Methane hydrate – a new energy source?
3.7 > In 2011, Japan and South Korea were among the 10 largest net importers of gas in the world, i.e. those countries that must import significantly more natural gas than they can produce or export themselves. Both countries bring the resources in by ship. Gas hydrates in their own territorial waters could offer an alternative.The abbreviation bcm stands for billion cubic metres.
Escape from dependence?The huge size of worldwide methane hydrate deposits is reason enough to make them economically interesting. Methane hydrates are especially attractive for countries with very limited fossil energy resources that must import them at great cost. Japan, for example, meets its energy needs for the most part with oil, coal and gas imports. Japan was a large importer of energy even before the accident at the Fukushima nuclear power plant. Its dependence on imports has become even greater with the shutdown of Japanese nuclear plants after the accident. Energy resources are all transported to Japan by ship, with natural gas taking the form of Liquefied Natural Gas (LNG). Because of the high costs of liquefaction and transport, gas is very expensive in Japan. The natural gas price there is around four times the price in the USA. The situation is similar in South Korea, where over 90 per cent of fossil fuels are imported, including natural gas and particularly coal for the production of electricity. Large consumers of electricity there include for example steel producers as well as the chip and electronics industries. Methane hydrates might also provide a way for other South-East Asian countries such as Taiwan or Vietnam to reduce their dependence on energy imports.
The first steps to methane hydrate productionFor more than 10 years international projects have been studying whether and how methane hydrate might be produced in the future. Scientists must first determine whether it is at all possible to release meth-ane from the hydrates in large amounts and, if so, which methods would be most practical. The production of methane hydrate is fundamentally different from the extraction of oil and natural gas. These conventional fuels flow naturally through the pores of the reservoirs to the well. Hydrates, on the other hand, are solid, and must first be dissociated before the methane gas can be extracted. Three different procedures are being considered for the recovery of methane: WATER CIRCULATION: Hot water is pumped into the methane hydrate deposits through a well, raising the temperature to the point that the hydrate breaks down and methane is released. DEPRESSURIZATION: High pressures prevail in the methane hydrate layers because of overlying water and sediment loads. Drilling into the deposits from above releases pressure like puncturing the inner tube of a bicycle tyre. With the drop in pressure the hydrate slowly dissociates and the methane is released. CARBON DIOXIDE INJECTION: Methane is released from hydrates when they are infused with a gas. Carbon dioxide displaces the methane in the clathrate, replac-ing it in the molecular cage. One result of this is a stronger bond of the water molecule with carbon di-oxide than it had with the methane. The carbon dioxide hydrate is thus significantly more stable than the methane hydrate. Researchers suggest that the carbon dioxide needed for injection could be obtained from the exhausts emitted by gas and coal power plants. Thus the carbon dioxide would not be released into the atmosphere, but transported in liquid form by ship or pipeline to the deposit and sequestered in the hydrates.
- 3.8 > Methane hydrate can be dissociated by pumping in hot water (a) or by reducing the pressure in the well using pumps (b). If carbon dioxide is injected into the hydrate (c), the carbon dioxide molecule replaces the methane. In this case the hydrate does not dissociate.
- Various projects have been carried out by researchers and commercial companies in the past to investigate whether methane can actually be produced on an industrial scale using these methods. Initial production tests were carried out around 10 years ago in the permafrost of the Mackenzie River Delta in northwest Canada by partners from Japan, Canada and Germany. These are considered to be a milestone because important knowl-edge for the future exploitation of methane hydrate was obtained. It was learned, for example, that the depressurization method is much simpler and more inexpensive than flushing with hot water. Additionally, filters were developed and tested to prevent sediments from flowing into the drill hole due to the high pressures. Though sand filters have long been available for use in the gas and oil industry, there has so far been no patent solution for the production of methane hydrates. In 2011 and 2012 a Japanese-American industrial consortium carried out the Ignik Sikumi Project in the permafrost of northern Alaska with support from the United States Department of Energy (DOE). Here, for the first time outside the laboratory under natural conditions, the exchange of carbon dioxide and methane was tested. After only a few days, injected carbon dioxide was already fixed in the hydrate. It was then possible to produce almost pure methane gas for several weeks, and the gas yield was greater than mathematical models had predicted. The first field test in the ocean was finally carried out in early 2013. Through a well in the Nankai Trough, an ocean basin 80 kilometres off the coast of Japan, Japanese researchers retrieved methane up to the surface over a period of one week from a water depth of 1000 metres. The gas hydrate was dissociated through depressurization. Japan has now set a goal to start the operation of a first large pilot production installation in 2018. The necessary technology for long-term operations, however, still has to be developed.
3.9 > In February 2012, using the research vessel “Chikyu”, a Japanese scientific team drilled for methane hydrates south of the Atsumi Peninsula. The following year, for the first time, the ship brought methane up to the ocean surface through a test well nearby.
Getting started is the hardest partRegardless of the method selected for methane extraction in the future, the production rates for all of them depend heavily upon how rapidly the hydrate disso-ciates under the sea floor. Laboratory experiments and test wells in the field have shown that presently all of the methods quickly reach their practical limits or have serious disadvantages:
- Flooding with water requires immense amounts of energy, which makes it uneconomical.
- With depressurization, dissociation of the hydrate decreases over time. This is due to a number of factors. Firstly, the methane gas that forms with the breakdown of the hydrate increases pressure in the deposit, which impedes continued breakdown of the hydrate. Secondly, with the dissociation of the hydrate, water molecules are also released. The deposit thus becomes less saline, which chemically hampers hydrate decomposition. Thirdly, energy is required to break down the clathrate and to destroy the hydrogen bonds between the molecules. Chemically this is known as an endothermic reaction – one that consumes energy. Because this energy is removed from the surroundings in the form of heat, the ambient environment cools down. This cooling down also has a negative effect on the hydrate breakdown process.
- The injection method, on the other hand, proceeds too slowly. Various research groups, therefore, are searching for ways to accelerate the exchange of carbon dioxide and methane. These attempts have led to some initial successes: The exchange of carbon dioxide and methane proceeds more rapidly when the CO2 is introduced into the reservoir as a warm supercritical fluid. In contrast to depressurization, the injection method has the advantage that some heat is released with the exchange of carbon dioxide and methane, which tends to sustain the dissociation process. This method is presently being advanced by German researchers.
Asia is heavily involvedWhich of these methods will be best suited for production at industrial scales in the future is still uncertain. For this reason large amounts of money continue to be spent on research. To date, close to 1 billion US dollars have been in-vested in gas hydrate research worldwide. Japan and South Korea are at the cutting edge. In the coming years these two countries will carry out additional production tests on the sea floor. Significant efforts are also being undertaken in Taiwan, China, India, Vietnam and New Zealand to develop domestic gas hydrate reserves in the sea floor.
Critical point When a gas is subjected to high pressure it normally liquefies. If both temperature and pressure are increased at the same time, however, the gas attains a kind of hybrid state between gas and liquid. Scientists refer to this as the critical point of a gas. At this point the substance is referred to as a fluid. If the temperature or pressure is further increased it reaches the supercritical state, and becomes a supercritical fluid. The supercritical fluid is especially reactive. Supercritical CO₂, for example, reacts intensively with the methane hydrates so that greater amounts of methane are released rapidly.
The search continuesThe present task for the energy industry and research scientists is to thoroughly investigate promising areas of the sea floor for methane hydrate deposits. Regions with favourable pressure and temperature conditions that also exhibit thick sediment packages are of particular interest. Specialists searching for natural resources generally distinguish two distinct phases, prospecting and exploration. Prospecting is the search for unknown deposits. Exploration follows this up with precise investigations and development of the reserves and deposits found. Development can only begin after exploration has demonstrated that sufficient amounts of resources can be extracted. Sites such as the Ulleung Basin off South Korea and the Nankai Trough off Japan have already been extensively explored. Many other areas in the world, such as the Exclusive Economic Zones (EEZ) of China, India, New Zealand or Taiwan are still in the prospecting phase. Prospecting and exploration methods being applied today to investigate methane hydrate deposits include a number of techniques already used in the gas and oil industry, as well as new technology developed over the past 5 years, in part by a German joint project involv-ing around 20 university and industry partners. >