was infused into the center part of the core while each end of the core about 0 10 pore volume on one end and about 0 20 pore volume on the other end of the core remained essentially free of the gel forming fluid flow was stopped for 4 hours and the gel was then allowed to form in the pores thereafter intermittent infusion with brine c was conducted over the next 50 days to determine the permeability of the thusly 34 gelled 34 core to brine c the results are given in table iii and shown in fig1 wherein the permeability of the core is expressed as the percent of the original core permeability as a function of time in this example the gel forming fluid was prepared by combining 100 cc of 2 5 wt with polyvinyl alcohol in brine c 1 cc of glacial acetic acid 0 365 gr of sodium acetate trihydate and 0 20 ml of malon aldehyde bisdimethylacetal the ph of the initial gel forming fluid was 3 4 another berea sandstone core was equilibrated to 82 u00b0 c and conditioned for residual oil formation simulation using brine b as described above an aliquot of gel forming fluid equal to 0 75 pore volume 13 7 ml was infused into the pores of the core the gel forming fluid in the lines leading to the core was then forced into the core by displacement with 7 0 ml of 2 5 polyvinyl alcohol the flow was stopped for hours to permit the gel to form thereafter intermittent brine infusion.