Massive global reserves of natural gas hydrates play a huge role in the carbon cycle and could be a bridge-fuel to renewable energy sources. Now a new generation of sophisticated models are offering new insights into how they are deposited in nature. A recent article in Reviews of Geophysics assesses two decades of methane hydrates research and suggests new avenues of research. Here, the article’s lead authors answer questions about why studying natural gas hydrates is more important than ever and how scientists hope to answer future questions about this mysterious substance.
What are natural gas hydrates and where are they found?
Natural gas hydrates are an ice-like solid composed of water and gas, most commonly methane. They only form at high pressure and low temperatures, in places where both water and gas are plentiful. Small changes in temperature and pressure can cause gas hydrates to abruptly separate into water and gas, which means they are very difficult to study.
Such conditions are surprisingly common in nature; for example, within sediment layers along the world’s continental margins, and within and beneath Arctic permafrost.
The most interesting gas hydrate deposits are those found within the pores of coarse-grained, sandy sediments or squeezed into rock fractures. These deposits have the highest concentrations of gas hydrates and are of particular interest because of their energy potential.
Why do we need to better understand the presence and behavior of gas hydrates in the environment?
Natural gas hydrates, which are derived from naturally occurring gas hydrocarbons, are an important part of the carbon cycle. Estimates vary, but they are thought to hold between 5 and 22 percent (approximately 500 to 2,500 Gigatons) of the Earth’s total organic carbon.
Methane itself is a powerful greenhouse gas, with a warming effect nearly forty times that of carbon dioxide. Methane escaping from large natural gas hydrate deposits has been linked to periods of past climate change.
Methane hydrate is energy-dense: at atmospheric pressure, each unit of frozen methane hydrate can produce 164 units of natural gas. Their abundance in nature means that methane hydrates represent one of the largest known unconventional energy sources – and a much cleaner alternative to crude oil or coal.
Methane hydrate deposits in the Gulf of Mexico alone have the potential to power the United States with natural gas for hundreds of years.
Important questions remain, however, because we do not fully understand how gas hydrates are distributed in nature and how these deposits evolve.
What is the best way to categorize the many different types of gas hydrate deposits?
Natural gas hydrate deposits can be categorized into several types depending on where they are found, how they form, and their physical properties.
Low concentrations are found almost ubiquitously throughout the first few hundred meters beneath the seafloor around continental margins.
If you were to dig into these muddy sediments you would find tiny pockets of methane hydrates in the microscopic pores between sediment grains or within thin, vein-like fractures in the rock.
Large fractures connecting the deep subsurface with the seafloor can also be rich in gas hydrates. At vent sites on the seafloor, methane gas escapes these fractures and seeps into the ocean, typically as a visible stream of bubbles.
Extraordinary concentrations (greater than 90 percent of the pore space) are found in coarse-grained, sandy sediments.
Has there been effective reconciliation between field observations and models?
Partly. Our review identifies six models explaining the presence and formation of different types of natural gas hydrate deposits. Each model differs according to where the gas comes from and how it is transported into the deposits.
Regardless of which model you subscribe to, we know that most natural gas found in methane hydrates is produced by microorganisms consuming organic matter. Scientists have long known that microorganisms living in the shallow, muddy subsurface produce methane, but this can account for only very low hydrate concentrations.
The exception lies within thin layers of coarse-grained sand, where, over time, hydrates collect through simple diffusion.
Lab experiments have shown hydrates migrating into centimeter-thick sand layers, leaving little or no hydrates in the surrounding muds. This matches well with observations of this kind of deposit.
We also expect to find moderately high concentrations deeper in the Earth. As sediments sink, the icy hydrates they carry melt, releasing their gas. This gas bubbles back up to form more gas hydrates just above the boundary, where they melt.
A lack of field observations means that we don’t fully know what goes on at this boundary, although emerging research could help quantify what we can expect to find.
The most exciting advances have come from recent models showing how methane flows as a free gas – rather than dissolved in water – through the deep subsurface. Free gas flow allows the low concentration methane produced by microbial biogenesis to be concentrated into high concentration deposits. These models have changed our thinking about how highly concentrated deposits form and so far, are supported by evidence from the field.
No single model perfectly matches observations, however, and often one type of hydrate deposit can usually match several formation models.
What are some of the unresolved questions where additional research, data or modeling is needed?
The methane in most natural hydrates comes from microbial biogenesis as microorganisms consume organic carbon found in sediments. However, our understanding of this process is still evolving. Under what conditions and how quickly can microbial organisms generate methane? Does most of it occur in cold, shallow, muddy sediments or deeper in the Earth where it has been heated for longer?
These questions are important because biogenic gas is already showing potential as a critical energy source. Leviathan, a massive biogenic gas field off the coast of Israel, contains enough natural gas to power Israel for hundreds of years. When it began commercial production last year it completely transformed Israel’s energy security.
Another big question is how methane finds its way into hydrate deposits. We now know that flowing bubbles of methane play an important role in this process. The next challenge is to understand how the bubbles move through different types of sediments near the seafloor, and how hydrate formation affects the flow of gas.
Finally, an exciting and complicated question is how deposits with mixed gas hydrates form and dissociate.
Models and observations tell us that layers of ethane hydrate can be found beneath pure methane hydrate deposits.
We are only just beginning to explore why and how this fractionation occurs.
The same process could allow us to ‘swap’ methane with carbon dioxide in hydrate deposits, raising the exciting prospect of simultaneous energy production and carbon storage for a nearly carbon-neutral system.
By answering these questions, we will be deciphering an important part of our world’s carbon cycle with implications on societal issues that span climate change, geohazards and energy security.
—Kehua You ([email protected]; 0000-0002-0194-8720) and Peter B. Flemings ( 0000-0002-5377-3694), Institute for Geophysics (UTIG), Jackson School of Geosciences, The University of Texas at Austin, USA