CCE and CVD results The usual PVT measurements on gas condensates are Constant Composition Expansion (CCE) – sometimes called Constant Mass Expansion Constant Volume Depletion (CVD) A CCE test is used to measure the dew point pressure, and to provide volumetric data above and below the dew point pressure. A CVD test measures data below dew point pressure. Below dew point pressure, a CVD test is much more valuable than CCE as it provides data on the equilibrium gas composition.

Constant Composition (Mass) Expansion Test Data Dew point pressure Z-factor Total volume (rel. to volume at dew point) Liquid phase volumes

CCE results

Constant Volume Depletion Test P1 = Pdew P3 < P2 P2 < Pdew Data Amount of gas removed (in moles) Equilibrium gas Z-factor Equilibrium gas composition and C7+ properties Liquid saturation

CVD results

Calculated recoveries from CVD results

Using CVD results The CVD test simulates closely the actual behavior of a gas condensate reservoir undergoing pressure depletion, and results from the lab measurements can be used directly to quantify recoveries of surface gas and condensate as a function of pressure below the dewpoint. Combined with single-phase Z-factors from the CCE test, a complete prediction of depletion behavior (recoveries and liquid- yield variation) can be accurately predicted from initial pressure to abandonment. Accurate measurement of the removed gas composition is very important to the prediction of condensate recovery and liquid-yield variation – much more important than accurate measurement of retrograde oil volumes. Special laboratory procedures should be followed to ensure accurate CVD compositional measurements (e.g. appropriate heating of tubing used to remove equilibrium gas from the cell).

Material Balance Checks of CVD results Measurement of the final low-pressure condensate composition allows an important material balance check of CVD results. The best technique is a “backward” material-balance. This starts with the final condensate composition and amount (using the reported final oil relative volume and properties), adding incrementally the removed gas from each CVD step, and ending up with a check of the original fluid composition. Material balance can also be used to calculate the liquid phase composition and density at each CVD step. Densities and K-values can then be compared with results of correlations.