What is a Gas Hydrate? Gas Hydrates refers to crystalline compound that are composed of water any of the following light molecule; methane, ethane, propane, nitrogen. Hydrates formation usually occurs when water molecule exists in the vicinity of these molecules at temperature above or below the ice point and relative high pressure.
Natural gas hydrate commonly called gas hydrates the crystalline compounds formed with water molecule and methane gas exist together at the right temperature and pressure. • The pressure and temperature of most ocean environment offer appropriate condition for methane hydrate stability but considerable amount of hydrate are formed at the continental shelf due to effect of geothermal gradient.
Structure of Gas Hydrate • The cage like structure formed is stabilized by the interactions that exists between the water molecule and methane gas. • Grey=carbon • Green=hydrogen in CH4 • Red = oxygen • White= hydrogen in H2O
Completely combustion of methane gas gives water, carbon dioxide and energy. • This makes natural gas more environmentally friendly than other fossil fuels because more energy is liberated and less CO2 produced.
Gas hydrate can be stored at equilibrium condition with either its saturation temperature or pressure.
Classification of Gas Hydrate. • Biogenic gas is formed at shallow depths and low temperatures by bacterial decomposition of sedimentary organic matter. Biogenic gas is very dry. It consist entirely of methane. • Thermogenic gas is formed by thermal craking of sedimentary organic matter into hydrocarbon liquids and gas. • Thermogenic gas can be dry or can contain significant amount of wet gas.
Why is it urgent to be studied? A future energy source. Climate change It can affect sediment strength, which can initiate landslides on the slope and rise.
A Future Energy Source When hydrate fills the pore space of sediment, it can reduce permeability and create a gas trap. The gas can continually migrate upwards to fill any open pore spaces. This process, in turn, causes the trap to become more effective, producing highly concentrated methane and methane hydrate reservoirs.
Energy stored in methane hydrates range from 350 to 3500 years’ supply
Climate Change Methane from the hydrate reservoir might significantly modify the global greenhouse, because methane is 20 times as effective a greenhouse gas as carbon dioxide, and gas hydrate may contain three orders of magnitude more methane than exists in the present day atmosphere. Because hydrate breakdown, causing release of methane to the atmosphere, can be related to pressure change by glacial sea level fluctuation, gas hydrate may play a role in controlling long term global climate change.
Methane is an environmentally cleaner fuel than oil, coal, or oil shale which all have an immense environmental impact during production and combustion.
Affects sediment strength • Gas hydrate apparently cements sediments, and, therefore, it can have a significant effect on sediment strength; its formation and breakdown may influence the occurrence and location of submarine landslides. • Such sandstones may release methane into the atmosphere, which may affect global climate.
Changes in water pressure due to sea level changes may generate landslides by converting the hydrate to gas plus water, causing significant weakening of the sediment, and generating a rise of pore pressure. Conversely, sea-floor landslides can cause breakdown of hydrate by reducing the pressure in sediments. • These interacting processes may caused cascading slides, which would result in breakdown of hydrate and release of methane to the atmosphere.
Production of Gas Hydrates • Depressurization • Thermal stimulation • Inhibitor Injection
Depressurization involves decreasing the reservoir pressure below hydrate equilibrium, thus allowing the gas to disassociate. • Thermal stimulation involves increase the temperature of reservoir high equilibrium. • Inhibitor injection involves injection of methanol or glycol, into the reservoir to decrease the stability of gas hydrate.
Problems with present day techniques • Thermal injection – Unavoidable heat losses due to host rock, economical infeasibility. • Depressurization – Endothermic nature causing decrease in reservoir temperature. • Inhibitors – methanol and ethylene glycol are expensive chemicals.
Gas Hydrate in India • The first Indian National Gas Hydrates Program (NGHP) explored four areas of Indian Ocean. • Krishna-Godavari • Kerala Konkan • Mahanadi • Andaman Islands
References • Aregba, A.G.(2017) Gas Hydrate—Properties, Formation and Benefits. http://www.scirp.org/journal/ojogas • Natural gas production from gas hydrates–an economic perspective https://www.researchgate.net/publication/228435274