Surreptitiously converting dead matter into oil and coal.
But to a small group of scientists studying how oil and coal form from carbon-rich decayed plants and algae, aqueous organic chemistry- reactions of carbon-based compounds in hot water - represents a better way of thinking about how the Earth created those vast underground energy reserves.
"We're promoting the idea that water is important in natural organic reactions," says Michael D. Lewan, a geochemist with the U.S. Geological Survey (USGS) in Denver.
In addition, sustained investigation into how hot water affects organic materials may lead to more efficient and environmentally friendly processes. Someday, water may aid in making -- and recycling or cleaning up -- plastics and other petroleum-based products.
Once again, this commonplace substance turns out to have some uncommon attributes. "We just take water for granted:' says Theodore P. Goldstein, an organic chemist at Mobile Research & Development Corp. in Princeton, N.J. "We don't think its properties can change."
Little did he and others realize how mutable water can be.
Until now, scientists thought that coal forms when dying plants in soggy marshes get buried, creating a peat that simmers in this soupy environment. If no oxygen is present, chemical events slowly change peat, first into lignite and then, millions of years later, into bituminous coal. If temperatures climb high enough, anthracite coal forms.
Oil formation was viewed similarly, Dead marine microorganisms sink to the seafloor, then become buried by silt washing out of a river. If enough silt piles up, it creates a geologic Dutch oven, in which high temperature and pressure cause the organic debris to condense. A source rock -- oil shale -- forms. In its pores, chemical processes continue until oil oozes forth. The key requirement is getting this "oven" hot enough for a long enough time - or so everyone thought.
These explanations did not satisfy Andrew Kaldor, a researcher at Exxon Research and Engineering Co. in Annandale, N.J. He realized that ideas about oil and coal formation had evolved many years ago and had not really been updated to include new chemical and biological knowledge.
So Kaldor and Exxon organic chemist Michael Siskin decided to reexamine these ideas by first determining the chemical composition of organic materials in source-rock shales - an awesome job given the complex and highly variable nature of this starting material and the cascade of molecular transformations that occurs in forming oil and coal.
In both, plant matter decays into a potpourri of molecules that, depending on the conditions at hand, break up and clump in any number of ways. Carbon atoms get rearranged into assorted rings and chains to create a complex, interlocking network. Hydrogen atoms join, leave, and sometimes rejoin this network. as do other elements such as oxygen, nitrogen, or sulfur, eventually forming giant, insoluble macromolecules. "It's everything that winds up in the sediments," notes Goldstein.
Few chemists would even know how to begin teasing out the right combination of hydrocarbons to create oil or coal, but somehow nature manages to break these giant molecules in just the right places.
To understand this process better, the Exxon group collected samples of oil shale from different parts of the world. The samples included a series from oil shale under the North Sea, where rocks in different locations exhibit different degrees of transformation. Siskin then placed the samples into a pressurized reaction vessel and heated them individually to temperatures ranging from 570[degree]C to 750[degree]C. These hotter-thannatural conditions sped up the transformation from a geologic time frame of millions of years to one measured in days and hours.
Over the course of about two years, these and other tests helped the scientists piece together the locations of various atoms and side groups in representative molecular structures and in the intermediate products created in the transformation from molecular glob to oil.
"Knowing the structures at different stages of maturation can help us see the pathways," notes Siskin. He and Kaldor began to realize that heat alone could not break some of the bonds between these atoms.
To examine this inconsistency further, the scientists decided to look at "model" molecules -- simple, commercially available hydrocarbon compounds that stood in for the organic matter in source rock or coal. Each represents a class of organic material --esters, amides, alkanes, for example -- found in nature. The researchers wanted to see what conditions dissolved the linkages in these molecules. The experiments confirmed their suspicions.
"A lot of the molecules present in the structures we had developed were thermally not reactive," says Siskin. Certain groups of atoms key to holding the macromolecules together just would not fall apart, no matter how hot the chamber got.
The results perplexed the Exxon scientists, because they knew these groups of atoms disintegrated naturally -- and at lower temperatures than those used in the experiments. Also, coal formed under similar conditions. They began to eye water for answers.
"We knew that oil forms in an aqueous environment," recalls Kaldor. "One natural question is, 'Is the water really benign, or does it play another role?'"
As early as 1979, Lewan, then working for an oil company, and his colleagues had demonstrated that they could simulate oil formation in the laboratory, but only if they added water to their system. Without water, "the products were seldom like that in natural crude oil," recalls Lewan. He and his colleagues then tried adding water to their source-rock samples. When they opened up their reactor at the end of the experiment, "we found a beautiful layer of oil on top of the water," he adds. He called this conversion hydrous pyrolysis.
Work at Exxon had also suggested aqueous influences on organic reactions. One group had discovered that water and carbon monoxide can enhance the liquefaction of certain types of coal. A different research team in Canada began using a mix of hot water and steam to increase the amount of heavy oil recovered from reservoirs buried under Cold Lake in Alberta. Also, Alan R. Katritzky, an organic chemist from the University of Florida in Gainesville, had decided to test down some of his reactions to test whether water could help remove sulfur, oxygen, and nitrogen contaminants from ringed hydrocarbon compounds.
In 1985, Katritzky teamed up with Siskin to investigate water's role more intensively, "The folklore would have it that organic molecules will not react with water," says Siskin. But the two researchers realized they could make water more amenable to organic materials by putting it under pressure and heating it up. As water molecules get hotter, they become less polar and so are more likely to interact with nonpolar organic molecules. At 300[degree]C, water acts like the organic solvent acetone at room temperature, Siskin adds.
The hot water molecules also tend to break apart, splitting into positive hydrogen and negative hydroxyl (OH') components. They become acidic and basic and therefore much more reactive. Also, keeping the water under pressure ensures that it remains liquid; as such, "it can act as a solvent, it can act as a catalyst, and it can act as a reagent:' Siskin says. Siskin and Katritzky started adding water to their reaction vessels and experimenting with different combinations of heat, water, brine (salt water), minerals, and clay - conditions that might exist deep within Earth's crust. They discovered that high temperatures cause an organic molecule to break into fragments - and so does water and brine, sometimes more effectively.
"In these model systems, the results are that water is not always benign," Kaldor says. Some classes of organic molecules proved very susceptible to water's influence. In fact, water sometimes causes organic material to disintegrate into fragments that then transform into oil's hydrocarbons more readily than heat-induced fragments do.
The results indicate that hot water becomes a catalyst for a series of ionic reactions -creating a second pathway for the cascade of molecular transformations that leads to oil. The acidic and basic nature of hot water -- rather than heat -- drives this cascade.
For example, water may function first as a base, nibbling away at certain linkages in the organic material. As new molecular fragments build up and modify the reaction environment, water can change its catalytic nature. It can then act as an acid, accelerating different reactions. The resulting products attack parts of the remaining molecules, further speeding the breakdown. Siskin and Katritzky described these processes in the Oct. 11, 1991 ScIENcE.
"What we have learned is that ionic chemistry predominates in most cases and opens up pathways not accessible by the thermal route:' Kaldor says.
These results bolster findings obtained by Lewan and complement research in hydrous pyrolysis by providing some details about the chemistry that could be occurring during these transformations. "It opens up the idea that you can't focus on one aspect, the organic aspect, of oil formation," says Goldstein. "You've got to focus on the chemistry of the whole system."
Lewan, too, has continued his investigations into the role of water. From his perspective, hydrous pyrolysis takes place because small amounts of water trapped in pores in source rock become awash in organic molecules, not because organic material dissolves in aqueous solution. Like Siskin, he runs his experiments at higher temperatures than exist naturally in order to speed up the process. Some of Lewan's ideas about the details of water's role differ from those of the Exxon group. Nevertheless, these and other results are building a convincing body of evidence.
"Over the past 10 years within the organic geochemistry community, it's been very controversial whether water is important:' says Lewan. "1 think what we'll argue over the next 10 years is how it's important."
Whatever the actual mechanism, the fact that water plays a role could wreak havoc on established ideas about oil formation. The results suggest that oil can mature faster than previously thought, says Kaldor. Consequently, not only do the ideas buck tradition, but, if right, they will require the revision of time parameters in computer programs now used to predict locations of new reserves. For that reason alone he expects the oil-exploration community to accept these ideas slowly.
"We have not convinced Exxon geoscientists that the model is correct:' Kaldor says.
"But these experiments are making believers out of them," Lewan adds, citing his work as well as that of Exxon.
To help convince their colleagues that such revision is warranted, Kaldor and Siskin are seeking ways to verify their ideas. "You can't do real-time experiments," Kaldor says, "so you have to begin to look for clues in the natural environment." Those clues could be molecules that form only through waterinitiated ionic reactions, for example. "But we don't have that yet:' he says.
Meanwhile, the concepts that have come out of this work may have broad impact. "We started this whole quest with just an interest in oil;' Kaldor says. "But it really has paid off in a generic way in natural processing. It enables people to begin to do things they wouldn't be able to do before:'
Already, Exxon foresees the possibility of using hot water to introduce more hydrogen into coal - to make it more amenable to liquefaction and to reduce the cost of this process. Also, hot water and steam might help add hydrogen to low-quality oil deposits, improving and loosening the oil from pores in source rock so it will move easily to the surface.
Hot-water chemistry promises to aid other chemical processes as well. For example, it could increase the efficiency of the production of isopropyl alcohol by providing a way for a waste product, an ether, to be converted into more alcohol. Hot-water processing also offers ways to break down petroleum-based materials that might otherwise contaminate the environment. Even the U.S. Army has expressed interest in aqueous organic chemistry -- as a way to destroy chemical warfare agents. In other instances, the use of hot water may eliminate the need for other catalysts that prove difficult to dispose of safely "It opens up an entire area of synthetic chemistry." Goldstein says.
"We're at the stage where it's becoming more and more routine:' adds Kaldor. "My guess is that in the next few years, we'll see commercial applications."
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|Title Annotation:||Water, Water Everywhere|
|Article Type:||Cover Story|
|Date:||Feb 20, 1993|
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