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).

Statement of problem Initial reservoir pressure = 5000 psig Estimate the fractional recovery of oil and condensate, assuming that the reservoir pressure at abandonment = 1200 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

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

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

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 0.853 times the volume at dew point. 1 res.bbl. of reservoir fluid at 5000 psi expands to 1/0.853 = 1.172 bbl at 3428 psig. At 5000 psig, the fractional recovery is 0.172 / 1.172 = 14.7%. This figure applies to all components in the reservoir fluid.

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.

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 (1-0.0181-0.0657) = 0.9162 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+}= 0.09095(1-0.0143-0.0400) And during the depletion from 3000 psig to 2440 psig it is (0.24702-0.09095)(1-0.0108-0.0226)

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+}= 0.5792 lb moles The fractional recovery of surface gas by depletion from 5000 psig to 1200 psig is then 0.147 + 0.853 * 0.5792 / 0.9162 = 68.6% below Pdew Above Pdew

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+}= 0.0181*86/(62.5*0.659) + 0.0659*140(62.5*0.774) = 0.229 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+} = 0.0125 cu ft.

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+} = 0.0418 cu ft. The fractional recovery of surface condensate by depletion from 5000 psig to 1200 psig is then 0.147 + 0.853 * 0.0418 / 0.229 = 30.3% Above Pdew below Pdew Spreadsheet with recovery factor calculations

Depletion recoveries

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.

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 = 347.74 cu ft Surface oil volume = 0.229 cu ft CGR = 117 stb/MMscf

Condensate Gas ratio of equilibrium gas

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