Behavior of the reservoir chalk rock surface during smart water flooding
Changing brine composition during water flooding for enhanced oil recovery is a commonly used tool, but the fundamental mechanism associated to this phenomena lesser known. We have developed a model for calculating the feasible surface reactions; based on free energy profiles of the different combinations of water, metal (Ca2+, Sr2+ and Mg2+) ions and hydrocarbons over a temperature regime of 273 K to 373 K .The approach uses Density Functional Theory based Local Density Approximation on molecular dynamic simulation. Herein we observe that a thin water film exist between Chalk and brine/hydrocarbons. The most stable adsorption complex for each ion for water-ion (M2+—[H2O]n) and for water-ion-acetate complex (CH3COO—M2+—[H2O]n) (where n =1-9, M2+= Ca2+, Sr2+ and Mg2+) was calculated at different temperature. And thereafter a free energy profile was developed for each of the adsorbing complexes. These calculations together propose a wettability model described as: Mg adsorption released energy. This energy helped in release of carboxyl ion. The model explains the temperature effect in SW-EOR. It also indicates Mg2+ behaves differently from Ca2+ and Sr2+.Mg2+ preferred to stay alone while Ca2+ preferred to adsorb Carboxyl ion. This wettability change in the polar hydrocarbon is proposed to be related to the bulk oil mobility around a snap off. The moving saline water through a thin film between the chalk and the bulk oil, and desorption of carboxyl groups together suggest the wettability change can act as a possible trigger mechanism to a successful snap off.
Further analysis was done through an electrolyte thermodynamic model GEM-Selector to calculate the amount and the pattern of the formation of these organic salts. Several experimental data including dissociation constant, activity coefficient, osmotic coefficient, specific heat and vapor pressure of organic acids and their salts was used for the optimization the parameters . The model showed good correlation with the experimental data. Thereafter for a 50 % water saturated (at 25 °C) oil-water-calcite system the equilibrium calculations where conducted for 10%, 5% and 1% polarity (acids), considering different oil types. And the equilibrium composition was calculated for 50°C to 150°C and 100 bars to 500 bars. Herein the formation of the additional salts because of the change in brine composition indicated that Ca2+ provided more active ions (Ca(RCOO)+) in the solution, and was relatively less effected to pressure change in comparison with Mg2+ ions. The relative concentration of the salt and ion (of the organic acids) increased with the increase in the length of the hydrocarbon chain. This particular increase was more evident beyond 100 °C, more so in case of salts with Ca. Further with decrease in polarity a relative increase in the salt concentration was observed, which all together indicated that the all over compositional change strongly dependent on the oil percentage polar fraction. But in any case oil did not undergo compositional change of more than 2 %, and this can only act as a trigger or complimentary mechanism. And its efficiency would be largely related to the pore structures and the flow pattern, like in case of snap off as discussed above.