RE03: Gas Condensate Reservoir Analysis


Phase Behavoir

Gas condensate field produce mostly gas, with some liquid dropout, frequently occurring in the separator. The phase diagram shows the retrograde gas field must have a temperature higher than the critical point temperature. The vertical line on the phase diagram shows the phase changes in the reservoir, while the curve line shows these changes as the fluid cools going up the wellbore and into the separator.

In both cases, liquids drop out as the pressure drops below dew point pressure.

With the retrograde condensate, the %liquid begins to increase to point "A" then decreases with further pressure declines. Thus the name "retrograde" meaning to retreat or go back. So first condensation and then vaporization occurs, and this vaporization can help in further recovery of liquids.

Any hydrocarbon above the dew point line is 100% gas. Any hydrocarbon above the bubble point line is 100% liquid. Hydrocarbons above the bubble point line and close to the critical point are volatile oils. The cricondentherm is the maximum temperature which two phase flow can exist (maximum temperature on the dew point line of the phase diagram). But a field may have both a oil leg and gas cap, which during depletion produces some condensate from the gas.

Fields with active water drive may experience little pressure declines, so condensation occurs only at surface and a constant GLR would be expected.

Specific gravity and hydrocarbons in place


Recovery estimates

In cases where an active water drive will support pressure, so condensation occurs in the surface separators, the conventional volumetric estimates of gas recovery are possible, where Er=(nitial gas volume - final gas volume)/ final gas volume. This is the ultimate or displacement recovery. A sweep factor can be added, since so: Er = (Gi-Gf)/Gi*F.

An example of material balance which includes gas condensate production is given in Craft and Hawkins, page 82.