A New Approach to Water Saturation Modeling and Distribution in Dynamic Models Using Log Derived Saturation Height Function (SHF) (A Case Study of Niger Delta Province)

Abstract

Abstract Complex variations in pore geometrical attributes, which in turn, defines the existence of distinct zones or hydraulic unit within a mappable geological facie or “rock type (RRT)' which have similar dynamic behavior is “masked” when “rock types (RRT's)” in reservoir simulators are assumed to be different geological facies of similar dynamic behavior. To effectively and accurately distribute water saturation (Swi) in the cells of dynamic model, capillary pressure (Pc) vs.Swi and relative permeability curves must be assigned such that they honor the pore system structure as well as the saturation from well logs. Therefore it might not be enough to assign one SHF for a given geological RRT. These poses a challenge, for example, in Niger Delta sandstone reservoir comprisingmainly alternating deltaic sandstone with shale suggesting two or three consistent RRT's with wide variability in Swi. The proposed approach tends to develop a correlation that will consistently replicate the behavioral relationship between reservoir quality index ((RQI=0.0314K/∅ϵ)), effective water saturation S* = (Sw − Swirr)/(1 − Swirr) and Height above free water level(HAFWL) of both routine core and log data. This correlation is used to develop drainage log derived Pc and relative permeability curves for each class of RRT. Rock classification employed in this work was based mainly on irreducible water saturation(Swirr) classes. The SATNUM keyword in ECLIPSE was used to assign each SHF and relative permeability curve to the grid cells of the dynamic model based on the rock classification. The developed SHF yielded an excellent match, both vertically and laterally, with log derived Swi and within 1% difference between static and dynamic hydrocarbon-in- place for some Niger-delta reservoirs studied. Also, a plot of Swi generated from the newly developed SHF at well points on the Y-axis and those from well logs on the X-axis yielded a 45° straight line through the origin with an R-square of 0.9935. The dynamic model stability and running performance was greatly improved using the developed SHF.