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.
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.
First prospecting …The following techniques and measurement tools, both proven and novel, are now being employed to prospect for methane hydrates: COMPUTER SIMULATION: For years now, computer simulation programs have been in use for the production of gas and oil which indicate the marine areas with potential reserves of oil and gas. Calculations by these programs take into account many variables, including the magnitude of plankton sedimentation in various ocean regions over millions of years, the thickness of sediment layers, and the prevailing pressures and temperatures at different depths. The simulations provide initial indications of where further prospecting with research vessels could be worthwhile. Over the past 5 years German scientists, together with a software producer, have expanded a proven and tested computer program used by the gas and oil industry to create a simulation module for methane hydrate. This newly developed module takes into account the special environmental conditions required for the formation of methane hydrate, and provides important clues to undiscovered hydrate occurrences. MULTIBEAM SWATH SOUNDER: This relatively new acoustic instrument can detect methane gas bubbles escaping from methane deposits through natural leaks. It is attached to the bottom of a ship and sends out fan-shaped ultrasound waves. It is thus able to scan a strip hundreds of metres wide on the sea floor. One of the challenges in using this instrument is to separate the reflection signal of the bubbles from numerous inter-ference signals in the depth sounder. Special software has been developed for this purpose by scientists using the system. The swath sounder can be deployed early in the prospecting phase. Methane gas bubbles detected in the water can provide the first indication that methane hydrate is located in the sediments.
- 3.11 > For 3-D seismics, multiple parallel streamers are towed behind the ship. Because the receivers pick up slightly offset signals from different angles, an overall 3-D image of the bottom is produced.
- METHANE SENSOR: Until recently no measurement technique was available for directly determining the concentrations of methane in sea water. Water samples from various depths had to be retrieved by researchers and examined in the laboratory on board. But now there is a submersible mini-laboratory on the market about the size of a roll of wallpaper. It sucks the seawater in and ascertains the methane concentration directly in the ocean. The measurement data are transferred to the ship via a cable. The sensor complements the multi-beam swath sounder because it can determine the deep methane concentrations with much greater accuracy. MULTICHANNEL SEISMICS: Seismic methods use airguns to produce acoustic waves that penetrate into the seabed, where they are reflected by the different layers at different strengths or refracted. Receivers mounted on a cable several kilometres long called a streamer are towed behind the ship and record the reflected waves. The data from all of the receivers (channels) are then processed to create an image of the sea floor. While a spacing of 12 metres between the receivers is sufficient when prospecting for oil and gas, streamers to search for methane hydrate deposits have been developed with receiver spacings of only 1.5 metres. This provides a higher resolution and makes it possible to obtain an image of the sea floor on a finer grid. Multichannel seismics are also employed in the early stages of prospect-ing. They can reveal the presence of the bottom-simulating reflector (BSR). This is a strong reflection of the acoustic waves that is recognized as a conspicuous lighter layer in the seismic image. This effect is seen in different types of sediments. In the case of methane hydrate the strong reflector is produced by free meth-ane gas below the gas hydrate stability zone. Below the GHSZ the temperature is too high for the formation of methane hydrate. Methane gas rising from greater depths in the sediments therefore collects here. Because it has a much lower density than the methane hydrate or the surrounding sediments, it is clearly distinguishable from other layers in the seismic image data as the bottom-simulating reflector.
- 3.12 > For multichannel seismics, airguns generate acoustic waves that are reflected differently by different layers in the sea floor. The reflections are picked up by receivers that are anchored on the sea floor (ocean-bottom seismometers) or towed on a streamer behind a ship. Higher-resolution seismic images can be obtained using deep-towed streamers.
- DEEP-TOWED STREAMER: To achieve a higher resolu-tion of the seismic image, streamers can be towed through the water closer to the seabed, for example 100 metres above the sea floor. The advantage of this is that proximity to the bottom gives the streamers a wider-angle image of the sea bed. This allows them to get a low angle view beneath hard bacterial crusts that form naturally in some marine regions. These bacterial crusts are normally impenetrable for seismic waves. 3-D SEISMICS: At the first indication of possible meth-ane hydrate presence, systems are employed to illustrate the depth and lateral extent of the deposits in the sea floor in three dimensions. For these 3-D systems, a parallel arrangement of several streamers is towed behind the ship. Because the individual streamers peer into the sea floor at slightly different angles, they provide a combined stereoscopic impression. The resolution of systems that have been developed over the past five years is remarkable. They create an image of the sea floor down to a depth of 500 metres in a 3 by 3 metre grid. A reservoir can thus be displayed as a large void. These 3-D methods can furthermore recognize fissures in the reservoir through which methane can escape, and detect large methane gas bubbles in the vicinity of the fissures. In addition, 3-D seismics can provide important information regarding favourable sites to take bottom samples during the subsequent exploration phase.
… then explorationWhether methane hydrate deposits exist at all in an area is first determined during the prospecting phase. When their presence is confirmed then exploration, the detailed study of the marine area, can begin. With exploration methods it is possible to assess fairly accurately how much methane or methane hydrate is present in a deposit. The following techniques and devices are presently being used: CORING: A classic method in the exploration of mineral resources is the drilling of cores. With a drill string lowered from a research ship, sediment cores are re-trieved from hundreds of metres below the sea floor. These long cores, with the approximate diameter of a rain gutter, are cut into a number of metre-long sections on board the research vessel and studied later in a labor-atory on land for the presence of methane hydrates. Special drilling tools that can maintain the high pres-sure as the methane hydrate sample is brought to the surface prevent dissociation of the methane hydrate until it is possible to analyse the core. OCEAN-BOTTOM SEISMOMETER: Ocean-bottom seismometers (OBS) function like conventional seismometers. The receivers, however, are not attached to a streamer but are stationed on the sea floor. This allows greater observational depth coverage. Acoustic waves travel through strata at different speeds depending on their densities. The waves accelerate in dense structures such as methane hydrates, but propagate more slowly through less dense structures such as muddy sediments or gas voids. The ocean-bottom seismometer system calculates an image of the sea floor from the lag of reflected waves. Because the instruments can detect at greater distances than a streamer, they can record signals from greater depths. The present record is 12 kilometres. Ocean-bottom seismometers will be deployed off Korea in 2014.
- 3.13 > Clump of methane hydrate in a drill core.
- ELECTROMAGNETICS: For the past ten years electromagnetic systems have also been employed by the gas and oil industries. These transmit electromagnetic impulses similar to those of a radio station antenna. Like acoustic waves for an ocean-bottom seismometer, different bottom structures change the electromagnetic signals to a greater or lesser extent. The physical principles of the two are not the same, however. This system takes advantage of the fact that different substances conduct electromagnetic impulses with varying levels of efficiency. Poorly conducting substances produce a resistance. Liquids, on the other hand, such as water, are very good conductors. The system very accurately senses these differences in conductivity or resistivity in the seabed. It is therefore possible to determine, using electromagnetic techniques, how much free methane gas is located below the GHSZ or how much is contained in the hydrates. The method, however, has disadvantages. For one, electromagnetic waves propagate in a circular pattern, in contrast to the directional explosion of the airgun. The conductivity values, and thus the methane deposits, are therefore difficult to pinpoint. Furthermore, the electromagnetic impulses weaken rapidly, so they cannot penetrate as deeply into the sea- bed as sound waves. In the past five years a mathema-tical technique has therefore been developed to combine the electromagnetic and seismic techniques. This method, called joint inversion, takes advantage of the strengths of both methods: the very high spatial resolution of the ocean-bottom seismometers and the precise conductivity values of the electromagnetic system, which provides information about the methane content. Much better characterizations of methane hydrate deposits can now be made than in the past, thanks to joint inversion methods. The joint inversion method will be used off Taiwan starting in 2014 to investigate the formation of gas hydrates there. Taiwan is especially interesting because it is located at a subduction zone where methane-rich water is squeezed out of the sediment. Even today it is still not known how much methane is released at subduction zones. This inhibits assessments of the total amounts of hydrates existing worldwide. A detailed analysis of the subduction zone off Taiwan and the amounts of meth-ane released there could thus help to make more accurate estimates of occurrences in the future.