Fluid Correlations

The Fluid Correlations dialog is split into four categories: Oil, Gas , Sorbed gas and Misc.

Simple fluid correlations for oil and gas are available when entering distributions for Bo and 1/Bg. The correlations are based on standard equations (Standing, McCord, etc.). They may or may not be valid for the hydrocarbons in your reservoir. Some of the correlations are based on a limited number of fluid samples, others may be incorrect above certain temperatures and pressures.

You are advised to use real data or analogous data rather than relying on these correlations.

In all these entries, the drop-down arrow at the right of the entry box allows you to change the unit, either converting the entered value (if it's there) or not.

Oil Properties

For oil formation volume factor you need to enter oil and gas gravity, gas/oil ratio, reservoir temperature and reservoir pressure. If you know the bubble point pressure, check the box and enter it, otherwise the correlation will estimate it.

The accuracy of the correlation is much improved if you know the bubble point pressure; but if you do, you probably have a laboratory measurement of Bo as well.

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Environment
Temperature	Enter the temperature
Pressure	Enter the pressure
Estimate	Click this button to estimate temperature and pressure from depth and gradients. See below

Properties
Oil gravity	Enter the oil gravity
Gas gravity	Enter the gas gravity
initial gas/oil ratio	Enter the initial gas-oil ratio

Bubble point pressure
Bubble point is known	If you check this, you must enter a value to be used in the correlation. Otherwise if unchecked it will be estimated from with the correlation.
Pb	Enter the bubble point if you know it.

Formation volume factor
Oil FVF	The estimated oil formation volume factor Bo. Bo is one of the input variables in a REP oil volume calculation

Units	Click to change units

Gas Properties

When estimating gas properties, you should first check the standard conditions (standard reservoir and pressure). Different countries (and in the USA, different states) can have different standard conditions and although the differences are usually small they can be both significant and irritating.

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Temperature	Enter the temperature defined for standard conditions
Pressure	Enter the pressure defined for standard conditions
Units	Click to change units

Another source of confusion comes when the reservoir gas has significant impurities (inerts) - nitrogen or carbon dioxide, for example. Do the properties you wish to use refer to the reservoir or the processed gas? In REP you should use the properties of the reservoir gas (i.e. including inerts).

For properties by correlation you need to know the specific gravity, mole percentages of CO2, N2 and H2S (if any) and the reservoir temperature and pressure. The correlations for gas are fairly accurate and most of the error comes from uncertainty in reservoir temperature and pressure.

Standard gas properties (Z factor, gas expansion factor 1/Bg and density at reservoir conditions) can be derived either from correlation or from composition.

The units of all entries can be changed by clicking the [Units] button.

pr_fcor3

Environment
Temperature	Enter the reservoir temperature
Pressure	Enter the reservoir pressure
Estimate	Click this button to estimate temperature and pressure from depth and gradients. See below

Properties
Gas specific gravity	Enter the gas specific gravity (air = 1)
Mole% CO2	Enter the mole percentage of carbon dioxide
Mole% N2	Enter the mole percentage of nitrogen
Mole% H2S	Enter the mole percentage of hydrogen sulphide

Results
FVF (1/Bg)	This is the gas expansion factor (1/Bg) which you will need in an evaluation of a gas reservoir
Density	The downhole gas density (i.e. the density at reservoir conditions)
Z factor	This is calculated Z factor.

Units	Click to change units