Collaborative Research: Deformation of poroelastic nanoporous materials of hierarchical structure upon adsorption of gas mixtures: theory, molecular modeling and experiments

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The increased use of hydraulic fracturing to recover oil and gas from shale deposits has become one of the most rapidly growing components of the US economy. Located deep underground, shales are complex geological media comprised of both organic (e.g. kerogen) and inorganic fractions (e.g. clays, silicates). Up to 85% of the shale fuel is encapsulated in nanometer sized pores in the form of adsorbed hydrocarbon mixtures. Adsorbed hydrocarbons exert a stress on the shale, and in the process of recovery, this stress is released, which may decompress the shale and induce its deformation. This compression/decompression has been observed to be over 20% in the kerogen fraction of the shale. Deformation of the pores influences both their adsorption capacity and subsequent permeation of the fuel through the shale reservoir as it is recovered. Capacity and recovery rate influence the energy efficiency of the recovery process, as well as the amount of fuel recovered. These effects can be exacerbated at the high temperatures and pressures typical of deep geological formations. An increased understanding of the interplay between fluid confinement in nanoporous media, the stresses this induces, and how these factors influence permeation and capacity will help optimize hydrocarbon recovery from shales. This project will combine statistical mechanics theory of poroelastic solids with novel high-pressure geophysical experimental measurements to develop a validated theory on adsorption-induced deformation of nanoporous media. The objective of the project is to couple the Gibbs theory of excess adsorption with the macroscopic Biot theory of poroelasticity. The Biot theory describes how a porous body saturated with a fluid deforms under the action of fluid pressure and external stresses, whereas the Gibbs theory of excess adsorption describes how fluids concentrate near a surface, particularly in nanoporous adsorbents. Molecular lever models and Monte Carlo simulation will be used to explore phase behavior and separation of typical mixtures of light hydrocarbons and carbon dioxide in nanopores of compliant adsorbents and predict the adsorption capacity and selectivity, as well as the adsorbent stress and strain at given external conditions of pressure, temperature, and adsorbate mixture composition. The combined theory will be validated with experimental demonstration of adsorption stress of model materials interacting with high pressure gas mixtures. The project will establish new techniques for measuring the adsorbent stresses and strains in the process of adsorption of hydrocarbon mixtures. If successful, the project will converge theories from adsorption science with geophysics, and have further implications for the design of flexible adsorbents and separation membranes, actuators, nanobumpers, and energy storage devices. Two PhD and three undergraduate students will be trained within this project. Educational and community outreach program facilitates student recruitment from underrepresented minority groups, summer opportunities for high school students and teachers, participation is special events such as Girl to Engineering Day and STEM Festival. Novel simulation methods and case-study topics will be incorporated into PIs graduate courses on Nanoscale Thermodynamics and Transport and Advanced Geomechanics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.