Effect of Pressure Gradient and Initial Water Saturation on Water Injection in Water-Wet and Mixed-Wet Fractured Porous Media

Abstract

Summary A systematic study of the effect of wettability and initial water saturation on water injection and imbibition is made in Kansas outcrop chalk samples. (Kansas outcrop chalk is very similar to the rock matrix of the North Sea fractured chalk reservoirs.) Water-injection tests were conducted at different pressure gradients to simulate the effect of gravity (that is, negative Pc) on recovery. Based on a large number of carefully conducted tests, the following conclusions are drawn: Initial water saturation has a pronounced effect on water injection in an intermediate-wet chalk. This effect is much less pronounced for a strongly water-wet chalk. The effects are also in opposite directions. Pressure gradients (which simulate the negative Pc effect) have a very strong effect on water-injection performance in an intermediate-wet chalk. Our interpretation of these experiments leads to the conclusion that recovery from the chalk reservoirs may be nearly independent of wettability state. The results from the experiments also reveal that there is no relation between laboratory measurements of spontaneous imbibition and field performance of mixed-wet reservoirs, even when the wettability state is perfectly restored in the laboratory.