In November 2000, somewhere off Canada’s Vancouver Island, the commercial fishing vessel Ocean Selector brought to the ocean’s surface an unusual “catch.” The trawl net that had been dragged near the seafloor to capture fish recovered more than 1000 kilograms of methane hydrates from a depth of 800 meters.
Methane hydrates are ice-like solids in which water molecules form cages around molecules of methane, the chief component of natural gas. Methane hydrates are ubiquitous and found in ocean sedimentsespecially in continental marginsand the Arctic permafrost.
The accidental mining of methane hydrates by a fishing vessel caught the attention of Rod Judkins, director of ORNL’s Fossil Energy Program. “This incident may suggest that some methane hydrates can be more easily recovered than we thought,” he says. “Also, although these materials could have been broken off of outcroppings, it could indicate that hydrates are not necessarily covered with much sediment, which would imply that their formation does not require as much time as we have previously believed.”
Judkins sees methane hydrates as the key to U.S. energy independence, which would give the nation energy security. “We must increase our primary energy sources to make us less dependent on foreign supplies of oil,” he says. “One way to do this is to tap the abundant natural-gas supplies in methane hydrates, which offer us more energy than we have in our 1500-year-supply of coal. Estimates by the U.S. Geological Survey and others place reserves of methane in methane hydrates as high as 46 x 1015 m3. This is an incredibly large potential energy resource, provided it can be safely and economically produced. Natural gas is a versatile fuel that can be used for generating electricity, heating homes, and fueling cars and trucks.”
GAS HYDRATE RESEARCH AT ORNL
ORNL researchers are supporting DOE’s study of four major issues regarding the ultimate use of methane hydrates for fuel. One issue is resource evaluationwhat are both the extent and the nature of methane hydrate deposits in the ocean and Arctic permafrost? Another is seafloor stability and safety: Will the harvesting of methane hydrates disrupt the stability of methane hydrate deposits and become a major safety hazard? For example, should an accidental release of methane gas from a methane hydrate deposit occur during drilling operations, the loss in buoyancy would have the potential to cause losses of expensive oil drilling platforms nearby. A third issue is the potential effect of methane hydrates on climate, especially if methane (a greenhouse gas) were to escape accidentally to the atmosphere as a result of a botched harvesting process. Finally, there is the question of production: How can methane be economically and safely extracted and captured from hydrates?
ORNL and the Idaho National Energy and Environmental Laboratory (INEEL) have commitments for funding from DOE’s Office of Fossil Energy to participate jointly in a ship cruise to recover and analyze methane hydrates as part of the U.S. Ocean Drilling Program. An INEEL scientist will take the ship cruise from July through September 2002. “Some methane hydrate samples will be brought back to ORNL for study and for comparisons with methane hydrates produced in our seafloor process simulator,” Judkins says.
Libby West of ORNL’s Environmental Sciences Division (ESD) is in charge of the day-to-day operations at the highly instrumented pressure vessel called the seafloor process simulator (SPS). She and Tommy Phelps, also of ESD, carry out research there. (See “Methane Hydrates: A Carbon Management Challenge,” ORNL Review, Vol. 33, No. 2, 2000). Experiments have shown that methane hydrates produced in the SPS will dissociate and release methane when the temperature of the water is raised above the temperature at which methane hydrates form when the pressure is right. “But,” she said, “depressurizing a methane hydrate field rather than raising its temperature may be a more economical way to harvest methane. We hope to do experiments later in the SPS to determine which harvesting schemes work best.”
Some scientists believe that many methane hydrate fields are covered by hundreds of meters of sediment consisting of sand, clay, and other materials. “In our SPS experiments we made methane hydrates in suspensions of geological particles,” West says. “We found that our surrogate sedimentsilica, which represents sand, and bentonite, which represents clayhas no chemical effect on the dissociation of methane hydrates but may affect their formation.”
West, together with Costas Tsouris of ORNL’s Nuclear Science and Technology Division and ESD’s Sangyong Lee and David Riestenberg, have conducted experiments in the SPS that demonstrate a possible approach for ocean sequestration of carbon dioxide (CO2) captured, say, from the stack emissions of coal-fired power plants. Carbon capture and sequestration are considered essential to ensuring the continued use of our abundant supply of fossil fuels for power production without increasing the threat of climate change.
“In a research effort started with a seed money project and now continuing in a program funded by DOE’s Office of Biological and Environmental Research, we found that intensely mixing water into liquid CO2 within a specially designed injector produces a paste-like, cohesive mass that contains CO2 hydrate,” West says. “The presence of CO2 hydrate, which is more dense than the seawater, caused this cohesive mass to be negatively buoyant, so it sank to the floor of the SPS vessel.”
Injecting liquid CO2 in this paste-like form may improve the efficiency and reduce the environmental impacts of ocean carbon sequestration by direct CO2 injection. ESD’s Liyuan Liang is studying the dissolution of this paste-like material and its subsequent geochemical interactions with the marine environment.
Claudia Rawn of the Metals and Ceramics Division, Bryan Chakoumakos of the Solid State Division, and Adam Rondinone of the Chemical Sciences Division have performed neutron scattering studies to characterize synthetic gas hydrates. Their efforts have been focused on determining how the atomic structure and physical behavior of various hydrates change as a function of temperature. While ORNL’s High Flux Isotope Reactor (HFIR) is being upgraded, their experiments are being performed using Japan Atomic Energy Research Institute and National Institute of Standards and Technology research reactors. Once the HFIR upgrades have been completed, the ORNL researchers will use a low-temperature pressure cell, which was constructed at HFIR using Laboratory Directed Research and Development Program funds. It will allow gas hydrates to be synthesized and studied in the neutron beam.
“Neutron scattering is well suited for the study of hydrates because it is sensitive to the hydrogen atoms that are a major part of these materials,” Chakoumakos says. “The neutron’s large penetration depth allows the use of complex sample environments, which are needed to simulate the conditions at which hydrates are stable.”
DISPLACING METHANE WITH CO2
Injecting CO2 into methane-rich coal seams hundreds or thousands of feet underground could have a double benefit: It could both boost energy production and, at the same time, reduce greenhouse gas emissions. Many unmineable coal seams have associated methane that has been adsorbed on the coal. As recent field tests by oil and gas companies have shown in New Mexico and Canada, CO2 pumped down an injection well into a deep, unmineable coal seam may be adsorbed on the coal bed, displacing the methane and forcing it to rise up through a production well. The use of CO2-enhanced coal-bed-methane (CBM) recovery for replacing one stored gas with another would allow long-term sequestration of CO2, slowing the atmospheric buildup of this “greenhouse gas” while enhancing production of methane, the main ingredient of natural gas. Through reforming processes, this gas could be a source of hydrogen for future fuel-cell-powered cars, which would reduce the need for imported oil for U.S. transportation vehicles.
Recently, Jim Blencoe, a geochemist in ORNL’s Chemical Sciences Division, had a revelation when he read that the Alberta Research Council is leading a consortium of Canadian and international companies in field tests of CO2-enhanced CBM recovery. Because of the basic research he has performed on the mixing of gases such as CO2 and methane at the elevated temperatures and pressures that exist below the earth’s surface, Blencoe believes his findings and analyses could contribute substantially to the development of this technology.
“In my opinion, this is the best geological method proposed for sequestering carbon dioxide,” he says. “The value added by CO2-enhanced CBM recovery is its ability to generate methane that is not recoverable by traditional methods of production.”
But, as it is with most technologies, implementation must be done right to ensure that the technology is safe and economical. That’s where basic research could provide needed guidance.
“If you inject carbon dioxide into coal layers where pressure and temperature are in a certain range, CO2 will strongly displace methane,” Blencoe says. “If CO2 is injected too rapidly, or the flow of gas toward a production well is impeded, gas pressure could rise enough to blow out seals around the injection well, damaging equipment and posing a serious safety hazard.”
Blencoe believes his basic research, funded by DOE’s Office of Basic Energy Sciences, is relevant to the DOE mission of developing appropriate carbon management strategies. He has recently received funding from DOE’s Office of Fossil Energy to determine experimentally the volumetric properties and viscosities of carbon dioxide–methane mixtures at temperatures and pressures achieved in coal beds buried 100 to 3300 meters (approximately 300 to 10,000 feet) below the earth’s surface.
“In 1996,” Blencoe says, “we measured the volumetric properties of carbon dioxide–methane mixtures at temperatures and pressures close to those encountered in field tests of CO2-enhanced CBM recovery. It turned out to be serendipity, because we did not foresee the relevance of the research to a practical application related to energy production and possible environmental protection. A particularly important finding was that, at 50°C and 100 atmospheres of gas pressure, CO2 repels methane, causing extensive and rapid expansion of the gas mixture.
“We will conduct laboratory experiments and develop mathematical expressions to accurately predict this behavior over a wide range of temperature, pressure, and gas composition. In addition to allowing gas pressure to be calculated reliably, this research would lead to more accurate predictions of the rates and levels of CO2 adherence (adsorption) on coal surfaces, which promotes release (desorption) of methane from those surfaces.”
Blencoe also investigates the properties of fluids that contain carbon, oxygen, hydrogen, and nitrogen. At the elevated temperatures and pressures in the earth’s crust, these elements form the stable gas species CO2, methane (CH4), water (H2O), and nitrogen (N2).
“We use special equipment to determine how gas species behave macroscopically and interact with each other,” he says. “The properties of pure and mixed CO2-CH4-N2-H2O gases are represented by equations of state, which describe the behavior of the gases over various ranges of temperature, pressure, and gas composition.
I do experiments to determine the thermodynamic mixing properties of gas species and use the results, along with suitable data from the literature, to develop more accurate and comprehensive equations of state.”
Thanks to the application of the results of basic research, ORNL scientists are discovering better ways to produce methane, an energy-rich gas, and sequester CO2, a major greenhouse gas.
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