Sealing Shales versus Brittle Shales: A Sharp Threshold in the Material Properties and Energy Technology Uses of Fine-Grained Sedimentary Rocks

Fine-grained sedimentary rocks (shale and mudstone) play important roles in global CO2 abatement efforts through their uses in carbon capture and storage (CCS), radioactive waste storage, and shale gas extraction. These different technologies, however, rely on seemingly conflicting premises regarding the sealing properties of shale and mudstone, suggesting that those 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, Xclay, is a very important variable the controls key material properties of these formations and that a remarkably sharp threshold at Xclay ∼ /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 uses of shale and mudstone formations for CCS, radioactive waste storage, and shale gas extraction. ■ INTRODUCTION Fine-grained sedimentary rocks (hereafter termed shales and mudstones) account for roughly two-thirds of the sedimentary rock mass. Vast formations of these rocks play important roles in three low-carbon energy technologies that have the potential to contribute up to 70% of global CO2 abatement efforts required to stabilize atmospheric CO2 levels over the next half-century (Figure 1). In carbon capture and storage (CCS), shale and mudstone are the predominant lithologies used or considered for use as caprocks of geologic CO2 storage sites. In nuclear energy production, they constitute a promising option for isolating radioactive waste on time scales greater than the half-lives of long-lived radioactive fission products. Finally, the transition from coal to natural gas as an energy source in North America is driven largely by hydrocarbons extracted from fine-grained sedimentary rocks. These multiple emerging roles of shale and mudstone have prompted concerns about the allocation of these rocks between hydrocarbon extraction, CCS, and nuclear waste storage. The technologies listed above rely on the ability of finegrained sedimentary rocks to essentially immobilize fluids (water, CO2, and hydrocarbons) in the subsurface. Chemical, isotopic, and fluid pressure gradients across shale and mudstone formations indicate that these rocks, at least in some cases, maintain their very low permeability (on the order of 10−20 m in core-scale laboratory experiments) over length scales of hundreds of meters and time scales of millions of years. Utilization of shale and mudstone in CCS and radioactive waste Received: July 9, 2015 Accepted: September 9, 2015 Figure 1. Contribution of different low-carbon energy technologies to global CO2 emission reductions over the next century according to one scenario proposed by the Intergovernmental Panel on Climate Change (IPCC). The checkered area highlights the technologies that rely on shale and mudstone. Figure modified from ref 4. Review pubs.acs.org/journal/estlcu © XXXX American Chemical Society A DOI: 10.1021/acs.estlett.5b00233 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX D ow nl oa de d by P R IN C E T O N U N IV o n Se pt em be r 11 , 2 01 5 | h ttp :// pu bs .a cs .o rg P ub lic at io n D at e (W eb ): S ep te m be r 11 , 2 01 5 | d oi : 1 0. 10 21 /a cs .e st le tt. 5b 00 23 3