The effect of bacterial solution on the wettability index and residual oil saturation in sandstone

Wettability is a major factor controlling the location, flow and distribution of fluids in a reservoir rock. The wettability of a core will have effects on almost all types of core analyses, hereby wettability changes and remaining oil saturation during microbial enhanced oil recovery processes. The objective of this study was to determine the influence of a bacterial solution on the Amott wettability index and the residual oil saturation resulting from the forced imbibition process in the wettability test. The experiments were performed both on water wet and neutral wet core plugs at laboratory temperature using NaCl brine and dodecane in one set of experiments and dodecane and a solution of brine and Rhodococcus sp 094 bacteria in another set of experiments. The Rhodococcus sp 094 bacteria were isolated from sea water and are alkane oxidizing bacteria that have the ability to form extremely stable crude oil-in-water emulsions. The core material was Berea sandstone from the same block where samples were cut to equal dimensions. To evaluate the effect of the bacterial solution on wettability in both water wet and neutral wet systems, some initially water wet Berea sandstone cores were chemically treated to obtain neutral wet properties. The wettability indices were determined by the Amott test involving spontaneous uptake of fluids and forced displacement steps. A comparison between wettability alteration and the remaining oil saturations by bacterial application on both water wet and neutral wet sandstone cores has been made. The results show that bacteria resulted in wettability changes on both systems, water wet and neutral wet, and influenced the remaining oil saturations. The change in wettability is probably due to adsorption of ingredients from the bacterial solution on the surface of the rock.