Subsurface Science and Engineering of Shale

Fundamental conceptual understanding and practical applications for successful subsurface science and engineering of shale is lacking. This volume will bring together a series of chapters from recognized experts from industry, academia, private research institutions, and national laboratories compiling of a state-of-the-art of current science and engineering practices involving this most enigmatic of rock types. This will include a basic overview of shale heterogeneity from nanometers to kilometers, the basic science of coupled multiphysics modelling of shale rock in the subsurface (i.e. flow, transport, geochemistry, geomechanics), and engineering practices associated with shale oil and gas extraction, seal integrity for carbon and other waste storage, and other shale-containing natural resources. The existing state of knowledge on shales will be surveyed and relevant research needs in relation to subsurface engineering endeavors will be organized for the coming decades.

The timing has never been more opportune for a summary treatise on the subsurface science and engineering of shale; which is supported by several factors. New imaging techniques, including dual focused ion beam-scanning electron and neutron scattering methods, have emerged in the last five years that allow unprecedented internal three-dimensional views into shale pore network topologies and pore lining phases at previously unobtainable lengths scales (i.e. nanometers). Shales have always been considered as a major sealing lithology and source for oil and gas reservoirs. Inasmuch as these seals already existed for trapping hydrocarbons, and hydrocarbon generation and migration was mostly ancient history, interest in shale specifically as an engineering target was never a priority for the oil and gas industry.

Industry was always more concerned with conventional sandstone and carbonate reservoirs that lie beneath the shales, and the shale formations themselves were mostly viewed as problematic from a drilling perspective and handled on a field-by-field basis. But recent occurrences have changed this perspective which is detailed in the key points below:

While shales were long-known reservoirs for oil and gas, and so-called hydrofracking was viable technology for reservoir stimulation since the 1950s, advances in horizontal drilling have only recently made extraction of oil and gas from shales economically viable--hence the shale gas "revolution."- Recent recognition of saline formations as a large potential reservoir for secure carbon sequestration brings the need for secure storage, and hence "caprocks" as sealing formations. Inasmuch as most saline formations are clastic reservoirs composed of sandstones, depositional predisposition led to shales as the dominant proposed caprock lithology. The question of storage security, and caprock integrity, for geologic carbon storage, is largely a question of shale performance assessment. Natural and induced fractures in caprock as potential leakage pathways is a difficult research topic from field, laboratory, and multi-physics reactive transport modeling perspectives, but key to regulatory and public acceptance of large-scale CO2 storage. The "shelving" of Yucca Mountain as the geologic repository for US nuclear waste has led to a reconsideration of shale formations as such in the US. The recognition that shale formation pore-water overpressure, resulting from ice sheet compression, has persisted at least for 10,000 years is strong evidence for the sealing potential for these rock types on repository-requisite time scales. Shales are the dominant lithology considered for proposed European repositories. Shales are under renewed scrutiny as hosts for light and heavy rare-earth elements. These are in high demand for aerospace and electronic industries and there is growing concern of a worldwide shortage. A newly released study by the US Geological Survey suggests that US reserves in predominately shale formations are larger than previously thought, with potential geopolitical consequences. These four factors, taken together, offer justification for a new summary treatise on shale as proposed herein.