Exploring wettability by imaging the adsorption of crude oil, re-dissolved asphaltene, and phenol solutions onto calcite. Implications to sorption mechanisms and molecular structure of surface-active compounds in crude oil

Abstract

Surface-active compounds in crude oil can change the wettability of reservoirs rocks from water-wet to oil-wet. Since neither the composition of any specific oil with respect to these compounds is known nor the exact mechanism of how they make a surface more amenable toward adsorption of non-polar fractions in oil, prediction of wettability changes is difficult. One group of such surface-active compounds are asphaltenes and this study focuses on how this fraction of the crude interacts with mineral-surfaces to change wettability.The structure of asphaltenes is complex, not fully deciphered, and the functional groups involved in the wettability changes are still uncertain. We know, however, that asphaltene molecules are small (~1.5 nm diameter), consist of a core of aromatic rings linked to outer alkane chains, and contain a variety of heteroatom functional groups, including hydroxyl.One method used here to determine where on the surface crude oil or asphaltenes therein may attack specific surface sites is atomic force microscopy (AFM). Calcite surfaces exposed to (1) crude oil, (2) asphaltenes previously extracted from crude oil, and (3) hydroxyl functional groups in phenol used as a minimal surrogate for asphaltene with just one benzene ring and hydroxyl group, were all imaged using AFM.At the microscopic scale, medium crude oil adsorption is observed onto [4¯41] and [481¯] steps on the {101¯4} face with nanoscopic droplets attaching every ~200 nm after 20 min reaction. Adsorption to terrace sites with no step preference is observed at longer reaction times. Seawater conditioning enhances wettability changes, while brine and deionized water have less of an impact under the conditions examined.Adsorbed UG8, an asphaltene separated from a crude oil similar to the one used here and dissolved in toluene, shows winding continuous adsorbate structures that are in general alignment with the [010] or [421¯] direction on the {101¯4} face. Nanoaggregation or clustering of asphaltene molecules was required for oriented adsorption to occur, suggesting that conditions amenable for molecular aggregation affect adsorption processes and products.Hydroxyl functional groups in the form of phenol in n-decane show adsorbates with no adsorption preference, e.g., to specific step edges. Phenol in water shows both individual droplets preferentially adsorbed to steps in cleavage pits and semi-polygonal features that lack clear orientation but are reminiscent of the adsorbates produced by asphaltenes previously extracted from crude oil.In order to relate adsorbate behavior and changes in wettability to physicochemical changes of the mineral surface, quantum-mechanical calculations were performed on surface electrostatic potential distribution. Results on the {101¯4} face show broad areas of excess charges with alternating signs along the steps where adsorption occurs. The resulting positive areas (1) are of greater charge magnitude, (2) are associated to obtuse steps, and (3) are likely more influential at initiating surface interactions.Surface-active compounds in crude oil and in asphaltenes extracted from crude oil adsorb on cleaved calcites following surface features that follow crystallographic directions on the {101¯4} face. These adsorption patterns occur despite differences in solvents and experimental procedures. Dominant electron density deficits along surface steps are likely to attract negatively charged functional groups to the surface. This interaction is predominantly responsible for initiating and guiding the adsorption process. Molecular aggregation in the fluid appears to be a critical factor in switching the dominant crystal direction of adsorption from periodic bond chains to somewhat closer to polar edges (likely Ca2+-terminated), such as [010] or [421¯].The lack of obvious orientation of phenol adsorbates suggests that the size of the molecules, aggregates, or clusters in the fluid is important in the array of nanoscopic drops or adsorbates. Both the surface and the fluid influence the adsorption process, but our results suggest that the aromatic solvents (i.e., crude oil and toluene) play a role in producing adsorbates on surface features that follow crystallographic directions, while polar solvents may reduce the control of the surface charge over the adsorption process.