Gas condensate reservoirs may exhibit very poor performance due to serious phase interference effects. Engineer1 describes the Cal Canal field which exhibited exceptional productivity decline incident to liquid drop out. The operator's recommendation for field development was to abandon the reservoir after the reservoir pressure reached the dew point; the forecasted productivity was so poor that most of the hydrocarbon was going to be left unrecovered.
Times and gas price have changed but gas condensate production can still be very challenging. Phase behavior, interfacial tension, velocity and pore size distribution all affect how a condensate reservoir will produce. Much can be done in the laboratory to gain insight as to how serious phase interference effects are going to be in the field, long before field problems are encountered. This paper describes experimentation that was done to quantify the impact of pressure depletion on well productivity. The fluid preparation for this project was described in part I of this two part series2.
The salient findings of this work indicate that:
Retrograde condensate resulted in very rapid decrease in gas permeability - 60 and 84% reduction in Krg by the attainment of critical condensate saturation.
End-point saturations such as trapped gas and residual condensate saturation are sensitive to the level of interfacial tension (therefore pressure). Critical condensate saturation was much less sensitive to level of IFT (pressure).
Two-phase testing as a function of capillary number indicated an effective gas permeability very close to that measured by the explicit relative permeability measurements at saturations just higher than the critical condensate saturation.
The condensate and gas permeabilities measured during the gas-phase hysteresis injection did not agree with those measured during the two-phase injection.
The two-phase injection approach and the protocol whereby explicit measurement of relative permeability is performed provide a very thorough gas-condensate reservoir data set, which are amenable for use in simulation and reservoir production forecasting.
When retrograde condensation occurs during pressure decrease the liquid that accumulates fills some of the pores of the porous media. As the liquid increases in saturation the cross-sectional area available for flow may reduce the permeability to gas. The condensate may be mobilized if the draw-down pressure is large enough to overcome capillary pressure; this case results in the liquid having less severe impact on gas production. In some cases, once liquids accumulate, the gas permeability drops quickly and is difficult to restore. Laboratory testing helps the operator to identify which of the two scenarios will be germane to his specific reservoir. If appropriate testing is done some of the uncertainty can be removed from field development (references 3 – 5).
To gain an idea of whether a reservoir is going to experience severe liquid phase interference effects one can saturate the core stack, in the laboratory, with equilibrium fluid and then measure regain gas permeability (defined as the percentage of the singlephase permeability that is achieved by flowing equilibrium gas through the core sample) as a function of drawdown pressure at difference absolute pressures.
