Residual Oil Saturation Determination - Case Studies in Sandstone and Carbonate Reservoirs

Teklu, Tadesse Weldu (Colorado School of Mines) | Brown, Jeffrey S. (Colorado School of Mines) | Kazemi, Hossein (Colorado School of Mines) | Graves, Ramona M. (Colorado School of Mines) | AlSumaiti, Ali M. (The Petroleum Institute)



In petroleum reservoirs only a small fraction of the original oil-in-place is economically recovered by primary, secondary, and tertiary recovery mechanisms. A considerable amount of hydrocarbon ends up unrecovered or trapped due to microscopic phase trapping in porous media which results in an oil recovery factor typically less than 50%. Waterflooding is by far the most widely used method to increase oil recovery. The oil that remains in the porous media after waterflooding is called remaining oil saturation (ROS) which is larger than the relative permeability residual oil saturation (Sorw or simply Sor). This residual oil saturation varies depending on lithology, pore size distribution, permeability, wettability, fluid characteristics, recovery method, and production scheme. The Sor represents a statistical average over a wide range of pore scale residual saturations with in a representative elementary volume (REV). Determination of the residual oil saturation of a reservoir is a key parameter for reserve assessment and recovery estimates. Further, reliable Sor data is important for investigation of potential incremental recovery under Enhanced Oil Recovery (EOR) methods.

Various residual oil saturation measurement techniques are available both at laboratory and field scale. None of the techniques can be regarded as a single best method of determining Sor. Depending on the complexity of the reservoir under study, combinations of methods are always advisable for appropriate Sor determination.  This paper catalogues a number of field case studies which use different techniques of determining Sor and ROS in sandstone and carbonate reservoirs.

1.     Introduction

Only part of the original oil-in-place is economically recovered by conventional methods. Due to macroscopic and microscopic phenomena, a considerable amount of hydrocarbon is unrecovered or trapped during multiphase flow in porous media. Determination of residual oil saturation (Sor) is a fundamental requirement for studying and understanding the behavior of a field during waterflooding and beyond. Especially, before embarking on a tertiary recovery scheme, it is imperative to know the Sor of the reservoir in order to assess its technical feasibility and profitability.

2.     Residual Oil Saturation Determination Techniques

There are several ways to determine or estimate the residual oil saturation (Sor ) or the remaining oil saturation (ROS).   These include core analysis methods, well log methods, and other methods.  Teklu et al., 2013 gives a more detailed review of the techniques and when they are applicable.  Table 1 lists the advantages and disadvantages of some of these techniques applied in the case studies.

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