Analysis for effect of variable formation water salinity on accuracy of water saturation

Key words:  formation water salinity  formation resistivity of mixed fluid  Archie formula  water saturation

Abstract:

Determining the formation resistivity of mixed fluid and the Archie 's parameters precisely is the core technique for the evaluation of remaining oil saturation. During the water-flooding development, the formation resistivity of mixed fluid is however a dynamic parameter. In addition, when the water salinity varies in a wide range, the cementation exponent (m) and the saturation exponent (n) increase with increasing water salinity, affecting the accuracy in calculating water saturation using the Archie formula. In this paper, a method to obtain high-precision formation resistivity of mixed fluid is proposed using the theoretical simulation material balance method of variable multiple injected water. Based on the W-S model and Archie 's formula, the formulas to calculate the dynamic cementation exponent (m) and saturation exponent (n) are derived, that potentially provides a series of geologic parameters for the calculation of water saturation. The study shows that the method can improve the accuracy of the resistivity of mixed liquid; and using dynamic values of the m and n can improve the accuracy of water saturation significantly.