A Current Appraisal of Field Miscible Slug Projects

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Abstract
In addition to experimental and mathematical studies of miscible slug flooding, more than seventy field tests have been initiated. Of those tests carried out where conditions are favorable to miscible flooding, more than forty percent have shown good or encouraging oil recovery efficiencies. Introduction: Over the past 20 years, research efforts have been directed toward improving conventional gas injection by increasing the oil displacement efficiency. This research has resulted in a family of so-called "improved gas drives". The common characteristic of these improved gas drives is that a solvent bank is developed or created within the reservoir. This solvent bank is miscible at its leading edge with the reservoir oil and miscible at its trailing edge with gas. This bank can be driven through the reservoir, displacing oil ahead of it, resulting in higher oil recoveries than those normally obtainable by conventional gas drive. This miscible bank can be created either from hydrocarbon components already present in the reservoir or from components contained in the injected fluid. The former is termed high-pressure or vaporizing gas drive and the latter is termed miscible slug flooding. The mechanism of high-pressure (miscible) gas drives has been described in the literature, and the steps involved in miscible slug flooding have been taught by Morse and Crump. In miscible slug flooding the constituents in the solvent bank are injected as a fluid - either as liquid at reservoir conditions (LPG slug flooding) or as liquefiable components contained in rich gas (condensing gas drive or enriched gas drive). In miscible slug flooding, mixing bones form between the reservoir oil and the solvent bank and between the solvent bank and the scavenging gas. Early research studies were devoted to a quantitative description of the factors affecting mixing-zone growth. Other studies have focused upon determining what composition of injected rich gas would develop within the reservoir a solvent bank that is miscible with the reservoir oil. The areal and vertical sweep efficiencies expected during miscible slug flooding have also received research attention. Engineering efforts have also been concerned with methods of predicting the oil recovery possible by miscible predicting the oil recovery possible by miscible slug flooding. Based upon these laboratory studies, the preferred reservoir and operating conditions for proper application of miscible slug flooding have been defined as follows: Reservoir oil viscosity of 5 cp or less. Formation with less than about 25 ft ofcontinuous vertical section. Permeability less than about 100 md. No continuous high permeability streaks. Reservoir pressure sufficient for miscibility, atboth the leading and trailing edges of the solventbank. An injected solvent slug volume of from 2 to 10 percent of the hydrocarbon pore volume involved. Dimensionally, scaled model test results have shown that, because of gravity segregation of the oil, solvent, and scavenging gas, the tendency for channeling and bypassing of oil is reduced in permeable, steeply dipping reservoirs. JPT P. 529

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