Wettability alteration is considered to be one of the important mechanisms that lead to increased oil recovery during microbial enhanced oil recovery (MEOR) processes. Changes in wettability will greatly influence the petrophysical properties of the reservoir rocks and determine the location, flow and distribution of different fluids inside the porous media. Understanding the active mechanisms of surface wettability changes by the bacteria would help to optimize the condition for more oil recovery. As the mechanisms behind wettability alteration are still poorly understood, the objective of this study is to investigate the wettability alteration at pore scale and find the most effective mechanism of wettability changes in different cases. The experiments were performed on different substrates at fresh condition or aged in crude oil to mimic various wetting conditions. Using an Enterobacter cloacae strain, the influence of bacterial metabolites, bacterial adhesion and bacterial solution with two different carbon sources on wettability were determined for different aging periods. Contact angle measurements were used to quantify the wettability alteration of the solid surfaces. Atomic force microscopy (AFM) experiments were also utilized to combine the macroscopic measurements of wettability with the microscopic study of the surface changes. It was found that the surface wettability could vary from neutral- or oil-wet to water-wet state. Bacterial adhesion and biofilm formation seems to be the dominant mechanism of wettability alteration. The aged glass surfaces regained their initial water wetness where the bacteria could remove the polar and asphaltene compounds from them.
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