Wettability and Adsorption Characteristics of Crude-Oil Asphaltene and Polar Fractions

Abstract

Summary. This study relates the chemical composition of the polar compounds of crude oil to the wettability of rock/oil/brine systems. Adsorption properties of polar and asphaltene fractions were evaluated to determine their effects on wettability. Polar compound fractions were found to cause an oil-wet state on Berea sandstone, but the effects were not a function of the polar-fraction concentration. The concentration of nitrogen/sulfur compounds in six crude-oil polar fractions correlated with the wettability of the polar fractions on Berea sandstone. Langmuir-type adsorption on Berea sandstone was observed in adsorption studies of the asphaltene and polar fractions. Additional analysis with brine-saturated Berea sandstone resulted in adsorption values up to three times less than that for dry Berea. The amount of polar fraction adsorbed on brine-saturated Berea sandstone correlated with crude-oil wettability. Introduction Wettability as applied to an oil reservoir describes the tendency of a fluid to adhere or adsorb to a solid surface in the presence of another immiscible fluid. It can be described as a measure of the affinity of the rock surface for the oil or water phase. A major role of wettability in a reservoir is that of determining the location and distribution of reservoir fluids that influence reservoir-fluid relative permeabilities and thus recovery efficiency. Therefore. wettability is a major factor in determining the degree of oil recovery from a reservoir. This importance has been noted by a number of authors when water-driven systems were evaluated. The amount of oil recovery, as a function of water injection, was found to be greater from water-wet systems than from oil-wet systems. The evaluation of reservoir wettability is also critical in the determination of specific EOR processes. The conditions that establish a given reservoir wettability are not well known. The fluid movement through a reservoir, temperature and pressure changes, fluid production, and injection of fluids and chemicals used to enhance production are factors that must be considered as affecting wettability. Research has indicated that surface-active constituents can be isolated from a crude oil. These constituents can be important in defining reservoir wettability. The properties of reservoir rock are also factors in determining wettability. Significant variations in wettability may be related to variations in pore-surface roughness and mineralogic composition. The presence of water or previously adsorbed organic films, possibility from contact with crude oil or other organic materials, is an additional factor that influences wettability. Only a fraction of crude-oil constituents are believed to be capable of reacting with the reservoir rock surface. Several researchers have indicated that the wettability of a reservoir is strongly related to the amount of adsorption by the heavy ends found in the oil. The heavy ends contain the most polar class of compounds found in the crude oil and are principally asphaltene and resin fractions. One approach to gain insight into wettability has been adsorption studies of crude-oil and oil components on reservoir rock and minerals. A study of adsorption of petroleum heavy ends onto clay minerals has been reported. Adsorption of heavy ends onto clays, a relatively reactive constituent of the reservoir rock surface. was found to depend on the cationic form of clay and on the solvent used for heavy-ends dissolution. Subsequent work found that adsorption of asphaltenes onto clays and minerals was reduced by the presence of water. Attempts have been made to identify more specific compound classes in crude oils that affect wettability. Some researchers believe that organic acids and bases can alter wettability, but one study concluded that low-molecular-weight acids and bases did not induce wettability changes in porous media. 13 Other tractors may alter wettability for example, it has been found that transition metal ions can affect wetting on high-energy surfaces. The purpose of this research was to relate changes in reservoir wettability produced by asphaltene- and polar-compound fractions to the chemical composition of the crude oils. Adsorption properties of asphaltene and polar fractions were evaluated in terms of their effect on wettability. Improving oil production from a reservoir depends on a good, fundamental understanding of- the interaction that occurs between the reservoir fluids and the reservoir matrix. Wettability and adsorption studies are a means to increase this understanding. Experimental The individual crude-oil fractions used in this study (asphaltenes and polars) were obtained with a modified version of ASTM procedure D-2007. With this procedure. a crude oil can be separated into four fractions [saturates, aromatics, polars (may also be referred to as resins). and asphaltenes] on the basis of polarity and solubility differences of the oil constituents. The asphaltene fractions are separated on the basis of their insolubility in n-pentane. The polar fractions are retained on Attapulgus clay-packed chromatographic columns after elution with n-pentane and then removed from the clay columns by an acetone : methylene chloride (1 : 1) elution. Wettability values of the crude oils and their asphaltene and polar fractions were determined by the USBM centrifuge method, in which negative values indicate oil-wet, while positive values indicate water-wet for a given fluid/rock system. The asphaltene and polar fractions are smaller percentages of the crude oil than the saturates and aromatics. Because these materials are quite viscous, the wettability values of the asphaltenes and polars were determined on dilute solutions of these fractions dissolved in a heavy mineral oil (73 cp). The core materials were pretreated before contact with the polar or asphaltene solution and were then subjected to an initial crudeoil saturation drive, Soxhet extracted with toluene, and vacuum dried at room temperature for 48 hours. This procedure made it possible to use the same cores for a series of wettability determinations, thereby eliminating core variations. JPT P. 470^