates gel in the pores it contacts, creating relatively unobstructed oil channels while water remains trapped in gel-filled pathways.
  • The asymmetry between oil and water behavior after gel placement is a direct consequence of gel’s physical response to oil contact: oil dehydrates the gel (reducing its volume and opening space for flow) while water cannot perform the same dehydration.
  • XMT provides direct quantitative evidence linking the pore-scale physical process (oil trapping and gel dehydration) to the macroscale permeability asymmetry — establishing the mechanistic basis for DPR in permeable sandstone.
  • This mechanistic understanding predicts that DPR will be strongest when gel placement results in the most complete blocking of water pathways — favoring high gel concentration, sufficient gelation time, and porous media with well-defined pore-throat networks.
Read Article

Related Articles