Base Number and Wetting Properties of Crude Oils
S.T. Dubey (Shell Development Co.)
P.H. Doe (Shell Development Co.)
Oil acid and base numbers influence wetting through their effect on electrostatic interactions with the mineral surface. An improved nonaqueous potentiometric titration has been developed that correctly quantifies weak bases in crude oils. In crude-oil/silica systems, wetting behavior correlates with base/acid ratio and is consistent with wetting theories based on disjoining pressure.
Reservoir oil recovery and the time scale over which recovery may be achieved are affected significantly by the extent to which the oil wets the reservoir minerals. Morrow’s1 recent review summarizes some important effects of wetting on oil recovery and emphasizes that most systems depart from what he calls very strongly water-wet conditions. If the departure from water-wet conditions is significant, special care is required in core analysis to duplicate reservoir wettability conditions as closely as possible.
The theoretical understanding of wettability has not advanced to the stage where these departures from water-wetness can be predicted reliably from first principles. Oil accumulation in a reservoir represents a process of partial displacement of reservoir brine with crude oil. During this process, oil approaches the mineral surfaces but remains separated from them by very thin (typically < 50-nm thick) water mms. Hirasaki2-4 argued that wetting in these oil/brine/mineral systems will be determined by the water-film thickness, which in turn is determined by the balance of forces within the film. These forces give rise to an excess pressure, the disjoining pressure, that acts to resist further thinning of the film. The film will drain (become thinner) until its disjoining pressure is equal to the applied capillary pressure.2,3
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