Composition and Properties of Hydrocarbons
Hydrocarbon CompositionThe petroleum fluids from reservoirs normally are multiphase and multicomponent mixtures, primarily consisting of hydrocarbons, which can be classified into the following three groups:
In addition to hydrocarbons, water (H2O), nitrogen (N2), carbon dioxide (CO2), hydrogen sulfide (H2S), salts, and solids are often found in petroleum mixtures.
An increase in the carbon number of the component formula. If the carbon number of a component is less than 5, the component is in the gas phase at atmospheric pressure. When the mixture contains larger molecules, it is a liquid at normal temperatures and pressures. A typical petroleum fluid contains thousands of different chemical compounds, and trying to separate it into different chemical homogeneous compounds is impractical.
Equation of State
For oil and gas mixtures, the phase behavior and physical properties such as densities, viscosities, and enthalpies are uniquely determined by the state of the system. The equations of state (EOS) for petroleum mixtures are mathematical relations between volume, pressure, temperature, and composition, which are used for describing the system state and transitions between states. Most thermodynamic and transport properties in engineering analyses are derived from the EOS. Since 1873, when the first EOS for representation of real systems was developed by the van der Waals, hundreds of different EOSs have been proposed and they are distinguished by Leland into four families:
1. van der Waals family;
2. Benedict-Webb-Rubin family;
3. Reference-fluid equations;
4. Augmented-rigid-body equations.
The cubic equations of the van der Waals family are widely used in the oil and gas industry for engineering calculations because of their simplicity and relative accuracy for describing multiphase mixtures. The simple cubic equations of state, Soave-Redlich-Kwong (SRK) and Peng-Robinson (PR), are used in flow assurance software (e.g., PIPESIM). They require limited pure component data and are robust and efficient, and usually give broadly similar results.
The SRK Equation
Oil and gas are very complex fluids composed of hydrocarbon compounds that exist in petroleum in a wide variety of combinations. Physical properties change with the component composition, pressure, and temperature. No two oils have the same physical properties. In considering hydrocarbon flow in pipes, the most important physical properties are density and viscosity.
DensityDead oil is defined as oil without gas in solution. Its specific gravity go is defined as the ratio of the oil density and the water density at the same temperature and pressure:
The dynamic viscosity of a fluid is a measure of the resistance to flowexerted by a fluid. The viscosity of Newtonian fluids is a function of temperature and pressure. If the viscosity of a fluid varies with the thermal or time history, the fluid is called non-Newtonian. Most of the hydrocarbon fluids are Newtonian fluid, but in some cases, the fluid in the flowline should be considered to be non-Newtonian. The viscosity of oil decreases with increasing temperature, while the viscosity of gas increases with increasing temperature. The general relations can be expressed as follows for liquids:
The viscosity of crude oil with dissolved gases is an important parameter for the calculation of pressure loss for hydrocarbon flow in pipeline. Pressure drops of high viscous fluid along the pipeline segment may significantly impact deliverability of products. The oil viscosity should be determined in the laboratory for the required pressure and temperature ranges. Many empirical correlations are available in the literature that can be used to calculate the oil viscosity based on the system parameters such as temperature, pressure, and oil and gas gravities when no measured viscosity data are available.
To minimize pressure drop along the pipeline for viscous crude oils, it is beneficial to insulate the pipeline so it can retain a high temperature. The flow resistance is less because the oil viscosity is lower at higher temperatures. The selection of insulation is dependent on cost and operability issues. For most developed fields insulation may not be important. However, for heavy oil, high-pressure drops due to viscosity in the connecting pipeline between the subsea location and the receiving platform, insulation may have a key role to play.
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