Effects and mechanisms of smart water-flooding in carbonate reservoir

Key words:  carbonate reservoir  smart water-flooding  wettability  crude oil-brine interaction  multiphase ion-exchange  salting in-salting out

Abstract:

On the basis of the long core flooding experiment, contact angle and interfacial tension measurements of the porous carbonate reservoir in Mishrif formation, Missan oil field, Iraq,the effects and mechanisms of smart water-flooding in carbonate reservoir were investigated. It is shown that the wettability improvement and the fluid-fluid interaction change act synergistically to enhance oil recovery in smart water-flooding of the carbonate reservoir, in which the influence of the wettability is more significant. The type and amount of potential determining ions have great influence on wettability and interfacial tension. Herein, Mg²⁺ and Ca²⁺ can effectively promote the change of wettability and interfacial tension. With the concentration increasing, the contact angle and interfacial tension decrease first and then increase afterward. There is an optimal concentration corresponding to the minimum contact angle and interfacial tension. It is also found that the effect of Mg²⁺ is greater than that of Ca²⁺. SO2-₄ promotes the potential determining cations effect on wettability, meanwhile, hinders their effect on interfacial tension. The higher concentration of SO2-₄ means greater effect. SO2-₄ is preferentially adsorbed on the calcite surface, and promotes the multiphase ion exchange reaction between cations and oil-wet calcite surface, which causes the wettability alteration. With the ionic composition of the solution changing the adsorbed polar crude oil molecules on the oil-water interface undergo salting-in and salting-out processes, resulting in the fluid-fluid interaction change.