Methane hydrate thermodynamics

Hydrates of Natural Gas

Natural gas and water combine chemically under pressure at a temperature that is significantly higher than the freezing point of water to form natural gas hydrates, which are solid, crystalline compounds. For natural gas hydrates, the chemical formulae are as follows:

Methane?????? CH4?7H2O

Ethane?????????? C2H6?8H2O

Propane??????? C3H4?18H2O

Carbon Dioxide?????? CO2?7H2O

When free water is present and the temperature drops below what is necessary for hydrate formation, hydrates typically form. This is typically a result of sudden expansion-related pressure drop in the surface line. Usually, when fluids pass through chokes, back pressure regulators, or orifices, this occurs.

Natural gas hydrates will undoubtedly form more readily in the system if there is a tiny "seed" crystal of hydrate or acid gas (H2S or CO2) present and the flow rate is high with agitation.

Hydrate formation is an unusual existence of hydrocarbon in which molecules of natural gas, typically methane, are trapped in ice molecules well above the freezing point of water at high pressures. Clathrate hydrates are non-stoichiometric mixtures of water and natural gas.? The gas molecules are trapped in a polygonal crystalline structure made of water molecules. The water molecules arrange themselves in an orderly fashion around the gas molecules, thus entrapping them.

High-pressure and low-temperature conditions favor hydrate formation

Air: Pressure dew point temperature

?Image credit: Google

Issues of gas hydrate

Gas hydrate deposits are found wherever methane occurs in the presence of water under elevated pressures and at relatively low temperatures.? Methane that forms hydrate can be both biogenic, created by biological activity in sediments, and thermogenic, created by geological processes deeper within the earth.

It has been reported that the maximum fast time of hydrate formation at T = 1°C and P = 10 MPa is about 2.5 hours. It has been determined that the greater the methane concentration in the mixture, the greater must be the pressure in the system for the gas hydrate formation.

Hydrates have a strong tendency to agglomerate and adhere to the pipe wall thereby plugging the pipeline.

Thermodynamic conditions favoring hydrate formation are often found in pipelines. The sequence of events leading to hydrate formation in gas pipelines involves water vapor condensation, accumulation of water at lowered sections of the pipeline, nucleation, and growth of hydrate particles ultimately blocking the pipeline.

Natural gas pipelines always have some moisture content as the gas reaches the wellhead saturated with water. During the cold season, the ground temperatures are lower resulting in the temperature drop of the pipeline content. If the temperature of the gas falls below the saturation temperature corresponding to the dew point of water, condensation of water vapors will start. In straighter sections of the pipeline, the condensate starts to accumulate at the base of the pipe due to gravity. The accumulation may increase at the base of the sagging/uphill sections of the pipe. If enough water is present and the thermodynamic conditions are suitable, hydrate nucleation processes are initiated. Generally speaking, compression increases the dew point, and expansion lowers the dew point of the air. Please refer to the above dew point graph. At atmospheric pressure, the dew point is about 10 degc. At 3 bar pressure, the dew point is about 35 degc and at 7 bar pressure the air dew point can increase to about 50 degc.

Methane hydrate phase diagram: Key points to note

When hydrocarbon contacts water, the two components separate into two phases in which the mutual component solubility is less than 1.0 mol% at ambient conditions. This splitting of phases affects almost all treatments of mixed water and hydrocarbon systems and is caused by the different molecular attractions within water and hydrocarbons. Hydrocarbon molecules have a weak, noncharged attraction for each other, while water attracts other water molecules through a strong, charged hydrogen bond. Because hydrogen bonds are significantly stronger than those between hydrocarbon molecules, hydrocarbon solubility in water (and that of water in hydrocarbons) is very small.

This phase diagram shows water depth (pressure) on the vertical y-axis and temperature on the horizontal x-axis. The dashed lines separate stability fields of water, water ice, gas, and gas hydrate. The line labeled "hydrate to gas. transition" is significant.

?Conditions for the formation of methane hydrate occur below this line. Above this line, methane hydrate will not form. The red line traces a geotherm (the change of temperature with depth at a

specific location). Note how, as depth increases, the geotherm crosses the hydrate-to-gas transition line.

Image credit: Google

Hydrate formation, prevention, and mitigation

Hydrates have a strong tendency to agglomerate and adhere to the pipe wall thereby plugging the pipeline. Once formed, they can be decomposed by increasing the temperature and/or decreasing the pressure. Even under these conditions, the clathrate dissociation is a slow process.

Therefore, preventing hydrate formation appears to be the key to the problem. Hydrate prevention could typically be based on three levels:

[1] Avoid conditions that might cause the formation of hydrates. Depress temperature using glycol dehydration;

[2] Change operating conditions to avoid hydrate formation;

[3] Add chemicals to (a) shift the hydrate equilibrium conditions towards lower temperatures and higher pressures or (b) increase hydrate formation time by inhibitors.

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