Oil Volumetrics

To use the Advanced Volumetrics Module, enter all the relevant numbers for your reservoir.  Petra creates a report on the right side of the screen.  This report screen is automatically updated to reflect the most recent volumetric parameters entered.  

While some entries are necessary for the calculation (like TVD), other values are optional and exist only to override values Petra calculates automatically (such as the Z Factor).  An “*” next to the entry name signifies mandatory entries.

Once all the entries are set, Petra automatically performs the reservoir calculation and displays a report on the right side of the screen.

1 - Reservoir Info

Though it’s optional, the first step is to enter the name of company, the field, the reservoir, and the county.  These entries show up on the top of all reports.

2 - Reservoir Properties

Drive Mechanism* -This dropdown menu governs how Petra treats reservoir depletion due to production.  

Water Drive (100 Percent Pressure Maintenance) – Use this method when an underlying aquifer is strong enough to replace a barrel of oil produced with a barrel of water, or when the rate of production is very small relative to the rate of water influx.  Importantly, this method assumes no pressure loss with production.   If the reservoir experiences a pressure drop with production, use a combination drive mechanism instead.  

Solution Gas Drive (Volumetric Depletion) – Use this method with undersaturated volumetric reservoirs with no water drive.  Solution gas drive reservoirs have two stages of production: the liquid expansion stage from initial discovery pressure to the bubble point and the gas expansion stage from bubble point to abandonment pressure.  This method assumes that all solution gas is either produced or remains in the pore space; there is no provision for the formation of a secondary gas cap.   The solution drive dive reserves calculation particularly depends on the accuracy of the Kg/Ko vs. gas saturation curve.

Gas Cap Drive – Use this method when an expanding gas cap forces the oil lower in the formation with production.

Combination Drive (Solution + Gas Cap + Water) – Use this method with reservoirs that do not produce exclusively by one of the previously discussed drive mechanisms.

Average TVD of Reservoir* - Enter the average true vertical depth of the horizon.  Petra accepts values between 300 and 35000’.

Initial Reservoir Pressure* - Enter the average reservoir pressure at initial conditions.  Petra accepts values between 14.7 and 20,000 psia.

Abandonment Reservoir Pressure* - Enter average reservoir pressure at final conditions. Petra accepts values between 14.7 and 500 psia.  This entry is not used for water drive reservoirs because abandonment reservoir pressure equals the initial reservoir pressure.

Temperature* - Enter the temperature of the producing horizon. Petra accepts values between 40 and 500 degrees.

Connate Water Saturation* - Enter the connate water saturation of the producing horizon in percent.  Petra accepts values between 1 and 99.99 percent.  Remember to use whole numbers instead of decimal percentages.

Vertical Conformance Factor - Enter the efficiency of the water sweeping the oil zone. By default, Petra assumes 85 percent efficiency.  Remember to use whole numbers instead of decimal percentages.  Note this box is grayed out for gas cap and solution gas drives, since they assume no water drive.

Average Porosity* - Enter the volume-weighted average porosity of the producing formation in percent. Petra accepts values between 0.01 and 100 percent.  Remember to use whole numbers instead of decimal percentages.  

Residual Oil Saturation* – Enter the residual oil saturation after production.  Petra accepts values between 1 and 100 percent.  Remember to use whole numbers instead of decimal percentages.  This entry is grayed out for gas cap and solution gas drives.

Net Interest* - Enter the company's working interest before royalty. Petra accepts values between .01 and 100 percent.  Remember to use whole numbers instead of decimal percentages.

3 - Oil Reservoir Volumes

Total Reservoir Volume* – Enter the size of the reservoir in acre-feet.  

Water Invaded Volume* - Enter the size of the water-invaded part of the reservoir in acre-feet.  

4 - Oil Fluid Properties

PVT correlation* – This dropdown governs the fluids in the system expand or contract under different PVT conditions.

Standing – This option uses the Standing PVT correlation. This correlation is based on data points from 22 California crudes of gravities ranging from 16.5 to 63.8°API.

Glaso – This option uses the Glaso PVT correlation. This correlation is based on 45 oils (mostly from the North Sea) with gravities ranging from 22.3 - 48.1°API.

Vazquez and Beggs – This option uses the Vazquez and Beggs PVT correlation. This correlation is based on an extensive amount of data, with oil gravities ranging from 15.3 to 59.5°API.

Separator Gas Gravity* - Enter specific gravity of the solution gas relative to air.  Petra accepts values between 0.55 and 10.  

Separator Oil Gravity* - Enter specific gravity of the oil.  Petra accepts values between 1 and 200∨dm; API.  

Initial Z-factor - Enter the initial z-factor to override the internally-calculated value.  By default, Petra calculates the z-factor with the Hall-Yarborough correlation, based on the Starling-Carnahan equation of state.  This method agrees very well with the Standing-Katz chart.  The program does not make corrections for impurities. Petra accepts values between 0 and 200.

The Hall-Yarborough correlation uses the following equations.

where:

Pr = Pseudo-reduced pressure

t = Reciprocal pseudo-reduced temperature

y = "Reduced” density

The “reduced density” or “y” term comes from the Newton-Ralphson technique:

Reference Pressure* - Enter the standard reference pressure for delivery point measurements.  Petra accepts values between 14 and 20 psia.  

Oil Compressibility - Enter the oil compressibility to override the internally-calculated value.  Petra accepts values between 0 and 10,000 1/psi E-6. By default, Petra uses the following Mobil Correlation chart.

Water Compressibility - Enter the water compressibility to override the internally-calculated value.  Petra accepts values between 0 and 200 1/psi E-6. By default, Petra calculates water compressibility using Meehan’s correlation.

where:

Cw = Water compressibility in microsips (psi-1 * 10-6)

A1 = - 0.00014765 * Pi + 3.916801021

B1 = (6.379 * 10-7)Pi - 0.011441478

C1 = (-1.3536114 * 10-10)Pi + 4.238314 * 10-5

Pi = Initial reservoir pressure, psia (Where does this show up in the equation?)

T = Temperature

Formation Compressibility - Enter the formation compressibility to override the internally-calculated value.  Petra accepts values between 0 and 200 1/psi E-6. By default, Petra calculates formation compressibility using Hall's method.

where:

Cf = Formation compressibility, microsips

φo = Oil zone porosity, fraction

Bubble Point Pressure* - Enter the bubble point pressure (Pb) for the reservoir.  Petra accepts values between 14.7 and 3000 psia. Typically, the ranges for Pb depend on drive type:

100% Water Drive: 14.7 < Pb < Pi (psia)

Solution Drive: 14.7 < Pb < Pi (psia)

Gas Cap Drive: This entry is not available for Gas Cap Drive because Pb = Pi is set automatically and the corresponding GOR at Pb is calculated.

Combination Drive (no gas cap): 14.7 < Pb < Pi (p150sia)

Combination Drive (with gas cap): Pb = Pi (where Pi is the Initial Reservoir Pressure)

5 - Gas Cap Properties

Gas Cap Porosity* – Enter the average porosity of the rock containing the gas cap.  Petra accepts values between .01 and 100 percent.  Remember to use whole numbers instead of decimal percentages.

Gas Cap Volume – Enter the volume of the gas cap.

Gas Produced from Gas Cap – Enter the volume of gas produced from the gas cap.

6 - Oil Cumulative Production

Oil* - Enter the total oil production from the discovery date to the date of reserve determination.

Gas* - Enter the total solution gas production from the discovery date to the date of reserve

Relative Permeability Input

The relative permeability input governs how the reservoir model handles the relative permeability of gas and oil versus different gas saturations.  Petra comes with three default curves, which are typical for unconsolidated south Louisiana sandstone.  To use a different curve, select the Corey Equation option, and enter in values for Eo, Sro, Smo, No, Eg, Srg, Smg, and Ng.  Select “Calculate” to calculate the Corey equation and use the values in the volumetrics calculation.