It is well established that treatment of porous rocks with gelled polymersystems often causes the permeability of water at residual oil saturation to bereduced by one to three orders of magnitude more than the permeability of oilat the water saturation that is immobile after treatment. This phenomenon iscalled disproportionate permeability reduction 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/HPPAM gel and why disproportionate permeability reduction is observed. Experimental data for the flow of oil and brine were obtained in unconsolidated sandpacks and in Berea sandstone coreswith and without residual oil saturation after a chromium acetate/Alcoflood 935 gelant was injected and gelled in situ.
We show that oil permeability develops as oil penetrates into the gel-filledpore space, dehydrating the gel by displacing brine from the gel structure andcreating "new flow" channels within the gel. The new pore space is a fraction of the original porosity, and the permeability to oil is reduced substantially from its initial value. 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 thedisproportionate reduction in brine permeability because the brine flows primarily in the pore channels created by dehydration of the gel even thoughthe gel has some brine permeability. When gelant is placed in matrix contain ingresidual oil, dehydration of the gel reconnects some of the trapped oil and theoil permeability increases. Brine displacement experiments at the same pressure drop showed that initial brine permeability was reduced by factors of 100–1000 more than the oil permeability, verifying the existence of disproportionate permeability reduction.
Increased water production is a world wide problem in mature fields producedby natural water drive or active waterflood. There are economic andenvironmental incentives to develop methods that have the potential to reducewater production without significantly affecting oil production. During the past 15 years, a number of polymer systems have been developed that when placedin a porous matrix reduce the permeability to water at residual oil saturation significantly more than the permeability to oil at the saturation where water is immobile. This phenomenon is termed disproportionate permeability reduction(DPR). Systems that exhibit this behavior are called relative permeability modifiers (RPM).
There are extensive investigations (Liang et al. 1, Dawe and Zhang2, Liang and Seright3–4, Thompson and Fogler 5, Nilsson et al. 6 and Al-Sharji et al.7) on the mechanisms that cause disproportionate permeability reduction but none have been demonstrated to be the primary cause of DPR. This paper describes an experimental study of chromium acetate polyacrylamide gels which demonstrate DPR when placed in sandpacks and Berea sandstone core material. The research was stimulated by experiments conducted by Dawe and Zhang2 who usedmicroscale models to observe mechanisms of oil and water flow through a gelplaced in a porous medium made by etching pore structure on glass plate. They observed that oil flowed through the gel by fingering through the gel and displacing some water from the gel, syneresing the gel. Water flowed through the gel by diffusing into the gel structure. A subsequent paper by Al-Sharji etal.7 provides additional support for the pore level mechanisms observed by Dawe and Zhang 2.