An Experimental Investigation of First and Multiple Contact Miscible Displacement in Arabian Carbonate Cores

Abstract

The application of enhanced oil recovery (EOR) for a Saudi Arabian carbonate reservoir was the subject of this study. Initial process screening revealed that miscible displacement was the only viable EOR process for recovering the relatively heavy oil (28 ° API). The use of separator gas, separator gas enriched with liquefied petroleum gas (LPG), pure LPG, CO2, and CO2 enriched with LPG as displacing fluids was explored. Some of the experimental results were presented in the 1987 Middle East Oil Technical Conference (see ref. 1). Further results obtained from a continuation of the study are described in this paper. The minimum miscible pressure of pure CO2 with oil was found to be higher than the reservoir pressure. CO2 enriched with 15 percent LPG gave optimal multiple-contact miscibility conditions. An unconventional apparatus consisting of a slim tube and a core holder joined in series was designed to study multiple-contact displacement. It was found to be very useful for investigating the effectiveness of enriched CO2 as a displacing fluid. The conclusions of this experimental study for the carbonate cores were:Oil displacement efficiencies by LPG and enriched CO2 were comparable, although the displacement mechanisms were different.The optimal size of LPG and enriched CO2 was 5 and 25 percent, respectively.Water alternating with gas (WAG) gave similar results when used in place of continuous CO2 as driving fluid.CO2 acted as an excellent buffer for separating the displacing fluid from the driving fluid.The amount of oil recovered by the miscible process was twice as much as that recovered by waterflooding. INTRODUCTION Several researchers have demonstrated that multiple-contact miscibility (MCM) tests require certain length or travel distance before the miscibility develops. Their reports2,3,4,5 showed that short cores could not be used in MCM displacement experiments to estimate the recovery from large scale systems. In this study, an improved linear model, called combined slim-tube coreflood apparatus, was employed to perform MCM enriched CO2 tests. This unconventional model consisting of a 41 foot (12.5 m) long slim-tube connected in series with a core holder, allows the miscible bank to develop in the slim-tube and then miscibly displace the crude out of a 12 inch (30.5 cm) composite core. Holm6 and Ehrlich et al7 have used similar experimental models for achieving the same purpose.