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A large source of frozen gas lies buried just under the seafloor off New Zealand’s East Coast and could provide a future resource of over eight trillion cubic feet of natural gas, writes Lindsay Clark.
Gas hydrates are a potential huge source of energy. The US Geological Survey estimates the worldwide amount of methane in gas hydrates to be twice the amount of carbon held in all fossil fuels on earth. The frozen energy could also be a potent ocean climate change mechanism functioning as a source, and possibly also a sink, for atmospheric methane that is 20 times as powerful a greenhouse gas as carbon dioxide. Dr Stuart Henrys, a senior research scientist at GNS Science, says that the country possesses a world class gas hydrate province covering an area of about 50,000 square kilometres along the offshore East Coast basin from about a 600 metre depth and below. There is also an area of gas hydrates about 2500 square kilometres known at the south west of the South Island near Fiordland. Dr Henrys says gas hydrates may also exist in other parts of New Zealand’s large offshore Economic Zone. Bubbles of methane gas have been recorded being released from the East Coast sea floor as the Pacific Plate is thrust up along the East Coast and begins its slow subduction under the northern half of New Zealand. A combination of very cold currents near the seafloor flowing from the Antarctic and pressure of seawater above leads to the formation of gas hydrates in sediments under the sea bottom off New Zealand. Henrys says research so far has pinpointed some “sweet spots” where gas hydrates exist in high concentration such as off Cape Palliser in southern Wairarapa and off Cape Campbell in Cook Strait. Hydrates can be mapped out by analysis of bottom simulating reflections in reflection seismic data. High gas hydrate concentrations are predicted to be associated with free gas immediately below. The frozen hydrates layer can also provide a seal for free gas trapped below. Study of the East Coast margin area suggests about foru percent of the whole area is sweet spots (indicated by strong bottom simulating reflections). Assuming the hydrates were present in a 10 metre thick layer with methane saturation of 30 percent of the pore space, this might contain about 8.5 to 21 trillion cubic feet of potentially recoverable natural gas. However Henrys says research carried out last year in the area suggested that these sweet spots may contain much more hydrate than assumed in the earlier calculation. More studies have begun on these sweet spots with 3D seismic surveys and drilling the long-term aim. A big question remains about whether gas hydrates can be technically and economically developed. Although the frozen gas is relatively more accessible lying just below the seafloor, it could prove hard to extract just because it is a solid and is at low pressure. This is unlike natural gas found in deep structures which easily flows through pores in reservoir rocks and is forced up well pipes by the pressure of rocks above. Large quantities of hot water or steam may be required to melt the hydrates to release the gas. Pure methane, the dominant component of natural gas, is also a relatively low-priced commodity, when it is not associated with the high value condensates found in most New Zealand gas fields such as Pohokura, Kupe and Maui. Deepwater developments in the petroleum industry are always very expensive. Gas hydrate fields using current petroleum well techniques would require more wells each producing less gas. Internationally Japan, which has extensive gas hydrates but little oil and gas, is leading research on hydrates development and aims for commercial development by 2017. India too has hydrate resources off its coasts and plans a production test well in 2009. Most testing to date has been in the arctic oil and gas regions of Alaska and northern Canada where hydrates and gas fields have been found together. Mac Beggs, Wellington petroleum consultant, advocates a cheaper way of mining gas hydrates than using traditional petroleum wells. Beggs says that the industry should investigate using subsea offshore mining techniques now being used to mine minerals on the seafloor. Subsea crawlers on the seafloor might use pumps to suck up the gas hydrates and sediment riser tubes to the surface where warming would allow the methane to expand to a gas. A notional gas hydrate development based on a sweet spot 20 kilometres off the south Wairarapa coast might ship the solid hydrate to a shore-based facility where it could be converted to natural gas, Beggs says. Or the hydrate to gas conversion might be done at sea and the gas piped ashore. Henrys estimates gas hydrates research could be handed over to commercially driven investment in the next 10-15 years.
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Scientific studies show that sheets of gas hydrates — made up of ice-like crystals of water and methane molecules intermixed with sediments — are found all the way from offshore Marlborough to offshore Gisborne.