Introduction reservoir engineering 3 (Gas Reservoirs)

 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