How Naturally Adsorbed Material on Minerals Affects Low Salinity Enhanced Oil Recovery

Laboratory core flood and field scale tests have demonstrated that about 5 to 40% more oil can be released from sandstone reservoirs by injecting low salinity water, rather than high salinity fluids such as seawater or formation water. The effect has been explained by a change in wettability of the minerals that form the pore wall, as a result of the decrease in divalent cation concentration. Using X-ray photoelectron spectroscopy, we have demonstrated that even for solvent cleaned core samples, mineral surfaces retain a significant quantity of carbon containing material. Thus, pore wall wettability is more likely dominated by tightly adsorbed organic material than by the character of the underlying minerals. To test this hypothesis, we used the chemical force mapping (CFM) mode of atomic force microscopy (AFM) to directly measure adhesion forces on individual quartz grains that were plucked from core plugs. We functionalized AFM tips with model oil compounds so they would represent tiny oil droplets, and we measured their ability to adhere to surfaces as salinity changed. We examined grains from a sandstone core plug that had been cut into segments, which had been stored in kerosene or solvent cleaned. On all samples, surfaces were more oil wet (higher adhesion) in artificial seawater (ASW; 35,600 ppm) than in ASW diluted with ultrapure deionized water to similar to 1,500 ppm. XPS demonstrated that solvent cleaned surfaces had less adsorbed organic material than the kerosene stored sample. AFM measurements showed that the low salinity effect, namely the change in adhesion caused by decreasing salinity, was twice as high on kerosene stored samples as on solvent cleaned surfaces. The organic material that is adsorbed on the pore surfaces in the preserved sandstone offer very sticky anchor points for adhering oil molecules. This suggests that in reservoirs, even hydrophilic minerals located at the pore-fluid interface have tightly adhering hydrocarbons and the low salinity response depends on the behavior of this adsorbed material.