In Phase 1 of “Japan’s Methane Hydrate R&D Program,” two events were executed. They were 1) the establishment of a method for exploring methane hydrate in the model offshore area, and 2) a survey of the original methane hydrate in place in the eastern Nankai Trough area (the deep water zone spanning from an area off the coast of Shizuoka Prefecture to an area off the coast of Wakayama Prefecture), which was set as the model offshore area.
It has been known the late 1960s to the early 1970s that “lines” appear in the seismic survey records that cannot be explained as an ordinary accumulation of layers.
Since these records appear in parallel to each other at the seafloor, they were called “BSR (Bottom Simulating Reflectors).” Later studies have determined that BSR is a reflector that indicates the existence of methane hydrate.
The following list shows the features of BSR.
- They are sonic reflectors that extend irrespective of the records of the neighboring orderly accumulated bedding plane
- They are in parallel with reflection records at the seafloor
- Their reflectional property is different from that of an ordinary layer reflector (reversed-phase in technical terminology)
The first step in exploring methane hydrate in an ocean is to find BSR by use of the seismic survey method.
In Japan, MH21 summarized the distribution charts of BSR offshore Japan in 2000 in the figure shown below. Based on this chart, “Japan’s Methane Hydrate R&D Program” started in FY2001.
In addition, the results of the FY2003 METI Exploratory Test Wells “Tokai-oki to Kumano-nada,” which was conducted in MH21 Phase 1, started to reveal the relationship between the BSR and methane hydrate-bearing layers.
METI Exploratory Test Wells “Tokai-oki to Kumano-nada”
The following describes the outline of the FY2003 METI Exploratory Test Wells “Tokai-oki to Kumano-nada.”
- The objective was to confirm the occurrences of methane hydrate in Tokai-oki to Kumano-nada (off the coast from Shizuoka Prefecture to Wakayama Prefecture) where its distribution was forecasted.
- The working period was from the end of January to the mid May of 2004.
- 32 wells were drilled at 16 locations in 3 areas in Tokai-oki to Kumano-nada.
- The target water depth was approximately 700 to 2,000 m and the drilling depth was 250 to 400 m from the seafloor.
- Riserless drilling vessel JOIDES Resolution was used for the drilling.
In the METI Exploratory Test Wells “Tokai-oki to Kumano-nada,” surveys were executed in line with the following five items.
- LWD: Logging While Drilling
- Wireline logging
- Coring (Geological sampling)
- Demonstration experiment
- Installation of the geothermometer
Methane hydrate layers and their upper and lower layers were sampled using the existing device (corer) and the PTCS (Pressure-Temperature Core Sampler) uniquely developed in Japan.
MH21 conducted demonstration experiments on basic technologies, including mud technology, cementing technology and horizontal well drilling technology, along with the measurement of various parameters in wells that are needed for the future production tests of methane hydrate.
Existence of methane hydrate stabilizes under a low temperature and high pressure environment. In particular, temperature is the key factor. The temperature distribution inside the methane hydrate-bearing layers was not clearly known to date. Therefore, MH21 has developed a unique optical-fiber distributed temperature sensor, and installed it in the wells after the wireline logging was finished.
Discovery of high grade methane hydrate layers
The wireline logging discovered many layers that exhibit extremely high specific resistivity. The thickness of some layers reached as deep as 105 m. MH21 has conducted coring and sampled the layers with high specific resistivity.
Subsequent analyses have determined that these are methane hydrate layers containing particles of methane hydrate between the sand particles of the sandy sediments. Since the space between sand particles is called the pore, this type of methane hydrate is named “pore filling type methane hydrate.”
In addition, since further analyses detected that methane hydrate layers are mainly contained in the sandy sediments, they are now called “pore filling type methane hydrate layers in sandy sediments.”
The ratio of methane hydrate volume to the volume of pore is called methane hydrate saturation. Detailed analyses found that methane hydrate saturation reaches a maximum of approximately 80%. These layers are evaluated as “high-grade methane hydrate layers.”
Establishment of a methane hydrate concentrated zone exploring method
After the METI Exploratory Test Wells “Tokai-oki to Kumano-nada” were finished, MH21 attempted to establish a method for displaying in three dimensions the spread of the pore filling type methane hydrate layers in sandy sediments, based on a re-examination and re-analysis of the seismic survey data, which was obtained before the METI Exploratory Test Wells were conducted.
Researchers and engineers of geophysics, geology, sedimentology and geochemistry were gathered to take on this big task and finally succeeded in establishing the method.
MH21 decided to call the thick and wide distribution of the pore filling type methane hydrate layers in sandy sediments as the “methane hydrate concentrated zone.” The methane hydrate concentrated zone is considered to be an ore deposit that can be developed using the currently available technologies.
It was the first time in the world for such methane hydrate layers that have potential for large-scale development as the methane hydrate concentrated zone to be found.
It was found from the studies performed so far that the following four indices are indispensable for estimating the distribution of methane hydrate concentrated zones from the seismic survey data.
- Existence of BSRs
- Distribution of turbidite sand and mud alteration layers
- High amplitude reflector
- High velocity anomaly
It is considered that the methane hydrate concentrated zone is a layer that meets the above four indices. Japan is the first country to exhibit the four indices systematically.
For details of the methane hydrate concentrated zone extraction method, refer to the following research papers.
Saeki, T. et. al, (2008): Extraction of Methane hydrate concentrated zone for resource Assessment in the Eastern Nankai Trough, Proceedings of the 2008 Offshore Technology Conference, OTC19311.
Resources assessment in the eastern Nankai Trough
A method for estimating the distribution of methane hydrate concentrated zones, which can potentially be developed, has been established. Thanks to the established method, MH21 is now able to determine the volume of the given methane hydrate concentrated zone.
The following equation is used when determining methane hydrate resources (the volume of methane gas in methane hydrate) in a methane hydrate concentrated zone. This method has been developed by modifying the “volumetric method” for obtaining the volume of oil and natural gas resources to be better suited for methane hydrate.
- Original methane gas in place = Total rock volume x Net/gross ratio x Porosity x Methane hydrate saturation x Volume ratio x Cage occupancy
MH21 discovered more than 10 methane hydrate concentrated zones in the eastern Nankai Trough, which was set as the model area, and evaluated the methane hydrate resources or methane gas volume in methane hydrate layers. The volume is called “original methane gas in place.”
This evaluation is based on a probabilistic approach similar to the evaluation used for oil and natural gas resources. “Pmean” in the following table denotes the most probable original methane gas in place.
MH21 is also determining the volume of the original methane gas in place in methane hydrate layers outside the methane hydrate concentrated zones using its own unique method.
For details of the evaluation of original methane gas in place in the eastern Nankai Trough, refer to the following literature.
Fujii, T. et. al, (2008): Resource Assessment of Methane Hydrate in the Eastern Nankai Trough, Japan, Proceedings of 2008 Offshore Technology Conference, OTC19310.
The volume of methane in methane hydrate concentrated zones is 573.9 billion m³. This volume roughly corresponds to the volume of natural gas consumed for approximately seven years in Japan. The volume for seven years may sound small, but the concentrated zones are ranked the largest among natural gas fields. Considering the small amount of oil and natural gas resources in Japan, it is surprising that such a large volume of methane exists in a limited area of the eastern Nankai Trough.
New BSR distribution chart
The content covered up to this point can be summarized as follows.
- Although BSR indicates the existence of methane hydrate, it does not tell how methane hydrate is stored on BSR.
- Studies conducted by MH21 in Phase 1 have discovered methane hydrate ore deposits with development possibility, which are called “methane hydrate concentrated zones.”
- MH21 has established a method for identifying the three-dimensional distribution of methane hydrate concentrated zones through a seismic survey.
- Using this method, MH21 has found more than 10 methane hydrate concentrated zones in the eastern Nankai Trough.
- MH21 has calculated the original methane gas in place within a methane hydrate concentrated zone in the eastern Nankai Trough using the probabilistic volumetric method that is normally used to assess oil and natural gas resources. The volume calculated corresponds to that of the largest gas fields.
- The original gas in place multiplied by the recovery factor is recoverable reserves. One of tasks of MH21 in the future is to find a production method with a higher recovery factor.
Reviewing seismic survey data obtained in the areas surrounding Japan, MH21 published in 2009 the new BSR distribution chart as shown below.