Experimental Study on the Effective Viscosity of Unstable CO₂ Flooding Produced Fluid with the Energy Dissipation Method

Abstract

As a kind of enhanced oil recovery technology, CO2 flooding is being employed more and more widely in oil fields. In CO2 flooding produced fluid, crude oil, water, and CO2 are transported simultaneously, and part of the fluid may be emulsified. The dissolving of CO2 may affect the kinetic stability of the emulsion formed in the mixed fluid and change the effective viscosity of the fluid as well. In this study, the solubility of CO2 in the water-in-crude oil emulsion was measured, and the variation of water/crude oil interfacial viscoelasticity was probed after the dissolution of CO2, so the influence of CO2 dissolution on emulsion stability and its mechanism could be uncovered. After the stability of the mixed fluid was verified, a viscosity measuring equipment was designed according to the principle of energy dissipation. With the help of this equipment, the effects of the CO2 solution pressure, temperature, oil/water ratio, and shear intensity on the effective viscosity of the mixed fluid were explored. The results reveal that the CO2 dissolution worsens the stability of the water-in-crude oil emulsion. The mechanism lies in the structural weakening of the interface by the action of the dissolved CO2 besides its viscosity-reducing effect. The effective viscosity of the CO2–water–crude oil mixed fluid decreases exponentially with the increasing CO2 solution pressure. Moreover, the viscosity reduction effect of the dissolved CO2 on the mixed fluid is more pronounced than on the pure crude oil.