Surface Complexation Modeling for Waterflooding of Sandstones

A theoretical surface coordination model of oil attraction to sandstone-reservoir surfaces confirms the two primary oil/mineral coordination reactions to be electrostatic linking of anionic kaolinite-edge sites to protonated nitrogen bases at pH<6 and calcium carboxylate groups at pH>6. Kaolinite basal planes are calculated to link to oil through oil -NH+ groups at pH<6-7 and through oil -COOCa+ groups at pH>6-7, and may be important to oil attraction where basal planes are more exposed than edges (the ranges shift, depending on the oil, acid, and base numbers). Model predictions are most sensitive to the dissociation constant of oil surface carboxylate groups but are relatively insensitive to other surface equilibria and temperature. The model shows that, although low-salinity, low-Ca waterfloods can enhance oil recovery by decreasing the number of Ca2+ bridges and anionic kaolinite-edge sites, dissolution of sandstone carbonate minerals dampens the low-salinity effect by buffering decreases in waterflood Ca2+ levels. Better model predictions require more-accurate predictions of Ca2+ levels during waterflooding, high-temperature sulfate-adsorption analyses, and more-precise measurements of oil acidity and basicity.