Kerogen is a waxy, insoluble organic substance dispersed in sedimentary rocks and is the precursor of oil and gas. In a new study, published in the Proceedings of the National Academy of Science, researchers from MIT and elsewhere have captured 3D images of kerogen’s internal structure, with a level of detail more than 50 times greater than has been previously achieved.
Using electron tomography, Pellenq et al probed a sample of kerogen to determine its internal structure. At left, the sample as seen from the outside, and at right, the detailed 3D image of its internal pore structure. Image credit: Pellenq et al, doi: 10.1073/pnas.1808402115.
Kerogen is a mixture of organic materials, primarily the remains of dead microbes, plants, and animals that have decomposed and been buried deep underground and compressed.
This process — a slow pyrolysis — forms a carbon-rich, rock-hard material riddled with pores of various sizes.
When transformed as a result of pressure or geothermal heat, hydrocarbon molecules in the kerogen break down into gas or petroleum. These flow through the pores and can be released through drilling.
“This process involves cooking oxygen and hydrogen, and at the end, you get a piece of charcoal,” said lead author Dr. Roland Pellenq, senior research scientist at MIT.
“But in between, you get this whole gradation of molecules, many of them useful fuels, lubricants, and chemical feedstocks.”
To get the detailed images of kerogen’s structure, Dr. Pellenq and co-authors used electron tomography, in which a small sample of the material is rotated within the microscope as a beam of electrons probes the structure to provide cross-sections at one angle after another.
These are then combined to produce a full 3D reconstruction of the pore structure.
“With this new nanoscale tomography, we can see where the hydrocarbon molecules are actually sitting inside the rock,” Dr. Pellenq said.
The team’s results show for the first time a dramatic difference in the nanostructure of kerogen depending on its age.
Relatively immature kerogen — whose actual age depends of the combination of temperatures and pressures it has been subjected to — tends to have much larger pores but almost no connections among those pores, making it much harder to extract the fuel.
Mature kerogen, by contrast, tends to have much tinier pores, but these are well-connected in a network that allows the gas or oil to flow easily, making much more of it recoverable.
The study also reveals that the typical pore sizes in these formations are so small that normal hydrodynamic equations used to calculate the way fluids move through porous materials won’t work.
At this scale the material is in such close contact with the pore walls that interactions with the wall dominate its behavior.
“There’s no fluid dynamics equation that works in these subnanoscale pores. No continuum physics works at that scale,” Dr. Pellenq said.
_____
Roland Pellenq et al. 2018. Mesoscale structure, mechanics, and transport properties of source rocks’ organic pore networks. PNAS, in press; doi: 10.1073/pnas.1808402115
