What’s the role of the compressibility factor? It’s ideal to gain a better understanding of this and how essential it is. Read on.
Oil and gas compressibility always plays a crucial role in material balance calculations, reservoir simulation, the breakdown of well test evaluation, and the design of high-pressure surface equipment, especially for systems set below the bubble point pressure. Accurate data on the compressibility of oil fluid – above and below bubble point pressure – is crucial for reservoir evaluation.
But what is the compressibility factor, and how important is it in the oil and gas industry? This is what you are about to find out in this post.
What is the Compressibility Factor in the Oil and Gas Industry?
At relatively high temperatures and low pressures, the volume of most gases known today is incredibly large. It is so large that the volume of gas molecules can easily be neglected. Moreover, the distance between these molecules is so considerable that even the presence of repulsive or incredibly strong attractive forces is not sufficient to affect the gas state’s behavior.
Nevertheless, as pressure steadily increases, the total volume the gas occupies becomes small enough that the volume of the molecules that make up the gas is highly appreciable and should, therefore, be considered.
Additionally, under these specific conditions, the distance between these molecules is so significantly decreased that the repulsive or attractive forces between them become essential.
This behavior easily weakens the assumptions demanded of ideal gas behavior, and critical errors are always detected when experimental volumes are compared to those calculated using the ideal gas law. Therefore, using a proportionality term, a real gas law was developed to correct the ideal gas law. The volume of natural gas – in the real world – is generally lesser than what the volume of an ideal gas is, at the same pressure and temperature. This makes a natural gas to be highly compressible.
Therefore, the compressibility factor has to do with the ratio of the real volume to the ideal volume. This is a measure of the amount of deviation of a gas from perfect behavior.
The compressibility factor is given the symbol ‘Z’ and is also known as the Z-factor or gas deviation. The gas deviation factor is the ratio of the volume that a gas occupies at a given temperature and pressure to the volume it would occupy if it behaves perfectly or ideally.
In simple terms:
Z = Actual volume of gas at specified T and p/Ideal volume of gas at same T and p. Bear in mind that the numerator and denominator refer to the same mass. This equation for the Z-factor can also be used for liquids.
Therefore, the real gas equation of state can be highlighted thus:
pV = ZnRT.
The compressibility factor should never be confused with the isothermal compressibility or coefficient of compressibility of a material, which measures the relative volume change of a solid or fluid in response to a pressure change. The gas deviation factor is often used as a correction factor to ideal behavior.
At high temperatures and low pressures, the compressibility factor is close to 1. This implies that in these conditions, gas behaves just like an ideal gas. At atmospheric or standard conditions, the compressibility factor is approximately 1.
But as the pressure steadily increases, the gas deviation factor, first of all, decreases to a minimum. This is approximately 0.27 for the critical pressure and critical temperature. The compressibility factor increases above 1 at high pressures, and this is where the gas becomes incompressible.
At these stated conditions, the specific volume of the gas becomes small, and the distance between its molecules has also become much smaller than it used to be. This is where the density of the gas is strongly affected by the overall volume that the individual molecules occupy. Therefore, the compressibility factor increases continuously.
The Importance of Compressibility Factor in the Oil and Gas Industry
The compressibility factor remains one of the most crucial parameters in the oil and gas industry. This is because it is directly connected with the density of a natural gas stream, hence its isothermal compressibility and flow rate.
It is often challenging to obtain accurate values of the compressibility factor for hydrocarbons and gas mixtures. This is because natural gas is a non-ideal, multicomponent system.
The overall process of obtaining the compressibility factor – in a traditional manner – involves simple empirical correlations. However, the results of this process were mostly weak, either as a result of calculation convergence difficulties or because of limited accuracy.
The compressibility factor is a vital tool for directly or indirectly computing reservoir fluid properties. The accurate estimation of the Z-factor is highly essential, especially when it has to do with the quick estimation of initial gas in place.
In other words, the compressibility factor is a correction factor that accurately describes the deviation of a real gas from the behavior of ideal gas. It is an incredibly useful thermodynamic property employed for modifying the ideal gas law to account for the behavior of real gas.
For a mixture of two – or more – pure gases (e.g., natural gas or air), it is highly essential to know the gas composition before calculating compressibility.
As another option, every specific gas has a compressibility factor that can easily be read from the comprehensive compressibility charts that plot the gas deviation factor of pressure at a constant temperature.
Relying on the compressibility factor is also essential, especially when it comes to gas metering. The accuracy of this factor plays a vital role in determining the volume flow realized from the orifice meter.
The gas deviation factor is one of the principal factors in the gas industry concerning the production and transportation of natural gas. Compressibility factors exemplify the formation as well as the behavior of hydrates.
Hydrate vs. Compressibility Factor
In the oil and gas industry, the crystallization of gas hydrate in pipelines is one of the central obstructions confronted by inflows known today. It is the source of the logjam experienced during oil and gas production, processing, and transportation.
With respect to thermodynamics, the crystallization of gas hydrate depends significantly on the changes of temperature and pressure, the number of gas moles the water molecules absorb, as well as the amount of temperature and pressure given to the system. Compressibility factors can accurately predict the overall behavior of a hydrate.
Conclusion
The role of the compressibility factor in the oil and gas industry cannot be overstated. It is the correction factor that accurately describes the deviation of a real gas from the overall behavior of ideal gas.
It is essential to know the gas composition of natural air or air or a mixture of two – or more – pure gases before calculating compressibility.
As an alternative, the gas deviation factor exists for specific gases. It can be read from the generalized compressibility charts that plot compressibility factor as a function of pressure at a constant temperature.