Crude Oil Properties

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Text of Crude Oil Properties

  • Reservoir Fluid Properties Course (2nd Ed.)

  • 1. empirical correlations for calculating z-factors

    2. Gas Properties: A. isothermal gas compressibility (Cg)

    B. gas formation volume factor (Bg) and gas expansion factor (Eg)

    C. Gas Viscosity correlations

  • 1. Crude Oil Properties: A. Density (rho), Gravity (gamma, API)

    B. Gas Solubility (Solution gas) (Rs)

    C. Bubble-point pressure (Pb)

  • Properties of Crude Oil Systems

    Petroleum (an equivalent term is crude oil) is a complex mixture consisting predominantly of hydrocarbons and containing sulfur, nitrogen, oxygen, and helium as minor constituents

    The physical and chemical properties of crude oils vary considerably and are dependent on the concentration of the various types of

    hydrocarbons and minor constituents present.

    An accurate description of physical properties of crude oils is of a considerable importance in the fields of both applied and theoretical science and especially

    in the solution of petroleum reservoir engineering problems

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 5

  • Physical Properties of Petroleum

    Main physical properties:Fluid gravitySpecific gravity of the

    solution gasGas solubilityBubble-point pressureOil formation volume factorIsothermal compressibility

    coefficient of undersaturated crude oils

    Oil densityTotal formation volume

    factorCrude oil viscositySurface tension

    Data on most of these fluid properties are usually determined by laboratory experiments

    performed on samples of actual reservoir fluids.

    In the absence of experimentally measured properties of crude oils, it is necessary for the

    petroleum engineer to determine the properties from empirically derived


    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 6

  • Crude Oil density and Crude Oil specific GravityThe crude oil density (o)

    is defined as the mass of a unit volume of the crude at a specified pressure and temperature.

    is usually expressed in pounds per cubic foot.

    The specific gravity of a crude oil (o)is defined as the ratio of the density of the oil

    to that of water. Both densities are measured

    at 60F and atmospheric pressure:

    the liquid specific gravity is dimensionless, but traditionally is given the units 60/60to emphasize the fact that both densities

    are measured at standard conditions.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 7


  • Crude Oil Gravity

    Although the density and specific gravity are used extensively in the petroleum industry, the API gravity is the preferred gravity scale.

    API gravityThis gravity scale is precisely related to the specific

    gravity by:

    The API gravities of crude oils usually range from 47 API for the lighter crude oils to

    10 API for the heavier asphaltic crude oils.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 8

  • Specific Gravity of the Solution Gas

    g is described by weighted average of (based on separator gas-oil ratio)

    the specific gravities of the separated gas from each separator.

    Where n = number of separators,

    Rsep = separator gas-oil ratio, scf/STB,

    sep = separator gas gravity,

    Rst = gas-oil ratio from the stock tank, scf/ STB,

    st = gas gravity from the stock tank

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 9

  • Gas Solubility definition

    The gas solubility Rs is defined as the number of standard cubic feet of gas

    that will dissolve in one stock-tank barrel of crude oil at certain pressure and temperature.

    The solubility of a natural gas in a crude oil is a strong function of the pressure,


    API gravity, and

    gas gravity.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 11

  • Gas Solubility variation with pressure

    For a particular gas and crude oil to exist at a constant temperature, the solubility increases with pressure

    until the saturation pressure is reached.

    At the saturation pressure (bubble-point pressure) all the available gases are dissolved in the oil and

    the gas solubility reaches its maximum value.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 12

  • Gas Solubility measurement with pressureRather than measuring the amount of gas

    that will dissolve in a given stock-tank crude oil as the pressure is increased, it is customary to determine the amount of gas that will come

    out of a sample of reservoir crude oil as pressure decreases.

    As the pressure is reduced from the initial reservoir pressure pi, to the bubble-point pressure Pb, no gas evolves from the oil and consequently

    gas solubility remains constant at its maximum value of Rsb.

    Below the bubble-point pressure, solution gas is liberated and Rs decreases with pressure

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 13

  • Gas-Solubility Pressure Diagram

    A typical gas solubility curve, as a function

    of pressure

    for an undersaturated crude oil

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 14

  • Empirical Correlations for Estimating the RsThe following five empirical correlations for

    estimating the gas solubility are given below:Standings correlation

    The Vasquez-Beggs correlation

    Glasos correlation

    Marhouns correlation

    The Petrosky-Farshad correlation

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 16

  • Standing (1947) Correlation

    Standing (1947) proposed a graphical correlation

    for determining the gas solubility as a function of pressure, gas specific gravity, API gravity, and system


    The correlation was developed from a total of 105 experimentally

    determined data points on 22 hydrocarbon mixtures

    from California crude oils and natural gases.

    The proposed correlation has an average error of 4.8%.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 17

  • Rs: Standings Correlation

    Standing (1981) expressed his proposed graphical correlation in more convenient mathematical form of:

    where T = temperature, R,

    p = system pressure, psia g = solution gas specific gravity

    Standings equation is valid for applications at and below the bubble-point pressure of the crude oil.

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 18

  • Rs: The Vasquez- Beggs (1980) CorrelationThey presented an improved empirical correlation

    The correlation was obtained by regression analysis using 5,008 measured gas solubility data points.

    predicting Rs with an average absolute error of 12.7%

    Based on oil gravity, the measured data were divided into two groups. (at a value of oil gravity of 30API)

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 19

  • gas gravity at the reference separator pressure (Vasquez-Beggs Correlation)the value of the specific gravity of the gas depends on

    the conditions under which it is separated from the oil,

    So the value of the gas specific gravity as obtained from a separator pressure of 100 psig must be used

    This reference pressure was chosen because it represents the average field separator conditions.

    Adjustment relationship for the gas gravity g to the reference separator pressure:

    gs = gas gravity at the reference separator pressureg = gas gravity at the actual separator conditions of psep and Tseppsep (Tsep)= actual separator pressure (Temperature), psia (R)

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 20

  • Rs: Glasos Correlation

    Glaso (1980) proposed a correlation for estimating the gas solubility as a function of

    API gravity, pressure, temperature, gas specific gravity.

    from studying 45 North Sea crude oil samples.

    an average error of 1.28%, a standard deviation of 6.98%

    p*b is a correlating number

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 21

  • Rs: Marhouns Correlation

    Marhoun (1988) developed an expression for estimating the saturation pressure of

    the Middle Eastern crude oil systems.

    The correlation originates from 160 experimental saturation pressure data.

    The proposed correlation can be rearranged and solved for the gas solubility:

    whereg = gas specific gravityo = stock-tank oil gravityT = temperature, Ra = 185.843208b = 1.877840c = 3.1437d = 1.32657e = 1.398441

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 22

  • Rs: The Petrosky-Farshad Correlation

    Petrosky and Farshad (1993) used a nonlinear multiple regression software

    to develop a gas solubility correlation.

    The authors constructed a PVT database from 81 laboratory analyses

    from the Gulf of Mexico crude oil system.

    p = pressure, psia, T = temperature, R

    Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 23

  • Rs: gas solubility calculation from the experimental measured PVT dataThe gas solubility can also be calculated rigorously

    from the experimental measured PVT data at the specified pressure and temperature.

    The expression relates the gas solubility Rs to o, Bo, o, g o = oil density, lb/ft3Bo = oil formation volume factor, bbl/STBo = specific gravity of the stock-tank oilg = specific gravity of the solution gas

    the weight average of separator and stock-tank gas specific gravities should be used

    The error in calculating Rs by using the equation will depend only on the accuracy of the available PVT data.

    Fall 13 H. Alami