New technology employed in offshore Japan’s Nankai Trough has helped recover and preserve rare subsea gas hydrate samples (pressure cores) and keep them in their natural and stable condition for scientific analysis.

An instrument called the Instrumented Pressure Testing Chamber (IPTC), part of a suite of Pressure Core Characterization Tools (PCCT), is the first device to measure the properties of gas hydrate core samples without depressurizing them, which is one of the keys to understanding the highly unstable compound.

During pressure cores sample production testing, the cores were depressurized to analyze the methane hydrate and the release of natural gas. Scientists believe that the gas volume and the production rate results will help them predict the potential behavior of the reservoirs during a planned field test. In a recent test, scientists were able to measure bioactivity, strength, compressability, gas and water permeability and sediment response during gas production.

The Nankai Trough test site is on the north slope of Daini Atumi Knoll off Atsumi and Shima peninsulas, where water depth is approximately 3,300 feet.

Results from the testing, according to a U.S. Geological Survey (USGS) report titled Groundbreaking Gas Hydrate Research, “advance understanding of the global distribution of gas hydrates as well as whether and how methane contained in gas hydrates can be used as a viable energy source.”

Why Study Gas Hydrates?

Gas hydrates are common in many parts of the world, including Arctic permafrost regions and in relatively shallow sediments in the ocean's deep waters, and are stable in these environments. Gas hydrates form under high fluid pressure and low temperature, where biogenic or thermogenic gases are available.

Gas hydrates are a naturally occurring, solid form of methane gas combined with water. They hold large amounts of methane, making them a potentially significant source for natural gas.

What makes them so tempting is that the estimated worldwide resource potential of methane in gas hydrate exceeds the combined worldwide reserves of conventional oil and gas reservoirs, coal and oil shale. A May 2010 Congressional Research Service report for Congress stated that if 1% of the estimated gas hydrate resources could be developed, it would provide enough natural gas for U.S. needs for approximately 100 years.

Recent Energy Information Administration (EIA) global estimates place the gas hydrate volume in ocean deposits from 30,000 trillion cubic feet (Tcf) to 49,100,000 Tcf. For the continental U.S., gas-hydrate deposit estimates range from 5,000 to 12,000,000 Tcf. Comparatively, current worldwide natural-gas resources are about 13,000 Tcf and natural gas reserves are about 5,000 Tcf.

A 2008 USGS assessment indicates that Alaska’s North Slope region alone may hold 25- to 158 Tcf of undiscovered, technically recoverable gas-hydrate resources.

In 1995, the first dedicated scientific effort to investigate marine methane hydrate was carried out by the Ocean Drilling Program (ODP), with an expedition to offshore South Carolina’s Blake Ridge area. Scientists drilled through prominent bottom simulating reflections (BSRs), which were observed in seismic profiles and thought to be indicative of hydrate in the subsurface. Scientists found lower than expected concentrations of methane hydrate in the Blake Ridge area.

This was followed by additional field testing in 1998 and 1999, when methane hydrate research wells were drilled at the Mallik site in the Canadian Arctic and in the Nankai Trough off the coast of Japan. Wells drilled at these sites showed high concentrations of methane hydrate, with saturations as high as 80% in Nankai Trough sandstones. This contrasted sharply with the dispersed, low-saturation accumulations identified in the Blake Ridge area.

How Could Gas Hydrates Be Produced?

Currently there are no active commercially producing gas hydrate wells, but work to develop new technologies and modify existing technologies for this unique resource is ongoing.

A thermal stimulation method is being researched that would use hot water, steam or other heated liquid. The heated fluid could be injected into to the hydrate stability zone which would raise the zone temperature and the hydrate would begin to decompose and the freed gas would move toward the well bore. Additionally, a chemical stimulation method would use a liquid-inhibiting chemical such as glycol.

Gas hydrates do pose uncommon challenges. Whether onshore or offshore, an uncontrolled gas release caused by a heated drill bit or by the rapid pressure decrease in a penetrated zone or reservoir could be a hazard.

Technology to create a downhole stability zone to overcome mechanical and chemical obstacles is under development and is outlined in a 2010 EIA report. Because of the thermodynamic instability associated with gas hydrates, one possible fix would be to create a stability zone beneath a pressurized zone, where pressurized gas could flow or be contained. This could cause the hydrate at the base of the zone to decompose and allow freed gas to flow towards the well bore.

International Gas Hydrates Project Team

The ongoing program is being conducted in Japan by a team of U.S. and Japanese researchers, including the USGS and Georgia Tech.

The program is being led by the Japan Oil, Gas and Metals National Corp. (JOGMEC) and Japan’s National Institute of Advanced Industrial Science and Technology (AIST). The project is being conducted in collaboration with the USGS Gas Hydrates Project and researchers from the School of Civil and Environmental Engineering at Georgia Tech. This project is one component of an ongoing Japanese collaboration on methane hydrate research with the U.S. Department of Energy (DOE) and the Gulf of Mexico Gas Hydrate Joint Industry Project (JIP).