![]() ![]() Therefore, atmospheric pressure must be added to gauge pressures to convert them to an absolute basis. The universal gas constant in practical units isįor economic reasons, most (but not all) pressure gauges read zero pressure when pressure is equal to the ambient atmospheric pressure. The basis of gas PVT behavior is the ideal gas law, and by extension the real gas law: Pressure/volume/temperature (PVT) behavior Retrograde systems require more-complex calculations using equations of state (EOSs) and other advanced engineering methods. The only issue is whether there is a sufficient volume of produced liquid to be considered in such calculations as material balance or wellbore hydraulics. For both wet and dry gases, reservoir engineering calculations are based on a single-phase reservoir gas. While the difference between retrograde-condensate and wet gases is notable, there is much less distinction between wet and dry gases. A somewhat arbitrary cutoff liquid yield of 10 STB/MMscf is sometimes used to distinguish dry-gas reservoirs. 3), resulting in little or no liquid production at the surface. In a dry-gas reservoir, the cricondentherm is much lower than the reservoir temperature ( Fig. Wet-gas reservoirs generally produce liquids with gravities similar to those for retrograde condensates, but with yields less than approximately 20 STB/MMscf. Wet-gas reservoirs can always be treated as containing single-phase gas in the reservoir, while retrograde-condensate reservoirs may not. However, the term "condensate reservoir" should be applied only to situations in which condensate is actually formed in the reservoir because of retrograde behavior. The term "condensate" is often applied to any light hydrocarbon liquid produced from a gas well. ![]() Considerable liquid can still form (condense) at surface conditions or even in the wellbore. Therefore, a liquid phase never forms in the reservoir. In a wet-gas reservoir, temperature is higher than the cricondentherm ( Fig. A large amount of the most valuable hydrocarbon components is left in the reservoir rather than produced.Liquid deposition near the wellbore causes a decrease in gas deliverability that can approach 100% in a reservoir with less than 50-md permeability.The primary difficulties in producing condensate reservoirs are as follows: McCain suggests that when yields are below approximately 20 STB/MMscf, even though phase-behavior considerations may show retrograde behavior, the amount of liquid dropout in the reservoir is insignificant. Producing-liquid yields can be as high as 300 STB/MMscf. Condensate liquids are generally "water white" or light in color (brown, orange, or greenish), with gravities typically between 40 and 60☊PI. ![]() As a result, a liquid phase will form in the reservoir as pressure declines, and the amount and gravity of produced liquids will change with time. Copyright Pennwell Books, 1990.)Ī retrograde-condensate fluid has a phase envelope such that reservoir temperature lies between the critical temperature and the cricondentherm ( Fig. (Source: The Properties of Petroleum Fluids, second edition, by William D. Typical phase diagrams for the gas categories are shown in Figs. Studies of retrograde-condensate gas reservoirs must consider changes in condensate yield as reservoir pressure declines, the potential for decreased well deliverability as liquid saturations increase near the wellbore, and the effects of two-phase flow on wellbore hydraulics.Ī widely accepted system for categorizing petroleum reservoir fluids is based on five classes: From a reservoir standpoint, dry and wet gas can be treated similarly in terms of producing characteristics, pressure behavior, and recovery potential. Additional condensate forms with changes in pressure and temperature in the tubing and during lease separation. A retrograde-condensate gas reservoir initially contains a single-phase fluid, which changes to two phases (condensate and gas) in the reservoir when the reservoir pressure decreases. Condensate will form either while flowing to the surface or in lease-separation equipment. A wet-gas reservoir is defined as producing a single gas composition to the producing well perforations throughout its life. Some liquids may be recovered by processing in a gas plant.
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