A decade of research by Rice University scientists has produced a two-dimensional model to show how gas hydrates—ice-like substances in which gas molecules are encased in cages of water molecules—are formed under the ocean floor. The research was published by the Journal of Geophysical Research: Solid Earth.
Gas hydrates represent a potential source of energy, if they can be extracted and turned into usable form. (Earlier post.)
The award-winning mathematical model created by Rice alumnus Sayantan Chatterjee, who earned his doctorate in chemical engineer George Hirasaki’s group, is intended to help pinpoint abundant pockets of hydrate by extrapolating data from several sources: one-dimensional core samples, seismic surveys that image the fractures as well as stratified layers of sand and clay that build up over millennia, and the geochemistry of sediment and water near the ocean floor, which offers chemical clues to what lies beneath.
|The model shows where hydrates are likely to be found based on extrapolating data from core samples, seismic signals and other geologic data. Graphics by Sayantan Chatterjee. Click to enlarge.|
There’s a lot at stake in finding hydrates in high concentrations, with as much as 20 trillion tons of methane under the sea. Japanese researchers are already testing production techniques in the Pacific (earlier post), but extraction without reliable exploration tools is too expensive, Chatterjee said.
The Rice researchers’ two-dimensional model draws upon a variety of survey techniques to envision a more accurate slice of the deep-sea formation.
The modeling incorporates geologic processes such as sedimentation and compaction that enable methane-rich fluids to flow through porous media, Chatterjee said. Methane degraded by microbes from organic matter or rising from the depths turns into hydrate when it encounters the necessary pressure, temperature and salinity conditions in the gas hydrate stability zone, which can be as shallow as a few hundred meters.
High-saturation hydrate deposits preferentially occur in fracture networks within fine-grained sediment and interbedded, permeable sand sequences, and we’re looking for such lithologic sweet spots.—Sayantan Chatterjee
The complex stratigraphy and lack of homogeneity of subsea formations limits the ability of one-dimensional modeling and core samples to scan a potential hydrate reservoir isolated in permeable sand sequences between dense layers of clay, Chatterjee said.
Marine lithologic layering is very complex, and we can’t replicate it in our models. But we have developed techniques to compute local fluid flow in lithologically complex reservoirs, which we correlate to local hydrate saturation.
When people seismically image the submarine formations and recover sediment cores dominated with faults and fractures, they find these fractures to be filled with hydrates. Our model has explained this observation. It shows that these fracture networks and sand layers are the sweet spots for hydrate occurrence, the ones we want to pinpoint when it comes to exploration. Only when a pore space is highly saturated with hydrate is it economically feasible to drill at that location to extract these trapped hydrocarbons. But first we have to estimate the fluid flow. No flow, no hydrates—Sayantan Chatterjee
The Rice team intends the model to locate these hydrate-rich pockets and estimate how saturated they’re likely to be based on the geologic setting and history.
Chatterjee presented his research at the seventh International Conference on Gas Hydrates in Edinburgh, Scotland, and won first prize at the prestigious Society of Petroleum Engineers’ Young Professionals meeting and second prize at the Gulf Coast Regional student paper competition.
Co-authors are Rice alumnus Gaurav Bhatnagar, now a scientist at Shell International Exploration and Production in Houston; Brandon Dugan, an associate professor of Earth science; Gerald Dickens, a professor of Earth science; and Walter Chapman, the William W. Akers Professor of Chemical and Biomolecular Engineering, all of Rice. Chatterjee is a scientist at Shell Global Solutions in Houston. Hirasaki is the A.J. Hartsook Professor Emeritus of Chemical and Biomolecular Engineering.
The Department of Energy, Rice’s Shell Center for Sustainability and Rice’s Consortium of Processes in Porous Media supported the research.
Sayantan Chatterjee, Gaurav Bhatnagar, Brandon Dugan, Gerald R. Dickens, Walter G. Chapman and George J. Hirasaki (2014) “The Impact of Lithologic Heterogeneity and Focused Fluid Flow upon Gas Hydrate Distribution in Marine Sediments,” Journal Of Geophysical Research: Solid Earth Accepted manuscript online: doi: 10.1002/2014JB011236