Effect of Laboratory Core Cleaning on Water-Oil Relative Permeability

Predictions of reservoir performance are usually based on laboratory measurements of core properties. Laboratory measurements, for example of oil recovery by waterflooding, are in turn related to the wettability of the rock surface. This has been shown by recent papers in the petroleum literature on the subject of wettability. Data have already been presented by this laboratory which show that wettability determines the shape of the water-oil relative permeability curves for a given porous medium. Thus, ideally, laboratory measurements should be made on cores which have the same wettability as the reservoir rock. This paper presents results of laboratory relative permeability measurements on oil field cores. Measurements were made in the water-oil system using the dynamic or "Penn State" relative permeability method. Saturations were obtained by using a radioactive oil soluble compound. Data were obtained on cores as taken from the field in an unextracted condition. Measurements were then made on the same cores after toluene extraction. A comparison of the relative permeability vs saturation curves obtained from the core samples before and after extraction showed a measurable change in the curves, but the changes were small and not due to significant changes in wettability. It can be concluded that the laboratory procedure of core cleaning with toluene extraction and subsequent handling during core analysis does not significantly change the relative permeability characteristics from those of the core material at the start of the core analysis operation. This conclusion is based on data from cores that were preferentially water-wet, cores that were preferentially oil-wet, and cores that were of intermediate wettability. Introduction Reservoir rocks are porous media with high specific surface area and exceedingly fine pore structure. It is not difficult to understand why a knowledge of the surface properties of reservoir rocks is fundamental to an understanding of how fluids flow in such porous media. The molecular processes that occur on the surface of the reservoir minerals and at the fluid interfaces have been shown to be a dominating factor when considering the recovery of oil from porous media. Intensive research is being conducted by most oil field research laboratories in an to evaluate qualitatively and quantitatively the role of wettability. The activity in this field can be verified by noting the number of recent publications and patents dealing with the subject of "wettability". Prior to these recent articles, only a meager amount of wettability data was available in the petroleum literature. These articles show that wettability can materially affect predictions concerning the volume of oil-in-place, and controls to a great degree the relative success of completion methods and several recovery processes.