Gas Reservoirs
Hydrocarbon reservoir can be called gas reservoir, if the temperature of the reservoir is greater than the cricondentherm of the hydrocarbon fluid. This is only applicable to non-associated gas reservoirs which can either be wet or dry gas depending on the phase present in the reservoir and at the surface separator. 1.4.5.5 Wet-Gas Reservoirs A natural gas system which contains a significant amount of propane, butane and other liquid hydrocarbons is known as wet gas or rich gas. It contains less amount of methane (85%) and more ethane than dry gas. Figure 1.13 shows a wet gas reservoir which exists solely as a gas in the reservoir throughout the reduction in reservoir
pressure. It temperature lies above the cricondentherm of the hydrocarbon mixture
similar to a dry gas reservoir. The reservoir fluid will always remain in the vapor
phase region as the reservoir is depleted isothermally, along with the production path
unlike retrograde condensate; no liquid is formed inside the reservoir. However,
separator conditions lie within the phase envelope, causing some liquid to be formed
at the surface. This surface liquid is normally called condensate. Wet-gas reservoirs
are characterized by gas oil ratios between 60,000 to 100,000 scf/STB, stock-tank oil
gravity above 60 API, the liquid is water-white in color and separator conditions lie
within the two-phase region.
1.4.5.6 Dry Gas Reservoir
The hydrocarbon mixture of a dry gas exists as a gas in the reservoir (even in the two
phase region) and in the surface separator characterized with a gas-oil ratio greater
than 100,000 scf/STB. It contains mainly methane with some intermediates. The
pressure or production path does not enter into the phase envelope (two phase
region) as shown in Fig. 1.14, this means that the surface separator conditions fall
outside the phase envelope which is in contrast to wet gas reservoir; hence there is no
traces of liquid formed at the surface separator.
Natural gas which occurs in the absence of condensate or liquid hydrocarbons, or
gas that had condensable hydrocarbons removed, is called dry gas. It is primarily
methane with some intermediates. The hydrocarbon mixture is solely gas in the
reservoir and there is no liquid (condensate surface liquid) formed either in the
reservoir or at the surface. The pressure path (line) does not enter into the phase
