Classifications of Gas Reservoirs

Based on the phase diagram of multi component system, the gas reservoirs can be divided into dry gas, wet gas, or condensate reservoirs.

Figure shows a phase diagram for a multi-component system. Within the phase diagram, the dotted lines represent the percentage of liquid as pressure and temperature are changed within the two-phase diagram. Point C represents the critical point and CT represents cricondentherm. The two vertical lines beginning from point C and CT respectively, separate the condensate reservoir from the gas reservoir.

Both the wet and dry gas reservoirs are represented by point A. AA' represents the phase behavior in the reservoir. As the reservoir pressure is depleted, under isothermal conditions, the path AA' is followed in the reservoir. The path AA' never intersects the two-phase diagram and, hence, throughout the depletion process, the reservoir fluid (gas) remains under single-phase flow conditions.

The path from reservoir to separator is represented by changes in the temperature. Typically, as the reservoir gas i brought to the surface, both pressure and temperature are reduced. If the path from reservoir to separator is represented by AA'', we call the reservoir a wet gas reservoir. This is because the reservoir fluid will produce liquid at the surface.

If the path from reservoir to separator is represented by AA''', we call this a dry gas reservoir. Both the initial reservoir stage, as well as the final stage at separator conditions, is outside the two-phase envelope. This is the easiest type of reservoir to produce and, for a depletion type of mechanism, has a recovery of 80 to 90%. In most cases, a dry gas reservoir represents very lean composition of gas with gas gravity less than 0.6 and comprised of mostly methane.

Condensate reservoir behavior is represented by the flow path BB'. Condensate reservoirs are initially present as single-phase gas in the reservoir. The phase diagram for condensate reservoirs indicates that the original reservoir temperature is higher than the critical point but lower than the cricondentherm. As the reservoir pressure is depleted, the pressure path passes through the dew-point curve resulting in the dropping of liquids, heavier components, which are commercially valuable. As the pressure depletes, more and more liquid drops out of the system. In addition, due to changes in the composition of the remaining fluid in the reservoir, the two-phase diagram shifts to the right, making it difficult to recover dropped liquid. If the composition in the reservoir would remain the same, the line BB' would eventually intersect the dew-point line below it and all the liquid would vaporize again. Unfortunately, as the composition gets heavier in the reservoir, the two-phase diagram shifts to the right, making it difficult for line BB' to intersect the dew-point line again. The process of liquid dropping as the pressure is reduced is called retrograde condensation.