By Jesse H. Ausubel, Rockefeller University, USA
The cynosure of the Deep Carbon Observatory is naturally carbon. But part of carbon’s brilliance is its ability to store hydrogen. A liter of a typical gasoline contains about 60% more hydrogen than a liter of pure liquid hydrogen. One of carbon’s most amazing feats of hydrogen storage is the reservoir of frozen natural gas or methane hydrates. Methane gas hydrates are solid, ice-like combinations of methane and water stable under conditions of relatively high pressure and low temperature. Widespread in the seafloor, they occur in the continents as well.
Recent estimates of the total amount of methane contained in the world’s gas hydrates range from 0.1 to 1.1 million exajoules, or 3,000 to 30,000 trillion cubic meters. As a point of comparison, annual global energy consumption from all sources is currently about 500 exajoules. A conservative estimate of 5,000 Gt of C in methane hydrates would size this reservoir about the same as coal resources or those of oil plus other forms of natural gas. Gas hydrates may contain a third of the world’s mobile organic carbon.
A new 2-part UN Environment Program (UNEP) report prepared by more than 30 experts from Norway, Canada, Japan, Korea, USA, and other nations on methane hydrates, Frozen Heat, shares current summaries of all aspects of this fascinating mineral. Leadership came from Yannick Beaudoin (GRID Arendal, Norway), Scott Dallimore (Geological Survey of Canada – Natural Resources Canada), and Ray Boswell (U.S. Department of Energy). The report includes superb visualizations. I had the privilege of contributing to sections of the report concerned with the role of methane in the evolution of the energy system. One-paragraph summaries of Volume 1 (on natural science aspects) and Volume 2 (on energy system and engineering aspects and social implications) are below.
The report stimulates opportunities for Deep Carbon Observers, for example, in improving estimates not only of reservoirs but also fluxes of deep carbon. The report also schematizes origins of the methane hydrates as shown in the figure below. Methane produced during microbial (or “biogenic”) and thermogenic decomposition can slowly migrate through overlying sediment with fluids or rise rapidly along faults or other permeable paths. As methane-saturated fluids rise and cool, excess methane forms gas bubbles below the base of gas hydrate stability or “BGHS.” Above the BGHS, excess methane generally forms methane hydrate.
Such a schema is testable with the new mass spectrometry and absorption spectroscopy instruments developed by the DCO community. The DCO community of researchers should be able to resolve definitively the formation temperatures and thus origins of methane hydrates. While humanity may worry about explosive releases of methane hydrates, science will enjoy some of the surprises sure to come from learning more about frozen heat.
Executive Summary: Frozen Heat: UNEP Global Outlook on Methane Gas Hydrates.
Beaudoin, Y. C., Waite, W., Boswell, R. and Dallimore, S. R. (eds), 2014. Frozen Heat: UNEP Global Outlook on Methane Gas Hydrates. Volume 1. United Nations Environment Programme, GRID-Arendal.
Beaudoin, Y. C., Dallimore, S. R., and Boswell, R. (eds), 2014. Frozen Heat: UNEP Global Outlook on Methane Gas Hydrates. Volume 2. United Nations Environment Programme, GRID-Arendal.
Brief summary - Volume 1 (Natural science aspects)
Chapter 1 of Volume 1 describes the crystal structures of gas hydrates, their stability requirements, and the environmental settings in which gas hydrates commonly occur. It also gives estimates of the global quantity and distribution of gas hydrates. These gas hydrate basics provide a context for the central message in Chapter 2: gas hydrates are a key part of the global carbon cycle, storing and releasing vast quantities of methane in response to changing environmental conditions. Chapter 2 summarizes how methane is generated, moved into and out of gas hydrates, and gets consumed. Chapter 2 also discusses the link between gas hydrates and deep marine ecosystems. For example, much of the methane released by gas hydrates into these ecosystems is consumed by microbes in the upper sediment layers and water column and never reaches the atmosphere. Understanding the behavior of gas hydrates over long time periods is an important step in understanding how Earth works. As discussed in Chapter 3, the breakdown of gas hydrates due to natural events, such as long-term increases in bottom-water temperature, could release large volumes of gas from marine sediments, potentially transferring significant amounts of methane into the oceans and, to a lesser degree, into the atmosphere. Chapter 3 considers models of past climate change and future climate conditions and how those models might be affected by potential feedbacks from gas hydrates. It is currently thought that methane from gas hydrates likely contributed to, but did not trigger, past global warming events. Chapter 3 notes that, in the near term, the direct contribution of methane from gas hydrates to Earth’s climate warming will likely be of minor significance. Despite the tremendous quantity of methane contained in gas hydrates globally, only a small fraction occurs in environments that will warm sufficiently over the next century to release methane capable of reaching the atmosphere. A more significant near term result of methane release, particularly in the ocean, may be the oxygen depletion and acidification of the deep ocean that occurs when methane is broken down by microbes. Baseline monitoring studies will be important for understanding the extent of these environmental degradation issues.
Brief summary - Volume 2 (on energy system and engineering aspects and social implications)
The central message in Volume 2 is that gas hydrates may represent both an enormous potential energy resource and source of greenhouse gas for a world with ever-increasing energy demands and rising carbon emissions. Even if no more than a small subset of the global resource is accessible through existing technologies, that portion still represents a very large quantity of gas. The accessible subset could include highly concentrated gas hydrate accumulations in locations where conventional hydrocarbon production is already planned or underway, and more diffuse deposits in areas with strong societal motivations for developing domestic energy resources. To date, a few short-term, pilot-scale methane production tests have been conducted in research wells. The results suggest that larger-scale exploitation may be feasible, but no commercial gas hydrate production has yet occurred. Several nations, however, are currently researching the energy potential of gas hydrates. Recent detailed assessments of the energy potential of methane-gas hydrates concluded that there are no anticipated technical roadblocks to producing gas from hydrate deposits. Ultimately, a combination of technological advances and favorable global/regional market conditions could make gas hydrate production economically viable. Therefore, Volume 2 provides a summary of gas-hydrate-based, energy-related information useful in evaluating future energy resource options. Topics addressed in Volume 2 include a review of likely future trends in energy supply, a characterization of prospective gas hydrate resources, technologies for exploration and development, and the potential environmental, economic and social implications of gas hydrate production.
The full report is available for download here.