Oil properties.pdf

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PROPERTIES OF CRUDE OIL

SYSTEMS

Fundamental of petroleum Engineering course

Peyman Maroufi

Petroleum Engineering department

Soran university

PROPERTIES OF CRUDE OIL SYSTEMS

� 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.

� Physical properties of primary interest in petroleum engineering studies include:

� Fluid gravity

� Specific gravity of the solution gas

� Gas solubility

� Bubble-point pressure

� Oil formation volume factor

� Isothermal compressibility coefficient of undersaturated crude oils

� Oil density

� Total formation volume factor

� Crude oil viscosity

� Surface tension


� Crude Oil Gravity:

� The crude oil density is defined as the mass of a unit volume of the crude at a

specified pressure and temperature.

� The specific gravity of a crude oil 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 density of the water is approximately 62.4 ��/��, or:


� Although the density and specific gravity are used extensively in the petroleum

industry, the API gravity is the preferred gravity scale.

� This gravity scale is precisely related to the specific gravity by the following

expression:

� 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.

� Example 1Calculate the specific gravity and the API gravity of a crude oil system with a

measured density of 53 lb/ft3 at standard conditions.


� Gas Solubility or Solution Gas-Oil Ratio:

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

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.

� 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.

� Rather 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.


� A typical gas solubility curve, as a function of pressure for an undersaturated crude

oil, is shown in Figure 2-7.

� 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 the gas solubility remains

constant at its maximum value of Rsb.

� Below the bubble-point pressure, the solution gas is liberated and the value of Rs

decreases with pressure.


� The 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’s Correlation:

Standing (1947) proposed the following correlation for determining the gas solubility

as a function of pressure, gas specific gravity, API gravity, and system temperature.

with

It should be noted that Standing’s equation is valid for applications at and below

the bubble-point pressure of the crude oil.


� Example2 :The following experimental PVT data on six different crude oil systems are

available. Results are based on two-stage surface separation.

Using Standing’s correlation,

estimate the gas solubility at the

bubble point pressure


� Formation Volume Factor, Bo:

� The oil formation volume factor, Bo, is defined as the ratio of the volume of oil

(plus the gas in solution) at the prevailing reservoir temperature and pressure to

the volume of oil at standard conditions.

� Bo is always greater than or equal to unity.

� The oil formation volume factor can be expressed mathematically as:


� A typical oil formation factor curve, as a function

of pressure for an undersaturated crude oil (pi >

pb), is shown in Figure 2-8.

� As the pressure is reduced below the initial

reservoir pressure pi, the oil volume increases due

to the oil expansion. This behavior results in an

increase in the oil formation volume factor and will

continue until the bubble-point pressure is reached.

� At Pb, the oil r\eaches its maximum expansion and consequently attains a maximum value of Bob for

the oil formation volume factor.

� As the pressure is reduced below pb, volume of the oil and Bo are decreased as the solution gas is

liberated.

� When the pressure is reduced to atmospheric pressure and the temperature to 60°F, the value of Bo is

equal to one.


� Most of the published empirical Bo correlations utilize the following generalized

relationship:

� Six different methods of predicting the oil formation volume factor are presented

below:

• Standing’s correlation

• The Vasquez-Beggs correlation

• Glaso’s correlation

• Marhoun’s correlation

• The Petrosky-Farshad correlation

• Other correlations


� Standing’s Correlation:

Standing (1981) showed that the oil formation volume factor can be expressed

more conveniently in a mathematical form by the following equation:

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Oil properties.pdf