Click to see subtopics:Gas correlations
There are three types of gas correlations: pressure volume temperature (PVT), viscosity, and gas type.
PVT
B.W.R. (Table): The Benedict-Webb-Rubin is an empirical relationship that predicts the state of liquids and gases. Tables were developed for approximately fifty substances, mostly hydrocarbons that related gas density, formation volume factor, and temperature to pressure.
B.W.R. (8-constant): The Benedict-Webb-Rubin (8-constant) equation of state developed a set of generalized coefficients in order to permit widespread application to all types of gases.
B.W.R. (11-constant): The Benedict-Webb-Rubin (11-constant) equation of state is a further refined method to predict the state of liquids and gases. Used for all types of gases, it uses 11 empirical constants to achieve a more accurate interpolation of the data.
AGA8 Detail: Calculates the Z-factor based on the physical chemistry of the gas components at specified temperatures and pressures. It can be used when the exact composition (fraction mole percents) of the gas is available.
Carbon Dioxide: The Carbon Dioxide tables are for computing the Z factor for 100% CO2 content.
Nitrogen: The Nitrogen tables are for computing the Z factor for 100% N2 content.
Hydrogen Sulfide: The Hydrogen Sulfide tables are for computing the Z factor for 100% H2S content.
Custom Table: Allows you to enter laboratory measurements for the gas Z-factor or formation volume factor. The Z factor is calculated from the formation volume factor or vice versa. Gas compressibility is subsequently calculated from these two values.
Viscosity
Carr et al: Developed to predict the viscosity of gas hydrocarbon mixtures for temperatures between 32ºF and 400ºF and pressures up to 12,000 psi. Applies to both sweet and sour gas and is designed to handle non-hydrocarbon components (CO2, H2S, N2) in concentrations of up to 15% each.
Lee, Gonzalez, Eakin: Applies to sweet gas and does not account for the presence of non-hydrocarbon components. Applicable for pressure ranges of 100 - 8,000 psi and temperatures between 100ºF - 340ºF. Less accurate for gases with specific gravity above 1.0.
Optimized Lee et al: Developed using the Lee, Gonzalez, and Eakin method, but uses a optimized temperature history to achieve more accurate results, thus resulting in different coefficients in the equations. Less than 5% difference in extreme cases from original correlations.
Lucas et al: Uses the method of corresponding states to calculate gas viscosity. Better suited for higher density gases at lower pressures.
Custom Table: Choosing this correlation allows you to enter custom gas viscosity values from laboratory measurements.
Gas type
Selecting wet or dry gas changes the results obtained from gas correlations, as different coefficients have been developed for these gas types. The Liquid-rich Gas option allows for the recombination of gas and condensate into a single gas stream. For more information, see reservoir fluid types.
Ovalle et al.'s correlation for Rv
Ovalle et al.'s correlation is based solely on commonly available field data. Required field data is as follows: the initial producing gas / condensate ratio from the first separator, the initial stock tank liquid gravity in API, the specific gravity of initial reservoir gas, and reservoir temperature. This correlation does not need require the dew point pressure to calculate the "Rv" curve vs. pressure.
| n | Varn |
COn | C1n | C2n | C3n | C4n |
|---|---|---|---|---|---|---|
| 1 | In P, psi | 20.809 | -6.7095 | 0.5136 | 0 | 0 |
| 2 | APId | 11.175 | -1.2965 | 0.042311 | -0.0005438 | 2.49E-06 |
| 3 |
|
-13.365 | 27.652 | -18.598 | 4.3658 | 0 |
| 4 | TR,°F | -1.5309 | 0.0058453 | 1.40E-06 | 0 | 0 |
| APId | Gravity of stock-tank liquid, oAPI, determined when P ≥ Pd |
| P | Pressure, psi |
|
Reservoir gas specific gravity at P≥Pd (This value is the recombined gas specific gravity |
| TR | Reservoir temperature, °F |
| RV | Vaporized oil ratio, Stb / MMscf |
Calculation of the vaporized oil ratio
The vaporized oil ratio (Rv) can be calculated from the liquid drop-out (VL / Vsat) measured in the lab.
where
(VL/Vsat): the liquid drop-out of a liquid-rich gas, meaning the retrograde liquid volume at any pressure (below the dew point pressure) and reservoir temperature relative to the volume of the gas sample at the dew point pressure and reservoir temperature.
Bgd,sat: the dry gas formation factor at the dew point pressure (saturation pressure)