JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III PETROLEUM PRODUCTION ENGINEERING AND DESIGN Previous Question Paper (August/September - 2021 - R18)

A natural gas is a naturally occurring hydrocarbon gas mixture consisting primarily of methane, with varying amounts of other alkanes and small percentages of nitrogen, carbon dioxide, hydrogen sulfide, or helium. It is a fossil fuel energy source that forms deep beneath the earth's surface.

A real gas is a gas that does not behave according to the ideal gas law because it occupies space and has intermolecular forces. Unlike ideal gases, real gases deviate from the equation of state

The properties of real gases are defined by the following key parameters:

• Compressibility Factor (z): This is the ratio of the actual volume of a real gas to the volume it would occupy if it behaved as an ideal gas. It is expressed as
  z = PV / nRT
  . A value of z ≠ 1 indicates deviation from ideal behavior.
• Gas Density (ρ): Real gas density is influenced by pressure and temperature, calculated using the formula
  ρ = PM / zRT
  , where M is the apparent molecular weight of the gas mixture.
• Gas Formation Volume Factor (Bg): This property relates the volume of gas at reservoir conditions to the volume at standard surface conditions, defined as
  Bg = 0.02827 (zT / P)
  (in units of bbl/scf).
• Viscosity (μg): Real gas viscosity is a measure of its internal resistance to flow. It is primarily a function of temperature and pressure, and for natural gases, it typically increases as temperature increases or as pressure increases above the critical point.
• Critical Properties (Pc and Tc): These are the pressure and temperature values above which a gas cannot be liquefied by pressure alone. For mixtures, these are determined using pseudo-critical properties based on the mole fractions of the components.

In single-phase liquid flow through a pipe, the total pressure drop (ΔP) is the sum of pressure losses due to friction, elevation changes, and acceleration. The governing equation for the total pressure drop over a length (L) is expressed as:

Where the components are defined as follows:

1. Frictional Pressure Drop (ΔP_f): This represents the energy loss due to fluid viscosity and pipe wall roughness. It is calculated using the Darcy-Weisbach equation:
   ΔP_f = (f × L × ρ × v²) / (2 × D × g_c)
   where f is the Fanning or Darcy friction factor, ρ is the liquid density, v is the average fluid velocity, D is the internal pipe diameter, and g_c is the gravitational conversion factor.
2. Elevational Pressure Drop (ΔP_el): This is the hydrostatic pressure change caused by the change in vertical height (Δh) of the fluid column. It is given by:
   ΔP_el = (ρ × g × Δh) / g_c
   where g is the acceleration due to gravity. For upward flow, this is a pressure loss; for downward flow, it is a pressure gain.
3. Accelerational Pressure Drop (ΔP_acc): This occurs when there is a change in fluid velocity along the flow path, typically due to changes in pipe diameter or fluid density (though density remains constant for incompressible liquids). It is expressed as:
   ΔP_acc = (ρ × v × Δv) / g_c
   where Δv is the change in velocity. In most steady-state single-phase liquid production scenarios with constant pipe diameter, this term is negligible.

In petroleum production engineering, the accurate calculation of these components is critical for designing tubing strings and flowlines to ensure that the reservoir pressure is sufficient to lift the fluid to the surface at the required flow rate.