The Effect of Wettability on Oil Recovery: A Review

Abstract Numerous early reports on experimental works relating to the role of wettability in various aspects of oil recovery have been published. Early examples of laboratory waterfloods show oil recovery increasing with increasing water-wetness. This result is consistent with the intuitive notion that strong wetting preference of the rock for water and associated strong capillary imbibition forces give the most efficient oil displacement. However, an increasing number of examples of improved waterflood recovery with shift from strongly water-wet conditions to intermediate, weakly water-wet and oil-wet conditions have also been reported. All the early and more recently observed experimental outcomes, on wettability effects on waterflood oil recovery, point to the divergence of conclusions regarding the optimum wetting condition for maximum waterflood oil recovery. The impact of the in-situ reservoir wetting state on miscible and /or immiscible gas flood oil recovery, though not as pronounced as that observed in waterflooding processes, is still nonetheless quite significant. It has been shown that wettability does indeed determine the gasflood oil recovery efficiency and changes in wetting states have resulted in observed improved gasflood recoveries. This paper examines the effect of wettability on waterflooding and gasflooding processes respectively. Waterflood oil recoveries were examined for the dual cases of uniform and non-uniform wetting conditions. The uniformly wetted systems examined in this work are considered to be those systems in which the observed wetting state is uniformly distributed and covers the range from strongly water-wet through the intermediate-wet to strongly oil-wet conditions. The non-uniformly wetted systems examined include the mixed-wet and the fractionally-wetted systems. The effects of wettability on miscible and immiscible gas recovery processes (including WAG) are also examined. Other factors such as the spreading coefficient, mobility ratio, crossflows, etc. were also reported to affect gasflood oil recoveries. Most experiments on wettability studies are performed on core samples and depending on the process of core sample recovery / retrieval, transportation and storage, the wetting state of the core sample may be altered. Consequently the importance of proper core handling procedure in ensuring that the reservoir native-state wettability is preserved is highlighted. In a situation where the native wetting state has been altered, the need for adequate care in the process of/approach to reproducing wettability is also mentioned.