Retrograde gas

The diagram (Fig. 5.21) shows that as the pressure is reduced below the dew point, the volume of liquid in the two phase mixture initially increases. This contradicts the common observation of the fraction of liquids in a volatile mixture reducing as the pressure is dropped (vaporisation), and explains why the fluids are sometimes referred to as retrograde gas condensates. 

The gas of Exercise 3-4 is a retrograde gas. However, discovery pressure in reservoir, 7000 psig, is higher than dew point pressure, 6010 psig. Reservoir temperature is 256°F. The reservoir lies under 7040 acres, has an average thickness of 13 ft, has a porosity of 11%, and has a water saturation of 40%. Calculate the mass of gas in the reservoir at initial conditions. Give your answer in lb moles. 

There are five types of reservoir fluids. These are usually called black oil, volatile oil, retrograde gas, wet gas, and dry gas. The five types of reservoir fluids have been defined because each requires different approaches by reservoir engineers and production engineers. 

Fig. 5-3. Phase diagram of a typical retrograde gas with line of isothermal reduction of reservoir pressure, 123, and surface separator conditions.

The third type of reservoir fluid we will consider is retrograde gas. Retrograde Gas Phase Diagram... 

The phase diagram of a retrograde gas is somewhat smaller than that for oils, and the critical point is further down the left side of the envelope. These changes are a result of retrograde gases containing fewer of the heavy hydrocarbons than do the oils. 

The phase diagram of a retrograde gas has a critical temperature less than reservoir temperature and a cricondentherm greater than reservoir temperature. See Figure 5-3. Initially, the retrograde gas is totally gas in the reservoir, point 1. As reservoir pressure decreases, the retrograde gas exhibits a dew point, point 2. As pressure is reduced, liquid condenses from the gas to form a free liquid in the reservoir. This liquid will normally not flow and cannot be produced. 

Producing gas-oil ratios for a retrograde gas will increase after production begins when reservoir pressure falls below the dew-point pressure of the gas. 

Retrograde gases are also called retrograde gas-condensates, retrograde condensate gases, gas condensates, or condensates.1,2 The use of the word condensate in the name of this reservoir fluid leads to much confusion. Initially, the fluid is gas in tire reservoir and exhibits retrograde behavior. Hence, the correct name is retrograde gas. 

Stock-tank liquid produced from retrograde gas reservoirs often is called condensate. The liquid produced in the reservoir is called condensate also. A better name is retrograde liquid. 

An initial producing gas-oil ratio of 3300 to 5000 scf/STB indicates a very rich retrograde gas, one which will condense sufficient liquid to fill 35 percent or more of the reservoir volume. Even this quantity of liquid seldom will flow and normally cannot be produced. 

A dry gas reservoir often is called simply a gas reservoir. This leads to confusion because wet gas reservoirs sometimes are called gas reservoirs. Further, a retrograde gas initially exists as gas in the reservoir. 

A reasonably accurate procedure for estimating the dew-point pressure of a retrograde gas is given in Appendix B. 

The surface gas from a retrograde gas reservoir is rich in intermediate molecules. Processing it through a plant to produce additional liquids is usually economically attractive. The calculation of plant liquids is the same as described previously. 

A reservoir containing retrograde gas has a reservoir pressure higher than the dew-point pressure of the gas as evidenced by constant producing gas-oil ratio. The gas is produced through... 

A retrograde gas is produced through a three-stage separator system. The primary separator at 625 psia and 75°F produces 8885 scf/STB of 0.676 specific gravity gas. The second separator operates at 60 psia and 75°F. Stock-tank liquid gravity is 54.8°API. Estimate the specific gravity of the gas in the reservoir,... 

Producing gas-oil ratio from a retrograde gas reservoir is also constant during early production and then increases. When reservoir pressure is above the dew point, the gas carries a constant quantity of components which will liquefy at surface conditions. However, at reservoir pressures below the dew point some of these components condense in the reservoir. This liquid does not flow to any appreciable degree. Thus, the produced gas carries fewer condensable components to the surface and producing gas-oil ratio increases. Dew point pressure cannot be determined from a plot of pressure versus cumulative production the change in slope at the dew point is rarely apparent. 

Compare your answers with the laboratory measured specific volume of reservoir liquid at bubble point, 0.0353 cu ft/Ib. 13-18, The gas-liquid equilibrium calculations for a two-stage separation of a retrograde gas have been completed. Results are given below. 

The gas-liquid equilibrium calculations for a three-stage separation of a retrograde gas have been completed. Results are given below. 

Fig. 14-1. Equilibrium ratios for a retrograde gas at various temperatures and pressures. (Roland, Smith, Kaveler, Oil and Gas Journal 39, No. 46, 128, with permission.)...

Fig. 14-7. Effects of errors in equilibrium ratios on the calculated quantity of liquid of a retrograde gas at reservoir conditions. (Standing, Volumetric and Phase Behavior of Oil Field Hydrocarbon Systems, SPE, Dallas, 1951. Copyright 1951 SPE-AIME.

Figure 14-7 shows the results of calculations for a retrograde gas at reservoir conditions. Inaccuracies in the equilibrium ratios for both the lightest component, methane, and the heaviest components, represented by heptanes plus, result in drastic error in the calculated liquid volume. In fact, an error of only 8% in the equilibrium ratio for methane results in the calculation of a dew point rather than the correct liquid volume. This indicates that calculations near the dew point of a mixture are highly dependent on the accuracy with which equilibrium ratios are known. 

Fig. 14-8. Experimentally determined equilibrium ratios for a black oil (gas-oil) and a retrograde gas (gas-condensate) at 201 °F. (Hoffman, Crump, Hocott, Trans., AiME, 198, 1. Copyright 1953 SPE-AIME.)...

Gas-liquid equilibrium has been calculated for a retrograde gas at 1500 psia and 125°F. A convergence pressure of 5000 psia was used to obtain K-factors. Results are given below. What value of convergence pressure should have been used for this mixture at... 

Constant volume depletion of a retrograde gas produced the following K-factors at 256°F. 

Gold et al. developed the following equation from three-stage separation of 237 retrograde gas samples.13 The average absolute error is 5.8%. 

Retrograde Gas Phase Diagram — Field Identification of Retrograde Gases — Laboratory Analysis of Retrograde Gases — Comments... 

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