Shales are the largest component of sedimentary basin fill, and they strongly affect the hydrology of groundwater systems. Shales play increasingly dominant roles in subsurface energy development and environmental protection, from hydrocarbon production to secure geologic storage of carbon dioxide and nuclear waste repositories in shale.
Shales—defined here as fine-grained clay-rich rocks—present great challenges for study and engineering because of their enormous heterogeneity in composition and material properties. These rocks exhibit multiscale structure, including nanoscale pores, nanoscale to macroscale natural and engineered fractures, and intricate textural architectures due to sedimentary and postdepositional (e.g., burial and tectonic) processes. To further complicate matters, these rocks undergo the full range of thermal-hydraulic-mechanical-chemical-biological (THMCB) coupled processes with multiscale rates of changes.
Understanding these systems, with their natural and engineered multiscale heterogeneity, has proven challenging and requires new fundamental knowledge, sophisticated tools, and advanced predictive capabilities. To discuss shale research and technology needs with a focus on coupled processes, the Shales at All Scales workshop gathered 47 researchers from academia, the U.S. Department of Energy, geological surveys, industry, and national laboratories.
Experts in geology, hydraulic and energetic stimulation (i.e., “fracking” and explosives use), hydrology, inorganic and organic geochemistry, geomechanics, nanotechnology, numerical modeling, and petroleum engineering made presentations on a variety of major themes:
- The complex and pervasive heterogeneity of shale limits understanding. This heterogeneity at all scales has an impact on field measurements, the usefulness of laboratory results, and the prediction of coupled processes.
- Intermolecular and surface forces of fluids under nanoconfinement in shale produce nonbulk and difficult-to-characterize phase behaviors and transport processes.
- Matric potential (the combined effects of capillarity and adsorptive forces), osmotic potential, and the activity of water may be as important as pressure gradients in controlling multiphase fluid flow.
- Not all fractures are equal—natural fractures manifest in a variety of configurations, each with their own connectivity and potential interaction with hydraulic fractures.
Break-out discussions revealed that a common language, or “mental picture,” among disciplines is needed. Shale does not even have a broadly accepted and useful definition. Attendees developed the following research priorities:
- Disciplines that study shale should develop their own clear conceptual models and analysis approaches. Collectively, the disciplines should then synthesize those models and strive for greater agreement on how to conceptualize and model coupled processes.
- Research should focus on coupled THMCB perturbations resulting from engineered activities that may have significant or disparate time scales for reequilibration and that may affect site performance.
- A new Shale Consortium should be created that would support a shale core repository, where carefully preserved cores and fluids could be accessed on the basis of user proposals.
The workshop program and selected presentations are available online.
Funding for the workshop was provided by Sandia National Laboratories. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
—Jason Heath, Department of Geomechanics, Sandia National Laboratories, Albuquerque, N.M.; email: [email protected]; and Anastasia Ilgen, Department of Geochemistry, Sandia National Laboratories, Albuquerque, N.M.
Citation: Heath, J., and A. Ilgen (2015), Understanding multiscale coupled processes in shale, Eos, 96, doi:10.1029/2015EO036447. Published on 5 October 2015.