NATURAL GAS HYDRATES A GUIDE FOR ENGINEERS

INTRODUCTION OF GAS HYDRATES. Hydrocarbon gas and liquid water combine to form solids taking after wet snow at temperatures above at which water solidifies – become a solid.material by freezing- These solids are called gas hydrates.  This phenomenon particularly interests those in the petroleum industry because these solids can form at temperatures and pressures normally encountered in producing and transporting natural gases. Naturally,  gas hydrates could be found on the seafloor- sea-, in ocean sediments, in deep lake sediments, as well as in the Permafrost regions. The amount of methane potentially trapped in natural methane hydrate deposits may be significant, which makes them interesting as a potential energy resource. In adittion, Methane is more efficient greenhouse gas than Carbon dioxide and then  the fast decomposition of such deposits is considered a geohazard due to its potential to trigger, or cause landslides, earthquakes and tsunamis. However, natural gas hydrates do not contain only methane but also other hydrocarbon gases, as well as H2S and CO2. Air hydrates are frequently observed in polar ice samples.

GAS HYDRATE

GAS HYDRATE FORMATION

Gas hydrates behave as gas solutions in crystalline solids rather than as chemical compounds. The major framework or skeleton of the hydrate crystal is formed with water molecules. The hydrocarbon molecules occupy empty spaces withing the lattice of water molecules. Hydrate formation is physical rather than chemical in nature.

Apparently, no strong chemical bonds are formed between the hydrocarbon and water molecules. Actually, the hydrocarbon molecules are free to rotate within the void spaces. Two types of hydrate crystal lattices are known. Each one contains void spaces of two different sizes. One lattice has voids sized to accept small molecules such as methane and larger molecules such as propane. The other lattice accepts methane molecules and medium– sized molecules such as ethane. Although gas hydrates appear to be solid solutions rather than chemical compounds, a specific number of water molecules is associated with each gas molecule. This is due to the framework of the crystal. The ratio depends primarily on the size of the gas molecules.

FORMATION CONDITIONS OF GAS HYDRATES

  • High pressures and low temperatures suit gas hydrate formations which could also form at higher temperatures than freezing water temperatures.
  • The pressence of liquid water ( therefore, the amount of water in natural gas must be gone down to avoid any system which reaches dew points especially gases with CO or H2S that will probably form acid with condensate water.
  • Turbulence, high flow rate, etc may well form the first crystals of hydrates; once it happens, the way on how crystals of hydrates form – that phenomenon- are fast.
  • The temperature at which hydrate formations begin is obtained by graphics, which  are built by experimental datas, at a specific pressure and a well-known  gas density.
  • As gas density increases the temperature of hydrate formation could be higher, once hydration occurs, making its pressure plummet could produce hydrate’s disolution. One of the results of that operation could cause gas losses that are finally vented towards the atmosphere.
  • The elimination of hydrates could really last a longer time than usual and certainly be difficult to attain or achieve. In adittion, microscopic crystals keep long periods after hydrates has been eliminated.

Now, analizing the phase diagram of a typical mixture of water and a light hydrocarbon.

The most important consideration in hydrate formation is that liquid water must be present for hydrate to form. Even with liquid water present a metastable equilibrium can exist between water and a gas condition of pressure and temperature for which hydrate formation could occur. But once seed crystals are formed, hydration occurs readily. Seed crystals start forming at 300 or more psi above hydrate-forming pressure. However, dust or rust particles may act like seed crystals in initiating hydrate formation.

  • The line Q2C  separates the region in which liquid water and hydrocarbon gas exist from the region in which liquid
    HYDRATE PORTION OF THE PHASE DIAGRAM OF A TYPICAL MIXTURE OF WATER AND A LIGHT HYDROCARBON

    water and hydrocarbon liquid exist. None of the phases is pure; all contain slight amounts of the other substance according to their mutual solubility.

  • Point C is the three-phase critical point at which the properties of the hydrocarbon gas and liquid merge to form a single hydrocarbon phase in equilibrium with liquid water.
  • The line Q1Q2 separates the area in which liquid water and hydrocarbon gas exist from the area in which liquid water and hydrate exist. This line represents the conditions at which gas and liquid water combine to form hydrate.
  • Point Q2 is a cuadruple point -four phases are in equilibrium ( liquid water, hydrocarbon liquid, hydrocarbon gas, and solid crystal)-And Besides, so is Q1 because this line represents the line at which ice, hydrate, liquid waterm and hydrocarbon gas exist in equilibrium.

GAS HYDRATE FORMATION CAUSED BY REDUCTION OF PRESSURE

Reducing pressure at normal surface conditions, such as across a choke, causes a reduction in the temperature of the gas. This temperature reduction could cause the condensation of water vapour from the gas. It also could bring the mixture of gas and liquid water to the ocnditions necessary for hydrate formation. The temperature reductions accompanying pressure reductions have been calculated for typical natural gases. These results combined with hydrate-formation conditions, given in the next picture, provide calculation charts giving the maximum reduction in pressure to which a natural gas can be subjected prior to the onset of hydrate formation.

HYDRATE-FORMING CONDITIONS FOR NATURAL GASES

Preventing hydrate formation appears to be the key to the problem. A hydrate prevention philosophy could typically be based on three levels of security, listed in order of priority:

  1. Avoid operational conditions that might cause formation of hydrates by depressing the hydrate formation temperature using glycol dehydration.
  2. Temporarily change operating condition in order to avoid hydrate formation.
  3. Prevent formation of hydrates by addition of chemicals that shift the hydrate equilibrium conditions towards lower temperatures and higher pressures, or increase hydrate formation time (inhibitors).

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