Improved determination of oil/water drainage and imbibition saturation functions of chalk core plugs from flooding experiments

This paper presents improvements to a recently developed experimental method, which is particularly suitable for the determination of oil/water capillary pressure and relative permeability for water wet samples of chalk for both drainage and imbibition. Inherently chalk has high capillary pressure, and conventional laboratory determinations of saturation functions on such material are complicated by strong scale effects, particularly by the capillary end effect. The end effect does, however contain detailed information about the saturation functions of the sample and this information is utilized in the method. The saturation functions are represented by analytical expressions that are determined by matching simulated results and measured data. A five-step core-flooding scheme provides the fluid distributions and production data from which the relative permeabilities and capillary pressures are computed for both drainage and imbibition by a least squares technique. A chemical shift NMR technique is used for fluid distribution determination while the simulation of the experiments is carried out by a commercial reservoir simulator. In our previous work with this method simple power functions were used to represent relative permeability functions. The present paper demonstrates the use of more flexible analytical expressions for the oil relative permeability that enables a better representation of certain function shapes. Also an improved representation of capillary pressure is incorporated in the method. The new technique is applied to a chalk sample from the North Sea.