Wettability Evaluation During Restored-State Core Analysis

Abstract

ABSTRACT Wettability changes during stages of restored state core analysis were evaluated for a North Sea reservoir. The goal was to determine the proper technique for estimating the remaining oil saturation after waterflooding from core analysis. The traditional methods for quantifying wettability, i.e., USBM and Amott indices and the Brownell-Katz correlation, were evaluated. It was found that the USBM and Amott procedures, when modified slightly, could be interpreted to give a good indication of wettability. The traditional method of cleaning cores by Dean-Stark extraction with toluene followed by chloroform/methanol was found to be ineffective in making a core water-wet. To overcome this difficulty, we evaluated core cleaning procedures that employ a sequence of solvents. The remaining oil saturation increased from 9% with crude oil in the "as received" state to 28% with refined oil after the third cleaning. Once the cores were made water-wet, aging the cores in a North Sea crude oil returned them to a mixed-wet state. However, the restored state cores were not as oil-wet as the "as received" state cores. Even after aging, there was a significant difference between the cores cleaned by Dean-Stark extraction and by a sequence of solvents. The mixed-wet condition with the North Sea crude oil is, therefore, expected to apply to the field: the crude oil base number, isoelectric point, effluent pH during core floods, and bottle wettability tests support this contention. Examination of the Brownell-Katz number correlation and the capillary pressure curves showed that much of the remaining oil saturation in a mixed-wet core, during core analysis could be due to retention by a capillary end effect. This was supported by a CT scan of the core. In this mixed-wet reservoir, the remaining oil saturation is not equal to the residual oil saturation, but must be calculated using the appropriate relative permeability curve and the effect of buoyancy.