Material Balance of Gas Condesate Reservoir
As reservoir pressure decreases below the dew point a gas condensate reservoir drops out liquid condensate in the reservoir. Considering the initial state and final state of the reservoir, we create the material balance equation. We should picture the phase diagram for a gas condensate as illustrated below:
Figure-2 States of Gas Condesate Reservoir
It can be shown from Figure-2 above, a gas condensate reservoir contains only free gas at initial state. When pressure decreases below the dew point condensate (liquid oil) drops out into the reservoir. In Figure-1, number 1 is initial point, number 2 is final state and "S" indicates the separator condition. We also think that the material balance equation by looking at the pressure decrease path in the phase diagram and reservoir states as shown above. Therefore, we may remove the terms and the results are shown below:
After eliminating the terms, the material balance equation for a wet gas reservoir becomes the following:
We initially have gas in the reservoir, thus 
is defined. Despite of the pressure declines, we always have two phase gas in the reservoir. Thus, two phase gas formation volume factor 
is defined. It can be seen form the phase diagram, the reservoir decline path enters the condensate region( grey region). Therefore 
is defined because liquid drops out of the gas. 
is defined because the condensate oil shrinks as it rises to the surface and produced two fluids at the surface. Because the pressure decline path never enters the region of retrograde, so 
, , and 
are defined. The is defined is because the pressure decline path enters the condensate region. Overall, the condensate that drops out at the surface does evaporize. The equation below shows which terms are eliminated if one assumes a volumetric reservoir:
is defined. Despite of the pressure declines, we always have two phase gas in the reservoir. Thus, two phase gas formation volume factor is defined. It can be seen form the phase diagram, the reservoir decline path enters the condensate region( grey region). Therefore is defined because liquid drops out of the gas. is defined because the condensate oil shrinks as it rises to the surface and produced two fluids at the surface. Because the pressure decline path never enters the region of retrograde, so , , and are defined. The is defined is because the pressure decline path enters the condensate region. Overall, the condensate that drops out at the surface does evaporize. The equation below shows which terms are eliminated if one assumes a volumetric reservoir:
By removing the terms with volumetric assumption, the material balance equation for a volumetric gas condensate reservoir is the following:
In conclusion, material balance equation combined with our knowledge of phase behavior results in expression to explain the production of gas condensate reservoir, which is one of the most difficult reserviors to characterize.
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