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 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 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 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 CIA 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. Introduction There are two approaches for analyzing cores under the proper wettability conditions. "native state" and "restored state" analyses. (See Reference 1 for literature survey.) Native state analysis requires that the wettability state of the core be preserved at the in situ state. This requires that the core be protected from drilling fluid contamination, evaporation, oxidation, and freezing. If this could be accomplished successfully, it would avoid the effort of the restored state analysis. Since core preservation is often questionable and difficult to verify, however, restored state analysis is often wed. Restored state analysis requires that the core be cleaned to the water-wet state that existed before oil accumulated in the formation. The core is then saturated with crude oil to a capillary pressure typical of the formation and the system allowed to equilibrate or "age" under conditions representative of those existing in the formation. SYSTEM EVALUATED Sandstone cores from a North Sea field, having brine permeabilities in the range 200-500 md and porosities of about 0.25. were used. P. 361