Experimental Investigation of the Effects of Temperature, Pressure, and Crude Oil Composition on Interfacial Properties

Abstract

Summary The wettability of the fluid/rock system affects the distribution of fluids within a porous medium; this distribution in turn strongly affects displacement behavior and oil recovery efficiency. This paper is an investigation of the influence of temperature, pressure, and oil composition on the wettability of a specific carbonate reservoir. Contact-angle measurements were used to quantify wettabilities on calcium-carbonate crystals. The experimental conditions included varying temperature and pressure for both dead crude oil and oil recombined to the original bubblepoint. In addition to quantification of wetting properties, interfacial tension (IFT) between oil and water was measured, and formation of rigid films was observed. A complete reversal from a predominantly oil-wet system at lower temperatures to a predominantly water-wet system at higher temperatures was found. Pressure alone had little effect on the wettability of the system. IFT between crude oil and brine showed an increase with increase in temperature under anaerobic conditions, whereas at aerobic conditions, IFT decreased with increase in temperature. The tendency to form rigid films at the crude-oil/brine interface was found to be temperature-dependent. The formation of rigid films was stronger at lower temperatures. Introduction For some time, researchers have recognized that wettability affects the distribution of fluids within a porous medium, which in turn strongly affects the displacement behavior, relative-permeability characteristics, and consequently, the oil production. The mechanisms that result from this phenomenon and the factors that influence wettability are not satisfactorily understood, possibly because of the lack of an accurate definition of wettability and a method to quantify it in porous media. The method most widely used to evaluate the wetting characteristics of a solid surface is contact-angle measurements, which can be related to processes taking place in the porous medium. This method is discussed place in the porous medium. This method is discussed in detail in Ref. 11. Considerable controversy exists, however, concerning the validity of using a smooth, flat, homogeneous solid surface to measure contact angles and then extrapolating the results to reservoir systems where the solid surface is rough and heterogeneous. Contact angles are a function of the IFT at the solid/ liquid and liquid/liquid interfaces. For reservoir fluids, IFT's are expected to be affected by changes in temperature that result from the presence of surface-active materials in the crude oil. Pressure may also be important in reservoir systems because of its effect on hydrocarbon and other gases in solution. The adsorption of surface-active material at the brine/oil interface will also cause film formation at the interface, and these films will possess properties that are different from those of the bulk fluids. It has been claimed that rigid films will have the greatest influence on the interfacial and flow properties of the fluids in porous media. A more detailed description of the dependence of contact angle on IFT and its application to oil recovery processes can be found in Ref. 11. As pointed out, wettability is closely related to oil/brine IFT; hence the study of IFT and film formation may explain changes in contact angle or wetting behavior of solid surfaces. Only a few investigations that deal with wettability of real reservoir systems at high pressure and temperature have been reported in the literature. Because we knew that temperature and pressure might play an important role in IFT and wettability, this study was undertaken to evaluate the effect of those factors on the interfacial properties of an Alberta carbonate oil reservoir. Experiments were carried out with reservoir-produced brine and both stock-tank oil and live oil. Calcium-carbonate crystals were used to represent the pore walls of the reservoir. The contact-angle method was used to quantify wettability, and the pendant drop method was used to study both IFT and film formation.