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Scientists study frozen seafloor 'energy' deposits - 17/02/1999

A group of Wellington scientists is studying frozen seafloor deposits that could become an important energy resource in the 21st century.

It is New Zealand first systematic evaluation of the seafloor phenomenon known as gas hydrates - a natural ice-like material made up of methane and water found in ocean sediments worldwide.

Hydrate deposits occur mainly in the top 500m of the seafloor at higher latitudes. Substantial volumes have been mapped on New Zealand continental shelf. They are particularly prominent off the North Island east coast and off Fiordland.

Geophysicist Stuart Henrys, of the Institute of Geological & Nuclear Sciences Limited (GNS), said the two-year study was aimed at better understanding the distribution and physical properties of gas hydrates, the origin of the methane, and ways that individual components change over time.

" Once we have a sound understanding of hydrates we can work on resource characterisation finding out how much is out there and locating the richest deposits.

Dr Henrysteam will be using a combination of geological, geophysical and geochemical techniques to characterise gas hydrates. He said the work fitted in well with GNS other marine activities which included evaluating oil and gas reservoirs, mapping geological structures on New Zealand continental margin, and assessing the mineral potential of the seafloor.

Dr Henrys said conservative estimates were that gas hydrates may hold twice the energy contained in all the world known reserves of oil, coal, and natural gas. The first serious attempts to drill and extract gas hydrates had been made recently by a consortium of Japanese oil companies and Canada national earth science organisation - the Canadian Geological Survey.

Small changes in temperature and pressure on the seafloor can cause hydrates to break down and the methane to be released as gas. This weakens seafloor sediment resulting in sudden slumping possibly triggering tsunamis. The methane released into the atmosphere is thought to contribute to greenhouse warming, he said.

Contact: Stuart Henrys


THE ICE THAT BURNS
A BACKGROUNDER

It looks remarkably like ice, yet it burns when ignited. Until recently, the natural gas and petroleum industries considered it a nuisance something that occasionally plugs up pipelines and causes a headache for drilling operators. But increasingly geologists believe that gas hydrates may be a major energy source in the 21st century, and governments are beefing up research programmes to evaluate its potential.

Gas hydrate deposits occur under the ocean floor worldwide, particularly in cooler latitudes. It is an ice-like material made up of water and highly concentrated methane. With some geologists predicting that oil supplies will start to dwindle in the next 15 years, the prospect of vast new fossil fuel deposits has fired the imagination of energy experts. According to some estimates, the energy locked up in gas hydrates amounts to more than twice the global reserves of all conventional gas, oil, and coal deposits combined.

As Richard Monastersky points out in Science News Online, no-one has yet been able to show that extracting natural gas out of gas hydrate deposits is commercially viable. While hydrates are abundant worldwide, scientists are unsure what percentage of deposits occur in commercially extractable concentrations.

Most gas hydrate deposits sit far offshore in water depths of at least 600m. In this environment, low temperatures and extreme pressure combine to squeeze methane and water into a crystalline structure. Each molecule of methane gets trapped in a cage-like lattice of frozen water molecules an arrangement that concentrates a large amount of methane into a small space. Hydrates also go by the name of clathrates, a term derived from the Latin word for lattice.

The methane in most hydrate deposits originally comes from bacteria living beneath the seafloor. As they consume plant and animal remains in the sediment, the bacteria excrete methane. In some deposits, the source of the gas comes from deeper sediments warmed by the Earth internal heat. Several kilometers below the seafloor, sediment temperature is so high that it cooks the buried organic debris. This produces petroleum and hydrocarbon gases that leak upward toward the seafloor. As the gases reach cooler sediments, they can form hydrates containing a mixture of hydrocarbons.

To find underwater hydrate deposits, geologists rely mainly on a technique called seismic reflection profiling, which is used in oil exploration. Blasts set off near the ocean surface send out sound waves that reflect off geological structures below the ocean floor and then return to the surface where they are recorded. This process sometimes picks up a distinct band in the sediments that runs parallel to the seafloor. Called "bottom simulating reflectors" this band marks the bottom of a hydrate deposit, where bubbles of methane gas in the sediment have become trapped below an impermeable frozen layer.

New Zealand has joined a growing list of countries with their own gas hydrate research programmes. Countries that have substantial programmes include the United States, Japan, Canada, India, Korea, and Norway.

Even though hydrates are uneconomical at present, there are other reasons for researching these deposits. As oil drilling operations move into deeper water, they are encountering hydrate deposits more often, raising safety concerns. Drilling through a hydrate deposit can cause it to break up. Each litre of melted hydrate releases about 160 litres of gas which can explode out of the drill-hole, causing drilling crews to lose control of a well. This can be an expensive problem to solve.

Piercing a hydrate zone can also cause unstable hydrate layers to give way beneath oil platforms. A sudden thawing of hydrate causes sediment to lose strength, resulting in large-scale slumping. Many scientists believe slumping can trigger tsunamis.

Climate researchers are also studying hydrates because the amount of methane they release into the atmosphere can accelerate greenhouse warming. Some scientists cite the breakdown of gas hydrates as the likely reason for the abrupt ending of glacial periods.

Before scientists can evaluate the influence of gas hydrates on global climate change and in altering the stability of the continental slope, they need a better understanding of the physical and chemical properties of hydrate reservoirs. This includes a meaningful estimate of methane content. International research is currently focused in five main areas resource characterisation, production, global climate change, safety, and seafloor stability.

Contact: Stuart Henrys or John Callan