The demand for energy worldwide is increasing day by day. On the contrary, conventional energy reserves are depleting at an alarming rate. This situation mandates unconventional and renewable sources of energy. Though renewable energy resources are gaining importance, these resources will need augmentation.
In recent years, the role of natural gas is increasing in the energy market due to its environment-benign nature over the conventional crude oil and coal. Various natural gas resources are being explored which include shale gas, coal bed methane and gas hydrates. Among these resources, gas hydrates stand at the top as they occur widely (Makogon et al., 2007).
The secrets of water never cease to astonish researchers. Water is a universal solvent, and scientists unravelled how water in contact with certain gas molecules at a very high pressure and low temperature, can form cage like structures which trap gas molecules forming a solid ice like compound. These were named clathrate hydrates or gas hydrates (Sloan and Koh, 2007).
Most low molecular weight gases such as hydrogen, nitrogen, methane, carbon dioxide, and hydrogen sulphide can be trapped to form gas hydrate under varying pressure and temperature conditions. Natural gases such as methane, when trapped in the hydrate cages, are referred to as natural gas hydrates or methane hydrates (Fig. 1). These natural gas hydrates can be dissociated, releasing methane.
Each cage of the hydrate accepts gas molecules of a particular size. Three main types of hydrate structures are known so far. These are structures I, II and sH. Each hydrate system derives its name by the type of gas present in the cage. Natural gas hydrates, if dissociated can produce 163 m3 of gas/m3 of solid hydrate, providing significant amount of energy in the form of methane gas (Sloan and Koh, 2007).
Joseph Priestley in the 18th century and Humphry Davy in the 19th century discovered the gas hydrate phenomenon in which the water forms cages, trapping gas molecules under high pressure and low temperature conditions. Subsequently, Hammerschmidt, in early 19th century, who was working as an engineer with a Texas based natural gas company, found out hydrates blocking up the natural gas pipelines in winters. These gas hydrates were found to be culprits for challenging the flow assurance of oil and gas from the subsea pipelines in upstream oil and gas industry.
In 1981, Makogon (1981) estimated the vast amount of hydrate reserves present around the world and paved the way for further research to produce gas from these reserves. Subsequently, huge amount of hydrate reserves were found on ocean beds and permafrost regions. This is because the ocean depth provides required high pressure conditions along with low temperatures suitable for hydrate formation. Exploration of gas hydrate and polymetallic nodule on deep sea beds are thus areas of active research in the global scientific community (Pic. 1 and 2). Also, in the permafrost regions, lower temperatures are sufficient to form hydrate at atmospheric conditions.
Contingent upon the temperature and pressure, these natural gas hydrates happen at depths ranging from 130 m to 2000 m beneath the ground at permafrost, while it is from 800 m to 3000 m underneath the seabed in offshore continental margins. Studies have estimated that USA has 3,200 to 19,000 trillion m3 of hydrate reserves (Moridis, et al., 2011).
Hydrate Prospects in India
India is expected to have about 1900 trillion m3 of hydrate reserves which is estimated to be 1500 times the known natural gas reserves (Sain et. al., 2012). These reserves have been confirmed in the Kerala-Konkan basin, the Krishna-Godavari basin, the Mahanadi basin and offshore Andaman-Nicobar Islands. These indicate a potential energy resource for India and offers a viable solution to meet the energy demand of India.
In view of this, the Indian government has started the National Gas Hydrate Programme (NGHP) under the ministry of earth sciences, involving National Institute of Oceanography, Goa, National Geophysical Research Institute, Hyderabad and National Institute of Ocean Technology including some of the major national oil and gas companies (Pic. 3).
To fulfil the current and future energy requirements, a huge capital investment is being made on this Programme. Based on seismic observations, gas hydrates were discovered in four offshore locations in the India—a few of them were found accidentally while drilling for conventional oil and gas resources. This confirmed the presence of several gas hydrate deposits in the country, out of which the most notable is the 130 m thick fractured-shale occurrence in the Krishna-Godavari basin (Sain et. al., 2012; Vedachalam, et. al., 2015).
Production of methane from hydrate reservoirs
Methane hydrate deposits can be classified as,
- Class 1 reservoirs with an overlying free gas layer,
- Class 2 reservoirs with an overlying water saturated layer, and;
- Class 3 reservoirs with two impermeable shale layers bounded together.
Another classification which is based on where the hydrate is being formed includes—pore filling type, naturally fractured type and massive/nodule of hydrate deposits. In pore filling type, which is like the conventional oil and gas reservoir, hydrate crystals are present in the pores of the rock such as sandstone and carbonate. In case of fractured reservoir formations, hydrate gets accumulated in the natural fractures, while in the third case, the hydrate deposits are found in loose sand on the surface of seafloor as vast nodules of methane hydrate accumulated in the form of lumps (Kurihara, et al., 2011).
Various methods have been suggested to produce gas from these hydrate reserves. These include, depressurisation (reducing pressure), thermal stimulation (increasing temperature) and chemical injection (using inhibitors). These methods primarily affect the phase stability conditions of hydrate so that they get dissociated. In depressurisation, the thermodynamic state of the hydrate vertically shifts below the equilibrium curve, while in thermal stimulation, it shifts towards the right. In both the cases, the near-hydrate zone conditions fall within the hydrate unstable region thereby releasing methane gas.
In case of inhibitor injection, the equilibrium curve shifts towards the low pressure and high temperature region and hence outside the hydrate stability zone releasing methane. In addition, CO2 sequestration, in which CO2 gas is injected into hydrate reserves, is one of the potential methods for gas production. In upstream oil and gas industry, CO2 is also being used for enhanced oil recovery to improve the recovery of oil from matured reservoirs. In case of gas production from hydrate resources, CO2 can be exchanged with methane gas from hydrate cages. It is a kind of zero energy scheme. This can help in CO2 sequestration which involves capturing CO2 and sequestering it back in the earth’s crust. This method of fixing CO2 has dual advantages. First, it helps in cleaning up the CO2 that has been emitted and second, it stores CO2 in the form of hydrates beneath the earth’s surface where the temperature is very low (Nair, et al., 2016).
It is not only due to its solid state that gas recovery from gas hydrate is difficult, but also due to its occurrence in harsh environments like remote permafrost regions and deep marine environments. From the recent successful field tests in Alaska, northern Canada, offshore Japan, and China, confidence to produce natural gas from gas hydrates with existing production technology has risen. Investigation efforts to confirm the occurrence of economically recoverable gas hydrate in marine settings have shown positive results in Gulf of Mexico, Japan, and South China Sea. This has accelerated the pace of gas hydrate energy-assessment projects. There has been a tremendous development in the field of gas hydrate, which can be mainly attributed to the improvements in laboratory work, numerical-simulation analyses, and national and international collaborative field experiments.
Regardless of its abundance as a promising energy resource, gas hydrate pose many hurdles to drilling and production operations. Methane gas liberated from an unconfined naturally de stabilised gas hydrate reservoir can contribute to the atmospheric methane, a potential greenhouse gas.
The challenge for exploitation of hydrate from ocean environment, particularly from Indian offshore, is that the hydrate reserves are mainly in unconsolidated formation and as part of clay or sand sediments. Hydrate dissociation kinetics depend upon various factors of such complex subsea environment. Simply increasing the temperature and lowering the pressure at production sites would not be sufficient to produce methane from natural gas hydrate. Seawater already contains many dissolved salts which influence the stability of gas hydrates. Their role needs to be understood in detail including various other factors suitable for gas production. Along with it, the impact of various types of porous medium in ocean environment on the dissociation of hydrates needs to be investigated.
Hydrates are indeed great source of energy. They can also show potential impact for various technological advancements in water desalination, refrigeration, carbon capture and sequestration, and gas storage applications. In days to come, hydrate will surely be an integral form of energy resource for humankind.