Numerical Evaluation of the Effect of Simultaneous Steam and Carbon Dioxide Injection on the Recovery of Heavy Oil
Authors: Leung Louis C. 
Published: December 2007

Summary
The immiscible displacement mechanism of CO2 in a simultaneous injection of CO2 and steam in a heavy-oil reservoir is evaluated with a numerical simulation model. In a steam stimulation process, the viscosity reduction effect of CO2 on heavy oil is the major contributor to increased recovery in a high-compressibility reservoir. In a normal-compressibility reservoir, the major benefit is derived from the solution gas effect of the injected CO2. Ignoring the solubility of CO2 in water can introduce an optimistic incremental recovery over steam-only injection. In a steamdrive process, the addition of CO2 to be injected steam improves the final recovery by only a small amount. However, the oil production rate before steam breakthrough appears to be production rate before steam breakthrough appears to be accelerated by the solution of CO2 in the heavy oil. The swelling effect of CO2 does not appear to play an important role in the incremental recovery, because the swelling effect of CO2 at high temperature is small when compared with the thermal expansion of the crude oil.

Introduction
The growing interest in the application of CO2 injection in EOR schemes has been concentrated so far on the miscible aspect of CO2. The CO2 miscible flood process generally is limited to the recovery of light-gravity process generally is limited to the recovery of light-gravity oil. Application of CO2 miscible flood to heavy-gravity oil has been hampered by the complex phase behavior, including the possible deposition of asphaltene, a solid phase, which could be detrimental to the permeability of phase, which could be detrimental to the permeability of the reservoir. In the immiscible aspect of CO2 injection, the viscosity reduction effect of CO2, generally unimportant in light-oil reservoirs, could play a significant role in the recovery of heavy oil. For all heavy-oil reservoirs, the major mechanism of enhanced recovery methods, which include steam injection and in-situ combustion process, is the reduction of reservoir oil viscosity to allow oil mobilization and to improve the mobility ratio between the reservoir oil and the displacement fluid. A study of the significant viscosity reduction effect of CO2 on high-viscosity crude oil was published as early as 1963. Welker and Dunlop showed that the viscosity reduction could be up to 98 % for a 4,800-cp (4.8-Pas) heavy crude oil at 80 deg. F (27 deg. C) and 800-psi (5515-kPa) carbonation pressure. A proposed field application of CO2 injection was reported to take advantage of this significant viscosity reduction aspect. It was also reported that supercritical CO2 was used to achieve solvent-reduced oil viscosity in some U.S. reservoirs in 1977. A study on the effect of normal reservoir parameters on a CO2 huff ‘n’ puff process was reported parameters on a CO2 huff ‘n’ puff process was reported in 1979. However, no field data have been published. In 1981, the first CO2 cyclic stimulation was conducted in Canada with interesting results. The experiment of solvent addition to steam process has been carried out in the field since probably as early as steam was applied successfully in oil recovery. No systematic evaluation was presented or published. Laboratory physical models were used to evaluate the benefit of solvent addition to steam and the results were reported by Redford.
JPT
P. 1591