01_Five Reservoir Fluids

# 01_Five Reservoir Fluids - The Five Reservoir Fluids...

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The Five Reservoir Fluids 1 The Five Reservoir Fluids Instructional Objectives: - List the five types of reservoir fluids. - Explain the difference between reservoir oils and reservoir gases. - Explain the differences between black oils and volatile oils. - Explain how to distinguish between black oils and volatile oils using initial production data, laboratory data, or production history. - Explain how to distinguish between volatile oils and retrograde gas condensates using initial production data, laboratory data, or production history. - Discuss wet gases, their occurrence in nature, the usefulness of the concept of wet gas in engineering calculations, and the identification of a wet gas using field data. - Discuss the unique feature of dry gases. Introduction: Phase Diagrams of Mixtures of Ethane and n-Heptane: 10 9 8 7 6 5 4 3 2 1 No. Wt % ethane 1 100.00 2 90.22 3 70.22 4 50.25 5 29.91 69 . 7 8 76 . 1 4 83 . 2 7 91 . 2 5 10 n-Heptane Composition 1400 1200 1000 400 600 800 200 0 200 300 400 500 100 Pressure, psia Temperature, °F The figure shows that the phase envelope changes with the composition of the fluid. The phase envelope shifts to the right when the fluid mixture becomes heavier. Types of Reservoir Fluids: There are five main types of reservoir fluids as follows: - Black oil. - Volatile oil. - Retrograde gas condensate. - Wet gas. - Dry gas.

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The Five Reservoir Fluids 2 Black Oil: Black Oil Critical point Pressure, psia B u b l e - p o i n t L Separator Pressure path in reservoir Dewpoint line 9 0 8 70 6 5 4 10 3 2 % Liquid Temperature, °F The lines within the phase envelope represent constant liquid volume, measured as percent of total volume. These lines are called iso-vols or quality lines. For a typical black-oil phase diagram, the iso-vols are spaced evenly within the envelope. A reduction in pressure below the bubble point at indicated reservoir temperature would release gas to form a free gas phase in the reservoir. As reservoir pressure declines more, additional gas is evolved in the reservoir. Additional gas evolves from the oil as it moves from the reservoir to the surface. This causes some shrinkage of the oil. However, separator conditions lie well within the phase envelope, indicating that a relatively large amount of liquid arrives at the surface. Volatile Oil: Pressure Temperature, °F Separator % Liquid De w in t lin Dewpoint line Volatile oil Pressure path in reservoir 3 2 1 1 7 Critical point The phase diagram for a typical volatile oil is somewhat different from the black-oil phase diagram. The critical temperature is much lower than for a black oil and, in fact, is close to reservoir temperature. Also, the iso-vols are not evenly spaced but are shifted upwards toward the bubble-point line. A small reduction in pressure below the original bubble-point, point 2, causes the release of a large amount of gas in the reservoir. Volatile oils may become as much as 50 percent gas in the reservoir at only a few hundred psi below the bubble-point pressure.
The Five Reservoir Fluids 3 Also, an iso-vol with a much lower percent liquid crosses the separator conditions. Hence the name volatile oil.

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## This note was uploaded on 06/12/2010 for the course PETROLEUM 1500 taught by Professor Ahmedalbamby during the Spring '10 term at University of Texas at Austin.

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01_Five Reservoir Fluids - The Five Reservoir Fluids...

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