Sealing shales versus brittle shales: A threshold in the properties and uses of fine-grained sedimentary rocks

Abstract:

Fine-grained sedimentary rocks (shale, mudstone) play important roles in global CO₂ abatement efforts through their importance in carbon capture and storage (CCS), radioactive waste storage, and shale gas extraction. These different technologies rely on seemingly conflicting premises regarding the sealing properties of shale and mudstone, suggesting that fine-grained rocks that lend themselves to hydrocarbon extraction may not be optimal seals for CCS or radioactive waste storage, and vice versa. In this paper, a compilation of experimental data on the properties of well-characterized shale and mudstone formations is used to demonstrate that clay mineral mass fraction, X_clay, is a master variable that controls key material properties of these formations and that a remarkably sharp threshold at X_clay ~ 1/3 separates fine-grained rocks with very different properties. This threshold coincides with the predictions of a simple conceptual model of the microstructure of sedimentary rocks and is reflected in the applications of shale and mudstone formations for CCS, radioactive waste storage, and shale gas extraction.