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, 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. (2025). 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, under review (manuscript available in this PDF).

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 bentonite buffers 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 

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

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