Abstract

Most of Saskatchewan's light and med1um oil (LMO) reservoirs have reached economic limits of production under waterflooding 1eaving seventy to eighty percent of the initial oil-in-place (IOIP) in the reservoirs. The miscible displacement process using carbon dioxide, hydrocarbon and other gases as solvents appears to be one of the most promising method for these reservoirs.

A laboratory study was conducted to evaluate the applicability of various solvents for the recovery of oil from a southeast Saskatchewan reservoir. The physical, chemical and phase behavior (PVT) properties of wellhead and reservoir fluids were determined.

Slim tube displacement tests were conducted between the reconstituted Pinto reservoir f1uid and carbon dioxide, ethane and wellhead gas from the Steelman gas plant as solvents at various pressures and the reservoir temperature of 56 °C. The minimum miscibility pressures (MMP) determined for the above three systems demonstrated that the MMP for ethane is much lower than that for carbon dioxide. which is lower than that for Steelman gas. The MMP for ethane and carbon dioxide are below the reservoir fracture pressure.

Introduction

Most of Saskatchewan's light and medium oil (LMO) reservoirs have reached their economic limit of production under current technology. Primary and secondary methods together recover about 21% of the initial oil-in-place (IOIP). Although the rema1ning 1ight and medium oil-in-place in Saskatchewan is 1135 million cubic metres, the remaining producible reserves under primary and secondary recovery are only 65 million cubic metres. 1 Medium oil in southwest Saskatchewan has not been considered here, but if it is 1ncluded then up to 17 million cubic metres of additional oil (through primary and secondary recovery) may be recovered. 2

The development of tertiary recovery techniques, therefore, is essential to increase the production 1ife of these reservoirs and to maintain current production. Application of the miscible oil displacement process using CO2, hydrocarbon and other gases as injection fluids is expected to increase the LMO reserves threefold and extend the production life of these pools by two decades.3 Miscible flooding with carbon dioxide and hydrocarbon solvents is considered to be one of the most effective enhanced oil recovery (EOR) processes applicable to LMO reservoirs. Current industry interest in CO 2 miscible flooding is high, as evidenced by the level of activity in field testing2 and CO2 source development in the United States.

It should be noted that since the world oil price collapse in 1986, the number of EOR projects in the United States has decreased markedly from 512 in 1986 to 366 in 1988, but the number of CO2 misib1e projects increased from 38 in 1986 to 49 in 1988, a 29% rise during these two years.3 Oil production in early 1988 by the CO2 miscible process showed a 125% increase over the same period in 1986.

In spite of the great potential of the CO2 process and the growing number of field CO2 projects, the mechanisms of CO2/crude oil displacement are not understood fully. 4

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