(169f) Wettability Analysis of Calcite Aged in Brine, Oil, and Smart Water

A significant portion of the world’s oil is produced from carbonate reservoirs. These reservoirs are known to be either mixed-wet or oil-wet with low oil recovery during waterflooding. However, carbonate rock is initially water-wet. Rupture of the brine film that separates oil from the rock and subsequent adsorption of polar components on to the rock surface transforms these reservoirs to either mixed-wet or oil-wet. Wettability is one of the critical parameters that determine the residual oil saturation. A slight alteration of reservoir rock wettability towards water-wet state decreases residual oil saturation and results in improved oil recovery during water injection. It has been observed by many researchers that smart water (high sulfate concentration) flooding in carbonate reservoirs results in higher oil recovery. This higher recovery is believed to be a function of the concentration of the potential determining ions (PDIs) Ca²⁺, Mg²⁺, and SO₄^2- which can alter the wettability of the rock surface towards water-wetness by assisting in the desorption of carboxylic material from the calcite surface.

In this work, wettability of pure calcite crystals was altered from the water-wet to the oil-wet state by aging in model oil (87 v/v% n-dodecane and 13 v/v% toluene) containing naphthenic acid. These oil-wet calcite crystals were aged in various brines of different compositions (NaCl, Na₂CO₃, NaHCO₃, MgCl₂, CaCl₂, MgSO₄ and Na₂SO₄) but of the same ionic strength to study the effect of PDIs on wettability alteration. Based on these data, we propose an optimum brine composition that results in improved oil recovery from carbonate reservoirs.