Hot Water, Hot Rocks, Hot Science

Of the numerous energy sources, several important ones come from deep inside the earth. Coal, oil, and natural gas have formed over time through the interaction of enormous pressures and temperatures on organic material. Geothermal energy--steam and hot water--is also a product of these subsurface forces. Even the raw material for nuclear energy, uranium, comes from ores deposited by hot water circulating through the earth's crust.  Geochemists in ORNL's Chemical and Analytical Sciences Division (CASD) focus on developing experimental and analytical methods to investigate and quantify the natural processes occurring below the surface. This earth science research examines processes that influence hydrocarbon and geothermal energy development, nuclear and toxic waste migration, and elemental cycling in the ocean-atmosphere-lithosphere system--research relevant to human use of the earth's energy and material resources.  The roots of geochemical research at ORNL reach all the way back to the late 1940s, when pioneering studies of the properties of high-temperature water and salt solutions were conducted in the old Reactor Chemistry Division. These seminal contributions, spanning more than four decades, were made by physical chemists Bill Marshall, Charlie Baes, Milt Lietzke, Arvin Quist, Fred Sweeton, Dick Busey, Howard Holmes, and many others. Their work has been continued by Bob Mesmer (leader), Don Palmer, Mike Simonson, Patience Ho, and Mirek Gruszkiewicz, all of the High Temperature Aqueous Chemistry Group. These scientists have firmly established ORNL as one of the world's leading centers for experimental studies of the properties of hydrothermal fluids, including water and steam used to generate energy in fossil and nuclear power plants. It was inevitable that a geoscience research program would grow from this fertile ground because water is the ubiquitous, premier solvent and transporter of matter and energy in the earth's crust.

ORNL is one of the world's leading centers for experimental studies of the properties of hydrothermal fluids.

Hydrothermal Geochemistry

Basic hydrothermal geochemistry research in Bob Mesmer's group began formally in 1975 and grew at a steady pace to a half-million-dollar per year program by 1984. It has been supported by DOE's Office of Basic Energy Sciences, Geoscience Research Program and the Division of Geothermal Technology Development. The group was able to determine precisely the acid-base properties of water in geothermal brines and the thermodynamics of dissolved silica species (the major component of geothermal reservoir rocks) at temperatures up to 300°C. They used unique systems developed by Mesmer and his colleagues for measuring acidity levels (pH) at high temperatures. Because another very important earth fluid, silicate magma, provides the primary heat source for geothermal energy resources, the Chemistry Division's first bona fide geochemist, Mike Naney, was hired in 1978 to study its properties. He was joined in the early 1980s by geochemists Ed Drummond, Frank Dickson, Dave Cole, Jim Blencoe, and me.

The Geochemistry Group was hatched from Mesmer's group in 1985. It was placed under the leadership of Ed Drummond, who has since left us for the more lucrative field of real estate development in Knoxville. Since 1989 I have had the great privilege of being the leader of this group, which includes Jim Blencoe (see photograph), Dave Cole (see photograph), and a number of visiting scientists, graduate students, postdoctoral fellows, and research faculty members at the University of Tennessee at Knoxville. It is interesting to note that Drummond, Cole, and I, as well as Gary Jacobs [head of the Earth and Engineering Sciences Section in the Environmental Sciences Division (ESD)], were fellow students together in the Geosciences Department at Pennsylvania State University at the same time that Jim Blencoe was a member of the faculty. We often call ORNL "Penn State South." Just to show how small the world is, Steve Stow (author of the previous article in this issue) was Drummond's master's thesis advisor at the University of Alabama!

Although the Geochemistry Group maintains strong ties to its past, its activities have broadened greatly in recent years. Much of my work involves continued collaboration with chemists Don Palmer and Bob Mesmer in experimental studies of the thermodynamic and kinetic properties of reactions and dissolved species in hot water, often using modified versions of the same high-temperature pH cell that Mesmer developed 26 years ago. We try to identify and study in detail aqueous reactions that have a controlling influence on subsurface solution chemistry; the solubilities and sorptive properties of minerals and waste forms; the stabilities of natural and man-made organic compounds in aqueous solutions; and permeability changes and reservoir characteristics in hydrocarbon, geothermal, and groundwater systems.

Stable Isotope Abundances

Dave Cole prepares a carbon dioxide gas sample extracted from soil carbonates from the southwestern United States for analysis of its 13C/12C and 18O/16O ratios in an ORNL mass spectrometer. He will use this information to assess the effects of rising atmospheric carbon dioxide content on plant communities some 9000 years ago.

Dave Cole is one of the world's leading experts in the study of geochemical controls on the relative abundances of the naturally occurring stable isotopes of oxygen (18O/16O), hydrogen (D/H), carbon (13C/12C), and sulfur (34S/32S) in minerals and fluids. These isotope ratios provide a wealth of information on the fluid sources and time-temperature histories of fluid-rock interactions in geological systems. Using state-of-the-art analytical facilities and a wide array of hydrothermal pressure vessels suitable for operation at temperatures up to 400°C and pressures up to 4000 atmospheres, Cole and his colleagues investigate the kinetics of isotope exchange reactions and the equilibrium partitioning of isotopes between brines and other phases, including minerals and steam.

Cole is also interested in the natural distributions of these isotopes in real rocks and fluids. He published a paper recently in Nature in which he and collaborators at New Mexico State University used the oxygen and carbon isotope ratios in desert soil carbonates (calcium carbonate, or "calcite"—CaCO3) to suggest that a major increase in atmospheric carbon dioxide some 9000 years ago may have resulted in a shift in plant communities of the southwestern United States from grassland to desert scrub. This research finding suggests that future productivity of certain food crops could decrease as carbon dioxide levels in the atmosphere continue to rise.

Probing Conditions Throughout Earth's Crust

Jim Blencoe has successfully completed development of two unique facilities that were initially conceived by Mike Naney and Ed Drummond. Our internally heated pressure vessel (IHPV) is capable of operation at pressures up to 10,000 atmospheres and temperatures up to 1200°C, conditions similar to those encountered at the base of the earth's crust. Blencoe and his colleagues are using this system to determine the thermodynamic properties of granite melts and coexisting aqueous fluids in a project funded by DOE's Office of Geothermal Technology Development. They also use the unique capabilities of the IHPV to study the properties of fluids containing water, carbon dioxide, methane, and nitrogen at high temperatures and pressures.

Jim Blencoe (left) and DOE Distinguished Postdoctoral Fellow Jeff Seitz (seated) "man the pumps" on ORNL's vibrating-tube densimeter, a facility that operates on the principle of a tuning fork. It enables geochemists to measure densities of fluid mixtures up to one part per million at temperatures up to 400°C and pressures up to 4000 atmospheres.

Blencoe and his co-workers greatly enhance these types of studies by using our newly developed vibrating-tube densimeter, which allows them to measure the pressure-volume-temperature relationships of fluid mixtures at temperatures up to 400°C and pressures up to 4000 atmospheres at unprecedented levels of accuracy. This work is contributing enormously to our current understanding of the behavior of natural gas in hydrocarbon reservoirs and gas pipelines, and the influence of natural fluids on geological processes.

New Analytical Capabilities

In addition to the experimental studies described here, the recent creation of CASD by merging the former Chemistry Division and Analytical Chemistry Division has further enhanced longstanding efforts to develop unique analytical capabilities for application in the earth sciences. Geochemist Lee Riciputi, who joined CASD's Inorganic Mass Spectrometry Group in 1991, has been helping the group use its unique capabilities to detect and measure concentrations of trace elements and radiogenic isotopes in rocks and soils for DOE's earth science missions. He is also collaborating extensively with members of the Geochemistry and Secondary Ionization Mass Spectrometry (SIMS) groups in the application of ion microprobes for studies of the distribution of trace elements and stable isotopes in rocks and minerals at the spatial resolution of individual mineral grains. This technique has been successfully employed to determine the origins of hydrogen sulfide-bearing, or "sour," natural gas in major hydrocarbon reservoirs. It has also been used to determine the partitioning of oxygen isotopes between water and magnetite, a natural iron oxide found in oil and gas fields as well as corroded power plant boilers. Finally, this approach proved useful in tracking the distribution of radioactive cesium and strontium in a recent ORNL test of in situ vitrification, a method of isolating and immobilizing radioactive waste at a burial site, in collaboration with Mike Naney and Gary Jacobs of ESD (see the preceding article by Steve Stow). Currently, Riciputi is working with Pete Todd and Tim Short of the SIMS Group, with support from the Laboratory Director's R&D program, to develop a unique ion microprobe specifically designed for high-precision stable isotope ratio measurements in rocks and minerals.

A large number of other basic and applied research and development activities within CASD have direct application in the earth sciences. To mention just a few: Gary Van Berkel of the Organic Mass Spectrometry Group has developed sophisticated analytical methods for detecting geoporphyrins, a class of "biomarkers" that indicate the biological origins of organic material in petroleum source rocks. Jack Young of the Optical Spectroscopy Group has collaborated with Larry Robinson of the Neutron Activation Analysis Group to determine whether dinosaur bones contain elevated concentrations of iridium, which might prove that their extinction was related to a large meteorite impact. The Analytical Methods and Environmental Monitoring groups have pioneered the development of instrumentation for subsurface soil gas analysis. Also, the Physical Organic Chemistry Group has for many years been studying the molecular properties of coal and related organic compounds.

CASD is home to a world-class program in experimental chemistry and geochemistry. The program provides the fundamental information needed to formulate accurate predictive models for the discovery of energy and material resources, development of these resources, and analysis of the consequences of their use. This work is enhanced by a parallel program to develop new analytical methods and instrumentation that will enable us to monitor geological processes more accurately or observe them in new ways. These capabilities will no doubt stand us in good stead as we move into the next century.

B I O G R A P H I C A L Sketches

David J. Wesolowski was leader of the Geochemistry Group in ORNL's Chemical and Analytical Sciences Division until recently when David Cole succeeded him. He is also the Laboratory's coordinator of Geosciences Research Programs, which are supported by DOE's Office of Basic Energy Sciences. A native of Canonsburg, Pennsylvania, he holds a Ph.D. degree in geochemistry and mineralogy from Pennsylvania State University. He has worked as an assistant geologist with the U.S. Bureau of Mines and exploration geologist for U.S. Steel Corporation. He joined ORNL's Chemistry Division in 1983 as a Eugene P. Wigner Fellow and attained his present position in 1989. Wesolowski serves as secretary of the Geochemical Society and is an associate editor of the society's international journal Geochimica et Cosmochimica Acta. He is an adjunct professor in the Department of Geological Sciences at the University of Tennessee at Knoxville, where he has taught graduate courses in aqueous and stable isotope geochemistry.

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