Environmental fields studied by MH21
“Japan’s Methane Hydrate R&D Program,” announced in July 2001, describes the following statement with respect to the environmental study.
“Establishment of a development system complying with the environment”
Considering “methane hydrate development and environmental assessment are inseparable,” MH21 focuses on environmental studies necessary for the commercialization of the development program. In addition, MH21 is a group consisting of methane hydrate specialists. It is one of the duties of MH21 to explore globally-published literature on natural events with environmental risks related to methane hydrate, and provide explanations regarding pertinent risks to the general public from a specialists’ standpoint.
Methane hydrate development and environmental impact
What kind of environmental risks will be induced by methane hydrate development? When we think about it, the following three points must first be determined.
- Methane hydrate layers existing in what conditions should be developed? (methane hydrate existence conditions)
- What methods should be used to produce methane gas from methane hydrate? (production methods)
- What equipment and facilities should be used for development? (development system)
The above three issues were included as study targets for Phase 1 when it was launched in FY2001, therefore, these questions had not been answered before Phase 1 was finished. In Phase 1, emphasis was placed on the following basic environmental monitoring-related studies.
- Environmental study of the ocean areas of the eastern Nankai Trough, which was selected as the model area
- Development of sensors to measure the anticipated environmental risks
- Simulations to predict the severity of the anticipated environmental risks
Environmental assessment in Phase 2
Thanks to the Phase 1 studies, MH21 learned the following, which were unknown at the time Phase 1 began:
- Occurrences of the methane hydrate targeted for development
- Production methods
- Development system
Methane hydrate concentrated zones consisted of methane hydrate existing in the sandy sediments
Depressurization method-based approach
Using the existing equipment and facilities for oil and natural gas development, methane hydrate development is possible with little or no modifications required.
From the above mentioned information, MH21 came to be able to forecast to some extent the possibility and scope of the potential environmental risks that can be induced by methane hydrate development.
(1) Leakage of methane gas from the seafloor
In the depressurization method, the pressure is low in the production well, and it is high in the layers. Consequently, methane gas dissociated from methane hydrate flows into the well.
When heat is applied, as in the heating method, pressure in the layers increases, thereby potentially allowing methane gas to move into various directions. The depressurization method, however, is not subject to this problem. Thus, the environmental risk of methane gas leakage from the seafloor during production is estimated to be minimal.
(2) Seafloor subsidence
In the methane hydrate concentrated zones targeted for development, methane hydrate is contained in sandy sediments, which means small methane hydrate particles fill the pore spaces between the sand grains in sandy sediments.
The backbone structure of sand grains remains almost unchanged after methane hydrate is dissociated. Therefore, ground subsidence that can seriously affect the deep-sea area where methane hydrate exists will not likely to occur.
(3) Submarine Landslide
It is known that ground characteristics such as the strength of methane hydrate-bearing layers change if methane hydrate is dissociated. However, no conclusion has yet been drawn anywhere in the world as to whether methane hydrate dissociation may possibly trigger landslides.
Landslides occur on inclined seafloors and do not occur on areas of the seafloor with virtually flat topology. MH21 considers that it should carry out tests and development on seafloor topology where landslides are unlikely to occur, until the relationship between methane hydrate dissociation and landslides is clarified.
There are many such places in the eastern Nankai Trough, which is the model area of MH21.
(4) Processing of production water
The dissociation of methane hydrate produces water as well as methane gas. The depressurization method produces water, as a result of methane hydrate dissociation.
The volume of the production water is estimated to be significant and it needs to be processed. Potential processing methods include transporting the production water to land, discharging it into the sea, and injecting it into the layers. Cost-wise, the best method is to discharge the water into the sea. When future methane hydrate development is taken into consideration, the best way would be to discharge the water into the sea.
Since methane hydrate-bearing layers are underwater sedimentary layers, the water contained in the layers was formerly sea water, and its components are almost the same as that of the water that exists today. It is also known that methane hydrate layers in the eastern Nankai Trough do not contain oil or heavy metals. Therefore, discharging the production water into the sea will not impact the environment.
However, the salinity concentration level and the temperature of the production water are estimated to be lower than that of marine water. In Phase 2, MH21 is going to study whether or not the environment will be affected when production water is discharged into marine water, and what processing should be carried out at the production facilities before production water is discharged.
Studies in Phase 2
As described above, environmental risks arising from the currently planned methane hydrate development are considered to be small. However, MH21 is determined to confirm through the Offshore Production Test whether the anticipated environmental impacts will actually occur, and if they will, what will be the scale of their occurrence.
Two Offshore Production Tests are scheduled in Phase 2 and these tests include the monitoring of the above mentioned environmental risks. The period of Offshore Production Tests will be very short and the scale of environmental risks will be minimal. However, we will pay attention to minute changes and the findings will be used as basic data for methane hydrate development.
The following shows the schedule of environmental studies in Phase 2.
Although methane hydrate-related research and development is carried out in many countries, systematically scheduled environmental studies for the development as described above are carried out only in Japan. MH21 is promoting a challenging task that is unprecedented in any part of the world.