Wettability alteration by surface adsorption of asphaltene molecular in carbonate porous media

Abstract

In this study, the effect of four asphaltene samples on the wettability alteration of carbonate reservoir rock is investigated. The wettability of each core, before and after the deposition of asphaltene, is evaluated using two methods: relative permeability and contact angle. Normally, core samples got oil-wet after the asphaltene deposition, and the contact angle on the surface increased. However, the degree of wettability of each crude oil was different based on the asphaltene composition. According to the elemental analysis of asphaltene samples, the H/C index alone cannot show the asphaltene adsorption potential. The sulfur is an element that enhances asphaltene-asphaltene interaction, but weakens calcite-asphaltene interaction. The oxygen can also enhance the adsorption of asphaltene to calcite, depending on the functional group and its location in the molecule. According to the results of the FTIR spectrum of asphaltene and rock powder, there are three main mechanisms for adsorption of asphaltene to calcite surface, followed by wettability alteration. The first mechanism is the electrostatic interaction between asphaltenes and calcite, which is because of the poly-aromatic nature of asphaltene. The intensity of the electrostatic interaction between asphaltene and rock is directly related to the aromaticity and condensation index of asphaltene. The second mechanism is dispersion, in which the side alkyl chain in the aromatic rings causes asphaltene to be adsorbed into the surface of the rock. The intensity of this mechanism is also proportional to the aliphatic index and the long chain index of asphaltene. The third mechanism, which is less important than the two above-mentioned ones, is the hydrogen bond that is proportional to the ROH index, calculated from the FTIR spectrum. To study the relationship between the electrostatic property of the surface after asphaltene adsorption with surface wettability, zeta potential was used. The results showed that asphaltene, because of its acidic nature, adsorbed to the calcite surface, would make its zeta potential negative. However, more negative zeta potential will not necessarily increase the contact angle. In other words, the surface wettability and electrostatic properties of the surface are two distinct categories that do not have a clear relationship. The surface wettability is more dependent on the amount of aromaticity index of asphaltene, whereas the electrostatic property of the surface is a function of the percentage of oxygen and sulfur compounds in the asphaltene structure. Finally, the scanning electron microscopy (SEM), along with the elemental analysis, was used to investigate the dispersion of various elements at the calcite surface after wettability alteration. The calcium and oxygen are the main elements of calcite production; therefore, the elemental mapping indicated that the surface would be covered with calcium and oxygen prior to the deposition of asphaltene (CaCO3). But by asphaltene deposition on the rock surface, calcium will be observed scattered in some points. For this reason, the amount of calcium in the elemental mapping can be related to the level of surface wettability, so that the lower the percentage of calcium in the surface, the more the asphaltene is adsorbed into calcite, and the changes in the contact angle is greater.