Gases Reservoirs Fluid Phase Behavior

2.1 Retrograde gas-condensate reservoirs

The retrograde gas-condensate reservoir is also called retrograde condensate gases, condensate, retrograde gas and gas condensates. In this type of natural gas, reservoir prefer called gas-condensate and not condensate only because this reservoir exhibits retrograde behavior [11]. In case of the reservoir temperature more than a critical temperature and less than a critical temperature, the reservoir is classified as a retrograde gas-condensate reservoir as shown in Figure 1. As a result of the critical point of the retrograde gas phase is further down the left side of the envelope as shown in Figure 2, heavy hydrocarbons will be fewer as compared with oils [12].

In the bagging of the reservoir, the hydrocarbon system will be totally one phase gas (i.e., vapor phase) because the reservoir pressure is above the dew-point pressure. As the reservoir pressure decrease from the initial formation pressure through the production until dew-point pressure, where the liquid starts to condense from the gas phase to form a free liquid in the reservoir as a result of molecules attraction between light and heavy components move further apart [13]. The condensate liquid still is inside the reservoir and cannot be produced from it. The condensate liquid volume not more than 15–19% of the pore volume, so this liquid still be inside the reservoir and cannot be produced as it is not large volume enough to flow. All of this indicates by reservoir pressure path as shown in the retrograde gas-condensate figure [14].

Physical characteristics identification:

• Gas-oil ratios (GOR): common gas-oil ratios between 8000 and 70,000 SCF/STB. But the lower gas-oil ratio is approximately 3300 SCF/STB and the upper limit is over 150,000 SCF/STB. In case of low gas-oil ratio condense the liquid may be reached to 35% or more. With time, the gas-oil ratio of condensate reservoir increases due to heavy components loss.

• Stock-tank gravity (API): is usually above 40° API stock-tank and increase as formation pressure decrease below dew point pressure.

• Heptane’s plus fraction: is less than 12.5-Mole% by laboratory analysis. But in case heptane plus fraction is less than one percent, the retrograde liquid volume is small so it is negligible.

• Color: may be slightly colored, orange, brown, greenish and water-white, so color is not depended on indicator if this reservoir gas condensate or oil.

Table 1 shows data of reservoir information and compositional analysis of reservoir fluid for three different examples of retrograde gas-condensate reservoirs.

2.2 Wet gas reservoirs

It is the second type of natural gas reservoir fluid. In this type, reservoir temperature exceeds hydrocarbon system cricondentherm, so the reservoir fluid always remains in the gas phase as the reservoir pressure decrease. No condensate liquid is formed in the formation as a result of the pressure path does not inside the phase envelope as shown in a wet gas phase diagram (Figure 3) [15]. Some of the liquid is formed at the surface due to separator conditions (separator pressure and temperature) still inside the phase envelope and is called condensate. The expression of “wet gas” does not mean that the gas is wet with water but means condensation that occurs at the surface [16].

Physical characteristics identification:

• Gas-oil ratios (GOR): is very high producing gas-oil ratios reached from 60,000 to 100,000 SCF/STB. During wet gas reservoir life, the gas-oil ratio does not change.

• Stock-tank gravity (API): as gravities of retrograde gas condensate reservoir and reach above 60° API. Also during wet gas reservoir life, stock-tank gravity of condensate liquid remains constant.

• Color: water-white.

Table 2 shows data for three different examples of wet gas reservoirs.

2.3 Dry gases reservoirs

This type is a gas phase in the reservoir and in the surface condition, where surface separator conditions located outside the phase envelope as given in Figure 4 [17]. This diagram also shows that no liquid is formed at stock-tank condition (temperature and pressure) as a result of no attraction between molecules. This type also simply called a gas reservoir. Dry gas is mainly methane component with some intermediates components. The expression of “dry gas” refers to does not have heavier molecules to form condensate liquid at the surface condition. In this case, gas-oil ratios are reached more than 100,000 SCF/STB [18]. Table 3 shows data for three different examples of wet dry reservoirs.