Interrelationship of Wettability, Initial Water Saturation, Aging Time, and Oil Recovery by Spontaneous Imbibition and Waterflooding
Authors: Zhou Xianmin , Morrow Norman R. , Ma Shouxiang 
Published: December 2007

Xianmin Zhou, SPE, Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY 82071, Norman R. Morrow, SPE, Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY 82071 and Western Research Institute, 365 N. 9th St., Laramie, WY 82070-3380, and Shouxiang Ma, SPE, Western Research Institute, 365 N. 9th St., Laramie, WY 82070-3380

Abstract
Previous studies of crude oil/brine/rock (COBR) and related ensembles showed that wettability and its effect on oil recovery depend on numerous complex interactions. In the present work, the wettability of COBR ensembles prepared using Prudhoe Bay crude oil, a synthetic formation brine, and Berea Sandstone was varied by systematic change in initial water saturation and length of aging time at reservoir temperature (88 C). All displacement tests were run at ambient temperature. Various degrees of water wetness were achieved and quantified by a modified Amott wettability index to water, the relative pseudo work of imbibition and a newly defmed apparent advancing dynamic contact angle.
Pairs of spontaneous imbibition (oil recovery by spontaneous imbibition of water) and waterflood (oil recovery vs pore volumes of water injected) curves were measured for each of the induced wetting states. Several trends were observed. Imbibition rate, and hence water wetness, decreased with increase in aging time and with decrease in initial water saturation. Breakthrough recoveries and final oil recovery by waterflooding increased with decrease in water wetness. Correlations between water wetness and oil recovery by waterflooding and spontaneous imbibition are presented.

Introduction
Wettability is an important factor in the performance of waterfloods. Final recovery of oil by waterflood is expressed in this paper as Rwf, the fraction of the original oil in place (OOIP) recovered. Review of literature showed variations in Rwf could be very large and were strongly dependent on how wettability was established. The wettability state induced by adsorption in the presence of an initial water saturation, Swi, is referred to as mixed. The main characteristic of mixed wettability is that only the areas of rock surface exposed to crude oil undergo changes in wetting that are significant to the displacement process. The term mixed wettability was adopted by Salathiel to describe this form of wettability distribution with strongly water-wet and strongly oil-wet surfaces. However, a wide range of wetting states can be induced by contact with crude oil and adsorption of oil components does not necessarily result in a strongly oil-wet surface. Nevertheless, the geometric description of mixed wettability envisaged by Salathiel is still appropriate and in this paper is applied to all possible types of mixed wetting that depend on the location of bulk water in the rock at the time of adsorption. The term bulk water is used here to identify water retained by capillary forces as distinct from water retained by adsorption at solid surfaces as thin films. The fraction of Swi existing as surface films is very small except at extremely low water saturations.
Jadhunandan and Morrow showed that wettability can be varied in Berea Sandstone by changing the conditions for adsorption from crude oil. Variations in the crude oil/brine/rock (COBR) ensembles that were studied included the crude oil, brine, initial water saturation, Swi, aging temperature of the rock in crude oil, Ta, and rate of flooding. The aging time, ta, was standardized at 10 days. From the results for over 50 waterfloods, a correlation was obtained between oil recovery and wettability defined by the Amott-Harvey wettability index, Iw-o, with the oil recovery peaking at close to neutral wettability (see Fig. 1). For all wettability conditions below Iw, o=0.9. with two exceptions, produced water was accompanied by a small and decreasing fraction of oil. This mode of production was postulated by Salathiel to result from a bi-continuous distribution of oil and water within the rock.
Jadhunandan and Morrow also investigated factors influencing the rate of oil recovery by spontaneous imbibition.
P. 325