Characterization of surface energetics for understanding wettability and asphaltene deposition in the wellbore
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The fundamental forces of adhesion are responsible for the spreading of fluids such as crude oil/brine on the reservoir rock surface. These physico-chemical interactions determine the surface energetics of a reservoir and thus their wetting phenomena. A systematic approach to characterize the mixed wet configurations of various reservoir rocks (sandstone and carbonates) by evaluating their surface energy distributions has been presented here. This approach was tested against macroscopic spatial distribution of oil-wet and water-wet sites and at different temperatures for validation. The new approach used to characterize the mixed wettability of a reservoir rock pertains to establishing a relation between the volume fraction of the mixed wet reservoir rocks and surface energy of the mixture. This approach is based on an accurate description of the various physico-chemical interfacial forces present at the reservoir rock surface using Inverse Gas Chromatography (IGC). Inverse Gas Chromatography is introduced to characterize the surface energy of several carbonate (Calcite and Dolomite) and sandstone rocks (Ottawa sand and Berea sandstone). The behavior of the Lifshitz-van der Waals and acid-base interaction forces acting on the rock surface was investigated at varying water coverage and at different temperatures in association with their surface chemistry. Mixed-wet configurations of various reservoir rocks are created by combining water-wet and oil-wet samples of the rock in different volume fractions and shaken together to establish uniform distribution. These samples were then subjected to the IGC analysis at different temperatures to deduce their surface energy distribution. The relation developed herein was tested against spatial heterogeneity by combining the oil-wet and water-wet rock samples in a layered fashion to validate the approach. A definite and conclusive relationship between the surface energy and mixed wettability of silica glass beads, calcite and dolomite samples was established in this study. The mixed wet configurations of the rock samples ranged from 0% oil wet (meaning water-wet samples) to 100% oil wet samples. The findings indicated that the Lifshitz-van der Waals component of the rock mixture did not undergo any change with change in the wetting state of the system under study. However the acid base components showed a marked decrease with increasing oil wetness before plateauing. Temperature was found to have a profound impact on the surface energy of a rock surface. Spatial heterogeneity by way of layered and segregated distribution of oil-wet and water-wet sites did not affect the eventual surface energy distribution and thus validating the new approach. The novelty of this work is that using this approach the mixed wettability any reservoir rock can be reliably and accurately determined in the shortest and most non-destructive manner using Inverse Gas Chromatography. Finally surface science concepts developed in this study were applied to study another adhesion problem pertaining to asphaltene deposition in the well-bore at varying flowrates, temperature and concentration.