Unlocking the secrets of gas hydrates: Bringing the laboratory undersea

By Keith Hester and Peter Brewer

In an effort to better understand gas hydrates and their makeup, scientists and engineers from the Center for Hydrate Research at the Colorado School of Mines and the Monterey Bay Aquarium Research Institute (MBARI) have for the first time obtained data from a Raman spectrometer deployed in the deep sea. With support from NOAA's Undersea Research Program and MBARI, scientists are exploring the composition and fate of gas hydrates in their natural environment using a modified bench-top Raman spectrometer at Hydrate Ridge (off the Oregon coast) and at Barkley Canyon (off Vancouver Island, British Columbia). This research has provided new insights into the stability, structure, composition and heterogeneity of these energy-rich, and potentially unstable methane hydrate deposits found in the deep sea (Figure 1).

Figure 1. Location of Hydrate Ridge off the coast of Oregon (A) and Barkley Canyon off the coast of Vancouver Island, British Columbia (B).

Before this project, experimental hydrate stability studies have been conducted in the laboratory, using synthetic materials and, more recently, naturally formed gas hydrates that have been recovered and returned to shore for analysis. While very valuable, these experimental results are difficult to relate to natural gas hydrate formed in the deep sea. There is a need for technological advances to enable in situ hydrate studies. The development of the undersea Raman spectrometer to directly measure in situ characteristics of natural hydrates begins to fulfill this need.

Figure 2. Gas hydrate from Barkley Canyon. As the hydrate's hydrocarbons that were trapped within the hydrate water cages escapes and burns.

What are gas hydrates?

Gas hydrates look like ice, but their chemical structure is quite different. Under the cold, high pressure conditions of the deep seafloor, carbon dioxide and methane gases interact with water to form crystalline "cages" that enclose and trap the gas molecules. The resulting "gas hydrates" can retain a phenomenal amount of methane -- one volume of solid hydrate breaks down at lower pressure to release 164 volumes of methane gas (Figure 2).

Submarine gas hydrates occur within sediment deposits at appropriate depths along most of Earth’s continental shelves. They can also occur beneath Arctic permafrost. Most known hydrates are white in color. They form as microbial activity changes sediment carbon to methane.

Current estimates of hydrate methane reserves greatly exceed all other fossil fuel reservoirs, representing a vast potential energy resource. Because they contain vast amounts of methane, gas hydrates are an important component of the global carbon cycle, with the potential to contribute large volumes of greenhouse gases and significantly impact global climate change. Because rapid chemical and physical changes in gas hydrates could potentially trigger seafloor collapse, followed by landslide and tsunami generation, they are also considered a potential geohazard. These remarkable deposits have focused the attention and imagination of scientists, resource managers and the public.

Advanced technology development

Figure 3. MBARI ROV Ventana deploying the Raman spectrometer (DORISS) and the micromanipulator (PUP) off the side of the R/V Pt. Lobos in Monterey Bay to measure the composition and structure of natural and synthetic gas hydrates at depth. (larger image)

Raman spectroscopy is a well-established laboratory tool that uses a laser beam to stimulate and measure changes in the characteristic vibrations of individual molecules. Raman measurements yield a distinct spectrum that is characteristic of the chemical species present and of their state (i.e., gas, liquid, and crystalline). The challenge for MBARI engineers was to enclose the Raman spectrometer within a deep sea pressure vessel, while maintaining the ability to focus the laser onto a sample several centimeters away. This delicate instrument package also needed to be robust enough to be launched and carried to the seafloor on an undersea robot (Figure 3). The team was able to overcome all these challenges and still maintain the laser focus on a spot that is only a few microns across. This precise focus allows for very detailed study of the internal structure and heterogeneity of seafloor hydrate outcrops.

Environmental applications

Figure 4. Raman spectra from natural hydrates measured on the seafloor at Hydrate Ridge. (larger image)

Figure 5. Yellow and white hydrates were observed at Barkley Canyon. The Raman probe (PUP) was positioned with the ROV manipulator to conduct in situ measurements. (larger image)

The first in situ measurements were made at Hydrate Ridge off the Oregon coast (Figure 4). At this site, gas hydrates are associated with cold seeps venting mainly methane gas from the seafloor. The Raman instrument determined that the bubbles from the cold seeps contain mainly methane, with small quantities of hydrogen sulfide. With the help of the Raman instrument the science team observed the formation of a hydrate coating around gas bubbles that were moving through the sediment. They discovered that these hydrate-encrusted bubbles can accumulate to form a hydrate bubble-fabric within the seafloor, trapping small pockets of free gas. Disturbance of a large bubble accumulation could destabilize the seafloor.

A second expedition, to Barkley Canyon, off the coast of Vancouver Island, Canada, located massive gas hydrate accumulations, several meters high, exposed on the seafloor at 850 meters depth. This hydrate deposit was discovered by a fishing trawler that pulled up over a ton of yellow, oil-stained hydrate. The yellow, oil-stained Barkley Canyon hydrates differ from common white hydrates of microbial origin. Yellow hydrates are formed from deeply buried organic matter that has been heated and "cracked" at high temperatures, perhaps derived from a petroleum reservoir deep beneath this site. Raman instrument measurements on the seafloor showed that these deposits contain mixtures of yellow and white hydrate. At "Double Mound," yellow hydrate, which overlies white hydrate (Figure 5), has Raman spectra that indicate a significant amount trapped oil within the hydrate structure. In contrast, the white hydrate includes a rich blend of entrapped hydrocarbons, including methane, ethane, propane, and butane.

Conclusion

Figure 6. The research team comprised of scientists and engineers on the R/V Western Flyer with the ROV Tiberon in the background.

The success of this project highlights the need for engineers and scientists with laboratory and at-sea experience to work together to develop new tools in order to solve tough problems (Figure 6). Further hydrate studies are essential to evaluate whether hydrates can alleviate growing energy demands, to understand their potential effects on slope stability and their importance for global carbon cycling, especially for greenhouse gas emissions. More work is needed to understand the complex pathways by which these multi-component hydrate systems are formed. In situ Raman spectroscopy is an important first step in an exciting new future for deep sea hydrate research.