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

越来越多地使用水力压裂技术从页岩矿床中开采石油和天然气，已成为美国经济增长最快的组成部分之一。页岩位于地下深处，是一种复杂的地质介质，由有机组分（如干酪根）和无机组分（如粘土、硅酸盐）组成。高达85%的页岩燃料以吸附碳氢化合物混合物的形式被封装在纳米大小的孔隙中。吸附烃对页岩产生应力，在开采过程中，该应力释放，使页岩减压并产生变形。这种压缩/减压在页岩的干酪根部分中被观察到超过20%。孔隙的变形既影响其吸附能力，也影响开采过程中燃料通过页岩储层的渗透性。容量和回收率影响回收过程的能源效率，以及回收的燃料量。这些影响在高温高压的深层地质构造中可能会加剧。进一步了解纳米多孔介质中流体约束之间的相互作用、由此产生的应力以及这些因素如何影响渗透率和产能，将有助于优化页岩油气采收率。本项目将孔隙弹性固体的统计力学理论与新型高压地球物理实验测量相结合，建立纳米多孔介质吸附诱导变形的验证理论。该项目的目标是将吉布斯过量吸附理论与宏观生物孔隙弹性理论相结合。Biot理论描述了饱和流体的多孔体如何在流体压力和外部应力的作用下变形，而Gibbs过量吸附理论描述了流体如何在表面附近集中，特别是在纳米多孔吸附剂中。分子杠杆模型和蒙特卡罗模拟将用于探索典型的轻烃和二氧化碳混合物在柔性吸附剂纳米孔中的相行为和分离，并预测在给定的外部条件下的吸附能力和选择性，以及吸附剂的应力和应变。该理论将通过模型材料与高压混合气体相互作用的吸附应力实验验证。本项目将建立测量烃类混合物吸附过程中吸附剂应力和应变的新技术。如果成功，该项目将把吸附科学与地球物理学的理论结合起来，并对柔性吸附剂和分离膜、致动器、纳米缓冲器和储能装置的设计产生进一步的影响。本项目将培养2名博士和3名本科生。教育和社区外展计划有助于从代表性不足的少数群体中招募学生，为高中学生和教师提供暑期机会，参与女生到工程日和STEM节等特殊活动。新颖的模拟方法和案例研究主题将被纳入PIs的纳米尺度热力学与输运和高级地质力学研究生课程。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.