Research

Bentonite clay is widely used as an engineered barrier for isolating high-level radioactive waste, where it experiences strong thermal-hydraulic-mechanical-chemical (THMC) gradients. We developed methodologies utilizing large-scale molecular dynamics (MD) simulations to predict the material properties of compacted bentonite. This research offers new insight into the coupled THMC properties of clay barrier systems and informs long-term performance assessments. 

Relevant  publications

1. Zheng, X. & Bourg, I. C. (2023). Nanoscale prediction of the thermal, mechanical, and transport properties of hydrated clay on 106- and 1015-fold larger length and time scales. ACS Nano. https://doi.org/10.1021/acsnano.3c05751

2. Zheng, X., Underwood, T. R., & Bourg, I. C. (2023). Molecular dynamics simulation of thermal, hydraulic, and mechanical properties of bentonite clay at 298 to 373 K. Applied Clay Science, 240, 106964. https://doi.org/10.1016/j.clay.2023.106964

3. Zheng, X., Harrington, J. F., Bourg, I. C. (2025). Nanoscale prediction and experimental verification of the properties of compacted bentonite clay at temperatures above 100 C. Applied Clay Science, to be submitted pending complementary experimental results by collaborator Jon Harrington of the British Geological Survey (manuscript available in this PDF).

We developed a coarse-grained (CG) model capable of simulating thousands of smectite clay platelets at micrometer scales. This model captures the formation of hierarchical clay aggregates and enables predictions of the microstructure, dynamics, and THMC properties of clay minerals. The approach provides new opportunities for understanding clay-rich materials including drilling fluids, soils, bentonite, shale, mudstone, and clay barrier. 

Relevant  publications

1. Shen, X., Zheng, X., & Bourg, I. C. (2025). A coarse-grained model of clay colloidal aggregation and consolidation with explicit representation of the electrical double layer. Journal of Colloid and Interface Science. https://doi.org/10.1016/j.jcis.2024.12.053

2. Zheng, X., Shen, X., Bourg, I. C. (2025). Coarse-grained simulation of colloidal self-assembly, cation exchange, and rheology in Na/Ca smectite clay gels. Journal of Colloid and Interface Science. https://doi.org/10.1016/j.jcis.2025.137573

3. Zheng, X. & Bourg, I. C. (2026). Microstructure, transport, and mechanics of compacted clay simulated at the 0.1 μm scale (1400 smectite clay particles) using a coarse-grained model with explicit counterions. Journal of Physical Chemistry C. https://doi.org/10.1021/acs.jpcc.6c00004.

The macroscopic strength, durability, and contaminant retention of fly-ash–stabilised marine clay are controlled by nanoscale chemical and mechanical interactions at the smectite–NASH interface. We propose a multiscale computational framework to quantify adsorption energies, activation barriers, gel polymerization, ion exchange, and heavy-metal binding. Simulation predictions will link nanoscale interfacial mechanisms to emergent mechanical properties and constitutive relationships of clay–binder assemblages.

Relevant  publications

1.  Zhan, W. & Zheng, X. (2026). Composition-topology coupling programs clay-gel interphase assembly in alkali-activated aluminosilicates, in preparation.

Despite the widespread presence of nanoplastics in waste streams and subsurface environments, their interactions with clay barriers remain poorly understood at the molecular and pore scales. Critical knowledge gaps persist: How do chemically distinct nanoplastics modify adsorption behavior and selectivity on clay mineral surfaces, and influence the retention or remobilization of co-occurring contaminants? How do these molecular-scale interactions propagate to alter clay aggregation, pore-size heterogeneity, and emergent microstructures that ultimately control the transport, mechanical integrity, and sealing performance of clay barriers? Addressing these questions requires modeling capabilities that bridge molecular interactions and collective mesoscale behavior.

Relevant  publications

1.  Dai, W. & Zheng, X. (2026). Nanoplastics as Sinks or Mobilizers: PFAS Partitioning, Mobility, and Retention in Clay–Water Systems. Environmental Science & Technology, in preparation.

PFAS are distributed among the dissolved phase, suspended particulate matter (SPM), and clay-rich sediments. Natural organic matter (NOM) and clay minerals collectively regulate PFAS adsorption, partitioning, and long-term retention through molecular-scale interfacial interactions. The environmental fate of PFAS is further influenced by organic matter chemistry, organic matter content, and salinity, which together determine contaminant mobility and persistence in sedimentary environments.

Relevant  publications

1.  Lu, M. & Zheng, X. (2026). Molecular controls on PFAS retention and remobilization in organic-rich sediments. Environmental Science & Technology, in preparation.

Understanding the coupling between multiphase fluid flow in pores with distinct sizes and solid deformation induced by flow or external stresses is crucial for the development of many important geotechnics. The Darcy-Brinkman-Biot (DBB) framework can capture capillary, viscous, inertial, interfacial, and gravitational forces at both the pore and Darcy scales. In this work, we extend the DBB framework to model non-isothermal fluid flow. The model’s numerical implementation, hybridBiotThermalInterFoam, is achieved in the Computational Fluid Dynamics (CFD) software OpenFOAM (https://github.com/xiaojinz/hybridBiotThermalInterFoam). Results show that the new solver is capable of predicting fracture propagation and healing in soft materials exposed to strong thermal fluxes and complex aqueous chemistry conditions. The development in this work creates the first model representing multiphase non-isothermal fluid flow in multiscale deformable porous media.

Relevant  publications

1. Zheng, X. & Bourg, I. C. (2025). A multiscale approach to simulate non-isothermal multiphase flow in deformable porous materials. Water Resources Research. https://doi.org/10.1029/2025WR041300 

Most current vadose-zone PFAS models assume a rigid soil skeleton and a passive air phase. In this work we develop hybridBiotThermalSoluteInterFoam (HBTSIF), a volume-averaged, single-field transport model. It embeds a two-sided interfacial adsorption closure, here exercised in its air–water, aqueous-only limit, within a two-phase Darcy–Brinkman solver. We verify the framework against the Ogata–Banks analytical solution: on resolvable fronts the single-field solution matches the analytical profile to within 0.1% and reproduces the closed-form retardation law. 

Relevant  publications

1.  Ning, Y. & Zheng, X. (2026). A single-field two-phase model for PFAS air-water interfacial retention in variably saturated soils, in preparation.

Disused sealed radioactive sources must be confined until their inventory decays. However, their decay heat lowers the sorption affinity of the surrounding clay, and its radiation field degrades the material. In this work we develop  hybridBiotThermalRadioInterFoam (HBTRIF), a two-phase Darcy–Brinkman reactive-transport solver with a complete radionuclide subsystem: multi-nuclide Bateman decay chains whose sink acts on the total (dissolved + sorbed) inventory; a volumetric decay-heat source generated by the transported inventory itself and feeding back on temperature-dependent diffusion, viscosity and sorption; screened-Poisson γ-dose transport with strength degradation; He-gas pore pressure; and Mohr–Coulomb stability screening. 

Relevant  publications

1.   Ning, Y. & Zheng, X. (2026). Coupled thermo–hydro–chemical–radiological modelling of radionuclide fate and near-field stability around a buried heat-emitting waste source in marine clay, in preparation.

2.  Ning, Y. & Zheng, X. (2026). An open-source, file-coupled two-way relay between a reactive-transport/decay-heat THC solver and elasto-plastic rock mechanics: verification and application to underground nuclear caverns in granite, in preparation.

Pressure monitoring above the injection zone is a potential method to detect potential CO2 leaks into overlying formations. We applied a compositional simulator coupled with geomechanics to predict pressure changes above the caprock due to both fast hydraulic communication and partially undrained loading. The results reveal that pressure monitoring above the caprock is a feasible technology to track the CO2 plume and interpretation of pressure signals in the field must account for partially undrained poroelastic loading. Moreover, the presence of fault heterogeneity can lead to different trapping capacities for the same structure. We developed two stochastic models including a continuous shale gouge model and a discrete smear model to statistically determine the possible range of CO2 column height in sand-shale sequences. The results enable effective prediction of CO2 column height in the presence of heterogeneous clay smears in faults.

Relevant  publications

1. Zheng, X. & Espinoza, D. N. (2022). Stochastic quantification of CO2 fault sealing capacity in sand-shale sequences. Marine and Petroleum Geology, 105961. https://doi.org/10.1016/j.marpetgeo.2022.105961

2. Zheng, X., Espinoza, D. N., Vandamme, M., & Pereira, J.-M. (2022). CO2 plume and pressure monitoring through pressure sensors above the caprock. International Journal of Greenhouse Gas Control, 117, 103660. https://doi.org/10.1016/j.ijggc.2022.103660

3. Zheng, X., Espinoza, D. N., Vandamme, M., & Pereira, J.-M. (2021). Pressure monitoring above the injection zone for CO2 geological storage. 55th U.S. Rock Mechanics/Geomechanics Symposium, OnePetro. https://onepetro.org/ARMAUSRMS/proceedings/ARMA21/All-ARMA21/ARMA-2021-1609/468260

Faults are key geologic components defining fluid migration pathways in sedimentary basins. We ran experiments to quantify the effects of grain size, porosity, and clay content on the transport properties of smectite-rich fault gouge in faults, including CO2 breakthrough pressure and post-breakthrough CO2 permeability. The results of this work helped quantitatively evaluate fault sealing capability and migration of buoyant fluids through faults in sand-shale sequences. In addition, the injection of fluids into a compartmentalized formation induces pore pressure buildup and may result in fault reactivation. We measured the new uniaxial strain unloading compressibility of unconsolidated Frio sand to predict the pressure increase during fluid injection. The results generate useful guidelines for subsurface fluid injections to prevent excessive pressure buildup in storage formations.

Relevant  publications

1. Zheng, X. & Espinoza, D. N. (2021). Measurement of unloading pore volume compressibility of Frio sand under uniaxial strain stress path and implications on reservoir pressure management. Rock Mechanics and Rock Engineering. https://doi.org/10.1007/s00603-021-02571-3

2. Zheng, X. & Espinoza, D. N. (2021). Multiphase CO2-brine transport properties of synthetic fault gouge. Marine and Petroleum Geology, 129, 105054. https://doi.org/10.1016/j.marpetgeo.2021.105054

3. Zheng, X., Sun, Z., & Espinoza, N. D. (2019). Uniaxial strain unloading compressibility of Frio sand: measurements and implications on reservoir pressure management for CO2 storage. 53rd U.S. Rock Mechanics/Geomechanics Symposium, OnePetro. https://onepetro.org/ARMAUSRMS/proceedings/ARMA19/All-ARMA19/ARMA-2019-0379/124732