Gas Hydrates, once thought to be structures blocking the flow in crude oil pipelines, are now proven to be large sources of natural gas. Here is a peek into research on gas hydrates and their applications.
Water – one of the basic necessities for life, holds secrets that never cease to astonish researchers. Its liquid form is denser than the solid form. It expands both when heated to 40C and cooled to 40C. It is a universal solvent, issolving a large variety of substances. In addition to all peculiar qualities of water, researchers have now understood that water at freezing temperatures and high pressures can store certain gases too. Dr. Jitendra Sangwai, a Professor at the Department of Ocean Engineering, IIT Madras has done research that reveals this unique aspect of water.
Water at very high pressures (70 to 80 bar) and chilling temperatures (-100C to 00C) turns into a new form of ice-like crystalline structure. This crystalline structure has small gaps in it making room for gas storage. This crystalline structure together with gas is known as a clathrate hydrate or a gas hydrate. A gas hydrate then, is just a cage made of water. Most low molecular weight gases such as hydrogen, oxygen, nitrogen, methane (natural gas), carbon dioxide, and hydrogen sulphide can be trapped inside the cage, but each gas needs different pressure and temperature conditions for this to happen. Each hydrate derives its name from the gas present in the cage. So if methane is present in the cage, then it’s called methane hydrate. With this interesting property of water, harmful gases can be stored or useful gases present in the existing hydrates can be extracted.
The property of water transforming into cages and trapping gases was discovered by an English chemist, Joseph Priestley in the 18th century and by another English chemist, Humphry Davy in the 19th century independently. But attention was drawn towards it in the 20th century when EG Hammerschmidt, an engineer working in a Texas-based natural gas company found that these hydrates were blocking natural gas pipelines in winters.
Clathrate hydrates were looked upon as obstacles to the flow of natural gas in pipelines until a Russian petroleum engineer turned professor, Yuri F Makogon discovered in 1966 that clathrate hydrates can be used as source of energy by extracting the natural gas that has accumulated inside them. By 1981, he had estimated the amount of natural gas in hydrates present worldwide and paved the way for present researchers to find ways to extract it and understand the behavior of hydrates.
The oceans are a conducive environment for the formation of methane gas hydrates. At the depths where hydrates are found, pressure is very high due to the sheer height of water above, and the temperature is low as the sun’s rays can’t penetrate such depths. Vast reservoirs of methane hydrates are found in marine sediments, at depths greater than 500 meters, close to continental margins and in onshore permafrost – soil, rock or sediment that is frozen for more than two consecutive years. Availability close to continental margins means reduced extraction and production costs, without having to spend on deepwater drilling which is a costly and risky afair. This is a boon for India, since it has a very long continental margin. In India alone, the natural gas present in methane hydrates is estimated to be about 1890 trillion cubic meters – 1500 times the currently known natural gas reserves from other sources.
“Natural gas hydrates ofer a realistic solution compared to other polluting fossil fuels,” says Prof. Sangwai. Methane, which is present in the cage-like structures of methane hydrates, can be exchanged with the greenhouse gas CO2 produced by burning the methane. This is kind of a zero carbon energy scheme. This helps in CO2 sequestration –capturing CO2 and burying it back in the earth as part of the hydrate. This method of fixing CO2 has two advantages. One is cleaning up the CO2 that has been emitted and the other is that it is unlikely that the CO2 stored in the form of hydrates will come back to the Earth’s surface.
While drilling, CO2 is injected into the well for recovering the oil trapped in the tiny pores of rocks. The injected CO2 pumps out the trapped oil and, in the process, can also be consigned to the cage of the hydrates. This idea led researchers to the concept of flue gas separation. The drilling environment is a dirty place, emanating many dangerous flue gases like oxides of carbon, nitrogen and sulphur. Gas hydrates can save us here. If the pressure and temperature at which flue gases form hydrates are known, then by sending in water at that precise temperature and pressure, one can trap the gases in the cage-like structures of the hydrates.
Gas hydrates ofer an alternative to the high expenses of transporting and storing Liquefied Natural Gas (LNG). LNG infrastructure oten adds to the cost of importing as it needs a special type of floating tanker and heavy refrigeration facilities whereas gas hydrates need very minimal storage space – 1 cubic meter of methane hydrate can store 170 cubic meters of methane. Transporting hydrates is quite simple – it can be done using existing pipelines, with the hydrates in the form of slurries.
Another potential application of gas hydrates is employing them in desalination. Here, seawater is taken in a chamber and CO2 gas is passed it at very low temperature and high pressure resulting in the formation of CO2 hydrates. By taking the formed CO2 hydrates into another chamber and dissociating them by increasing the temperature and decreasing the pressure, pure water that is free from salts can be obtained. This happens because salts cannot form hydrates. Desalination using this technique is considerably cheaper than other conventional techniques.
The problem is that little is known about the stability of the structure of the cages which form at different combinations of pressures and temperatures. The environment where hydrates are found is very harsh. It is not possible to obtain 100% pure methane hydrates. Seawater already contains many dissolved salts that inhibit the formation of the gas hydrates by taking away the required water. Effects of these salts on gas-hydrate formation are not completely understood. Further, the impact of different types of porous medium in the ocean like silica gel, silica sand, activated carbon on the formation of hydrates are to be studied.
Apart from understanding what promotes the formation of hydrates, the study of substances which dissociate these hydrates are quite important. Gas hydrates are notorious for flow path blockage of pipelines in the oil and gas industry, which is how they were initially noticed. “Both promoting and disassociating of hydrates are to be mastered,” says Dr. Sangwai.
Dr. Sangwai and his team have been working with a variety of additives to improve the stability of gas hydrates at different temperatures and pressures. Rather than conducting experiments piecewise, which is expensive and time-consuming, they are developing a model to predict their behaviour.
Additives are of two types. They can either promote or inhibit the gas hydrates formation. Inhibitors help in freeing the pipelines, and promoters help in trapping gases in their cages. Additives can be classified on how they work with hydrates. Certain additives that tweak the temperature and pressure conditions at which hydrates form or dissociate are known as thermodynamic additives. Other additives that do not affect the temperature and pressure conditions but still affect the formation of these hydrates and are known as kinematic additives. New hybrid additives are emerging which serve the purpose of both thermodynamic and kinematic additives.
Safety is paramount while extracting methane hydrates. During the drilling operation of hydrates, if huge amounts of methane gas are released suddenly from the drilling site, an explosion under water is highly probable. To prevent this, a cheaper – highly volatile methanol is used as drilling fluid. This acts as a thermodynamic inhibitor. Prof. Sangwai and his team are looking for options to replace the highly volatile methanol with polyethylene glycol.
If gas hydrates have so many advantages, then why aren’t we seeing natural gas extracted from methane hydrates in the mainstream industry? The main problem is that the transportation of large amounts of water from the recovery site to the extraction site is a costly afair. Moreover, formation of gas hydrates is a very slow process which forms a bottleneck in the supply chain. The extra transportation costs coupled with the kinetically slow process has proven to be one of the deterrents in adoption of this technique by the industry, which is why LNG still dictates the natural gas industry. Dr. Sangwai states cheerfully, “This is where we come into the picture.” Gas hydrates can be made competitive by minimising the transportation between recovery site and extraction site. Dr. Sangwai’s aim now is to develop new kind of additives that will reduce the cost of extracting methane from methane hydrates.
When asked about how he decided to work in the field of gas hydrates, Dr. Sangwai says “Right ater my PhD, I wanted to work in the area which will be the future of oil and gas industry. I believe gas hydrates will be our future energy sources.” With pilot production already started in Japan, though at a slow pace and small scale, we can expect the production of natural gas from gas hydrates soon to begin in India.
Dr. Jitendra Sangwai is an Associate Professor in the Department of Ocean Engineering at IIT Madras. He received his PhD in Chemical Engineering from IIT Kanpur and is the founder of Gas Hydrate and Flow Assurance Laboratory at IIT Madras. He holds eight patents in the field of gas hydrate, enhanced oil recovery and flow assurance. His research interest lies mainly in the field of gas hydrates, enhanced oil recovery, rheology of drilling fluids, flow assurance, and polymer and nanotechnology applications for upstream oil and gas engineering.
Nikhil Mulinti is a final year Dual Degree (B.Tech. – M.Tech.) student in the Department of Ocean Engineering at IIT Madras. He is fond of science and his fascinations range from the cosmology to anthropology. He is currently working on the bubbly flow technology, a trending research area in marine hydrodynamics. For any comments or criticism, the easiestway is to drop a mail on firstname.lastname@example.org.
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