Abstract

In the immiscible displacement of oil by carbon dioxide gas, the solution and diffusion of carbon dioxide are important factors that determine the efficiency of the process, since an increase in the carbon dioxide solubility and diffusivity into oil leads to an increase in oil recovery because the oil phase left behind contains more carbon dioxide and less oil. It is shown by experimental studies that the solubility and diffusivity of carbon dioxide into oil are governed by the saturation pressure, reservoir temperature I composition of the oil and purity of the gas. The solubility and diffusivity of carbon dioxide into Aberfeldy heavy oil were measured, using impure carbon dioxide gas containing nitrogen as the main ontaminant gas. It was noted that increasing the concentration of nitrogen in the carbon dioxide stream ecreased the solubility and. diffusivity of carbon dioxide into oil, consequently leading to a reduction in the swelling oil of by carbon dioxide.

Displacement experiments were also conducted to observe the effect of using impure carbon dioxide in place of pure carbon dioxide in the immiscible displacement WAG process. It was noted that the presence of nitrogen in carbon dioxide adversely affected oil recovery by the process and that increasing the nitrogen concentration up to 30 mole% could result in 10% loss in oil recovery.

Introduction

The solubility of carbon dioxide is the most important effect in the immiscible displacement of oil by carbon dioxide gas since it is theorized that among other mechanisms, an increase in the carbon dioxide solubility in oil leads to an increase in oil recovery because the oil phase left behind contains more carbon dioxide and less oil.

Early work in 1926 by Beecher and Parkhurst1 showed that carbon dioxide was more soluble on a molar basis in a 30.2 °API oil than air and natural gas. Svreck and Mehrotra's data2 also showed that, among the three gases: carbon dioxide methane, and nitrogen, carbon dioxide is the most soluble and nitrogen the least soluble in bitumen.

The solubility of carbon dioxide in oil is governed by the saturation pressure, reservoir temperature, composition of the oil and purity of the gas. Miller and Jones3 and Chung, Jones, and Nguyen4 measured the solubility of carbon dioxide n Canyon and Wilmington heavy oils and found that the solubility of carbon dioxide in heavy crude oils increased with pressure but decreased with temperature and reduced API gravity. Later, Sayegh and Sarbar5 established that carbon dioxide is more soluble in oil at lower temperatures than at higher ones. Patton, Coats, and Spence6, Holm and Josendal7, and Chung et al4 showed that the solubility of carbon dioxide reduced with me presence of methane in oil since carbon dioxide had to displace methane before dissolving in oil Holm and Josendal7 also mentioned that carbon dioxide did not displace all of the methane when it came into contact with oil. Spivak and Chima noted that the solubility of pure carbon dioxide in oil was higher than that of a carbon dioxide-nitrogen mixture.

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