Abstract
Background and hypothesis: In the oil industry, asphaltenes are known for their tendency to aggregate in solution and to deposit on surfaces, with both properties being connected with operational problems associated with recovery, production and refining. Interactions involving asphaltenes and other crude oil components have been a major aspect of investigation in attempting to understand the full nature of these problems. Water is implicated in asphaltene behavior in solution, for example, where it has been found to delay the deposition of asphaltene aggregates. At interfaces, there is evidence that water-in- crude oil emulsion stability is enhanced through asphaltene-water interactions, and there have been a few reports that asphaltene adsorption on mineral surfaces is reduced in the presence of water. We consider the latter aspect, because the instances of reduced adsorption to date have not attempted to quantify the effect. Previous studies showed that vapor-phase adsorption of organic molecules is reduced in the presence of pre-adsorbed water and we were interested to determine whether the same is true for liquid-phase adsorption of asphaltenes. Experiments: The surface of quartz sand was controlled by pre-adsorption of water from different relative humidity (RH) environments as the water adsorption isotherm is known from previous studies. These pre-conditioned sand samples were used as substrates for the adsorption from toluene solutions of n-heptane-precipitated asphaltenes (C7A) from an Athabasca oil sands bitumen. To supplement the adsorption behavior, atomic force microscopy (AFM) images were taken of the resultant sand grains at 0 and 80% RH, and low-field NMR was used to estimate the sand wettability. Findings: Asphaltene adsorption on sand is sensitive to surface-adsorbed water, with 4-fold reduction in adsorption when increasing the RH from 0 to 100%. This is in general agreement with previous vapor-phase adsorption of small organic molecules, and is therefore believed to be the first demonstration of the effect of pre-adsorbed water on adsorption from solution. Asphaltene adsorption as a function of RH is the converse of literature water adsorption isotherm data. Three asphaltene adsorption regions have been tentatively identified based on water adsorption behavior and the literature interpretation of water structure on quartz: the highest asphaltene adsorption occurs at very low RH, decreasing to a near-constant value in the range 40–80% RH, followed by a rapid decrease beyond 80% RH. Further analysis strongly suggests that asphaltene adsorption decreases linearly with the thickness of the adsorbed water film. The effects of RH on asphaltene adsorption are also reflected in AFM images and NMR wettability results.