Adsorbed Organic Material and Its Control on Wettability

摘要

Laboratory core flood and field scale tests have demonstrated that 5-40% 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 surfaces, as a result of the decrease in divalent cation. concentration. We have previously demonstrated that, even for solvent cleaned core samples, mineral surfaces retain a significant quantity of carbon containing material and this affects wettability and response to changed salinity. Here we quantified the response of sandstone core plug material in its preserved state (i.e., after storage in kerosene) and after the same core plug material was treated with ethanol and ozone to remove adsorbed organic compounds. We used the chemical force microscopy (CFM) mode of atomic force microscopy (AFM) to directly measure the adhesion force for two types of molecules pore surfaces of individual sand grains that were plucked from an oil reservoir core plug. We functionalized AFM tips with alkane Or carboxylate, so they resembled tiny oil droplets and measured adhesion to the sand grain surfaces in artificial seawater (ASW; 35,600 ppm) and in ASW diluted to -similar to 1,500 ppm (ASW-low). Both before and after the ethanol/ozone treatment, and for both the alkane and the carboxylate functionalized tips, the adhesion was lower in ASW diluted to,similar to 1,500 ppm than in ASW. For both alkarie and carboxylate, the difference in adhesion between ASW and ASW-low was higher before the ethanol/ozone treatment. We attribute this change in response to the salinity dependent force caused mainly by the electric double layer (EDL) at the sand grain surfaces. We interpret the higher adhesion difference, before a very thorough ethanol/ozone treatment, to be a result of the loss of the organic material that was originally adsorbed on these surfaces, which adds to the charge density and therehy to the salinity dependent EDL force. Investigating the same area on the same pore surface, before and after removal of the organic material, demonstrates without doubt that it is organic material that causes the low salinity response, not the underlying mineral surface.