What is the production of methane hydrate?
When using methane hydrate – existing as a solid substance in layers – as an energy resource, we must dissociate methane hydrate to methane gas and water and collect only the methane gas.
This process is called “the production of methane hydrate”or “the methane hydrate production.”
Since methane hydrate is a solid substance, you can think it can be mined as coal. However, it is not efficient to mine methane hydrate because it is contained in geologic layers under a deep ocean.
The production of methane hydrate means dissociating methane hydrate in the layers and collecting the resultant methane gas through wells and production systems.
Once methane gas is generated from dissociated methane hydrate, then the above approach may be used to collect it using a similar principle, methods, equipment and facilities as employed for the development of natural gas.
To dissociate methane hydrate that is stable at low temperature and under high pressure, we must (1) increase the temperature or (2) decrease the pressure.
Therefore, the operation of the “increasing temperature” or “decreasing pressure” of layers bearing methane hydrate is the actual way of the production of methane hydrate.
The production method that involves increasing the temperature is called the “heating method,” and another that involves decreasing the pressure is called the “depressurization method.”
Heating method-based methane hydrate production test
Immediately after Phase 1 of “Japan’s Methane Hydrate R&D Program” started in 2002, the First Onshore Production Test was carried out at the Mallik site in the Mackenzie Delta in the Northwest Territories of Canada.
The Mallik site, which is located onshore, has damp ground in summer and is frozen during winter. In these surrounding areas, the ground is permanently frozen to a depth of approximately 600 m. As the pressure increases, underground temperature becomes lower. Therefore, this region provides methane hydrate with a low-temperature and high-pressure environment with which it can exist stably.
The First Onshore Gas Hydrate Production Test was carried out as a collaborative research project among five countries (Japan, Canada, USA, India, and Germany) and seven research institutes (the former Japan National Oil Corporation – the predecessor of JOGMEC, Geological Survey of Canada (GSC), US Department of Energy (DOE), US Geological Survey (USGS), German Research Center for Geosciences (GFZ), Indian Ministry of Petroleum and Natural Gas – Oil and Natural Gas Corporation (ONGC) in India, and Project of BP-ChevronTexaco Mackenzie Delta Joint Venture).
In this test, the “hot water circulation method” – a type of heating method – was selected for producing methane gas from methane hydrate. In this method, hot water heated up to 80℃ was fed into test wells to heat methane hydrate layers existing approximately 1,100 m below ground so that methane hydrate can be dissociated. The temperature of hot water was estimated to be around 50℃ when it came near the methane hydrate layers.
This test succeeded in producing approximately 470 m³ of methane gas over the five-day production period. This was the first time in the world that anyone had ever produced methane gas from methane hydrate layers.
However, the hot water circulation method and the heating method have to use another form of energy as a means of producing the energy resource, which is methane hydrate. As you can easily imagine, the energy efficiency of these approaches is poor.
In order to come up with a more efficient production method to replace the heating method, MH21 started to probe scientific approaches based on laboratory experiments.
State and physicality of methane hydrate-bearing layers
In order to scientifically conclude which production methods are effective, we must know in detail the physical properties of methane hydrate itself, and the state and physical properties of methane hydrate-bearing layers.
Since methane hydrate is a substance that is stable at low temperatures and high pressure, investigating its state and measuring its physical properties requires us to fabricate, from scratch, a special experimental system capable of observing and analyzing targets with variations in a low-temperature and high-pressure environment.
The establishment of analytical methods was also started from scratch because a method accepted on a global basis was not available.
The Research Group for Production Method and Modeling of MH21 grappled with the difficult task of fabricating the experimental system and establishing the analytical methods, and finally succeeded in establishing world-class analytical technologies.
Measuring unit of micro-focus X-ray CT device (left)
Nondestructive visualization of pore filling type methane hydrate bearing sediments on which coring was conducted in the eastern Nankai Trough (right)
Dissociation behavior of methane hydrate-bearing layers
Although we began to glean knowledge of the state and physical properties of methane hydrate-bearing layers, we must also know how methane hydrate dissociates in the layers.
The Research Group for Production Method and Modeling has manipulated an X-ray CT device normally used in the medical field to enable the visualization of the dissociation processes in the simulated methane hydrate sedimentary layers, and carried out methane hydrate dissociation experiments by simulating various production methods, including the depressurization and heating methods.
The following figure shows how methane hydrate dissociates when the depressurization method is applied to the simulated methane hydrate sedimentary layers. The warm color (red type) indicates the area in which methane hydrate is contained in pores of sand grains, and the cold color (yellow type) suggests an increase in the resultant gas due to methane hydrate dissociation.
Methane hydrate production simulator and depressurization method
Data acquired from the state and physical properties analysis of methane hydrate-bearing layers were loaded into methane hydrate production simulator “MH21-HYDRES,” uniquely developed in Japan. Then, a simulation of the laboratory-level dissociation experiments was carried out to tune the simulator function through a comparison of the result of the simulation with that of the dissociation experiments.
MH21-HYDRES is highly evaluated worldwide for its capability to simulate new production methods from the more than 10 production methods currently available based on the development-level scope and time scales. The production methods include the depressurization method, thermal stimulation method, inhibitor injection method, heterogeneous gas injection method, and a combined method of those.
Using MH21-HYDRES, MH21 searched for a suitable method for methane hydrate production with variations of methane hydrate occurrences and production methods. The optimum production method derived from the above study was the one based on the “depressurization method.”
For details of MH21-HYDRES and the rationales for selecting the depressurization method, refer to the following research papers.
Kurihara, M., Sato, A., Ouchi, H., Narita, H., Masuda, Y., Saeki, T., and Fujii, T. (2009): Prediction of gas productivity from Eastern Nankai Trough methane-hydrate reservoirs. SPE125481. SPE Reservoir Eval. Eng., 12, 477-499.
Field-level verification of depressurization method
The optimum methane hydrate production method derived by the production simulator MH21-HYDRES was the one based on the “depressurization method.”
In order to verify the accuracy of the result of this simulation at field level, MH21 carried out the Second Onshore Gas Production Test at the Mallik site in the Mackenzie Delta in the Northeast Territories of Canada, where they tested the hot water circulation method in 2002.
This testing was conducted twice, once in 2007 and again in 2008. The tests conducted in 2007 and 2008 are called the First Winter Test and the Second Winter Test, respectively.
The depressurization method in the field was executed by draining water in the well using the pump to alleviate the pressure put on the methane hydrate layers. The following figure shows the principle of the depressurization method.
In the First Winter Test in 2007, methane gas was collected from methane hydrate being dissociated with the depressurization method. However, since methane hydrate layers are unconsolidated sediments, sand was also collected (sand production) along with methane gas and water, and the sand stalled the pump. As a result, the test had to stop 12.5 hours after it began.
Although the test was terminated within a very short time, it was the first time in the world that methane gas had ever been successfully collected from methane hydrate layers using the depressurization method.
After developing measures to prevent sand production (sanding), MH21 reattempted the depressurization method-based production test again in the Second Winter Test in 2008. In this test, MH21 achieved continuous production over approximately 5.5 days. The amount of methane gas produced during the test period was approximately 13,000 m³, much larger than the approximately 470 m³ in the First Onshore Gas Hydrate Production Test. It demonstrated that the depressurization method is effective for producing methane hydrate.
The following summarizes the contents and the significance of the Second Onshore Gas Production Test.
Yamamoto, K., and Dallimore, S. R. (2008): Aurora-JOGMEC-NRCan Mallik 2006-2008 Gas Hydrate Research Project Progress. Fire in the Ice, Methane Hydrate Newsletter, National Energy Technology Laboratory, Summer 2008.
Tasks in and after Phase 2
Through the studies and tests in Phase 1, the “depressurization method” is now considered a more suitable method for producing methane hydrate. MH21, however, uses the term “depressurization method-based.”
The production simulation performed by MH21-HYDRES has proved that the depressurization method is superior to other production methods. It is, however, not known whether the monolithic depressurization method (simple depressurization method) alone can ensure cost-effectiveness in development.
The production volume available using the monolithic depressurization method is still small. The task in Phase 2 is, therefore, to increase the production volume by finding something extra to be added to the depressurization method-based production.
MH21 is also required to come up with economical and effective measures for preventing and solving productivity reduction, such as sand production (sanding) encountered in the Second Onshore Gas Production Test.