Pore scale observation of low salinity effects on outcrop and oil reservoir sandstone

Enhanced oil recovery has been observed in sandstone reservoirs when the salinity of the injected water is decreased during water flooding. In core plug tests, enhancement ranges from 5 to 38% over traditional water flooding using sea water or formation water. Previous work suggests that clays must be present in the reservoir, the oil must have polar components and divalent cations must be present in the formation water, but there is no clear explanation of exactly what controls the molecular level processes at the interface between the fluid and the sand grain surfaces. We have used a new method, atomic force mapping, where functionalised atomic force microscopy (AFM) tips feel a surface, to simulate the interaction between mineral, water and crude oil, under a variety of salinity conditions. We used contact mode AFM to image the internal pore surfaces from fresh outcrop sandstone and from reservoir sandstone that had been cleaned with strong solvents, the traditional treatment method for samples used in core plug testing. We see features that suggest clay nanoparticles attached to the sand grain surfaces. The tips were functionalised with carboxylic acid to model adhesion of polar components in crude oil to pore surfaces. Adhesion forces were extracted from force maps generated while the samples were exposed to solutions of high salinity (artificial seawater, ASW, 36,500ppm) and low salinity (diluted ASW, 1400ppm). The maps displayed heterogenous adhesion, which in some cases could be linked to features on the surface. The low salinity effect was observed on most sand grains from the sandstones; there was no significant difference in behaviour for the outcrop and solvent cleaned reservoir samples. Adhesion decrease ranged from 0 to 57% with an average of about 30%. For most cases, the effect was reversible when the salinity was changed, even through several salinity change cycles, and the effect was consistent on several individual sand grains.