Gas compressibility in hydrate formation in oil and gas industries: Fundamentals

The Z values for CH4 range from 0.85 to 0.91, while the Z values for CO2 range from 0.73-0.79, which are lower than the Z values for CH4 due to the lower pressure requirement for CO2 hydrates. CO2's intermolecular forces are stronger than those of CH4. Methane is a more ideal gas. The greater the deviation from the ideal gas, the greater the likelihood of hydrate formation.

Oil and gas compressibility is important in reservoir simulation, material balance calculations, high-pressure surface equipment design, and well-test analysis interpretation, particularly for systems below the bubble point pressure. Accurate information on the compressibility of oil fluids above and below the bubble point pressure is critical for reservoir evaluation. The natural gas compressibility factor (z) is also a key factor in the gas industry for natural gas production and transportation. Compressibility factors can characterize the formation and behaviour of hydrates.

What exactly is compressibility?

Most gases have such large volumes at low pressures and relatively high temperatures that the volume of the molecules themselves can be ignored. Furthermore, the distance between molecules is so great that even moderately strong attractive or repulsive forces are insufficient to affect the behaviour in the gas state. However, as the pressure rises, the total volume occupied by the gas shrinks to the point where the volume of the molecules themselves becomes significant and must be considered. Furthermore, the distance between the molecules is reduced under these conditions to the point where the attractive or repulsive forces between the molecules become significant.

This behaviour contradicts the assumptions required for ideal gas behaviour, resulting in significant errors when comparing experimental volumes to those calculated using the ideal gas law. As a result, a proportionality term was used to formulate a real gas law (as a correction to the ideal gas law).

A real gas's volume is usually less than the volume of an ideal gas at the same temperature and pressure; thus, a real gas is said to be compressible. The compressibility factor is defined as the ratio of the real volume to the ideal volume, which is a measure of how far the gas deviates from perfect behaviour. It is also known as the gas deviation factor and is denoted by the symbol, z

?The gas deviation factor is by definition the ratio of the volume actually occupied by a gas at a given pressure and temperature to the volume it would occupy if it behaved ideally,

or:

Note that the numerator and denominator in this equation refer to the same mass. (This equation for the z factor is also used for liquids.) Thus, the real gas equation of state is written as

PV = ZnRT

At low pressures and high temperatures, the gas deviation factor, z, is close to one, indicating that the gas behaves as an ideal gas. The gas z factor is always close to one under standard or atmospheric conditions. The z factor first decreases to a minimum as pressure increases, which is approximately 0.27 for the critical temperature and critical pressure. The minimum z factor is approximately 0.77 for temperatures 1.5 times the critical temperature, and 0.937 for temperatures twice the critical temperature. At high pressures, the z factor exceeds one, indicating that the gas is no longer compressible. Under these circumstances, the specific volume of the gas is becoming so small, and the distance between molecules is much smaller so that the density is more strongly affected by the volume occupied by the individual molecules. Hence, the z factor continues to increase

Why is gas compressibility important?

The compressibility factor of natural gas is a measure of how far the gas deviates from perfect gas behaviour. It is proportional to the density of a gas stream, and thus to its flow rate and isothermal compressibility. It is a useful tool in the gas industry for calculating reservoir fluid properties either directly or indirectly. Accurate estimation of compressibility factor (z) is critical, especially when estimating initial gas in place quickly. It is also an important factor to consider when dealing with gas metering, as the volume flow of gas obtained from the orifice metre is dependent on the z-accuracy. factors.

The natural gas compressibility factor (z) is also a key factor in the gas industry for natural gas production and transportation. Compressibility factors can characterize the formation and behaviour of hydrates.

Hydrate vs compressibility factor (z)

One of the most significant flow obstructions encountered in the petroleum industry is the formation of gas hydrate in pipelines, which is the root cause of the oil and gas production, transportation, and processing bottleneck. The formation of gas hydrate is highly dependent on pressure and temperature changes, the amount of pressure and temperature applied to the system, and the amount of gas moles absorbed by the water molecules. As previously stated, the compressibility factor can predict the behaviour of a hydrate.

Z value plays important role in characterizing a hydrate

Compressibility Factor (Z) vs Temp for CH4 hydrate

Compressibility Factor (Z) vs Temp for CO2 hydrate?

Y axis is onset of hydrate formation X axis is the temperature

Two typical hydrates look completely different with different properties

Compressibility Factor (Z) vs Temp for CH4 hydrate???????????????????

In the case of methane, it is indicated that the value of Z increased when hydrate formation started, y-axis while Z values depressed x-axis in the dissociation of hydrates.?The values of Z for CH4 are between 0.85 to 0.91.

Compressibility Factor (Z) vs Temp for CO2 hydrate

In the case of carbon dioxide, compressibility factor Z increased when hydrate formation started while Z values were depleted in the dissociation of CO2 hydrate as well. However, the Z values for CO2 range between 0.73-0.79 which were lower than CH4 compressibility values due to the lesser pressure requirement for CO2 hydrates.

CO2's intermolecular forces are stronger than those of CH4. Methane is a more ideal gas. The greater the deviation from the ideal gas, the greater the likelihood of hydrate formation.

Credit: Google