Forecasting Gas Condensate Systems Using Advanced Analytical Models

Analytical models only analyze single-phase systems. For condensate systems, gas and condensate rates measured at the surface are recombined into a single-phase rich gas for analytical modeling. When forecasting however, the recombined gas rate must be split back out into gas and condensate rates for estimating production measured at the surface. The procedure to split the recombined condensate-rich gas into separator gas and condensate is as follows:

  • Calculate the recombined gas rate factor (RGRF), which is dependent on the instantaneous value of the condensate gas ratio (CGR)
  • qsep gas = qrec / RGRF
  • qcond = qsep gas x instantaneous CGR
Note:    RGRF is calculated from separator conditions, condensate gravity, gas gravity, and CGR.

There are two options that control how CGR is calculated over the forecast period:

1. CGR changes with time:

in the Properties tab, Gas Properties Input pane, select Liquid-Rich Gas from the Gas Type drop-down list, and Constant Rv from the Vaporized Oil Ration Correlation drop-down list.




in the Forecast and Test Design tab (see the Advanced Analytical Models Tab or Advanced Numerical Models Tab), under Forecast Options, select the Separate Recombined Gas checkbox.

This results in additional columns (i.e., Initial CGR and Final CGR) being added to the Ratios section of the forecast table.

  • Fill in the required inputs in the forecast table.
  • To enter a CGR that varies linearly over the forecasted period, select Ramp for the Ratio Interpolation method; then enter a Final CGR value.

2. CGR changes with flowing pressure according to the Rv correlation:

  • in the Properties tab / Gas Properties Input pane, select Gas Liquid Content from the Gas Type drop-down list, and Ovalle et al from the Vaporized Oil Ration Correlation drop-down list.
  • In the Forecast and Test Design tab, select the Separate Recombined Gas checkbox to split the calculated rich-gas rate into separator gas and condensate using the Rv functionality that controls the rate by bottom hole flowing pressure.

The resulting forecast graph and table show recombined gas, gas (dry), and condensate rates.

In splitting the forecast of analytical models, the assumption is that these effects / properties in the reservoir can be ignored:

  • liquid drop-out on material balance
  • liquid drop-out on skin
  • liquid drop-out on gas composition (recombined gas-specific gravity)
  • pressure volume temperature (PVT) properties – viscosity and compressibility factors
Note:    The effect of liquid blockage on well productivity can be modeled by increasing skin with time.