Probing sedimentary rock porosity with neutrons

Whether sedimentary rocks store fossil hydrocarbons or act as impermeable layers to prevent the rise of oil, natural gas, or stored carbon dioxide—all depends on their porosity. The size, shape, structure and connectivity of the pore spaces are decisive.

At the Heinz Maier-Leibnitz Research Neutron Source (FRM II) at the Technical University of Munich (TUM), networks of micropores were characterized using small and very small angle neutron scattering.

Dense, dark, and compact—at first glance, the sedimentary rock samples that Dr. Amirzaman Rezayan keeps on his lab desk look a little different. Holes are invisible to the naked eye.

Yet precisely what gives pores their special properties is the pores: the pores, a few micrometers to sub-nanometers in size, form during sedimentation and shrink over time, determining permeability. These pores are a decisive factor in a rock's ability to host oil and natural gas or form impermeable layers that collect fossil fuels.

„Depending on the distribution, size and structure of pores, fine-grained sedimentary rocks are suitable for removing radioactive waste or sealing for carbon dioxide storage,” explains researcher Dr. Amirsaman Rezaian of the University of Calgary in Canada. „Its influence on the pore structure of rocks and permeability to fluid flow has not been studied to date, but it is important if you want to assess the potential of manrocks as oil reservoirs or impermeable layers.”

But how do you measure pores that are no bigger than bacteria? A variety of methods can actually be used to estimate pore size, but most of them can only detect large structures or limited pore sizes.

„Only small and very small-angle neutron scattering is suitable for fully measuring apertures between a few nanometers and micrometers,” says Rezayan, who, together with an international team at the Heinz Maier-Leibnitz Research Neutron Source (FRM II) at TUM, from Europe and the US studied the porosity of a dozen sedimentary rocks.

Measuring pores with nanometer precision

There are only a few measurement facilities for small angle neutron scattering (SANS) and very small angle neutron scattering (VSANS) worldwide. Two of them, KWS-1 and KWS-3, are operated by the Forschungszentrum Jülich at the Heinz Maier-Leibnitz Zentrum (MLZ).

MLZ is a scientific collaboration between TUM, Forschungszentrum Jülich and Helmholtz-Zentrum Hereon, which makes the neutrons of FRM II available to guest researchers in the form of scientific instruments.

Rezayan, of the Lyell Center at Heriot-Watt University in Edinburgh, Scotland, where he was working at the time, took his rock samples—all finely polished and without gas or liquid inclusions—to Karching to find micropores.

The samples were irradiated with neutrons from the reactor in the small-angle scatterers at FRM II. Since neutrons interact only with the nuclei of atoms, the diffraction pattern recorded by the detector is used to deduce the structure of the atoms.

In Scotland, researchers correlated measurements with microscopic properties of rock samples. As a result two articles have now been published, one in the Journal Energy And another one Energy and Fuels.

The researchers found that the porosity of fine sand depends on the proportion of clay minerals in the sediments: the more clay, the greater the probability of small pores with a diameter of less than 50 nanometers. Rocks with a high clay content are therefore more suitable for covering an underground disposal or storage site as an impermeable layer.

„However, clay content is only one part of the puzzle: there is a whole range of factors that must be taken into account when selecting suitable mudrock layers for oil and gas or CO production.₂ storage,” emphasizes Rezayan. „So we included other factors in the data analysis, such as rock compaction and organic matter. By doing this, we were able to establish highly statistically significant correlations.”

With the help of these correlations, it will be possible in the future to evaluate the physical properties of fine-grained sedimentary rocks based on depositional conditions and to determine their impermeable layers for nuclear waste repositories and CO.₂ Storage sites.