Microscopic Investigation of CO2, Flooding Process
Authors: Wang G.C. 
Published: December 2007

Summary
Visual examinations of microscopic behavior Of CO2 flooding processes were conducted with a high-pressure glass-bead-packed transparent flow tube. The apparatus and techniques developed made it possible to observe the physical phenomena of the displacement of oil by CO2 under both miscible and immiscible conditions. Effects of CO2 slug size and formation dip on oil recovery also were investigated. The test results were recorded in a series of magnified color photographs for after-run study and interpretation.
Three types of displacement were observed within the testing pressure range. These wereimmiscible,semimiscible, andmiscible.
Type 2 is an evaporating process in which oil is disintegrated into microscopic particles and transported in the CO2 stream. It was observed that more than one type of displacement could coexist in a flooding system. Under dynamic displacement and constant temperature conditions, miscibility could occur within a wide range of pressures, with higher pressures promoting earlier development of miscibility, thus resulting in better oil recovery.
A CO2 slug that is too large is wasteful and may cause early CO2 breakthrough. A slug too small would allow the trailing water to channel through the oil bank and degenerate the CO2 process to ordinary waterflooding; nevertheless, the oil recovery would be increased significantly over that obtained from a plain waterflood.
The water phase in the waterflooded area can be displaced by CO2 and recovery of residual oil may be accomplished by either semimiscible or miscible displacement. Oil swelling appears to play a major role in the formation of an oil bank.
Oil recovery could be increased considerably with downdip displacement. The increase is derived primarily by reducing CO2 override and expediting miscibility development.

Introduction
The use of CO2 for enhanced oil recovery (EOR) is considered one of the most promising methods for commercial application. It has been estimated that some 12 billion bbl (1.9 × 10–9 m3) of oil, or more than one-third of future U.S. tertiary oil reserves, could come from CO2 flooding methods.
Despite the great potential of the CO2 process and the rapidly -growing number of field CO2 projects, the mechanisms of CO2/crude oil displacement are not understood frilly. The interactions among the oil, CO2 and aqueous phase and the recovery of the residual oil have been interpreted largely from knowledge previously obtained from waterflooding and hydrocarbon injection, which may not hold true in the case of CO2 flooding. For that reason, controversies and disagreements have evolved in the petroleum industry.
Visual investigations of fluid displacement were used successfully for waterflooding studies by several investigators. Their work has yielded valuable information and new concepts in the areas of displacement mechanisms and multiphase flow characteristics that would not have been realized by the use of conventional core or slim-tube testing methods. With these similar approaches and objectives, a high-pressure transparent flow model, in conjunction with a photomicrographic device, was constructed, and a series of CO2 flooding experiments were performed at various pressure levels. This paper presents the results of this investigation.

Experimental Equipment
Basically, the flow tube is a five-section Jerguson high pressure liquid level gauge with an overall length of 62 in. (158 cm). Each section has two 0.5- x 10-in. (1.27- x25.7-cm) parallel glass windows, one in front and one in the rear. The distance between the two glass windows is 0.01 in. (2.2 mm). This clearance will accommodate two layers of 0.04-in. (1 -mm) diameter glass beads and permit adequate light transmission for visual investigation and photography.
JPT
P. 1789^