Revealing the molecular structure of kerogen in gas shale

04-02-2016

Fracking is a controversial recovery method to extract oil or gas from shale rock. There was not much known about kerogen, the organic matter which hosts hydrocarbons in gas shale structures – until now. Researchers from CNRS/MIT and the University of Haute-Alsace are reconstructing the complexity of such geological structures and have used the ESRF in their quest. Such molecular models of kerogen, which have been derived and validated against experimentally determined properties, can be used to unravel the behaviour of this complex organic matter. The results are published this week in Nature Materials.

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Oil and natural gas are produced from the decomposition of organic matter, known as kerogen, in shale rocks. In such structures, the kerogen is compressed within rocks a million times less permeable than in conventional hydrocarbon reservoirs. Owing to such ultra-low permeability and kerogen’s complex properties, the hydrocarbons are recovered through a severe and controversial extraction technique known as hydraulic fracturing, or fracking. In order to be able to develop new techniques, which would be both effective and with a lower environmental impact, scientists need to know more about the shale structures.

The international group of scientists studied four different kerogen samples of different origin and degree of maturation. They used beamlines ID11 and ID27 at the ESRF to study their chemical composition, texture and density. At ID27, the sample was placed in a diamond anvil cell with a pressure of up to 5.1 GPa to reconstruct the conditions underground. They also used neutrons at the Oak Ridge National Laboratory in the US.

X-ray microscopy image of an untreated sample of gas shale

X-ray microscopy (XRM) image of an untreated sample of gas shale, showing inclusions of pyrite, clay, organic matter and other minerals. Copyright: M. Hubler (MIT) and J. Gelb (Carl Zeiss X-ray Microscopy).

Using a hybrid method, combining an arsenal of experiments and molecular simulations, the team developed molecular models of kerogen with different maturities based on their chemical composition, texture and density. These atomistic models were then validated by comparing them with experimentally accessible kerogen properties.

These molecular models, which provide a new molecular view of kerogen, should now help understand the microscopic behaviour of this disordered and heterogeneous matter.  “These molecular models are a fundamental building block which allows unravelling the adsorption, mechanical, and transport properties of this complex organic matter known as kerogen”, explains Benoît Coasne, director of research at the CNRS and visiting scientist at the MIT.

 

Molecular model of a sample of the Marcellus kerogen

Molecular model of a sample of the Marcellus kerogen studied (the organic phase constitutes the source of hydrocarbons in shale gas). Carbon, hydrogen and oxygen atoms are shown in grey, white and red respectively. The size of the image is 5 × 5 nm. Four samples with different maturities, that is, with different times and conditions of formation, were considered. Copyright: Colin Bousige (CNRS).

 

Reference
Bousige C. et al., Realistic molecular model of kerogen’s nanostructure, Nature materials, 1 February 2016; doi: 10.1038/nmat4541.

 

Top image: Molecular model of a sample of the Marcellus kerogen. Copyright: Colin Bousige (CNRS).