Making gas by squeezing wet rocks

Dr. Dudley Herschbach, the head of the chemistry department at Harvard University in Harvard, Massachusetts, gave a guest lecture at the Colorado School of Mines for the semi-annual Lucas Lecture. The Lucas Lecture series, started by a George Lucas 22 years ago, is an opportunity to welcome faculty and staff from other schools across the nation to share their expertise and knowledge.

Last week, Herschbach presented to the chemistry department at Mines on the production of hydrocarbons in rocks. 60 years ago, Thomas Gold postulated that gasses such as methane and ethane were generated deep in the Earth, not by microorganisms decaying, but by the immense heat and pressure on the rocks thousands of feet underground. At the time, he was laughed at by his peers in the scientific world. Herschbach showed that, in fact, he was right. Under extremely high pressure, things happen that cannot occur any other way. “Chemistry would become extremely easy, were [experiments] conducted under high pressure. Almost anything under the Sun is possible.”

Not very many years after Thomas Gold made these bold statements, Percy Williams Bridgman designed a device, called a diamond anvil cell, that would be able to exert massive pressures on small objects. Diamond anvil cells work by using two diamonds spaced just 0.2 millimeters apart mounted in a stainless-steel gasket and forcing the diamonds together. This can produce roughly 3,000,000 atmospheres of pressure on the contents of the cell. The limitation of this device is in its necessarily diminutive size, which allows for only very small things to be placed inside. Herschbach added that, with synthetic diamonds getting bigger and more pure, the size of these cells will increase and allow for better and more diverse experiments. “Eventually, they will be able to make diamonds the size of a coffee cup. Of course, you would not want a coffee cup made out of diamonds because it is known for its heat conductivity. You would not be able to hold on to it.”

The advent of high-pressure experimentation allowed people such as Dimitri Mendeleev to study what happens to rocks under very high pressure, and they were able to see that iron oxide, calcite, and water together produced Methane and Ethane when compressed. In essence, we can now show that indeed, hydrocarbons can be produced by squeezing wet rocks. Maybe Thomas Gold was not so crazy after all, and even though his science was not very thorough, he had the right idea.

The understanding of pressure is not a product of modernity, as it started with Aristotle and the suction pump. Aristotle thought that “nature abhors a vacuum,” which is why he could draw water up a suction pump on a well. The difference in the pressures inside the suction pump and outside are what allows water to be drawn up the tube as far down as 30 feet.

Herschbach explained that atmospheric pressure is really much greater than we realize. If you add up the weight of all the air in the atmosphere pushing down on the Earth, it equals roughly 1 kilogram per square centimeter, which is about the size of your thumbnail. As a point of reference, the average person exerts a pressure of roughly 0.20 of a kilogram per square centimeter. We are lightweights, even compared to air.