Fluid Phase Equilibria Involving Water

Water is almost always associated with the acid gas resulting from the sweetening of natural gas. This is not produced water, which contains dissolved salts, which causes additional problems - this is condensed water and contains no dissolved solids. Since the water in the acid gas is present originally in the gas phase, there is no concern regarding brine. 

The presence of water in the acid gas poses several problems. These include corrosion and hydrate formation. Hydrates are discussed in detail in the next chapter and corrosion is mentioned throughout this textbook, where appropriate. However, at this point it is important to state that dry acid gas, that is acid gas with no free water present, is not corrosive to common steels, even carbon steel. Thus it is important to 1. Predict where liquid water may form in the system and 2. Design to prevent water formation form occurring. 

The water content of a fluid is the amount of water that it can hold at the given temperature and pressure. If the overall mixture contains less than this amount, then it is said to be undersaturated and a water-rich phase will not form. If the stream contains more than this amount, then it is over saturated and a water-rich phase 

Many of the results regarding the phase equilibria presented in this chapter are from Carroll (1999b, 2002). The reader is referred to that paper for a detailed review of the experimental investigations and for additional calculations. A recent study by Marriott et al. (2009) also provides an interesting review of the water content of acid gas mixtures. Valtz et al. (2004) provide a thorough review of the phase equilibra in mixture of C02 + water, including water content. Chapoy et al. (2005) Provide a similar study for the equilibria in H2S + water. 

In this chapter, the solubility of acid gas in water and brine is also discussed. Although brine is not a problem in the injection system, the reservoir may contain brine and thus the solubility in brine is important in reservoir modeling. 

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An important aspect of the design of an acid gas injection scheme is the non-aqueous phase equilibrium. Fluid phase equilibrium involving water, which is also very important, will be discussed in chapter 4 and hydrates in chapter 5. 

A- -Vi = B- -V2 (where A, B are volatile free phases and Vi, V2 are hydrous phases or carbonates), involve hydrates and/or carbonates and change the mineralogy of a rock volume according to the stability fields of the minerals, but do not liberate a fluid. Prograde subduction zone metamorphism (as is true for any type of prograde metamorphism) generally reduces the amount of H2O that can be stored in hydrous minerals with depth. Thus, almost any part of the oceanic crust sooner or later becomes fluid saturated. In an equilibrium situation, the volatile content bound in hydrous phases and carbonates remains constant until fluid saturation occurs. Either continuous or discontinuous reactions may lead to fluid saturation in a rock. The point at which this occurs depends on initial water content, and pressure and temperature, and somewhat counter-intuitively, initial low water contents do not cause early complete dehydration, but delay the onset of fluid production to high pressures. 

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