Promising source of fossil energy

New quest

MSS Murthy discusses the different methods that are used to look for methane hydrate, a potential alternative source of energy

In late July 2016, newspapers carried reports that a team of scientists from India, USA and Japan have discovered a large, highly enriched and one of the most comprehensive methane hydrate deposits in the world along the Krishna-Godavari (KG) Basin along the eastern coast of India. Methane hydrate deposits, found deep in ocean sediments and in permafrost, are increasingly attracting attention as an alternative source of fossil fuel.

An earlier survey in 2006 had revealed the possible existence of large deposits of methane hydrate in four locations along the Indian coastal lines — Goa, Krishna-Godavari and Mahanadi Basins and the Andaman Islands. The present expedition has confirmed the findings in the KG Basin. This is expected to open up a new source of energy to meet the country’s ever growing demands.

What is methane hydrate? On the geological scale, millions of years ago, the decaying remains of plants and animals built up into thick layers, sometimes mixed with sand and silt, on the ocean floor. Over aeons, they sank deeper and deeper. Anaerobic bacteria found in ocean sediments break down the organic matter to crude oil and a mixture of gases like methane, ethane, propane, isobutene, nitrogen, carbon dioxide and hydrogen sulfide. In due course, the gas molecules, being light, work their way up to where it is much cooler.

At a depth of about 500 to 1,000 metres below the seabed, under the influence of low temperature (0 to 20 degree Celsius) and high pressure (about 50 Atmospheres), water crystallises to a cage-like structure around gas molecules forming what are known as ‘gas hydrates’. Since methane forms the major gaseous component of the hydrates, they are also called ‘methane hydrates’. A similar process occurs in the permafrost in arctic regions, about 2,000 metres below the ground surface. Generally, below the gas hydrate stability zone (GHSZ), which contains highly concentrated dense layers of gas hydrates, the temperature is quite high. Therefore, methane, rising from greater depths in the sediment, collects here as free gas.

Thus, in nature, methane exists not only as free gas along with crude oil deposits, but also as solid, icy crystalline structure. In a rich sample, about one litre of methane hydrate holds about 169 litres of the gas. At normal temperature and pressure, methane hydrate separates into water and methane gas, which can be burnt. Hence, the name ‘combustible ice’. It burns, combining with oxygen, releasing carbon dioxide, water and 891 kJ/mole.

Though free methane gas is generally retrieved along with crude oil and used as a fuel in many situations, not much attention was being paid to methane hydrate deposits until recently. Experts estimate that there is two to three times as much carbon stored in methane hydrates as there is in all other forms of fossil fuels such as coal, natural gas and petrol.

Techniques for exploration

Prospecting for methane hydrates under the ocean is done in many stages, using both geophysical and geothermal techniques. The geophysical techniques generally employ seismic methods to characterise the areas under the seabed. Air guns attached to the bottom of an exploration ship produce acoustic waves that penetrate the seabed, where they are reflected differently from different layers.

Acoustic waves pass faster through dense hydrate depths than through lighter free gas zone below. They also reflect less from the hydrate zone than from the free gas zone.

Acoustic receivers mounted on long cables called streamers towed behind the ship record the reflected waves to produce a high resolution image of the sea floor.

The interface between the two zones which results in strong reflection of the acoustic waves is known as the bottom simulating reflector (BSR). Hence, whenever a BSR is detected in the acoustic tests, it indicates the presence of gas hydrates sedimentary layers. Thus, by analysing the reflected acoustic signals, the dense gas hydrate zone and the underling free gas zone can be mapped.

The geothermal methods trace the temperature variations to estimate the depths under the seabed where methane hydrates can exist in stable structures. Once mapping is completed, the prospective zones are drilled to retrieve sediment samples from hundreds of metres from below the seabed for quantitative determination of methane hydrate contained therein. The present expedition, the second in the series, which started in 2015, reported discovering about 1,894 trillion cubic metres of coarse grained sand-rich deposits of methane hydrate in the Krishna-Godavari Basin.

Though extensive methane hydrate deposits have been mapped along the sea coast of many countries, the challenge is to extract the gas safely and economically. Since methane hydrate occurs at low temperature and high pressure, the obvious methods to release the gas are either to raise the temperature (heating method) or to lower the pressure (depressurisation method) to a point where the hydrate breaks down.

However, both these methods have serious limitations and the technology is not yet well established. Hence, the next step in the Indian expedition is to address these questions to develop suitable methods for safe and economical means of extracting methane gas from these newly discovered reservoirs along its coastlines.