Expert Knowledge
Gas Hydrates
Gas hydrates are inclusion compounds from the clathrate group in which the host component water binds a guest molecule (e.g. methane, nitrogen) in cage-like crystal structures and precipitates as a solid. They represent a challenging problem in the oil and gas industry. This is particularly true when wet gas or multiphase mixtures of water, gas and alkane mixtures are kept under high pressure and low temperatures.
The formation of gas hydrates can lead to blockages in various conveying systems, such as pipelines, valves or production facilities. The addition of additives such as methanol, glycol or diethylene glycol causes the thermodynamic limit of gas hydrate formation to be shifted to lower temperatures and higher pressures (thermodynamic inhibition).
Background Knowledge Gas Hydrates
What is the clathrate?
A clathrate (lat.: clatratus = lattice) is a inclusion compound of a solid phase which forms a lattice-like crystall structure with cavities. In these cavities a second (or more) liquid or gaseous guest substance gets included. The compound is bonded only via intermolecular forces and not via chemical bonds. This differs clathrates from interstitial solution of metals and nonmetals and from complex compounds.
Clathrates are non-stoichiometric, they do not have a fix composition because not every cavity of the lattice has to be or could be occupied. The structure of the lattice depends on the size of the included guest molecule, the bigger it is the more voluminous the lattice polyhedron has to be. Typical lattice-forming host molecules besides water are urea, thiourea, hydroquinone and other organic molecules.
Clathrates are non-stoichiometric, they do not have a fix composition because not every cavity of the lattice has to be or could be occupied. The structure of the lattice depends on the size of the included guest molecule, the bigger it is the more voluminous the lattice polyhedron has to be. Typical lattice-forming host molecules besides water are urea, thiourea, hydroquinone and other organic molecules.
What are gas hydrates?
Inclusion compounds of gases in a lattice of water molecules are called gas hydrates. The structure of the lattice depends strongly on the included guest molecule, the physical conditions (pressure and temperature) and the chemical conditions (chemical composition of the alloy). The single cells of the lattice form a polyhedron which is assambled from regular polygons. The nomenclature of these polyhedron according to Jeffrey is , where ni is the number of angles of the polygonic face i and mi is the number of faces with ni angles. For example a cell with 12 similar pentagons as faces is denoted as 512.
At gas hydrates three different structures can be found, they are denoted as S-I, S-II and S-H. These structures differ in the number and form of included polyhedrons. In the following grafic the included polyhedrons of one unit cell for every structure are shown. Structure S-I contains for example 2 polyhedrons of the type 512 (=dodecahedron) and 6 polyhedrons of the type 51262 (= hexagonal truncated trapezohedron).
What is Methan hydrate?
Methane hydrate is a clathrate compound of water as lattice and methane as guest molecule. The density of methane hydrate is about 0.9 g/cm3 with a maximum mole ratio of 5.75 : 1 water to methane. One liter of hydrate contains therefore up to 168 liters of methane at standard temperature and pressure. The hydrate forms structures of the type S-I, an unit cell contains 46 water and 8 methane molecules.
Methane hydrate poses a major problem in the extraction and especially transportation of natural gas. The conditions of temperature and pressure inside pipelines mainly in cold areas and submarine allow the formation of hydrates. These hydrate agglomerate after their formation to bigger clusters and could plug valves, pumps and other narrow parts or even the whole pipeline.
Avoidance of gas hydrates with inhibitors
Avoidance of hydrate formation is preferable to removal of excisting hydrate from an economical view and safety concerns. To achieve this, the hydrate formation process can be influenced on several points:
- Dehydration of the gas before transport
- Controlling of pressure and temperature during the transport
- Use of thermodynamic hydrate inhibitors THI
- Use of kinetic hydrate inhibitors KHI
- Use of anti agglomerate AA
Dehydration of the gas
The less water the transported gas contains the lower is the risk of hydrate formation. Natural gas is dehydrated with triethylene glycol (TEG) or with molecular sieves. The chosen desiccant depends of the wanted dew point. Disadvantageous at the dehydration is the extensive needed technic directly or near by the drill hole.
Pressure and temperature control
If the conditions can be held outside of the limits for hydrate formation, wet gas can be transported, too. But it is to be considered, that local pressure and particularly temperature fluctuations can occure. The required technic for temperature control of the whole pipeline or for larger sections is costly, reduced pressure leads directly to lower feed rates.
Thermodynamic hydrate inhibitors THI
With thermodynamic inhibitors the chemical potential between the water moleculs is changed. The pressure and temperature range in which hydrate formation is possible gets shift to higher pressure and to lower temperature. Used as inhibitors are salts (esp. alkali halogenides), alcohols (methanol) and glycols.
The advanages of THI are their proven application, their recycling possibilities, their availabilities and that they works with any type of hydrocarbon.
Disadvantages are the required high volume percentages of THI. For example the ratio methanol to water is between 0.5 : 1 and 1:1. This leads to an extensive logistic support for the transport of THI to the drill hole, the feed rate decreases due to the added THI volume and the recycling of THI is costly. Also lead the chemical properties of the THI to an higher corrosion rate and to higher safety requirements. Too small amounts of THI can support the hydrate formation instead of preventing it.
The advanages of THI are their proven application, their recycling possibilities, their availabilities and that they works with any type of hydrocarbon.
Disadvantages are the required high volume percentages of THI. For example the ratio methanol to water is between 0.5 : 1 and 1:1. This leads to an extensive logistic support for the transport of THI to the drill hole, the feed rate decreases due to the added THI volume and the recycling of THI is costly. Also lead the chemical properties of the THI to an higher corrosion rate and to higher safety requirements. Too small amounts of THI can support the hydrate formation instead of preventing it.
Kinetic hydrate inhibitors KHI
Kinetic inhibition prevents or greatly slows down the formation of hydrates. The inhibitors interfere with the chemical equilibrium of the formation reaction and reduce the reaction rate. They are therefore the opposite of catalysts, which reduce the activation energy of reactions and thus increase reaction rates.
The main components of KHIs are water-soluble polymers. The retarding effect of KHIs is limited to a certain temperature range below the actual formation temperature of the gas hydrates. This sub-cooling can be up to approx. 10 °C. Above this range, inhibition is still possible, but the holding time (delay until gas hydrate formation) is increasingly reduced. Subsea and polar pipelines in particular, where sub-cooling is quite high, are often very long, so that shorter holding times do not guarantee safe, hydrate-free transportation.
Effectiveness of kinetic gas inhibitors
Rocking Cell RC5
Use the Rocking Cell RC5 to investigate the formation of gas hydrates and test the effectiveness of kinetic gas hydrate inhibitors (KHI) and the influence of corrosion inhibitors.
Gas Hydrate Autoclave GH200 + GH350
Use the GHA 200 and GHA 350 gas hydrate autoclaves to investigate the mode of action of kinetic and thermodynamic gas hydrate inhibitors and anti-agglomerates.
Antiagglomerate AA
Antiagglomerants do not prevent the formation of gas hydrates, but rather prevent the coalescence of small clusters of a few cages into larger agglomerates. No deposits are formed, the existing clusters are small enough to be transported in the gas/liquid flow. This relatively new approach to cold flow still requires very extensive development work on the AAs used.
Low Dosage Hydrate Inhibitors LDHI
KHI and AA are combined denoted as Low Dosage Hydrate Inhibitors, LDHI. Advantages of these inhibitors are their low dosage rates in a range of only a few mass or volume percentages what causes lower costs for chemicals, injection technique and logistics. Also higher feed rates can be obtained. Unfavourable is, that the LDHI can only be used under limited conditions and that they mostly have higher environmental risks and therefore require higher safety standards.
Verification in the experiment
Various measurement methods are available for testing the different inhibitors before field trials are carried out.
Sapphire Rocking Cell RCS
The Saphirglas Rocking Cell laboratory device investigates the effectiveness and mode of action of gas hydrate inhibitors. Gas hydrate inhibitors prevent or restrict the formation of gas hydrates.
Do you also want to measure the viscosity or various inhibitors? Or do you have a high measurement volume? Then learn more.
Trial phases for rocking cell tests
A typical test consists of three steps:
- Flowing conditions: The test cells are rocked back and forth at the adjustable rocking rate and rocking angle. During this process, the temperature is adjusted via a ramp or directly to a specified setpoint.
- Shut-in: The test cells are held in the desired resting position (adjustable up to 45° angle). The temperature is set to the specified setpoint.
- Restart flowing conditions: The test cells are moved again at the specified rocking rate and rocking angle.
Removal of gas hydrates
The removal of existing hydrate plug is very extensive in time and money. The hydrates are very stable after their formation and break down only slowly. The easiest way for decomposing hydrates is a pressure reduction and if possible an increased gas or pipeline wall temperature. This process requires due to the hydrate stability a high expentiture of time.
With chemical additives the hydrates can be dissolved but at completely plugged parts the contact between chemical and hydrate is possible only at the surface layer of the plug and the mixture is insufficient.
It must allways be kept in mind that the abrupt decomposition of hydrates release enormous amounts of included gas. The appearing pressure waves can cause great damage at the pipeline and at technical installations up to pipeline raptures.
Artificial gas hydrates
The production of artificial gas hydrates can be done in special autoclaves like the PSL Gas Hydrate Autoclave System. The formation of gas hydrate only occur under specific pressure and temperature conditions (formation envelope). At standard conditions (room temperature and normal atmospheric pressure) the methane hydrate is unstable and it dissociates into water and gas. The released methane forms an inflammable gas-air mixture over the surface, this can be ignited.
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