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  • Reservoir Fluid Properties Course (2nd Ed.)

  • 1. Reservoir Fluid Behaviors

    2. Petroleum ReservoirsA. Oil

    B. Gas

    3. Introduction to Physical Properties

  • 1. Gas Behavior

    2. Gas Properties: A. Z Factor:

    a. Calculation for pure components

    b. Calculation for mixture componentsI. Mixing rules for calculating pseudocritical properties

    II. Correlations for calculating pseudocritical properties

    c. Nonhydrocarbon adjustment

    d. High molecular weight gases adjustment

  • Reservoir Fluid Properties

    To understand and predict the volumetric behavior of oil and gas reservoirs as a function of pressure, knowledge of the physical properties of reservoir fluids must be gained.

    These fluid properties are usually determined by laboratory experiments performed on samples of actual reservoir fluids.

    In the absence of experimentally measured properties, it is necessary for the petroleum engineer to determine the properties from empirically derived correlations.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 5

  • Natural Gas Constituents

    A gas is defined as a homogeneous fluid of low viscosity and density that has no definite volume but expands to completely fill the vessel in which it is placed.

    Generally, the natural gas is a mixture of hydrocarbon and nonhydrocarbon gases. The hydrocarbon gases that are normally found in a

    natural gas are methanes, ethanes, propanes, butanes, pentanes, and small amounts of hexanes and heavier.

    The nonhydrocarbon gases (i.e., impurities) include carbon dioxide, hydrogen sulfide, and nitrogen.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 6

  • Properties of Natural Gases

    Knowledge of PVT relationships and other physical and chemical properties of gases is essential for solving problems in natural gas reservoir engineering. These properties include:Apparent molecular weight, MaSpecific gravity, gCompressibility factor, zDensity, gSpecific volume, v Isothermal gas compressibility coefficient, cgGas formation volume factor, BgGas expansion factor, EgViscosity, g

    The above gas properties may be obtained from direct laboratory measurements or by prediction from generalized mathematical expressions.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 7

  • equation-of-state

    For an ideal gas, the volume of molecules is insignificant compared with the total volume occupied by the gas.

    It is also assumed that these molecules have no attractive or repulsive forces between them, and that all collisions of molecules are perfectly elastic.

    Based on the above kinetic theory of gases, a mathematical equation called equation-of-state can be derived to express the relationship existing between pressure p, volume V, and temperature T for a given quantity of moles of gas n.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 8

  • The basic properties of gases

    Petroleum engineers are usually interested in the behavior of mixtures and rarely deal with pure component gases. Because natural gas is a mixture of hydrocarbon

    components, the overall physical and chemical properties can be determined from the physical properties of the individual components in the mixture by using appropriate mixing rules.

    The basic properties of gases are commonly expressed in terms of the apparent molecular weight, standard volume,

    density, specific volume, and specific gravity.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 9

  • Behavior of Ideal Gases

    The gas density at any P and T:

    Apparent Molecular Weight

    Standard Volume

    Specific Volumethe volume occupied by

    a unit mass of the gas

    Specific Gravity

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 10

  • ideal gas behavior

    Three pounds of n-butane are placed in a vessel at 120F and 60 psia.

    Calculate the volume of the gas assuming an ideal gas behavior.

    calculate the density of n-butane.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 11

  • ideal gas behavior

    Step 1. Determine the molecular weight of n-butane from the Table to give:M = 58.123

    Step 2. Solve Equation for the volume of gas:

    Step 3. Solve for the density by:

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 12

  • Ideal Gases vs. Real Gases

    In dealing with gases at a very low pressure, the ideal gas relationship is a convenient and

    generally satisfactory tool.

    At higher pressures, the use of the ideal gas equation-of-state may

    lead to errors as great as 500%,

    as compared to errors of 23% at atmospheric pressure.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 13

  • Behavior of Real Gases

    Basically, the magnitude of deviations of real gases from the conditions of the ideal gas law increases with increasing pressure and temperature and varies widely with the composition of the gas. The reason for this is that the perfect gas law was

    derived under the assumption that the volume of molecules is insignificant and that no molecular attraction or repulsion exists between them. Numerous equations-of-state have been developed in the

    attempt to correlate the pressure-volume-temperature variables for real gases with experimental data.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 14

  • Gas Compressibility Factor Definition

    In order to express a more exact relationship between the variables p, V, and T, a correction factor called the gas compressibility factor,

    gas deviation factor, or simply the z-factor, must be introduced to account for the departure of gases from

    ideality.

    The equation has the form of pV = znRT

    Where the gas compressibility factor z is a dimensionless quantity and is defined as the ratio of the actual volume of n-moles of gas at T and p to the ideal volume of the same number of moles at the same T and p:

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 16

  • Corresponding States Principle for Pure componentsthe critical point of a fluid is where the liquid and

    vapor molar volumes become equal; i.e., there is no distinction between the liquid and vapor

    phases. above Tc the two phases can no longer coexits.

    Each compound is characterized by its own unique (Tc), (Pc) and (Vc)

    Corresponding States Principle (CSP): All fluids behave similarly when described in terms of

    their reduced temperature and pressureTr=T/Tc and Pr=P/Pc

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 17

  • Deviation from law of ideal gases

    Theory of Corresponding states

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 18

  • one-fluid theory (mixtures)

    Generally, we apply exactly the same equations for mixtures by treating the mixture as a hypothetical "pure" component whose properties are

    some combination of the actual pure components that comprise it.

    We call this the one-fluid theory.

    To apply CSP, we use the same plot or table as pure components but we make the temperature and pressure dimensionless

    with pseudo criticals for the hypothetical pure fluid instead of any one set of values as scaling variables from pure component values.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 21

  • mixing rules

    Mixing rules form the pseudocritical of the hypothetical pure component (the mixture) bytaking some composition average of

    each component's critical properties.

    Many mixing rules are commonly used and provide more accuracy than kays mixing rule.You see other mixing rules in your thermodynamics class.

    Kay's mixing rules is the simplest possible, It obtains the pseudocritical for

    the hypothetical pure component.

    It use a simple mole fraction average for both Tc and Pc: (,) = ,, Ppc( ,) = ,

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 22

  • Pseudo-Reduced Properties Calculation (for mixtures)Studies of the gas compressibility factors for natural

    gases of various compositions have shown that compressibility factors can be generalized with sufficient

    accuracies for most engineering purposes when they are expressed in terms of the following two

    dimensionless properties: Pseudo-reduced pressure and Pseudo-reduced temperature

    These dimensionless terms are defined by the following expressions:

    ppc and Tpc, do not represent the actual critical properties of the gas mixture and are used as correlating parameters in generating gas properties.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 23

  • Standing and Katz Compressibility Factors Chart

    low pressure values (0

  • Standing and Katz Compressibility Factors Chart

    for higher pressure values (15

  • Graphical method for Pseudo-Critical Properties approximationa graphical method for

    a convenient approximation of the ppc and tpc of gases when only the specific

    gravity of the gas is available.

    Brown et al. (1948) Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 27

  • Mathematical method for Pseudo-Critical Properties approximationIn cases where the composition of a natural gas is

    not available, the pseudo-critical properties, i.e., Ppc and Tpc, can be predicted solely from the specific gravity of the gas.

    Standing (1977) expressed brown et al graphical correlation in the following mathematical forms:Case 1: Natural Gas Systems

    Case 2: Gas-Condensate Systems

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 28