When oil was injected after gel placement in polyethylene, the overall Swr value was 83.5% (Figure 19). For comparison, the overall Swr value was only 16.5% before gel placement (Figure 8). Presumably, the 67% difference was due to gel that was not dehydrated during oil injection. Put another way, oil injection reduced the gel volume by 16.3% (99.8%-83.5%)—a reduction that was substantially less than that seen in Berea (35% from Table 2). If the reductions are attributed entirely to gel dehydration, the gel would have been concentrated by factors of 2.2 in Berea (i.e., 63.7/28.7 and only 1.2 in polyethylene (i.e., 99.8/83.5).
The oil saturation increased 63% for pores that were smaller than 10-4 mm3 but only by 11% for pores that were larger than 10-3 mm3 (see Figure 20). Thus, compared to Berea, reduction of gel volume during oil injection into gel-filled porous polyethylene was more likely in small pores and less likely in large pores. If the losses are attributed to gel dehydration, the gel would have been concentrated by factors of 2.8 in small polyethylene pores and 1.1 in large polyethylene pores. Pressure gradients were not allowed to exceed 35 psi/ft during any stage of the polyethylene experiments. So, it seems unlikely that reduction of gel volume occurred because of exposure to excessive pressure gradients. If high pressure gradients were responsible, gel damage should have been greater in larger pores than in smaller pores
A comparison of Figures 6, 7, and 19 shows strong similarity between the saturation distributions at Swr after gel placement in polyethylene and those at Sor before gel placement. Why should this similarity occur? Before gel placement, residual oil preferentially located in films and small pores. Medium to large pores provided the path of least resistance during oil displacement. However, after gel placement, medium to large pores were filled with immobile gel—providing substantial flow resistance for both oil and water. During oil injection after gel placement, capillary forces favored imbibition of oil through films and small pores. When water was subsequently injected, these oil-filled films and small pores apparently provided an easier flow path for water than that through the gel in the medium to large pores. Nevertheless, it is interesting that water could displace oil from the small pores after gel placement but not before gel placement. Pressure gradients after gel placement were no greater than before gel placement.
Saturations for the final water injection are plotted in Figure 21; notice the similarity to Figure 13. At Sor, before and after gel placement, very little oil (less than 0.3%) remained in the imaged region. The small oil saturation that was present generally existed in the smallest pores.
For polyethylene, the oil residual resistance factor (Frro) was 24. The mobile oil flows through films and the smallest pores. One might have expected much higher resistance to flow for low oil saturations and such narrow flow paths. The measured value for Frrw was 2,130. Based on the Sw changes, the primary flow path for water is expected to be through the previously open oil paths, especially if little or no residual oil blocks the paths. However, if this were the case, Frrw should not be 89 times greater than Frro. On the other hand, the high water residual resistance factor could be explained if the oil paths closed up. During oil injection, paths may open by partial dehydration of the gel. During subsequent water injection, the paths could partially close when the gel rehydrates.