Mechanisms causing disproportionate permeability reduction in porous media treated with chromium acetate/HPAM gels

It is well established that treatment of porous rocks with gelled polymer systems can cause the permeability of water at residual oil saturation to be reduced by one to three orders of magnitude more than the permeability of oil at the water saturation that is immobile after treatment. This phenomenon is called disproportionate permeability reduction (DPR) and is of interest because application of gel treatments in production wells has potential to reduce water production. The mechanisms that cause this phenomenon are not well understood. This paper describes how permeability to oil and water is developed in pore space that is filled with a chromium acetate/partially hydrolyzed polyacrylamide (HPAM) gel and proposes a mechanism for DPR based on the interpretation of the experimental data. Experimental data for the flow of oil and brine were obtained in unconsolidated sandpacks and in Berea sandstone cores with and without residual oil saturation after a chromium acetate/Alcoflood 935 gelant was injected and gelled in situ. Interpretation of the experimental data suggests that oil permeability develops as oil penetrates into the gel-filled pore space, dehydrating the gel by displacing brine from the gel structure and creating "new flow channels" within or around the gel. The "new pore space" is a fraction of the original porosity, and the permeability to oil is reduced substantially from its value before placement and in-situ gelation of the gelant. Subsequent brine injection displaces oil from these flow channels but traps some of the oil in the new pore space as a residual saturation. The trapping of residual oil in the new pore space causes the disproportionate reduction in brine permeability because the brine flows primarily in the pore channels created by dehydration of the gel even though the gel has some brine permeability. When gelant is placed in a matrix containing residual oil, dehydration of the gel reconnects some of the trapped oil, and the oil permeability increases. Subsequent brine displacement experiments conducted at the same pressure drop showed that initial brine permeability was reduced by factors of 100 to 1,000 more than the oil permeability, verifying the existence of DPR.