Hydrogen geo-storage is attracting substantial interdisciplinary interest as a cost-effective and sustainable option for medium- and long-term storage. Hydrogen can be stored underground in diverse formations, including aquifers, salt caverns, and depleted oil and gas reservoirs. The wetting dynamics of the hydrogen-brine-rock system are critical for assessing both structural and residual storage capacities, and ensuring containment safety. Through molecular dynamics simulations, we explore how varying concentrations of cushion gases (CO2 or CH4) influence the wetting properties of hydrogen-brine-clay systems under geological conditions (15 MPa and 333 K). We employed models of talc and the hydroxylated basal face of kaolinite (kaoOH) as clay substrates. Our findings reveal that the effect of cushion gases on hydrogen-brine-clay wettability is strongly dependent on the clay-brine interactions. Notably, CO2 and CH4 reduce the water wettability of talc in hydrogen-brine-talc systems, while exerting no influence on the wettability of hydrogen-brine-kaoOH systems. Detailed analysis of free energy of cavity formation near clay surfaces, clay-brine interfacial tensions, and the Willard-Chandler surface for gas-brine interfaces elucidate the molecular mechanisms underlying wettability changes. Our simulations identify empirical correlations between wetting properties and the average free energy required to perturb a flat interface when clay-brine interactions are less dominant. Our thorough thermodynamic analysis of rock-fluid and fluid-fluid interactions, aligning with key experimental observations, underscores the utility of simulated interfacial properties in refining contact angle measurements and predicting experimentally relevant properties. These insights significantly enhance the assessment of gas geo-storage potential. Prospectively, the approaches and findings obtained from this study could form a basis for more advanced multiscale simulations that consider a range of geological and operational variables, potentially guiding the development and improvement of geo-storage systems in general, with a particular focus on hydrogen storage.
Conceptual models of smectite hydration include planar (flat) clay layers that undergo stepwise expansion as successive monolayers of water molecules fill the interlayer regions. However, X-ray diffraction (XRD) studies indicate the presence of interstratified hydration states, suggesting non-uniform interlayer hydration in smectites. Additionally, recent theoretical studies have shown that clay layers can adopt bent configurations over nanometer-scale lateral dimensions with minimal effect on mechanical properties. Therefore, in this study we used molecular simulations to evaluate structural properties and water adsorption isotherms for montmorillonite models composed of bent clay layers in mixed hydration states. Results are compared with models consisting of planar clay layers with interstratified hydration states (e.g. 1W–2W). The small degree of bending in these models (up to 1.5 Å of vertical displacement over a 1.3 nm lateral dimension) had little or no effect on bond lengths and angle distributions within the clay layers. Except for models that included dry states, porosities and simulated water adsorption isotherms were nearly identical for bent or flat clay layers with the same averaged layer spacing. Similar agreement was seen with Na- and Ca-exchanged clays. In conclusion, while the small bent models did not retain their configurations during unconstrained molecular dynamics simulation with flexible clay layers, we show that bent structures are stable at much larger length scales by simulating a 41.6×7.1 nm2 system that included dehydrated and hydrated regions in the same interlayer.
Solvent expulsion away from an intervening region between two approaching particles plays important roles in particle aggregation yet remains poorly understood. In this work, we use metadynamics molecular simulations to study the free energy landscape of removing water molecules from gibbsite and pyrophyllite slit pores representing the confined spaces between two approaching particles. For gibbsite, removing water from the intervening region is both entropically and enthalpically unfavorable. The closer the particles approach each other, the harder it is to expel water molecules. For pyrophyllite, water expulsion is spontaneous, which is different from the gibbsite system. A smaller pore makes the water removal more favorable. When water is being drained from the intervening region, single chains of water molecules are observed in gibbsite pore, while in pyrophyllite pore water cluster is usually observed. Water-gibbsite hydrogen bonds help stabilize water chains, while water forms clusters in pyrophyllite pore to maximize the number of hydrogen bonds among themselves. This work provides the first assessment into the energetics and structure of water being drained from the intervening region between two approaching particles during oriented attachment and aggregation.
Gas molecule clustering within nanopores holds significance in the fields of nanofluidics, biology, gas adsorption/desorption, and geological gas storage. However, the intricate roles of nanoconfinement and surface chemistry that govern the formation of gas clusters remain inadequately explored. In this study, through free energy calculation in molecular simulations, we systematically compared the tendencies of H2 and CO2 molecules to aggregate within hydrated hydrophobic pyrophyllite and hydrophilic gibbsite nanopores. The results indicate that nanoconfinement enhances gas dimer formation in the nanopores, irrespective of surface chemistry. However, surface hydrophilicity prohibits the formation of gas clusters larger than dimers, while large gas clusters form easily in hydrophobic nanopores. Despite H2 and CO2 both being non-polar, the larger quadrupole moment of CO2 leads to a stronger preference for dimer/cluster formation compared to H2. Our results also indicate that gases prefer to enter the nanopores as individual molecules, but exit the nanopores as dimers/clusters. This investigation provides a mechanistic understanding of gas cluster formation within nanopores, which is relevant to various applications, including geological gas storage.
Pyrimidine has two in-plane CH(δ+)/N̈(δ–)/CH(δ+) binding sites that are complementary to the (δ–/2δ+/δ–) quadrupole moment of CO2. For this study, we recorded broadband microwave spectra over the 7.5–17.5 GHz range for pyrimidine-(CO2)n with n = 1 and 2 formed in a supersonic expansion. Based on fits of the rotational transitions, including nuclear hyperfine splitting due to the two 14N nuclei, we have assigned 313 hyperfine components across 105 rotational transitions for the n = 1 complex and 208 hyperfine components across 105 rotational transitions for the n = 2 complex. The pyrimidine-CO2 complex is planar, with CO2 occupying one of the quadrupolar binding sites, forming a structure in which the CO2 is stabilized in the plane by interactions with the C–H hydrogens adjacent to the nitrogen atom. This structure is closely analogous to that of the pyridine-CO2 complex studied previously by (Doran, J. L. J. Mol. Struct. 2012, 1019, 191–195). The fit to the n = 2 cluster gives rotational constants consistent with a planar cluster of C2v symmetry in which the second CO2 molecule binds in the second quadrupolar binding pocket on the opposite side of the ring. The calculated total binding energy in pyrimidine-CO2 is –13.7 kJ mol–1, including corrections for basis set superposition error and zero-point energy, at the CCSD(T)/ 6-311++G(3df,2p) level, while that in pyrimidine-(CO2)2 is almost exactly double that size, indicating little interaction between the two CO2 molecules in the two binding sites. The enthalpy, entropy, and free energy of binding are also calculated at 300 K within the harmonic oscillator/rigid-rotor model. This model is shown to lack quantitative accuracy when it is applied to the formation of weakly bound complexes.
Understanding pure H2 and H2/CH4 adsorption and diffusion in earth materials is one vital step toward a successful and safe H2 storage in depleted gas reservoirs. Despite recent research efforts such understanding is far from complete. In this work we first use Nuclear Magnetic Resonance (NMR) experiments to study the NMR response of injected H2 into Duvernay shale and Berea sandstone samples, representing materials in confining and storage zones. Then we use molecular simulations to investigate H2/CH4 competitive adsorption and diffusion in kerogen, a common component of shale. Our results indicate that in shale there are two H2 populations, i.e., free H2 and adsorbed H2, that yield very distinct NMR responses. However, only free gas presents in sandstone that yields a H2 NMR response similar to that of bulk H2. About 10 % of injected H2 can be lost due to adsorption/desorption hysteresis in shale, and no H2 loss (no hysteresis) is observed in sandstone. Our molecular simulation results support our NMR results that there are two H2 populations in nanoporous materials (kerogen). The simulation results also indicate that CH4 outcompetes H2 in adsorption onto kerogen, due to stronger CH4-kerogen interactions than H2-kerogen interactions. Nevertheless, in a depleted gas reservoir with low CH4 gas pressure, about ∼30 % of residual CH4 can be desorbed upon H2 injection. The simulation results also predict that H2 diffusion in porous kerogen is about one order of magnitude higher than that of CH4 and CO2. This work provides an understanding of H2/CH4 behaviors in deleted gas reservoirs upon H2 injection and predictions of H2 loss and CH4 desorption in H2 storage.
Conceptual models of smectite hydration include planar (flat) clay layers that undergo stepwise expansion as successive monolayers of water molecules fill the interlayer regions. However, X-ray diffraction (XRD) studies indicate the presence of interstratified hydration states, suggesting non-uniform interlayer hydration in smectites. Additionally, recent theoretical studies have shown that clay layers can adopt bent configurations over nanometer-scale lateral dimensions with minimal effect on mechanical properties. Therefore, in this study we used molecular simulations to evaluate structural properties and water adsorption isotherms for montmorillonite models composed of bent clay layers in mixed hydration states. Results are compared with models consisting of planar clay layers with interstratified hydration states (e.g. 1W–2W). The small degree of bending in these models (up to 1.5 Å of vertical displacement over a 1.3 nm lateral dimension) had little or no effect on bond lengths and angle distributions within the clay layers. Except for models that included dry states, porosities and simulated water adsorption isotherms were nearly identical for bent or flat clay layers with the same averaged layer spacing. Similar agreement was seen with Na- and Ca-exchanged clays. While the small bent models did not retain their configurations during unconstrained molecular dynamics simulation with flexible clay layers, we show that bent structures are stable at much larger length scales by simulating a 41.6×7.1 nm2 system that included dehydrated and hydrated regions in the same interlayer.
During this LDRD project, our team developed a technology which enables the fabrication of novel nanostructures replicating seashell – “nature’s toughest material”. The resulting coatings exhibit high thermal stability up to 1650°C, which exceeds the hardness of Spectra® by ~44%, as well as the compressive strength of aluminum by ~57%. Coatings made with this technology are stronger, environmentally friendly, more sustainable, and more versatile than other comparable materials. Beryllium wafers, the current, most favorable shielding material in terms of thermal and mechanical properties, are very toxic and cost hundreds of times more than the new material developed in this project. The coatings on silicon wafer and stainless steel, respectively, have been tested as ride-along on the Z machine and clearly outperform the bare substrate. Use of this technology will have a profound global impact for pulsed power and fusion energy development, debris mitigation for spacecraft and satellites, durability of drill bits used in deep well drilling and tunnel boring operations, thermal protection of aircraft and manned spacecraft, and various other thermal and mechanical protection applications.
Jove-Colon, Carlos F.; Ho, Tuan A.; Lopez, Carlos M.; Rutqvist, Jonny; Guglielmi, Yves; Hu, Mengsu; Sasaki, Tsubasa; Yoon, Sangcheol; Steefel, Carl I.; Tournassat, Christophe; Mital, Utkarsh; Luu, Keurfon; Sauer, Kirsten B.; Caporuscio, Florie A.; Rock, Marlena J.; Zandanel, Amber E.; Zavarin, Mavrik; Wolery, Thomas J.; Chang, Elliot; Han, Sol-Chan; Wainwright, Haruko; Greathouse, Jeffery A.
This report represents the milestone deliverable M2SF-23SN010301072 “Evaluation of Nuclear Spent Fuel Disposal in Clay-Bearing Rock - Process Model Development and Experimental Studies” The report provides a status update of FY23 activities for the work package Argillite Disposal work packages for the DOE-NE Spent Fuel Waste Form Science and Technology (SFWST) Program. Clay-rich geological media (often referred as shale or argillite) are among the most abundant type of sedimentary rock near the Earth’s surface. Argillaceous rock formations have the following advantageous attributes for deep geological nuclear waste disposal: widespread geologic occurrence, found in stable geologic settings, low permeability, self-sealing properties, low effective diffusion coefficient, high sorption capacity, and have the appropriate depth and thickness to host nuclear waste repository concepts. The DOE R&D program under the Spent Fuel Waste Science Technology (SFWST) campaign has made key progress (through experiment, modeling, and testing) in the study of chemical and physical phenomena that could impact the long-term safety assessment of heat-generating nuclear waste disposition in clay/shale/argillaceous rock. International collaboration activities comprising field-scale heater tests, field data monitoring, and laboratory-scale experiments provide key information on changes to the engineered barrier system (EBS) material exposed high thermal loads. Moreover, consideration of direct disposal of large capacity dual-purpose canisters (DPCs) as part of the back-end SNF waste disposition strategy has generated interest in improving our understanding of the effects of elevated temperatures on the engineered barrier system (EBS) design concepts. Chemical and structural analyses of sampled bentonite material from laboratory tests at elevated temperatures are key to the characterization of thermal effects affecting bentonite clay barrier performance. The knowledge provided by these experiments is crucial to constrain the extent of sacrificial zones in the EBS design during the thermal period. Thermal, hydrologic, mechanical, and chemical (THMC) data collected from heater tests and laboratory experiments have been used in the development, validation, and calibration of THMC simulators to model near-field coupled processes. This information leads to the development of simulation approaches to assess issues on coupled processes involving porous media flow, transport, geomechanical phenomena, chemical interactions with barrier/geologic materials, and the development of EBS concepts. These lines of knowledge are central to the design of deep geological backfilled repository concepts where temperature plays a key role in the EBS behavior, potential interactions with host rock, and long-term performance in the safety assessment.
Using compressive mechanical forces, such as pressure, to induce crystallographic phase transitions and mesostructural changes while modulating material properties in nanoparticles (NPs) is a unique way to discover new phase behaviors, create novel nanostructures, and study emerging properties that are difficult to achieve under conventional conditions. In recent decades, NPs of a plethora of chemical compositions, sizes, shapes, surface ligands, and self-assembled mesostructures have been studied under pressure by in-situ scattering and/or spectroscopy techniques. As a result, the fundamental knowledge of pressure-structure-property relationships has been significantly improved, leading to a better understanding of the design guidelines for nanomaterial synthesis. In the present review, we discuss experimental progress in NP high-pressure research conducted primarily over roughly the past four years on semiconductor NPs, metal and metal oxide NPs, and perovskite NPs. We focus on the pressure-induced behaviors of NPs at both the atomic- and mesoscales, inorganic NP property changes upon compression, and the structural and property transitions of perovskite NPs under pressure. We further discuss in depth progress on molecular modeling, including simulations of ligand behavior, phase-change chalcogenides, layered transition metal dichalcogenides, boron nitride, and inorganic and hybrid organic-inorganic perovskites NPs. These models now provide both mechanistic explanations of experimental observations and predictive guidelines for future experimental design. We conclude with a summary and our insights on future directions for exploration of nanomaterial phase transition, coupling, growth, and nanoelectronic and photonic properties.
Gas intercalation into clay interlayers may result in hydrogen loss in the geological storage of hydrogen; a phenomenon that has not been fully understood and quantified. Here we use metadynamics molecular simulations to calculate the free energy landscape of H2 intercalation into montmorillonite interlayers and the H2 solubility in the confined water; in comparison with results obtained for CO2. The results indicate that H2 intercalation into hydrated interlayers is thermodynamically unfavorable while CO2 intercalation can be favorable. H2 solubility in hydrated clay interlayers is in the same order of magnitude as that in bulk water and therefore no over-solubility effect due to nanoconfinement is observed - in striking contrast with CO2. These results indicate that H2 loss and leakage through hydrated interlayers due to intercalation in a subsurface storage system, if any, is limited.
Oriented attachment (OA) of nanoparticles is an important crystal growth pathway in the synthesis of hierarchical structures. Although a significant understanding of OA has been made, the effect of atomistic misalignment and the roles of solvent/particle and particle/particle interactions on the structure-energy relationship during an OA remain elusive. In this study, we perform molecular dynamics simulations to calculate the potential of mean force (PMF) profile for gibbsite particle translation on a gibbsite slab with 1 or 2 intervening water layers (1W or 2W). The structures of the gibbsite surfaces and the confined water are analyzed to determine how the number and type of hydrogen bonds (H-bonds) influence the free energy profile during the translation. The PMF profile exhibits a periodicity of length 5.078 Å, consistent with the gibbsite unit cell size along the translation direction. The changes in the surface-water and water-water hydrogen bond network and water and surface OH groups’ orientations during the translation are strongly coupled with the changes in the PMF profile in the 1W case. However, when increasing the number of intervening water layers from 1W to 2W, the particle/slab misalignment becomes a dominant factor controlling the behavior of the PMF profile. We also establish a method to quantify misalignment between the particle and the slab, which exhibits a strong correlation with the free energy for the 2W case. These results shed more light into the roles of particle/slab misalignment and hydrogen bond network in the OA of mineral particles in aqueous solution.
Numerous experimental investigations indicated that expansive clays such as montmorillonite can intercalate CO2 preferentially into their interlayers and therefore potentially act as a material for CO2 separation, capture, and storage. However, an understanding of the energy-structure relationship during the intercalation of CO2 into clay interlayers remains elusive. Here, we use metadynamics molecular dynamics simulations to elucidate the energy landscape associated with CO2 intercalation. Our free energy calculations indicate that CO2 favorably partitions into nanoconfined water in clay interlayers from a gas phase, leading to an increase in the CO2/H2O ratio in clay interlayers as compared to that in bulk water. CO2 molecules prefer to be located at the centers of charge-neutral hydrophobic siloxane rings, whereas interlayer spaces close to structural charges tend to avoid CO2 intercalation. The structural charge distribution significantly affects the amount of CO2 intercalated in the interlayers. These results provide a mechanistic understanding of CO2 intercalation in clays for CO2 separation, capture, and storage.
Understanding the formation of H2CO3 in water from CO2 is important in environmental and industrial processes. Although numerous investigations have studied this reaction, the conversion of CO2 to H2CO3 in nanopores, and how it differs from that in bulk water, has not been understood. We use ReaxFF metadynamics molecular simulations to demonstrate striking differences in the free energy of CO2 conversion to H2CO3 in bulk and nanoconfined aqueous environments. We find that nanoconfinement not only reduces the energy barrier but also reverses the reaction from endothermic in bulk water to exothermic in nanoconfined water. Also, charged intermediates are observed more often under nanoconfinement than in bulk water. Stronger solvation and more favorable proton transfer with increasing nanoconfinement enhance the thermodynamics and kinetics of the reaction. Here our results provide a detailed mechanistic understanding of an important step in the carbonation process, which depends intricately on confinement, surface chemistry, and CO2 concentration.
Heavy metals released from kerogen to produced water during oil/gas extraction have caused major enviromental concerns. To curtail water usage and production in an operation and to use the same process for carbon sequestration, supercritical CO2 (scCO2) has been suggested as a fracking fluid or an oil/gas recovery agent. It has been shown previously that injection of scCO2 into a reservoir may cause several chemical and physical changes to the reservoir properties including pore surface wettability, gas sorption capacity, and transport properties. Using molecular dynamics simulations, we here demonstrate that injection of scCO2 might lead to desorption of physically adsorbed metals from kerogen structures. This process on one hand may impact the quality of produced water. On the other hand, it may enhance metal recovery if this process is used for in-situ extraction of critical metals from shale or other organic carbon-rich formations such as coal.
Swelling clay hydration/dehydration is important to many environmental and industrial processes. Experimental studies usually probe equilibrium hydration states in an averaged manner and thus cannot capture the fast water transport and structural change in interlayers during hydration/dehydration. Using molecular simulations and thermogravimetric analyses, we observe a two-stage dehydration process. The first stage is controlled by evaporation at the edges: water molecules near hydrophobic sites and the first few water molecules of the hydration shell of cations move fast to particle edges for evaporation. The second stage is controlled by slow desorption of the last 1-2 water molecules from the cations and slow transport through the interlayers. The two-stage dehydration is strongly coupled with interlayer collapse and the coordination number changes of cations, all of which depend on layer charge distribution. This mechanistic interpretation of clay dehydration can be key to the coupled chemomechanical behavior in natural/engineered barriers.
Using molecular dynamics simulations, we investigate the molecular scale origin of crystal face selectivity when one gibbsite particle attaches to another in water. A comparison of the free energy per unit surface area of particle–particle attachment indicates that particle attachment through edge surfaces, where the edge surfaces are either (1 0 0) or (1 1 0) crystal faces, is more energetically favorable compared to attachment between two basal surfaces (i.e., (0 0 1) crystal faces) or between the basal surface of one particle and the edge surface of another. This result suggests that gibbsite crystals with low basal/edge surface area ratio will preferentially attach through edge surfaces, potentially helping the crystals grow laterally. However, for larger gibbsite particles (high basal/edge surface area ratio) the total free energy, not normalized by surface area, of particle attachment through the basal surfaces is lower (more negative) than attachment through the edge surfaces, indicating that larger gibbsite particles will preferentially aggregate through basal surface attachments. The short-range electrostatic interactions including the interparticle hydrogen bonds from surface –OH groups drive particle attachment, and the dominant contribution to the free energy minimum is enthalpic rather than entropic. However, the enthalpy of basal-edge attachment is significantly offset by the entropy leading to a higher free energy (less negative) compared to that of basal-basal attachment. Study of the free energy for a few imperfect attachments of two particles indicates a higher free energy (i.e., less negative, less stable), compared to a perfect attachment
Upon extraction/injection of a large quantity of gas from/into a subsurface system in shale gas production or carbon sequestration, the gas pressure varies remarkably, which may significantly change the wettability of porous media involved. Mechanistic understanding of such changes is critical for designing and optimizing a related subsurface engineering process. Using molecular dynamics simulations, we have calculated the contact angle of a water droplet on various solid surfaces (kerogen, pyrophyllite, calcite, gibbsite, and montmorillonite) as a function of CO2 or CH4 gas pressure up to 200 atm at a temperature of 300 K. The calculation reveals a complex behavior of surface wettability alteration by gas pressure variation depending on surface chemistry and structure, and molecular interactions of fluid molecules with surfaces. As the CO2 gas pressure increases, a partially hydrophilic kerogen surface becomes highly hydrophobic, while a calcite surface becomes more hydrophilic. Considering kerogen and calcite being the major components of a shale formation, we postulate that the wettability alteration of a solid surface induced by a gas pressure change may play an important role in fluid flows in shale gas production and geological carbon sequestration.
The DOE R&D program under the Spent Fuel Waste Science Technology (SFWST) campaign has made key progress in modeling and experimental approaches towards the characterization of chemical and physical phenomena that could impact the long-term safety assessment of heatgenerating nuclear waste disposition in deep-seated clay/shale/argillaceous rock. International collaboration activities such as heater tests, continuous field data monitoring, and postmortem analysis of samples recovered from these have elucidated key information regarding changes in the engineered barrier system (EBS) material exposed to years of thermal loads. Chemical and structural analyses of sampled bentonite material from such tests as well as experiments conducted on these are key to the characterization of thermal effects affecting bentonite clay barrier performance and the extent of sacrificial zones in the EBS during the thermal period. Thermal, hydrologic, and chemical data collected from heater tests and laboratory experiments has been used in the development, validation, and calibration of THMC simulators to model near-field coupled processes. This information leads to the development of simulation approaches (e.g., continuum and discrete) to tackle issues related to flow and transport at various scales of the host-rock, its interactions with barrier materials, and EBS design concept.
This interim report is an update of ongoing experimental and modeling work on bentonite material described in Jové Colón et al. (2019, 2020) from past international collaboration activities. As noted in Jové Colón et al. (2020), work on international repository science activities such as FEBEX-DP and DECOVALEX19 is either no longer continuing by the international partners. Nevertheless, research activities on the collected sample materials and field data are still ongoing. Descriptions of these underground research laboratory (URL) R&D activities are described elsewhere (Birkholzer et al. 2019; Jové Colón et al. 2020) but will be explained here when needed. The current reports recent reactive-transport modeling on the leaching of sedimentary rock.
Water flow in nanometer or sub-nanometer hydrophilic channels bears special importance in diverse fields of science and engineering. However, the nature of such water flow remains elusive. Here, we report our molecular-modeling results on water flow in a sub-nanometer clay interlayer between two montmorillonite layers. We show that a fast advective flow can be induced by evaporation at one end of the interlayer channel, that is, a large suction pressure created by evaporation (∼818 MPa) is able to drive the fast water flow through the channel (∼0.88 m/s for a 46 Å-long channel). Scaled up for the pressure gradient to a 2 μm particle, the velocity of water is estimated to be about 95 μm/s, indicating that water can quickly flow through a μm-sized clay particle within seconds. The prediction seems to be confirmed by our thermogravimetric analysis of bentonite hydration and dehydration processes, which indicates that water transport at the early stage of the dehydration is a fast advective process, followed by a slow diffusion process. The possible occurrence of a fast advective water flow in clay interlayers prompts us to reassess water transport in a broad set of natural and engineered systems such as clay swelling/shrinking, moisture transport in soils, water uptake by plants, water imbibition/release in unconventional hydrocarbon reservoirs, and cap rock integrity of supercritical CO2 storage.
Smectite (e.g., Montmorillonite): phyllosilicate minerals found in bentonites. Bentonites have been considered as key backfill barrier materials in deep geological nuclear waste repository concepts. Swelling/shrinking of montmorillonite (MMT) occurs with increasing/decreasing relative humidity. Our research question is, "Microscopically, how does the hydration/dehydration process occur?"
Smectite (e.g., Montmorillonite): phyllosilicate minerals found in bentonites. Bentonites have been considered as key backfill barrier materials in deep geological nuclear waste repository concepts. Swelling/shrinking of montmorillonite (MMT) occurs with increasing/decreasing relative humidity. Microscopically, how does the hydration/dehydration process occur?
In shale gas production, gas composition may vary over time. To understand this phenomenon, we use molecular dynamics simulations to study the permeation of CH4, C2H6 and their mixture from a source container through a pyrophyllite nanopore driven by a pressure gradient. For a pure gas, the flow rate of CH4 is always higher than that of C2H6, regardless of pore size. For a 1:1 C2H6: CH4 mixture, however, C2H6:CH4 flow rate ratio is higher than the compositional ratio in the container (i.e., 1:1) when the pore size is smaller than ~1.8 nm. The selective transport is caused by the competitive adsorption of C2H6 over CH4 in the nanopore. The selectivity is also determined by the interplay between the surface diffusion of the adsorbed molecules and the viscous flow in the center of the pore, and it diminishes as the viscous flow becomes to dominate the surface diffusion when the pore size becomes larger than 1.8 nm. Our work shows that compositional differentiation of shale gas in production is a consequence of nanopore confinement and therefore a key characteristic of an unconventional reservoir. The related compositional information can potentially be used for monitoring the status of a production well such as its recovery rate.
The DOE R&D program under the Spent Fuel Waste Science Technology (SFWST) campaign has made key progress in modeling and experimental approaches towards the characterization of chemical and physical phenomena that could impact the long-term safety assessment of heat-generating nuclear waste disposition in deep clay/shale/argillaceous rock. International collaboration activities such as heater tests and postmortem analysis of samples recovered from these have elucidated key information regarding changes in the engineered barrier system (EBS) material exposed to years of thermal loads. Chemical and structural analyses of sampled bentonite material from such tests has as well as experiments conducted on these are key to the characterization of thermal effects affecting bentonite clay barrier performance and the extent of sacrificial zones in the EBS during the thermal period. Thermal, hydrologic, and chemical data collected from heater tests and laboratory experiments has been used in the development, validation, and calibration of THMC simulators to model near-field coupled processes. This information leads to the development of simulation approaches (e.g., continuum vs. discrete) to tackle issues related to flow and transport at various scales of the host-rock and EBS design concept. Consideration of direct disposal of large capacity dual-purpose canisters (DPCs) as part of the back-end SNF waste disposition strategy has generated interest in improving our understanding of the effects of elevated temperatures on the EBS design. This is particularly important for backfilled repository concepts where temperature plays a key role in the EBS behavior and long-term performance. This report describes multiple R&D efforts on disposal in argillaceous geologic media through development and application of coupled THMC process models, experimental studies on clay/metal/cement barrier and host-rock (argillite) material interactions, molecular dynamic (MD) simulations of water transport during (swelling) clay dehydration, first-principles studies of metaschoepite (UO2 corrosion product) stability, and advances in thermodynamic plus surface complexation database development. Drift-scale URL experiments provides key data for testing hydrological-chemical (HC) model involving strong couplings of fluid mixing and barrier material chemical interactions. The THM modeling focuses on heater test experiments in argillite rock and gas migration in bentonite as part of international collaboration activities at underground research laboratories (URLs). In addition, field testing at an URL involves in situ analysis of fault slip behavior and fault permeability. Pore-scale modeling of gas bubble migration is also being investigated within the gas migration modeling effort. Interaction experiments on bentonite samples from heater test under ambient and elevated temperatures permit the evaluation of ion exchange, phase stability, and mineral transformation changes that could impact clay swelling. Advances in the development, testing, and implementation of a spent nuclear fuel (SNF) degradation model coupled with canister corrosion focus on the effects of hydrogen gas generation and its integration with Geologic Disposal Safety Assessment (GDSA). GDSA integration activities includes evaluation of groundwater chemistries in shale formations.
This interim report is an update of the report Jove Colon et al. (2019; M4SF-19SN010301091) describing international collaboration activities pertaining to FEBEX-DP and DECOVALEX19 Task C projects. Although work on these two international repository science activities is no longer continuing by the international partners, investigations on the collected data and samples is still ongoing. Descriptions of these underground research laboratory (URL) R&D activities are given in Jové Colón et al. (2018; 2019) but will repeated here for completeness. The 2019 status of work conducted at Sandia National Laboratories (SNL) on these two activities is summarized along with other international collaboration activities in Birkholzer et al. (2019).
A molecular-scale understanding of the transition between hydration states in clay minerals remains a challenging problem because of the very fast stepwise swelling process observed from X-ray diffraction (XRD) experiments. XRD profile modeling assumes the coexistence of multiple hydration states in a clay sample to fit the experimental XRD pattern obtained under humid conditions. While XRD profile modeling provides a macroscopic understanding of the heterogeneous hydration structure of clay minerals, a microscopic model of the transition between hydration states is still missing. Here, for the first time, we use molecular dynamics simulation to investigate the transition states between a dry interlayer, one-layer hydrate, and two-layer hydrate. We find that the hydrogen bonds that form across the interlayer at the clay particle edge make an important contribution to the energy barrier to interlayer hydration, especially for initial hydration.
Understanding the viscosity and friction of a fluid under nanoconfinement is the key to nanofluidics research. Existing work on nanochannel flow enhancement has been focused on simple systems with only one to two fluids considered such as water flow in carbon nanotubes, and large slip lengths have been found to be the main factor for the massive flow enhancement. In this study, we use molecular dynamics simulations to study the fluid flow of a ternary mixture of octane-carbon dioxide-water confined within two muscovite and kerogen surfaces. The results indicate that, in a muscovite slit, supercritical CO2 (scCO2) and H2O both enhance the flow of octane due to (i) a decrease in the friction of octane with the muscovite wall because of the formation of thin layers of H2O and scCO2 near the surfaces; and (ii) a reduction in the viscosity of octane in nanoconfinement. Water reduces octane viscosity by weakening the interaction of octane with the muscovite surface, while scCO2 reduces octane viscosity by weakening both octane-octane and octane-surface interactions. In a kerogen slit, water does not play any significant role in changing the friction or viscosity of octane. In contrast, scCO2 reduces both the friction and the viscosity of octane, and the enhancement of octane flow is mainly caused by the reduction of viscosity. Our results highlight the importance of multicomponent interactions in nanoscale fluid transport. The results presented here also have a direct implication in enhanced oil recovery in unconventional reservoirs.
We present a measurement cell that allows simultaneous measurement of second harmonic generation (SHG) and streaming potential (SP) at mineral-water interfaces with flat specimen that are suitable for non-linear optical (NLO) studies. The set-up directly yields SHG data for the interface of interest and can also be used to obtain information concerning the influence of flow on NLO signals from that interface. The streaming potential is at present measured against a reference substrate (PTFE). The properties of this inert reference can be independently determined for the same conditions. With the new cell, for the first time the SHG signal and the SP for flat surfaces have been simultaneously measured on the same surface. This can in turn be used to unambiguously relate the two observations for identical solution composition. The SHG test of the cell with a fluorite sample confirmed previously observed differences in NLO signal under flow vs. no flow conditions in sum frequency generation (SFG) investigations. As a second test surface, an inert (“hydrophobic”) OTS covered sapphire-c electrolyte interface was studied to verify the zeta-potential measurements with the new cell. For this system we obtained combined zeta-potential/SHG data in the vicinity of the point of zero charge, which were found to be proportional to each other as expected. Furthermore, on the accessible time scales of the SHG measurements no effects of flow, flow velocity and stopped flow occurred on the interfacial water structure. This insensitivity to flow for the inert surface was corroborated by concomitant molecular dynamics simulations. Finally, the set-up was used for simultaneous measurements of the two properties as a function of pH in automated titrations with an oxidic surface. Different polarization combinations obtained in two separate titrations, yielded clearly different SHG data, while under identical conditions zeta-potentials were exactly reproduced. The polarization combination that is characteristic for dipoles perpendicular to the surface scaled with the zeta-potentials over the pH-range studied, while the other did not. The work provides an advanced approach for investigating liquid/surface interactions which play a major role in our environment. The set-up can be upgraded for SFG studies, which will allow more detailed studies on the chemistry and the water structure at a given interface, but also the combined study of specific adsorption including kinetics in combination with electrokinetics. Such investigations are crucial for the basic understanding of many environmental processes from aquatic to atmospheric systems.
We report a fluid flow in a nanochannel highly depends on the wettability of the channel surface to the fluid. The permeability of the nanochannel is usually very low, largely due to the adhesion of fluid at the solid interfaces. Using molecular dynamics (MD) simulations, we demonstrate that the flow of water in a nanochannel with rough hydrophilic surfaces can be significantly enhanced by the presence of a thin layer of supercritical carbon dioxide (scCO2) at the water–solid interfaces. The thin scCO2 layer acts like an atomistic lubricant that transforms a hydrophilic interface into a super-hydrophobic one and triggers a transition from a stick- to- a slip boundary condition for a nanoscale flow. Here, this work provides an atomistic insight into multicomponent interactions in nanochannels and illustrates that such interactions can be manipulated, if needed, to increase the throughput and energy efficiency of nanofluidic systems.
Jove-Colon, Carlos F.; Payne, Clay; Knight, A.W.; Ho, Tuan A.; Rutqvist, Jonny; Kim, Kunwi; Xu, Hao; Guglielmi, Yves; Birkholzer, Jens; Caporuscio, Florie; Sauer, Kirsten B.; Rock, M.J.; Houser, L.M.; Jerden, James; Gattu, Vineeth K.; Ebert, William
The DOE R&D program under the Spent Fuel Waste Science Technology (SFWST) campaign has made key progress in modeling and experimental approaches towards the characterization of chemical and physical phenomena that could impact the long-term safety assessment of nuclear waste disposition in deep clay/shale/argillaceous rock. Interactional collaboration activities such as heater tests, particularly postmortem sample recovery and analysis, have elucidated important information regarding changes in engineered barrier system (EBS) material exposed to years of thermal loads. Chemical and structural analyses of bentonite material from such tests has been key to the characterization of thermal effects affecting clay composition, sorption behavior, and swelling. These are crucial to evaluating the nature and extent of bentonite barrier sacrificial zones in the EBS during the thermal period. Thermal, hydrologic, and chemical data collected from heater tests and laboratory experiments has been used in the development and validation of THMC simulators to model near-field coupled processes affecting engineered and natural barrier materials, particularly during the thermal period. This information leads to the development of simulation approaches (e.g., continuum vs. discrete) to tackle issues related to flow and transport depending on the nature of the host-rock and EBS design concept. This report describes R&D efforts on disposal in argillaceous geologic media through developments of coupled THMC process models, hydrothermal experiments and characterization of clay/metal barrier material interactions, and spent fuel and canister material degradation. Currently, the THM modeling focus is on heater test experiments in argillite rock and gas migration in bentonite as part of international collaboration activities at underground research laboratories (URLs). In addition, field testing at an URL involves probing of fault movement and characterization of fault permeability changes. Analyses of barrier samples (bentonite) from heater tests at URLs provide compositional and structural data to evaluate changes in clay swelling and thermal behavior with distance from the heater surface. Development of a spent fuel degradation model coupled with canister corrosion effects has been centered towards its integration with Generic Disposal System Analysis (GDSA) to describe source term behavior. As in previous milestone deliverables, this report is structured according to various national laboratory contributions describing R&D activities applicable to clay/shale/argillite media.
The adsorption equilibrium constants of monovalent and divalent cations to material surfaces in aqueous media are central to many technological, natural, and geochemical processes. Cation adsorption-desorption is often proposed to occur in concert with proton transfer on hydroxyl-covered mineral surfaces, but to date this cooperative effect has been inferred indirectly. This work applies density functional theory-based molecular dynamics simulations of explicit liquid water/mineral interfaces to calculate metal ion desorption free energies. Monodentate adsorption of Na+, Mg2+, and Cu2+ on partially deprotonated silica surfaces are considered. Na+ is predicted to be unbound, while Cu2+ exhibits binding free energies to surface SiO- groups that are larger than those of Mg2+. The predicted trends agree with competitive adsorption measurements on fumed silica surfaces. As desorption proceeds, Cu2+ dissociates one of the H2O molecules in its first solvation shell, turning into Cu2+(OH-)(H2O)3, while Mg remains Mg2+(H2O)6. The protonation state of the SiO- group at the initial binding site does not vary monotonically with cation desorption.
Classical molecular dynamics simulation was used to study the adsorption of Na+, Ca2+, Ba2+, and Cl- ions on gibbsite edge (1 0 0), basal (0 0 1), and nanoparticle (NP) surfaces. The gibbsite NP consists of both basal and edge surfaces. Simulation results indicate that Na+ and Cl- ions adsorb on both (1 0 0) and (0 0 1) surfaces as inner-sphere species (i.e., no water molecules between an ion and the surface). Outer-sphere Cl- ions (i.e., one water molecule between an ion and the surface) were also found on these surfaces. On the (1 0 0) edge, Ca2+ ions adsorb as inner-sphere and outer-sphere complexes, whereas on the (0 0 1) surface, outer-sphere Ca2+ ions are the dominant species. Ba2+ ions were found as inner-sphere and outer-sphere complexes on both surfaces. Calculated ion surface coverages indicate that, for all ions, surface coverages are always higher on the basal surface compared to those on the edge surface. More importantly, surface coverages for cations on the gibbsite NP are always higher than those calculated for the (1 0 0) and (0 0 1) surfaces. This enhanced ion adsorption behavior for the NP is due to the significant number of inner-sphere cations found at NP corners. Outer-sphere cations do not contribute to the enhanced surface coverage. In addition, there is no ion adsorption enhancement observed for the Cl- ion. Our work provides a molecular-scale understanding of the relative significance of ion adsorption onto gibbsite basal versus edge surfaces and demonstrates the corner effect on ion adsorption on NPs.
Classical molecular dynamics simulation was used to study the adsorption of Na+, Ca2+, Ba2+, and Cl- ions on gibbsite edge (1 0 0), basal (0 0 1), and nanoparticle (NP) surfaces. The gibbsite NP consists of both basal and edge surfaces. Simulation results indicate that Na+ and Cl- ions adsorb on both (1 0 0) and (0 0 1) surfaces as inner-sphere species (i.e., no water molecules between an ion and the surface). Outer-sphere Cl- ions (i.e., one water molecule between an ion and the surface) were also found on these surfaces. On the (1 0 0) edge, Ca2+ ions adsorb as inner-sphere and outer-sphere complexes, whereas on the (0 0 1) surface, outer-sphere Ca2+ ions are the dominant species. Ba2+ ions were found as inner-sphere and outer-sphere complexes on both surfaces. Calculated ion surface coverages indicate that, for all ions, surface coverages are always higher on the basal surface compared to those on the edge surface. More importantly, surface coverages for cations on the gibbsite NP are always higher than those calculated for the (1 0 0) and (0 0 1) surfaces. This enhanced ion adsorption behavior for the NP is due to the significant number of inner-sphere cations found at NP corners. Outer-sphere cations do not contribute to the enhanced surface coverage. In addition, there is no ion adsorption enhancement observed for the Cl- ion. Our work provides a molecular-scale understanding of the relative significance of ion adsorption onto gibbsite basal versus edge surfaces and demonstrates the corner effect on ion adsorption on NPs.
Methane (CH4) and carbon dioxide (CO2), the two major components generated from kerogen maturation, are stored dominantly in nanometer-sized pores in shale matrix as (1) a compressed gas, (2) an adsorbed surface species and/or (3) a species dissolved in pore water (H2O). In addition, supercritical CO2 has been proposed as a fracturing fluid for simultaneous enhanced oil/gas recovery (EOR) and carbon sequestration. A mechanistic understanding of CH4-CO2-H2O interactions in shale nanopores is critical for designing effective operational processes. Using molecular simulations, we show that kerogen preferentially retains CO2 over CH4 and that the majority of CO2 either generated during kerogen maturation or injected in EOR will remain trapped in the kerogen matrix. The trapped CO2 may be released only if the reservoir pressure drops below the supercritical CO2 pressure. When water is present in the kerogen matrix, it may block CH4 release. However, the addition of CO2 may enhance CH4 release because CO2 can diffuse through water and exchange for adsorbed methane in the kerogen nanopores.
Kerogen plays a central role in hydrocarbon generation in an oil/gas reservoir. In a subsurface environment, kerogen is constantly subjected to stress confinement or relaxation. The interplay between mechanical deformation and gas adsorption of the materials could be an important process for shale gas production but unfortunately is poorly understood. Using a hybrid Monte Carlo/molecular dynamics simulation, we show here that a strong chemo-mechanical coupling may exist between gas adsorption and mechanical strain of a kerogen matrix. The results indicate that the kerogen volume can expand by up to 5.4% and 11% upon CH4 and CO2 adsorption at 192 atm, respectively. The kerogen volume increases with gas pressure and eventually approaches a plateau as the kerogen becomes saturated. The volume expansion appears to quadratically increase with the amount of gas adsorbed, indicating a critical role of the surface layer of gas adsorbed in the bulk strain of the material. Furthermore, gas uptake is greatly enhanced by kerogen swelling. Swelling also increases the surface area, porosity, and pore size of kerogen. Our results illustrate the dynamic nature of kerogen, thus questioning the validity of the current assumption of a rigid kerogen molecular structure in the estimation of gas-in-place for a shale gas reservoir or gas storage capacity for subsurface carbon sequestration. The coupling between gas adsorption and kerogen matrix deformation should be taken into consideration.
Using molecular dynamics simulation, we studied the density fluctuations and cavity formation probabilities in aqueous solutions and their effect on the hydration of CO2. With increasing salt concentration, we report an increased probability of observing a larger than the average number of species in the probe volume. Our energetic analyses indicate that the van der Waals and electrostatic interactions between CO2 and aqueous solutions become more favorable with increasing salt concentration, favoring the solubility of CO2 (salting in). However, due to the decreasing number of cavities forming when salt concentration is increased, the solubility of CO2 decreases. The formation of cavities was found to be the primary control on the dissolution of gas, and is responsible for the observed CO2 salting-out effect. Our results provide the fundamental understanding of the density fluctuation in aqueous solutions and the molecular origin of the salting-out effect for real gas.
The porosity of clay aggregates is an important property governing chemical reactions and fluid flow in low-permeability geologic formations and clay-based engineered barrier systems. Pore spaces in clays include interlayer and interparticle pores. Under compaction and dewatering, the size and geometry of such pore spaces may vary significantly (sub-nanometer to microns) depending on ambient physical and chemical conditions. Here we report a molecular dynamics simulation method to construct a complex and realistic clay-like nanoparticle aggregate with interparticle pores and grain boundaries. The model structure is then used to investigate the effect of dewatering and water content on micro-porosity of the aggregates. The results suggest that slow dewatering would create more compact aggregates compared to fast dewatering. Furthermore, the amount of water present in the aggregates strongly affects the particle-particle interactions and hence the aggregate structure. Detailed analyses of particle-particle and water-particle interactions provide a molecular-scale view of porosity and texture development of the aggregates. The simulation method developed here may also aid in modeling the synthesis of nanostructured materials through self-assembly of nanoparticles.
Molecular dynamics simulation was implemented using LAMMPS simulation package (1) to study the diffusivity of He3 and CO2 in NaCl aqueous solution. To simulate at infinite dilute gas concentration, we placed one He3 or CO2 molecule in an initial simulation box of 24x24x33Å3 containing 512 water molecules and a certain number of NaCl molecules depending on the concentration. Initial configuration was set up by placing water, NaCl, and gas molecules into different regions in the simulation box. Calculating diffusion coefficient for one He or CO2 molecule consistently yields poor results. To overcome this, for each simulation at specific conditions (i.e., temperature, pressure, and NaCl concentration), we conducted 50 simulations initiated from 50 different configurations. These configurations are obtained by performing the simulation starting from the initial configuration mentioned above in the NVE ensemble (i.e., constant number of particles, volume, and energy). for 100,000 time steps and collecting one configuration every 2,000 times step. The output temperature of this simulation is about 500K. The collected configurations were then equilibrated for 2ns in the NPT ensemble (i.e., constant number of particles, pressure, and temperature) followed by 9ns simulations in the NVT ensemble (i.e., constant number of particles, volume, and temperature). The time step is 1fs for all simulations.
Despite massive success of shale gas production in the US in the last few decades there are still major concerns with the steep decline in wellbore production and the large uncertainty in a long-term projection of decline curves. A reliable projection must rely on a mechanistic understanding of methane release in shale matrix-a limiting step in shale gas extraction. Using molecular simulations, we here show that methane release in nanoporous kerogen matrix is characterized by fast release of pressurized free gas (accounting for ∼30-47% recovery) followed by slow release of adsorbed gas as the gas pressure decreases. The first stage is driven by the gas pressure gradient while the second stage is controlled by gas desorption and diffusion. We further show that diffusion of all methane in nanoporous kerogen behaves differently from the bulk phase, with much smaller diffusion coefficients. The MD simulations also indicate that a significant fraction (3-35%) of methane deposited in kerogen can potentially become trapped in isolated nanopores and thus not recoverable. Our results shed a new light on mechanistic understanding gas release and production decline in unconventional reservoirs. The long-term production decline appears controlled by the second stage of gas release.