Published: Jan 2007
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As discussed in chapter 1, reservoir fluids are in the forms of natural gases, gas condensates, volatile oils, and black oils. As shown in Table 1.1, these fluids contain hydrocarbons from C1 to compounds with carbon number greater than 50. Composition of a reservoir fluid is generally expressed in mol% of nonhydrocarbon compounds (i.e., H2S, CO2, N2), C1, C2, C3, nC4, iC4, nC5, iC5, C6, and C7+. The boiling range of reservoir fluids can be greater than 550°C (∼>1000°F). Crude oil is produced by separating light gases from a reservoir fluid and bringing its condition to surface atmospheric pressure and temperature. Therefore, crude oils are generally free from methane and contain little ethane. The main difference between various reservoir fluid and produced crude oil is in their composition, as shown in Table 1.1. Amount of methane reduces from natural gas to gas condensate, volatile oil, black oil, and crude oil while amount of heavier compounds (i.e., C7+) increase in the same direction. Characterization of reservoir fluids and crude oils mainly involves characterization of hydrocarbonplus fractions generally expressed in terms of C7+ fractions. These fractions are completely different from petroleum fractions discussed in Chapter 3. A C7+ fraction of a crude oil has a very wide boiling range in comparison with a petroleum product and contains more complex and heavy compounds. Usually the only information available for a C7+ fraction is the mole fraction, molecular weight, and specific gravity. The characterization procedure involves how to present this mixture in terms of arbitrary number of subfractions (pseudocomponents) with known mole fraction, boiling point, specific gravity, and molecular weight. This approach is called pseudoization. The main objective of this chapter is to present methods of characterization of hydrocarbon-plus fractions, which involves prediction of distribution of hydrocarbons in the mixture and to represent the fluid in terms of several narrow range subfractions. However, for natural gases and gas condensate fluids that are rich in low-molecular-weight hydrocarbons simple relations have been proposed in the literature. In this chapter types of data available for reservoir fluids and crude oils are discussed followed by characterization of natural gases. Then physical properties of single carbon number (SCN) groups are presented. Three distribution models for properties of hydrocarbon plus fractions are introduced and their application in characterization of reservoir fluids is examined. Finally, the proposed methods are used to calculate some properties of crude oils. Accuracy of characterization of reservoir fluids largely depends on the distribution model used to express component distribution as well as characterization methods of petroleum fractions discussed in Chapter 2 to estimate properties of the narrow boiling range pseudocomponents.