Q922+rfp+l05 v1

Reservoir Fluid Properties Course (3rd Ed.)

mailto:[email protected]

http://bit.ly/Q922-RFP

mailto:[email protected]

http://bit.ly/Q922-RFP

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 (ρo), Gravity (γo, 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

Spring14 H. AlamiNia Reservoir Fluid Properties Course (3rd Ed.) 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

correlations.


Crude Oil density and Crude Oil specific GravityThe crude oil density (ρo)

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 (γo)is defined as the ratio of the density of the oil

to that of water. Both densities are measured

at 60°F and atmospheric pressure:

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

are measured at standard conditions.


𝜌𝑜 =𝑚

𝑉

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.


Specific Gravity of the Solution Gas

γg 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,

γsep = separator gas gravity,

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

γst = gas gravity from the stock tank


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,

temperature,

API gravity, and

gas gravity.


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.


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


Gas-Solubility Pressure Diagram

A typical gas solubility curve, as a function

of pressure

for an undersaturated crude oil


Empirical Correlations for Estimating the RsThe following five empirical correlations for

estimating the gas solubility are given below:Standing’s correlation

The Vasquez-Beggs correlation

Glaso’s correlation

Marhoun’s correlation

The Petrosky-Farshad correlation


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

temperature.

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%.


Rs: Standing’s Correlation

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

where T = temperature, °R,

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

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


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 30°API)


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 γg to the reference separator pressure:

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


Rs: Glaso’s 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


Rs: Marhoun’s 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:

whereγg = gas specific gravityγo = stock-tank oil gravityT = temperature, °Ra = 185.843208b = 1.877840c = −3.1437d = −1.32657e = 1.398441


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


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 ρo, Bo, γo, γg ρo = oil density, lb/ft3Bo = oil formation volume factor, bbl/STBγo = specific gravity of the stock-tank oilγg = 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.


Bubble-Point Pressure

The bubble-point pressure Pb of a hydrocarbon system is defined as the highest pressure at which

a bubble of gas is first liberated from the oil.

can be measured experimentally for a crude oil system by conducting a constant-composition expansion test.

In the absence of the experimentally measured bubble-point pressure, it is necessaryto make an estimate of this crude oil property

from the readily available measured producing parameters


Pb Correlations

Several graphical and mathematical correlations for determining Pb have been proposed during the last four decades. They are essentially based

on the assumption that the bubble-point pressure is a strong function of gas solubility Rs, gas gravity γg, oil gravity API, and temperature T, or:

Pb = f (RS, γg, API, T)

Several ways of combining the above parameters in a graphical form or a mathematical expression are proposed by numerous authors, including:Standing

Vasquez and Beggs

Glaso

Marhoun

Petrosky and Farshad


Pb: Standing’s Correlation

Standing (1947) graphical correlationBased on 105 experimentally measured Pb on 22 hydrocarbon systems from California oil fields,The correlating parameters are Rs, γg, API, and system TThe reported average error is 4.8%

Standing (1981) mathematical correlation

pb = bubble-point pressure, psia, T = system temperature, °R

Standing’s correlation should be used with caution if nonhydrocarbon components are known

to be present in the system.


Pb: The Vasquez-Beggs Correlation

Vasquez and Beggs’ gas solubility correlation can be solved for the pb

The coefficients C1, C2, and C3 have the following values:


Pb: Glaso’s Correlation

Glaso (1980) used 45 oil samples, mostly from the North Sea hydrocarbon system, to develop an accurate correlation for Pb

Glaso proposed the following expression:

p*b is a correlating number and defined by:

Rs = gas solubility, scf/STB, t = system temperature, °F,

γg = average specific gravity of the total surface gases,

a = 0.816, b = 0.172, c = −0.989• For volatile oils, the temperature exponent b, be 0.130.


Pb: Marhoun’s Correlation

Marhoun (1988) correlation for estimating pbused 160 experimentally determined bubble-point pressures from PVT analysis of 69 Middle Eastern hydrocarbon mixturesThe correlating parameters are Rs, γg, γo, and Taverage absolute relative error of 3.66%

when compared with the experimental data used to develop the correlation.

T = temperature, °Rγo = stock-tank oil specific gravityγg = gas specific gravitya=5.3809×10−3, b=0.71508, c=−1.8778, d=3.144, e=1.3266


Pb: The Petrosky-Farshad Correlation

The Petrosky and Farshad gas solubility equation, can be solved for the Pb to give:

where the correlating parameter x is previously defined by.

the correlation predicts measured bubble point pressures with an average absolute error of 3.28%.


1. Ahmed, T. (2010). Reservoir engineering handbook (Gulf Professional Publishing). Chapter 2

1. Crude Oil Properties: A. Formation volume factor for P=<Pb (Bo)

B. Isothermal compressibility coefficient (Co)

C. Formation volume factor for P>Pb (Bo)

D. Density

Q922+rfp+l05 v1

Education

Transcript of Q922+rfp+l05 v1