Effects of layer-charge distribution on swelling behavior of mixed-layer illite-montmorillonite clays: A molecular dynamics simulation study

Microscopic Characterization of Deformation Behavior during Kerogen Evolution: Effects of Maturity and Skeleton Moisture Content

The CH4 storage and seepage capacity of shale kerogen are the main controlling factors of the natural gas production rate, and the porosity and permeability of kerogen are greatly affected by kerogen deformation. Therefore, the study of the deformation rule and CH4 adsorption characteristics of kerogen at different maturities and skeleton moisture contents has an important impact on the proper understanding of the development potential of shale gas reservoirs. In this paper, kerogen maturity (II-A, II-B, II-C, and II-D) and skeleton moisture content (0.0, 0.6, 1.2, 1.8, and 2.4 wt %) were considered. The deformation of kerogen, the adsorption of CH4 after deformation, and the quadratic deformation induced by CH4 were studied by using Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD). The results show that the kerogen volume strain increases with increasing skeleton moisture content, following the order II-A < II-B < II-C < II-D for the same moisture content. The density of the kerogen matrix decreases, and porosity increases with rising moisture content. The void fraction of immature kerogen decreases with increasing water content, while the opposite is true for postmature kerogen. The presence of skeleton moisture decreases the CH4 adsorption capacity of immature kerogen and increases the CH4 adsorption capacity of postmature kerogen. The chemical structure of immature kerogen is relatively soft, making its volume more affected by CH4 adsorption compared with postmature kerogen. In the same water environment, postmature kerogen has greater CH4 storage, diffusion, and seepage capacity compared to those of immature kerogen, suggesting that reservoirs with high organic matter maturity should be prioritized for development.

Effects of type and distribution of clay minerals on the physico-chemical and geomechanical properties of engineered porous rocks

The study of the properties of engineered rocks is of great importance to researchers in engineering sciences such as petroleum, mining, and civil engineering owing to their wide application in these fields. In the present study, a physico-chemical and geomechanical investigation was carried out on the effects of different clay minerals on porous rocks. Various chemical products formed during chemical interactions between cement, clay minerals, and water can change the pore structure and thus the rock characteristics. The results of the current study showed that increasing the clay content could remarkably reduce the porosity and permeability of the rock by an average of 86% and 6.76%, respectively. In this regard, samples containing kaolinite were further influenced due to their new pore structure. Moreover, a power relationship was found between sonic velocity and porosity, which can be used to predict rock properties. Chemical analysis indicated an amplification in quantities of chemical products, particularly calcium silicate hydrate and portlandite, due to an increase in clay content. The impacts of porosity and cementation quality as two main factors on rock strength have also been studied. The outcomes revealed that a reduction in porosity could compensate for detrimental effects of poor bond quality and consequently improved UCS by up to 30% in samples containing kaolinite, while decreasing the degree of cementation prevailed over the porosity reduction in specimens including illite and resulted in a 14% decrease in UCS. The effects of porosity and bond quality on UCS would cancel each other out in samples containing bentonite. It is worth noting that when it comes to changes in geomechanical characteristics, the dominant factor (i.e., porosity reduction or cementation quality) determines the ultimate effect of clay minerals on the properties of engineered porous rocks.

A novel expansive soil hardener: performance and mechanism of immersion stability

Aiming at the existing problems of poor treatment effect and immersion stability of expansive soils, a slag soil hardener (SSH, developed by Wuhan University, China) was combined with different additives to dispose in this study. The free expansion rate, compressive strength, and immersion stability of samples were compared, and the influences of different additives, curing age, and dry density on the process and mechanism of improvement were discussed. The experimental results indicated that SSH combined with quicklime had the best improvement effect on expansive soils, in which the mass ratio of raw materials was: expansive soil/SSH/quicklime = 92/4/4, and the free expansion rate decreased from 45.90% to 4.4%, compressive strength increased from 2.53 MPa to 6.69 MPa, and there was no splitting after immersion under this ratio. FTIR spectroscopy, XPS and SEM were performed to analyze the characteristic functional groups, structural forms, and morphology of samples to study the mechanism of improvement, which showed that SSH greatly reduced the proportion of montmorillonite in the whole system and further enhanced the mechanism of ion exchange, soil particle connection, and coating protection. The research can provide theoretical reference for engineering the application of expansive soil area in rainy climate and has dual economic and environmental benefits.

Effects of surface property of mixed clays on methane hydrate formation in nanopores: A molecular dynamics study

HYPOTHESIS

Mixed clays (e.g. montmorillonite, illite and kaolinite) are ubiquitous in hydrate-bearing sediments under seafloor, and their surfaces inevitably affect the formation of natural gas hydrates therein. Nevertheless, the actual effects of clay surfaces on hydrate formation remain elusive.

EXPERIMENTS

Systematic molecular dynamics simulations have been performed to investigate CH₄ hydrate formation in mixed clay nanopores of montmorillonite, illite and kaolinite, to examine the effects of surface property and layer charges of mixed clays.

FINDINGS

Simulation results indicate that the surfaces of mixed clays affect CH₄ hydrate formation in the nanopores by changing the CH₄ concentration (x_CH4) and ion concentration (x_ions) in the middle region of the nanopores via surface adsorption for CH₄, H₂O and ions. Specifically, the surfaces of montmorillonite and illite, the siloxane and gibbsite surfaces of kaolinite show different affinities for adsorbing CH₄, H₂O and ions, which can significantly affect the x_CH4 and x_ions in the interfacial and middle regions of the nanopores. Moreover, hydrate growth shows certain surface preference. These molecular insights into the effect of mixed clay surfaces on CH₄ hydrate formation can help to understand the formation mechanism of natural gas hydrate in marine sediments.

Nanoarchitectonics of Illite-Based Materials: Effect of Metal Oxides Intercalation on the Mechanical Properties

Clay minerals inevitably interact with colloidal oxides (mainly iron and aluminum oxides) in the evolution of natural geomaterials. However, the interaction between the clay minerals and the colloidal oxides affecting the stability and the strength of geotechnical materials remains poorly understood. In the present work, the interaction between the clay minerals and the colloidal oxides was investigated by reaction molecular dynamics simulations to explore the mechanical properties of illite-based materials. It was found that the metal atoms of the intercalated amorphous iron and aluminum oxides interact with oxygen atoms of the silica tetrahedron at the interface generating chemical bonds to enhance the strength of the illite-based materials considerably. The deformation and failure processes of the hybrid illite-based structures illustrated that the Al–O bonds were more favorable to the mechanical properties’ improvement of the hybrid system compared with Fe–O bonds. Moreover, the anisotropy of illite was greatly improved with metal oxide intercalation. This study provides new insight into the mechanical properties’ improvement of clay-based materials through metal oxides intercalation.

A systematic and critical review of application of molecular dynamics simulation in low salinity water injection

Low Salinity Water Injection (LSWI) has been a well-researched EOR method, with several experimental and theoretical scientific papers reported in the literature over the past few decades. Despite this, there is still an ongoing debate on dominant mechanisms behind this complex EOR process, and some issues remain elusive. Part of the complexity arises from the scale of investigation, which spans from sub-pore scale (atomic and electronic scale) to pore scale, core scale, and reservoir scale. Molecular Dynamics (MD) simulation has been used as a research tool in the past decade to investigate the nano-scale interactions among reservoir rock (e.g., calcite, silica), crude oil, and brine systems in presence of some impurities (e.g., clay minerals) and additives (e.g., nanoparticles). In this paper, fundamental concepts of MD simulation and common analyses driven by MD are briefly reviewed. Then, an overview of molecular models of the most common minerals encountered in petroleum reservoirs: quartz, calcite, and clay, with their most common types of potential function, is provided. Next, a critical review and in depth analysis of application of MD simulations in LSWI process in both sandstone and carbonate reservoirs in terms of sub-pore scale mechanisms, namely electrical double layer (EDL) expansion, multi-ion exchange (MIE), and cation hydration, is presented to scrutinize role of salinity, ionic composition, and rock surface chemistry from an atomic level. Some inconsistencies observed in the literature are also highlighted and the reasons behind them are explained. Finally, a future research guide is provided after critically discussing the challenges and potential of the MD in LSWI to shed more light on governing mechanisms behind LSWI by enhancing the reliability of MD outcomes in future researches. Such insights can be used for design of new MD researches with complementary experimental studies at core scale to capture the main mechanisms behind LSWI.