A Laboratory Investigation of the Carbon Dioxide Immiscible Process
Authors: Mangalsingh D. , Jagai T. 
Published: December 2007

D. Mangalsingh, and T. Jagai, SPE

Abstract
The CO2 immiscible process is a potentially viable method of EOR for local reservoirs. Although this type of flood is being conducted on a pilot scale in Trinidad, no laboratory work has been done to support this field effort. This paper presents the results of the first laboratory investigation of CO2 immiscible displacement of local crudes using both the continuous injection method and the water alternating gas method (WAG).

Introduction
The continuous depletion of Trinidad's oil reserves necessitates the development and improvement of thermal and non thermal enhanced recovery techniques. Approximately 60% of the present oil reserves cannot be exploited because of present technical and economic constraints. The use of steam as a recovery agent loses it's economic viability when the reservoir is deeper than 1000m and when the formation is less than 10m thick. This is as a result of the heat loss associated with such conditions. The problem of poor areal and vertical sweep efficiency has limited the recovery in many heavy oil reservoirs. During the period 1920-1930 papers and patents were published indicating that carbon dioxide was a fluid capable of recovering oil from hydrocarbon reservoirs. In 1945 Poettman and Katz discussed the phase behaviour of CO2 and paraffin systems. Their studies indicated that a 10% to 20% increase in oil volume and a viscosity reduction too less than 0.1 of the original value was due to the solubility of the CO2 in the paraffin. The 1950's and 1960's saw emphasis in the application of CO2 as a miscible displacing fluid. At that time researchers understood the limitations of miscible CO2 flooding to be:High pressures that were required to achieve miscibility;Reservoir depths greater than 1000m;Crude oils of 30 API gravity and over gave high miscible pressures.
These restrictions prompted researchers to investigate the effectiveness of CO2 immiscible displacement for oil recovery. Four U.S. patents were issued to Martin et al, in 1959 relating to immiscible use of CO2 for oil recovery. The early 1970's saw field applications of immiscible CO2 which showed potential performance. The methods of CO2 injection were mainly continuous and huff and puff until the identification of WAG in the 1980's.
Continuous CO2 was deficient in areal sweep efficiency which resulted in early carbon dioxide breakthrough. Research also indicated that the production gas oil ratio (GOR) for continuous injection was very high. WAG injection resulted in lower mobility of carbon dioxide, thereby increasing sweep efficiency and lowering produced GOR. Rojas and Farouq Ali performed a scaled model study of carbon dioxide/brine injection strategies for heavy oil recovery from thin formations and disclosed the potential of WAG application.
CO2 immiscible displacement is a well established technique for increasing the recovery of crudes. This has been applied to reservoirs throughout the world. CO2 injection into Trinidad reservoirs started a few decades ago. Recovery as a result of this effort has been insignificant. Within the last few years, a more determined effort to implement CO2 immiscible displacement on a pilot scale has been attempted. However, no laboratory work has been conducted to support this field effort.
This paper describes the laboratory work that is being conducted to investigate the behaviour of local crudes when subjected to immiscible displacement by CO2 flooding. It presents the findings to date and compares them with those published. It also attempts to identify the important parameters that enhance recovery and seeks to optimize these parameters. The effect of continuous injection of CO2 on recovery has been studied. This has been compared to the recovery due to injecting alternate slugs of CO2 and water.

Recovery Mechanism
Immiscible CO2 flooding is a technique in which the flow properties of the oil in the reservoir are improved. The concept of immiscible CO2 flooding implies that CO2 is injected at subcritical pressures. In addition to providing energy to the reservoir, four mechanisms which contribute to increased oil recovery have been documented.
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