Abstract
Mudrocks, ubiquitous yet poorly understood sedimentary rocks with significant variations in composition and physical properties, form seals for geological carbon dioxide and energy (e.g., hydrogen and methane) storage, repositories for radioactive waste disposal, and reservoirs for natural gas. Understanding the controls on mudrock pore structure is essential for evaluating their porosity. The identification and quantification of controls depend on the nano-to micron scale pore network, which are the subject of this study. Small-angle (SANS) and very small-angle neutron scattering (VSANS) experiments were conducted on 13 diverse mudrock sets, characterised by differences in mineralogy, stratigraphy, maturity, and depositional environment. We performed multivariate statistics to systematically characterise the pore structure in 71 samples cross a 5 μm–2 nm pore size range. Our results indicate a multivariate approach more effectively captures the complex controls on porosity rather than single parameters. Compaction and clay content emerge as key primary and secondary controls on mudrock porosity, respectively, upon which we introduce a new porosity classification. Our complementary experimental-statistical assessment involving SANS-derived multiscale porosity sheds new light on the influence of structural controls on storage or production capacity in mudrocks.
| Original language | English |
|---|---|
| Article number | 129966 |
| Journal | Energy |
| Volume | 289 |
| DOIs | |
| State | Published - Feb 15 2024 |
Funding
Porosity and SSA for pore sizes between 5 μm–2 nm are illustrated in Fig. 1-A, subdivided into macropore and mesopore sizes according to IUPAC [7] as well as pores from 10 nm–2 nm that will be further referred to as nanopores in the following. Nanopores are taken to observe their contribution to porosity. Mudrocks show a log-normal distribution for both porosity (total and subgroups) and SSA. For all mudrocks except Carmel, the macroporosity is larger than the corresponding mesoporosity. In general, macropores make up only ∼1 % of the total SSA. The majority of SSA and total porosity resides in nanopores, which makes up ∼98 % and ∼60 %, respectively – see supporting information (SI, S3.1), Table S8. Nanopores are well-connected and oriented along bedding, resulting in low and anisotropic permeability [14], influencing gas flow, which ranges from transitional to diffusional regimes [31]. This work is based upon experiments performed at the KWS-1 and KWS-3 instruments operated by Julich Centre for Neutron Science at the Heinz Maier-Leibnitz Zentrum, Garching, Germany. We are very grateful for the possibility to conduct measurements at these instruments and thank Artem Feoktystov for helping with the data collection. We thank SCK-CEN for providing Boom Clay samples, Swisstopo, Switzerland for providing Opalinus Shale samples, and Norske Shell, Norway for making Våle shale samples available to this study. We are thankful to Pieter Bertier, Leon Leu, and Timo Seemann for useful discussions and support in acquiring SANS data. Contributions to the manuscript by Gernot Rother were supported by the Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division.
Keywords
- Mudrock
- Multivariate statistics
- Porosity
- Small-angle neutron scattering (SANS)
- Very small-angle neutron scattering (VSANS)
