Both water and hydrocarbon dewpoints are represented as the maximum solubility of each phase in the other. Because F 2, two intensive variables are needed to specify the system. At a given temperature and pressure, the user can determine the saturated water content of gases, the point at which a liquid water phase will precipitate. For this reason Figure 1 frequently is called the water dewpoint chart. Despite its limitations, Figure 1  is very useful and provides a check against high water content values calculated by commercial phase equilibria computer programs.
Hydrates are a possibility in oil/gas exploration, production, transportation, or processing, which involves water and molecules smaller than n-pentane. When small ( 9 Å) nonpolar molecules contact water at ambient temperatures (typically 100 F) and moderate pressures (typically 180 psia), a water crystal form may appear--a clathrate hydrate. These individual polyhedra then combine to form specific crystalline lattices. Such solids can be formed with N2, H2S, CO2, C1, C2, C3, and iso-butane. Larger molecules like n-butane and cyclopentane require the presence of some smaller molecules.
The prevention of hydrate-plug formation and safe removal of hydrate plugs represent 70% of deepwater flow-assurance challenges; the remaining 30% deal with waxes, scale, corrosion, and asphaltenes. Before considering prevention of hydrate plugs, it is important to consider safety problems involving hydrate plug removal. What is a typical pressure at which hydrates will form? Hydrate-formation data, at a typical deep seafloor temperature of 39 F, were averaged for 20 natural gases (listed in Chap.
For systems containing both water and small ( 9Å) hydrocarbons, hydrates are an important part of the phase diagram. More information about the impact of hydrate formation can be found beginning at Hydrates. Hydrocarbon guest repulsions prop open different sizes of water cages, which combine to form the three well-defined unit crystal structures shown in Figure 1. The smallest hydrated molecules (Ar, Kr, O2, and N2), with diameters of less than 4.0 Å, form sII; still smaller molecules cannot be enclathrated except at extreme pressures. These three common hydrate structures each have large and small cavities.
In order to provide the best possible strategy in dealing with hydrate formation, it is important to have a comprehensive understanding of the underlying conditions that lead to initial hydrate formation. While commercial software programs are available to examine phase equilibria, it is useful to understand the basics as a means to evaluate the computer results. The most accurate predictions of hydrate formation conditions are made using commercial phase equilibria computer programs. Of these two program types, the flash/Gibbs type is gaining pre-eminence because its predictions are available in the phase diagram interior (where many systems operate), whereas the incipient type provides the pressure/temperature (P/T) points of hydrate initiation. State-of-the-art programs are transitioning to the flash/Gibbs free-energy type.
Phases are homogeneous regions of matter--gas, liquid, or solid--that can be analyzed using common tools such as pressure gauges, thermocouples, and chromatographs. In this chapter, phases are distinct homogeneous regions larger than 100 μm. The order of phase listing is by decreasing water concentration. For example, the listing order LW H V LHC means that hydrates (H) contain less water than the liquid water phase (L W), but more water than vapor (V), which in turn contains more water than liquid hydrocarbon (LHC). The Gibbs phase rule for nonreacting systems provides the most convenient method for determining how many intensive variables are important in phase equilibria. For example, when excess gas (excess so that its composition does not change) contacts water to form hydrates, there are three phases (P 3, namely LW H V) and two components (C 2, namely water and a gas of constant composition), so that F 1; only one intensive variable (either pressure, temperature, or one phase composition) is needed to define the system.
The concept is that the hydrate concentrates gas by a factor of about 164, without the cost of compressing and transporting gas at high pressure. In shipping, preservation of hydrated gas is vital to prevent losses. Recent measurements have shown that only a mild amount of refrigeration (e.g., to 20 F) will prevent rapid hydrate dissociation, with rates that are several orders of magnitude less than values interpolated from lower or higher temperatures. The cause for this anomalous hydrate self-preservation effect is most likely uncertain because of an outer ice barrier that prevents inner-particle dissociation. The ice protective shell happens because the water from the melted hydrate surface, caused by endothermic hydrate melting, freezes.
Natural-gas hydrates are ice-like solids that form when free water and natural gas combine at high pressure and low temperature. Detailed reviews of gas-hydrate chemistry, physics, and oilfield engineering are found in Makogon and Sloan. This page focuses on prevention, inhibition, and removal of hydrates in production. Other pages provide more detail on hydrate formation and predicting hydrate formation. Shut-in gas wells are particularly prone to serious hydrate problems, if the well has been producing some water. Subsequent equilibration of the tubular and its contents with cold zones of the rock can lower the temperature into the hydrate-formation region.