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

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 (S_orw or simply S_or). This residual oil saturation varies depending on lithology, pore size distribution, permeability, wettability, fluid characteristics, recovery method, and production scheme. The S_or 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 S_or 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 S_or. Depending on the complexity of the reservoir under study, combinations of methods are always advisable for appropriate S_or determination. This paper catalogues a number of field case studies which use different techniques of determining S_or and ROS in sandstone and carbonate reservoirs.