Rice reinforces gas hydrate
strategy
New study goes deeper
in proving simple technique to pinpoint valuable energy source

BY MIKE WILLIAMS
Rice News staff

Their critics weren’t convinced the first time, but Rice University
researchers didn’t give up on the “ice that burns.”

A paper by a Rice team expands upon previous research to locate and
quantify the amount of methane hydrates — a potentially vast source of energy —
that may be trapped under the seabed by analyzing shallow core samples. The
paper published this week by the Journal of
Geophysical Research — Solid Earth should silence the skeptics, the
researchers said.

	JEFF FITLOW
	Rice University chemical engineer George Hirasaki, left, and graduate student Sayantan Chatterjee have expanded upon techniques to locate vast reserves of gas hydrates under the seafloor.

Chemical engineers George Hirasaki and Walter Chapman and oceanographer
Gerald Dickens headed the team.

In 2007, Hirasaki and former graduate student Gaurav Bhatnagar theorized that
gas hydrates — methane that freezes at low temperatures and high pressures —
could be detected via transition zones 10 to 30 meters below the seafloor near
continental shores; at that level, sulfate (a primary component of seawater) and methane react and consume each
other.

As sulfate migrates deeper into the sediment below the seafloor, it
decreases in concentration, as evidenced by measurements of pore water (water
trapped between sediment particles) from core samples. The depth at which the
sulfate in pore water gets completely consumed upon contact with methane rising
from below is the sulfate-methane transition (SMT) zone.

In the 2007 paper, Bhatnagar argued the depth of this transition zone
serves as a proxy for quantifying the amount of gas hydrates that lie beneath;
the shallower the SMT, the more likely methane will be found in the form of
hydrates in abundance at greater depth.

Though hydrates may be as deep as 500 meters below the seafloor,
locating deposits through shallow coring using such proxies should aid
selection of deep, expensive exploratory drilling sites, the researchers said.

The controversy that followed the publication of the original paper
focused on sulfate consumption processes in shallow sediment and whether
methane or organic carbon was responsible. Skeptics felt the basis of
Bhatnagar’s model, which assumes methane is a dominant consumer of pore-water
sulfate, was not typical at most sites.

“They believed that particulate organic carbon (primarily from
ocean-borne dead matter) was responsible for reducing sulfate,” said
Sayantan Chatterjee, lead author of the new paper. “According to their
assumption, the depth of the SMT, upward methane flux and hydrate occurrence
cannot be related. That would nullify all that we have done.”

So Chatterjee, a fifth-year graduate student in Hirasaki’s lab, set out
to prove the theory by bringing more chemical hitchhikers into the mix.

“In addition to methane and sulfate profiles, I added bicarbonate,
calcium and carbon isotope profiles of bicarbonate and methane to the
model,” Chatterjee said. “Those four additional components gave us a
far more complete story.”

By including a host of additional reactions in their calculations on
core samples from the coastline of Oregon and the Gulf of Mexico, “we can
give a much stronger argument to say that methane flux from below is
responsible for the SMT,” said Hirasaki, Rice’s A.J. Hartsook Professor of
Chemical and Biomolecular Engineering. “The big picture gives more
evidence of what’s happening, and it weighs toward the methane/sulfate reaction
and not the particulate organic carbon.”

The work is important not only for a natural gas industry eyeing an
energy resource estimated to outweigh the world’s oil, gas and coal reserves —
as much as 20 trillion tons — but also for environmental scientists who see
methane as the mother of all greenhouse gases, Hirasaki said.

“There’s a hypothesis by Dickens that says if the ocean
temperature starts changing, the stability of the hydrate changes. And
instability of the hydrates can release methane, a more severe greenhouse gas
than carbon dioxide.

“That can create more warming, which then feeds back on itself,”
Hirasaki said. “It can have a cascade effect, which is an implication for
global climate change.”

Chatterjee had the chance to discuss his results with his peers in July
at the seventh International Conference on Gas Hydrates in Edinburgh, Scotland,
where he presented a related paper that focused on the accumulation of hydrates
in heterogeneous submarine sediment.

Chatterjee said a number of eminent experts commended him after his
talk. “I got a chance to show my recent findings on our 2-D model. This
will simplify the search and locate isolated pockets where hydrates have
accumulated in deep ocean sediments,” he said.

Chatterjee’s conference paper was awarded a first prize at the
prestigious Society of Petroleum Engineers’ Young Professionals meeting and
second at the Gulf Coast Regional student paper competition.

Co-authors include Chapman, the William W. Akers Professor of Chemical
Engineering; Brandon Dugan, an assistant professor of Earth science; Glen
Snyder, a research scientist in Earth science; Dickens, a professor of Earth
science, and Bhatnagar, all of Rice.

Rice’s Shell Center for Sustainability and the Department of Energy
supported the research.