Nano-scale structure of crude oil deposits on water-wet substrates: Dependence on aqueous phase and organic solvents

Wettability alteration of glass substrates due to displacement of the surrounding salt solution by crude oil, and its reversal via salt solution flooding, were investigated as a function of salt concentration and solution pH, and the sample preparation procedure. The resulting wettability states were analyzed by microscopy of the substrate-bound oil residues and contact angle goniometry. Atomic force microscopy of the flooded state under its salt solution showed partial coverage of the substrate by adhering oil nano-blobs. Scanning electron microscopy provides a simpler means to acquire images of residues, but the need to rinse the bulk liquids and dry can introduce preparation artifacts. Three organic solvent rinsing procedures were examined, all showing that oil residue coverage decreases with increasing NaCl concentration and pH, and also decreases after flooding to follow these same trends. However, procedures in which bulk oil was rinsed from the flooded state using the solvents decalin and heptane created extraneous deposition; removal of the salt solution by methanol better preserved the oil residues. After flooding, even the salt solution allowing greatest adsorption/deposition of polar oil components, and strongest oil-substrate adhesion, had reduced coverage to only 20%. Analogous experiments on sandstone rock supported these findings. This suggests that local wettability of reservoirs can fluctuate in response to the oil and water slowly passing through pores during oil recovery by flooding.