Contract company for operation: J APEX
Location of the drill: Off Omaezaki Spur (About 50 km off from the mouth of Tenryu River, Shizuoka Prefecture), Depth 945 m
Planned depth of the drill: 2,800 m from the sea surface (1,855 m from the seafloor). (All depths are from the sea surface)
Drilling rig: " M. G. Hulme Jr. ", owned by the American company, Reading & Bates Falcon Drilling.
Total budget (planned): 5,000,000,000 yen Position: 34-12-56N,137-45-03E (Tokyo datum) 34-13-08N, 137-44-52E (WGS-84)
November 12-14, 1999: The first pilot hole (Water depth: 945 m, Drilled depth: 1,600 m) was drilled to investigate shallow gas. For the purpose of the prediction of possibly sudden expulsion of gas produced by dissociation of hydrate produced from the heat of drilling, the entry point of the drill hole on the seafloor was monitored by an instrumented Remotely Operated Vehicle (ROV). No escaping gas was recognized during the drilling operation.
November 14 to 16, 1999: The second pilot hole was drilled (Water depth: 945 m, Drilled depth: 1,486 m) was also drilled to check the existence of methane hydrate and to forecast the depth of the hydrate based on the data (resistivity, density, etc.) obtained by LWD (Logging While Drilling). High resistivity was recognized at the depth where methane hydrate was anticipated from analysis of the reflection seismic records.
Nov. 16,1999: The main well was spudded in.
Nov. 19 to Dec. 2, 1999: Conventional coring operations were carried out to obtain cores that might contain methane hydrate. Cores were obtained 5 times between 1,110 m to 1,146 m and 1,151 m to 1,175 m. (Recovery: 35.5 m/60 m, 59 %). Cores using PTCS (Pressure-Temperature Core Sampler, which is designed to maintain the pressure and temperature at the sampled location) were also attempted between 1,254 m to 1,272 m. (Recovery: 5.5 m/18 m, 31 %). The JNOC-developed PTCS and equipment was used for the first time in Japanese waters. However, PTCS cores were not achieved owing to operational difficulties. PTCS coring was attempted 27 times in the interval between 1,175 m to 1,254 m. (Recovery: 29.1 m/79 m, 37 %)
Core samples were obtained from sandy layers distributed in the depth from 1,110 m to 1,272 m from the sea surface by PTCS and other samplers. Based on the analysis of the large amount of gases generated from the samples, anomalous low temperature of the sample and anomalous low chloride ion concentration in the pore water, etc., the existence of the methane-hydrate was proved in three layers (total thickness was about 16 m) between 1,152 m and 1,210 m from the sea surface.
By the time the cores were recovered, no hydrate remained and some of the sediment appeared somewhat distorted, presumably by gas flow and consequent dewatering. Thus, no solid hydrate was recovered. Estimates of gas hydrate volume from all the drilling to date is based on measuring chlorinity and calculating the dilution factor of normal seawater by hydrate dissociation and the volume of hydrate required to provide the dilution effect.
Overall interpretation using core sample analysis data and petrophysical data revealed that gas hydrate occurs in three intervals (net 16 m) at total depths below sea level between 1152 m and 1210 m. It was reported that gas hydrate content within this zone is 20% in bulk rock volume, or about 80% of total pore space, which is almost 10 times more than the samples taken from ODP Leg 164
on the Blake Ridge (Chapters 13, 20, 23). Thus, there is about 525 million m3 per km2 methane in this hydrate (at STP), taking a uniform thickness and bulk composition percentage for the purpose of demonstrating a representative hydrate volume only. These drilling results have yielded information that makes the concept of extraction of methane from hydrate promising.
Although the zone in which hydrate is found is thinner than was hoped, the hydrate concentration in the zone was very rich ~ about 80% of pore space. These drilling results are regarded as being very promising and substantiate further research and development activities.
To date, no clear and unequivocal identification of recoverable reserves has been achieved. Concentrations of hydrate have been identified, but there has been little quantification as yet.
Dedicated geophysical survey and carefully selected drilling locations that will provide calibration of the survey data are required in the 'Nankai Trough' study area and elsewhere.
No dependable and safe technology for producing methane from seafloor hydrate has so far been proven for use in recovering methane from hydrate deposits around Japan.
The hydrate is present in geological horizons that probably have enough strength and porosity to allow for spontaneous gas flow without collapse of geological strata when hydrate is dissociated, so long as pressures are managed adequately.
Because of the relatively shallow water depth, gas pressures produced during dissociation will be relatively high (Dillon and Max, 1998; 1999). At water depths less than 1.6 km (approximately), the volume of methane concentrated in the hydrate is greater than the compression factor of free gas at the pressure depth of the 'Nankai Trough' hydrate deposit. Dissociation will produce substantial overpressures.
The earliest date that commercial production of methane hydrates is estimated to possibly begin, if sufficient hydrate is identified and extraction methods are perfected, is estimated to be about the year 2010.
Commercial recovery of one-tenth of the methane hydrate is currently regarded as being a fully economic target.
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