Dynamics of Subcritical CO2/Brine Floods for Heavy-Oil Recovery

Abstract

Summary Immiscible CO2, flooding is an important, field-proven heavy-oil recovery method, particularly suited for thin, marginal, or otherwise poor heavy-oil reservoirs, where thermal recovery processes are likely to be uneconomical. This paper processes are likely to be uneconomical. This paper describes the dynamics of this recovery technique on the basis of experiments conducted in a scaled model. The experiments represent a medium-heavy oil (1032 mpa.s at 23 degrees C [1,032 cp at 73 degrees F]) occurring in a shallow, thin sand. CO, was injected together with brine at subcritical conditions (5.5 MPa and 21 to 23 degrees C [800 psi and 70 to 73 degrees F]). The CO, and brine superficial velocities were varied from 0.18 to 2.9 m/d [0.6 to 9.5 ft/D]. This broad range of velocities permitted the study of the effect of the viscous, diffusive, and permitted the study of the effect of the viscous, diffusive, and gravitational forces on the CO, slug (20% HCPV) process for heavy-oil recovery. It was found that viscous forces completely dominated CO, injection. Also, the mass transfer between CO, and oil had a stabilizing effect on the brine injection. Although the molecular diffusion of CO2 in oil was high, it was not high enough to mobilize appreciable amounts of oil from uninvaded zones. The scaled experiment results showed that oil recoveries at CO, and brine breakthroughs were rate-dependent. While recovery at CO, breakthrough decreased with increasing rate, recovery at brine breakthrough increased. Reduction of interfacial tension (IFT) between brine and oil, leading to the formation of brine-in-oil emulsions, was found to be an additional effective mechanism of heavy-oil recovery by CO2/brine injection. Introduction A literature survey reveals that up to now all the experimental studies of immiscible displacement of heavy oil by CO, gas or liquid have been conducted in unscaled models of cylindrical geometry, possibly because "it has been impossible to fabricate a rectangular possibly because" it has been impossible to fabricate a rectangular model free of mechanical problems. Those studies provided insight into the microscopic displacement mechanisms of the process. Because the two-dimensional geometry of the reservoirs was process. Because the two-dimensional geometry of the reservoirs was not simulated, however, the viscous and gravitational instabilities in many cases were restricted, resulting in optimistically high oil recoveries. To overcome this problem, a rectangular. high-pressure, scaled model was constructed to simulate a typical thin heavy-oil reservoir in western Canada. Five types of forces control the displacement of heavy oil by CO2 and water: viscous. capillary. gravitational, diffusive, and inertial forces. Depending on the type of model and on the rock and reservoir-fluid properties, a few of these sources may be neglected. Laminar flow of CO2 is likely to occur in unconsolidated sand reservoirs (and rectangular models) at low or moderate superficial velocities, and if this is the case, the influence of inertial forces is not significant and can be neglected. Also, in dealing with highly viscous oil displacement from unconsolidated sands several authors, have pointed out that the oil recovery is only a weak function of the capillary forces. Neglecting capillary and inertial forces, the immiscible displacement of heavy oil by CO2 and water is controlled by viscous, gravitational, and diffusive forces. Mass Transfer Between CO2 and Heavy Oil In the zone invaded by CO2, mass transfer takes place as a result of solution, diffusion, and dispersion of CO2 into heavy oil, solution being the most important. The difference among these masstransfer mechanisms can be visualized as follows. According to Hildebrand, the solubility of one substance in another fundamentally depends on the case with which the two molecular species are able to mix. Thus solubility is proportional to the attractive forces (van der Waals-London forces) between the molecules of substances under consideration- Early work of Beecher and Parkhurst in 1926 showed that the solubility of CO2 in crude oil is higher than that of natural gas and air, Since that time, several investigators have shown that CO2 dissolves readily in heavy oil and bitumen, increasing the volume (10 to 20%) and reducing the viscosity (less than 90%). CO2 mixes with viscous oils not only by solution but also by diffusion. Crank defined diffusion as the process by which matter is transported from one part of a system to another as a result of random molecular motions. The difference between diffusion and solution can be summarized as follows: "It is significant that diffusion is not related to attractive forces as measured by solubility. The effect of temperature on solution and diffusion of gases into liquids has the opposite character. While solution decreases, diffusion increases with increasing temperature. Very little information is available on the molecular diffusion coefficient of CO2 into oils at reservoir conditions. Most experiments have been performed at atmospheric pressure.