Evaluation of Residual Oil Saturation and Recovery Efficiency of Two Distinct Arabian Carbonate Reservoirs

Abstract

Abstract Saudi Arabia has a variety of carbonate reservoirs of different geologic ages. Such reservoirs are characterized by heterogeneous rock properties. These heterogeneities are caused by wide spectrum of environments in which carbonates are deposited and subsequent alteration of original rock fabric. Extensive SCAL work was carried out on preserved core plugs recovered from two distinct carbonate reservoirs. The first reservoir is a Late Jurassic Arab-D offshore carbonate reservoir (Abu Safah field). The second one is Shu'aiba reservoir (Lower Cretaceous) located in Rub’ Al-Khali in southeastern Saudi Arabia. The purpose of this study is to provide and evaluate waterflood recovery efficiency and residual oil saturations of the two distinct Arabian reservoirs. Variation of oil recovery and residual oil saturation between the two distinct reservoirs is due to variations of rock characteristics especially the relationship of textural and diagenetic features which affect the size and distribution of pore throats. Shu'aiba reservoir rock could be classified as wackstone which demonstrates a higher ratio of lime mud to detrital grains (pore sizes ~ 0.27 to 1.5 microns). On the other hand, Arab-D reservoir is classified as grainstone and consists of oolitic limestone and dolomitic limestone with larger pore sizes (0.5 to 5.5 microns). The results indicated that pore structure, pore size distribution, rock fabric, and environment of deposition are important factors that affect microscopic oil and water flow in porous media and the development efficiency of an oil field developed by water injection. Oil recovery from Arab-D reservoir is slightly higher than that of Shu'aiba reservoir. The residual oil saturations values for Arab-D reservoir were also found to be slightly higher than those of Shu'aiba reservoir. Arab-D reservoir showed less permeability dependence for both (Swir and Sor) end point saturations.