Basic Principles

2.1 Introduction

Before discussing gas-handling, conditioning, and processing equipment and design techniques, it is necessary to review some basic principles and fluid properties. We will also discuss some of the common calculation procedures, conversions, and operations used to describe the fluids encountered in production operations.

2.2 Fluid Analysis

An example fluid analysis of a typical gas well is shown in Table 2.1. Note that only paraffin hydrocarbons are shown. This is not correct, even though only paraffin hydrocarbons may be the predominant series present. Also note that all molecules of heptane and larger ones are lumped together as heptanes plus fraction.

Sometimes operators request more complete analysis with many more components listed. Process engineers often like to input all of this data into their simulation models, but from a practical standpoint the exact composition of these higher-end components makes little difference in sizing equipment. In addition, fluid samples often are not totally representative of what is eventually produced through the facility as new wells are drilled and reservoir conditions change. Optimizing a design to a precise set of “given” fluid properties may result in a facility that is not flexible enough to handle the inevitable changes that occur in oil and gas field developments.

2.3 Physical Properties

An accurate estimate of physical properties is essential if one is to obtain reliable calculations. Physical and chemical properties depend on:

• Pressure

• Temperature

• Composition

Most hydrocarbon streams are mixtures of hydrocarbons that may contain varying quantities of contaminants such as:

• Hydrogen sulfide

• Carbon dioxide

• Water

The more similar the character of the mixture molecules, the more orderly is their behavior. A single-component system composed entirely of a simple molecule, such as methane, behaves in a very predictable, correctable manner.

The accuracy of calculations decrease in the following order:

• Single-component system

• Mixture of molecules from the same homologous series

• Mixture of molecules from different homologous series

• Hydrocarbon mixtures containing sulfur compounds and/or carbon dioxide

Performance data for a single-component system can be accurately correlated in graphic or tabular form. For all others, one must use either pressure/volume/temperature (PVT) equations of state or the weighted average. The weighted average assumes that the contribution of an individual molecule is in proportion to its relative quantity in the mixture. The more dissimilar the molecules, the less accurate the prediction becomes. Table 2.2 lists some of the physical properties of some of the paraffin hydrocarbon series.