Material Balance of Wet Gas Reservoir

  A reservoir that primarily stores gas is known as a dry gas reservoir. Furthermore, there is no liquid condensate that emerges at the surface; all that is created is gas. Once, we require to take account the reservoir's starting and present states in order to generate the material balance equation for a dry gas reservoir. It is also necessary to able to depict a dry gas reservoir's phase diagram. Evaluate the following states of a wet gas reservoir:

Figure-1 Wet Gas Phase Diagram

 

Figure-2 States of Wet Gas Reservoir

In Figure- As can be seen from Figure 2 above, a wet gas reservoir only contains gas at both the initial and final pressures, and at the separator, only gas emerges as the fluid rises to the surface. Number "1" shows the initial state, number "2" denotes the final state, and number "S" indicates the separator condition in Figure 1. We also take into account the material balance equation since, as shown in the phase diagram above, we are aware of the reservoir states and the wet gas reservoir's pressure decrease path. Therefore, we may start eliminating the phrases that don't apply, 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 single phase gas in the reservoir. Thus, two phase gas formation volume factor  gets converted to the single phase gas formation volume factor . It can be seen form the phase diagram, as the fluid goes to the surface, oil condensate and gas are produced. Therefore  is defined because liquid drops out of the gas.  is defined because the condensate oil shrinks as it rises to the surface. Because the pressure decline path never enters the region of retrograde, so , , and  are defined. The  is not defined is because the pressure decline path never enters the retrograde region (highlighted in gray). Overall, the condensate that drops out at the surface does not revaporize. The equation below shows which terms are eliminated if one deduces a volumetric reservoir:

By removing the terms with volumetric assumption, the material balance equation for a volumetric gas condensate reservoir is the following:

Overall, material balance equation combined with our knowledge of phase behavior results in a simple expression to describe the production of a wet gas reservoir and we have applied the single material balance equation to describe several different reservoir types.