A numerical approach to accurately estimate water resistivity (Rw) and saturation (Sw) in shaly sand formations

Abstract

In hydrocarbon reservoirs, the accuracy of hydrocarbon saturation depends on the precision of the water saturation and resistivity (Sw and Rw). A significant interpretation parameter is the resistivity of formation water (interstitial water or connate water which is, uncontaminated by the drilling mud that saturates the porous formation rock) because it is appropriate for the calculations of saturation (water and/or hydrocarbon) from basic resistivity logs. The most reliable way to determine this value is through the determination of the chemical composition or resistivity of uncontaminated connate water inside the formation. The saturation of water is the ratio between water volume to total pores volume and its determination accuracy has a great role in estimating hydrocarbon volume. However, the aims of this paper have two main folds, firstly, to introduce a complete review on water resistivity, saturation, and shale volume. Secondly, it presents a numerical method for the determination of resistivity of connate water and water saturation from the true resistivity of the formation using Schlumberger (1975) in shaly sand formations which require critical treatment compared to the clean sand reservoirs. To ensure its ability to determine both the resistivity and saturation of formation water, the technique was tested using synthetic and real field data.

Keywords

• shaly-sand reservoirs, water resistivity, water saturation