Effect Of Column Length On Co-Crude Oil Miscibility Pressure
Authors: Omole O. , Osoba J.S. 
Published: March 2010

The effect of column length on CO2-crude oil minimum miscibility ressure (MMP) has been investigated in linear porous media. Sand packs were placed in a vertical position and crude oil was displaced from them by CO2 at pressures ranging from 5.17 MPa to 12.41 MPa. A CO2-crude oil miscibility pressure was established in each of the sand packs.

A 34 ° API gravity (at 25.6 °C) Foster Crude Oil, and three 1.59 cm diameter sand packs, 0.5 m, 1.5 m and 6 m long respectively. were used/or the study. The investigation was conducted at 48.8 °C.

The results of the tests showed that the CO2-crude oil MMP was independent of the length of the sand pack in the range of lengths used. The crude oil was miscibly displaced from each of the three sand packs at 10.34 MPa. The hydrocarbon pore volume of CO2 required to achieve similar oil recoveries (per cent oil-in-place) from the sand packs was higher for the shorter sand pack and lower for the longer sand packs at corresponding flooding pressures. While the breakthrough oil recoveries (per cent oil-in-p/ace) from shorter columns were less than that from the longer ones. More oil was recovered from shorter than from longer columns after breakthrough at similar flooding pressures.

Introduction

A major factor to be considered when planning CO2 miscible flooding is the CO2-crude oil miscibility pressure. Earlier reports(1, 2) have demonstrated 'that displacing crude oil by CO2 will result in high oil recovery if carried out above the miscible pressure. The CO2-crude oil MMP is usually determined (3–5) by performing CO2-crude oil displacement tests at different pressures in the laboratory.

Menzie and Nielsen(2) and Holm and Josendale(3) have studied the mechanism by which CO2 generates miscibility with crude oil. They showed that the process is due to the ability of CO2 to extract hydrocarbons (C5 through C30) from crude oils. The study also demonstrated that, for a given crude oil, more hydrocarbons are extracted/vapourized as the flooding pressure is increased. The factors that affect CO2-crude oil miscibility pressure have been described in the literature as temperature, crude oil composition, and the purity of carbon dioxide(4).

Holm and Josendal(3) related the length of the CO2-crude oil transition zone to the flooding pressure. Low flooding pressures (near the miscible pressure) are less efficient and will produce ong transition zones while high flooding pressures (above the miscible pressure) are more efficient and will result in short transition ones. Adamson and Flock(6) reported that the length of the CO,-crude oil transition zone (as in other miscible displacement processes) depends on the stability of the transition zone. The transition zone is stable if it is devoid of viscous fingers and if there is a gradual change in fluid properties from that of the in-place reservoir oil to that of the CO2 throughout the flooding period.

The result of the study on Leveland crude oil reported by Yellig(5) demonstrated that the length of CO2-crude oil transition zone depends upon the type of phase equilibria exhibited by the CO2-crude oil mixtures at the time that the miscible zone is being formed.