A Comprehensive Review on the Capillary Desaturation Curves for Sandstone and Carbonate Reservoirs

Abstract

Various enhanced oil recovery (EOR) methods are applied after primary and secondary recovery stages to target remaining oil saturation (ROS). This remaining oil is divided into bypassed oil and capillary-trapped residual oil. Mobilizing the residual oil in the reservoir is usually achieved when viscous or gravity forces exceed capillary forces. The recovery of the microscopically trapped residual oil is mainly studied using capillary desaturation curve (CDC). To optimize the design of various EOR methods in carbonate and sandstone reservoirs, a fundamental understanding of CDC is needed. A thorough and well-documented research study has been performed for determining the residual oil and generating CDC in sandstone rocks. However, a very limited amount of work has been reported on carbonate rocks. Thus, the main objective of this paper is to provide the recent development made over the last few decades on the CDC studies for carbonate and sandstone reservoirs. Different CDC studies were discussed based on the trapping/bond/capillary number and were critically analyzed. Furthermore, the effects of different controlling factors—wettability, permeability, interfacial tension (IFT), and heterogeneity—on CDC were investigated. This review analysis indicates that CDC in carbonate rocks is broader as opposed to sandstone rocks. This is because of the presence of micropores, large pore size distribution, complex geological characteristics, chemical reactivity, mixed-to-oil-wet characteristics, and heterogeneity of carbonate rocks. Moreover, the critical capillary number for water injection in carbonate rocks reported in the literature lies between 10–8 and 10–5. On the other hand, for sandstone rocks, the number ranges between 10–5 and 10–2. Furthermore, a major influence of wettability on the shape of the CDC was observed. The CDC shape is broader for oil-wet rocks, and capillary number values are higher compared to water-wet and mixed-wet rocks. On the other hand, the lowest capillary number values are observed in water-wet rocks. The outcome of this research study will provide a way forward for CDC studies in both sandstone and carbonate rocks. Additionally, it will serve as a baseline for understanding various CDCs and hence better screening of various EOR methods for different types of reservoir rocks.