Water Imbibition into Berea: Oil Was Trapped

During water imbibition after gel placement, the average increase in Sw was 20.3%—much smaller than the 65.6% average change that occurred before gel injection (Figure 11). As was the case before gel injection, Figure 17 reveals that a wide range of changes occurred for most pore sizes. The average saturation change was not particularly sensitive to pore size (although the average saturation changes were close to zero for the smaller pores). In contrast to the results before gel injection, 22.6% of the pores gained oil even though water was injected. Evidentally, a significant degree of rearrangement occurred for water and oil saturations during imbibition post gel placement.

Figure 17 Changes in Berea: Swr after gel to Sor after gel.

Commensurate with this, Figure 18 reveals that at Sor after gel placement, 93.3% of the pores had higher oil saturations than at Sor before gel placement. Again a wide range of changes occurred for all pore sizes, and the average change was not sensitive to pore size.

Figure 18 Changes in Berea: Sor before gel to Sor after gel.

Residual Resistance Factors for Berea.

The measured residual resistance factors were 15 for Frro and 1,220 for Frrw. Thus, gel presence reduced permeability to water 81 times more than to oil. As mentioned earlier, much of the gel was reduced in volume during oil injection after gel placement. Why was the final permeability to water so much lower than that to oil? Previous analysis1,7 indicated that the gel trapped significantly more residual oil. Our new analysis, in which Sor jumped from 18.4% before gel placement to 51% after gel placement (i.e., Sw decreased from 81.6% to 49%; see Table 2 and Figure 18), confirmed this conclusion. With many pores permanently occupied by oil, water was forced to flow through narrow films and through the gel itself—explaining the large value for Frrw. In contrast, oil pathways were reopened by the oil, so Frro was much less.