1. Gas Condensate PVT A worked example to estimate
Condensate and gas recovery from a gas condensate reservoir produced by depletion
CGR as a function of reservoir pressure
The calculations use results from CCE and CVD experiments.
The results assume no aquifer influx and ignore the effect of rock and water compressibility.
They also assume that the condensate phase is immobile in the reservoir. This is a reasonable assumption for almost all gas condensate fluids (except in the near-well region, and this does not affect ultimate recovery).

2. Statement of problem Initial reservoir pressure = 5000 psig
Estimate the fractional recovery of oil and condensate, assuming that the reservoir pressure at abandonment = psig.
Define recoveries in terms of volumes at standard conditions.
Assume that the ‘condensate’ is the C6+ fraction of the reservoir fluid.
Which data have the most impact on liquid recovery and gas recovery?
Estimate the producing CGR as a function of reservoir pressure.
PVT data for this problem are taken from the 3rd SPE Comparative Solution Project

3. PVT Data - CCE Dew point pressure = 3428 psig
Spreadsheet with measured data

4. PVT Data - CVD
Assume that C6 MW = 86, and C6 SG = 0.659

5. Calculating recovery factors – above Pdew
First consider depletion from initial reservoir pressure (5000 psig) to dew point pressure (3428 psig), using the CCE data.
The reservoir fluid will be single phase gas during this period.
The CCE data shows that the relative volume at 5000 psig is times the volume at dew point.
1 res.bbl. of reservoir fluid at 5000 psi expands to 1/0.853 = bbl at 3428 psig.
At 5000 psig, the fractional recovery is / = 14.7%. This figure applies to all components in the reservoir fluid.

6. Calculating recovery factors – below Pdew
Now consider depletion from dew point pressure (3428 psig) to abandonment pressure (1200 psig), using the CVD data.
The reservoir fluid will be two- phase during this period.
The recoveries need to be calculated by summing over each step in the CVD analysis.

7. Calculating gas recovery
factors below Pdew
For gas, we can base the recovery factors on the number of lb moles, as this is proportional to the volume at standard conditions.
CVD data are based on 1 lb mole of reservoir fluid at Pdew. This contains ( ) = lb moles of surface gas. (i.e. all components except C6 and C7+)
The number of lb moles of surface gas produced during the depletion from 3428 psi to 3000 psi is
Dnp{1-yC6+}= ( )
And during the depletion from 3000 psig to 2440 psig it is
( )( )

8. Calculating gas recovery
factors
Starting with 1 lb mole of reservoir fluid at Pdew, the amount of surface gas produced during the depletion from 3428 psi to 1200 psi is
Dnp{1-yC6+}= lb moles
The fractional recovery of surface gas by depletion from 5000 psig to 1200 psig is then
* / = 68.6%
below Pdew
Above Pdew

9. Calculating condensate recovery
factors below Pdew
Need to convert CVD composition data (in lb moles) to surface volumes. This volume is given by (no of lb moles) * MW / density.
CVD data are based on 1 lb mole of reservoir fluid at Pdew. This contains Dnp{yC6 MWC6 / rC6 + yC7+ MWC7+ /rC7+}= *86/(62.5*0.659) *140(62.5*0.774) = cu ft of surface condensate.
The amount of surface condensate produced during the depletion from 3428 psi to 3000 psi is
Dnp{yC6 MWC6 / rC6 + yC7+ MWC7+ /rC7+}
= cu ft.

10. Calculating condensate recovery factors
Starting with 1 lb mole of reservoir fluid at Pdew, the surface condensate produced during the depletion from 3428 psi to 1200 psi is
Dnp{yC6 MWC6 / rC6 + yC7+ MWC7+ /rC7+}
= cu ft.
The fractional recovery of surface condensate by depletion from 5000 psig to 1200 psig is then
* / = 30.3%
Above Pdew
below Pdew
Spreadsheet with recovery factor calculations

11. Depletion recoveries

12. Which PVT data affect gas and condensate recovery factors?
Key data for gas recovery are the relative volumes above dew point, and the ‘wellstream produced’ data from the CVD.
Condensate recovery depends on the gas recovery, and also on the C6+ mole fractions in the produced gas.
The liquid saturations during the CVD (liquid drop out curve) are not relevant – they are not used in the recovery calculations.

13. CGR calculation
The CGR can be estimated from the equilibrium gas compositions in the CVD.
From 1 lb mole of equilibrium gas, the surface gas volume is
(1-yC6 -yC7+)RTstd/Pstd cu ft
The surface condensate volume is
(yC6 MWC6 / rC6 + yC7+ MWC7+ /rC7+) cu ft
At a reservoir pressure of 3428 psig,
Surface gas volume = 0.916*10.732*520/14.7 = cu ft
Surface oil volume = cu ft
CGR = 117 stb/MMscf

14. Condensate Gas ratio of equilibrium gas

15. References Notes Gas Condensate PVT – What’s Really Important and Why?
SPE Phase Behavior Monograph, Ch. 6
Notes