Efficient flocculation of dolomite-rich phosphate tailings is crucial for safe disposal and water recovery. In this study, four representative polymeric flocculants-non-ionic polyacrylamide (PAM), anionic poly(acrylic acid) (PAA), cationic poly(2-(methacryloyloxy)ethyl trimethylammonium chloride) (PDMC) and zwitterionic poly(acrylic acid-co-(3-acrylamidopropyl)trimethylammonium chloride) (PAA-APTAC)-were systematically evaluated through sedimentation tests and molecular dynamics (MD) simulations. The sedimentation experiments showed distinct performance among the four flocculants. PAA exhibited the most efficient clarification through electrostatic neutralisation and bridging. PAM produced the densest flocs via hydrogen bonding. PDMC demonstrated rapid but less compact flocculation, whereas PAA-APTAC showed poor performance as intramolecular charge pairing limited surface anchoring. MD simulations further revealed distinct adsorption modes and chain conformations: PAA and PDMC formed extended conformations mediated by counter-ions. PAM exhibited a single-layer hydrogen-bonded adsorption, while PAA-APTAC maintained a stretched conformation without effective surface binding. These results highlight that effective flocculation depends on balancing polymer chain extension for bridging with strong surface affinity, and they provide a mechanistic framework for selecting flocculants tailored to carbonate-dominated tailings.

Tight gas reservoirs are characterized by low porosity, low permeability, and strong heterogeneity. CO2 flooding, as an important approach for enhancing gas recovery while achieving carbon sequestration, is often restricted by gas channeling. Based on the sandstone reservoir parameters of the Shihezi Formation in the Ordos Basin, a two-dimensional fracture–matrix coupled numerical model was developed to systematically investigate the effects of fracture number, fracture inclination, fracture width, injection pressure, and permeability contrast on gas breakthrough time and sweep efficiency. A second-order regression model was further established using response surface methodology (RSM). The results show that a moderate fracture density can extend breakthrough time and improve sweep efficiency, while permeability contrast is the fundamental factor controlling gas channeling risk. When the contrast increases from 0.7 to 9.9, the breakthrough efficiency decreases from 88.5% to 68.9%. The response surface analysis reveals significant nonlinear interactions, including the coupled effects of fracture number with fracture width, injection pressure, and inclination angle. Under the optimized conditions, the breakthrough time can be extended to 46,984 h, with a corresponding sweep efficiency of 87.7%. These findings provide a quantitative evaluation method and engineering optimization guidance for controlling CO2 channeling in tight gas reservoirs.

Achieving the self-assembly of lamellar liquid crystals (LLCs) at sub-zero temperatures and elucidating their structure-assembly interplay are crucial for understanding cryobiological processes and facilitating cryogenic soft materials; however, this remains a formidable challenge. Herein, six alkyl alkanolamide amphiphiles are designed, and their self-assembly behavior in 1,2-propanediol/water cosolvent is investigated from 80 to -20 °C. The hydrocarbon chain length exerts a significant influence on self-assembly behavior at both short-range and long-range scales. Amphiphiles with hydrocarbon chains shorter than C16 (i.e., the number of carbon atom is 16) exhibit limited solubility and cannot form LLCs at low temperatures, while longer chains enhance cryo-solubility and self-assembly capabilities, contradicting conventional assumptions. Notably, amphiphiles with chains of C18 or longer require only 0.3 wt.% for LLCs formation. These LLCs exhibit intriguing temperature-dependent phase transitions, including a liquid-like lamellar phase, a tilted gel phase, and a distinct phase characterized by tighter alkyl chain packing. The hydrocarbon chain length directly governs the transition temperatures and further influences the long-range orientational ordering of lamellar sheets. Additionally, the tightly-packed configuration confers exceptional rheological properties, including ultra-high viscosity, shear-thinning behavior, and elasticity. These findings provide important insights for the design and engineering of high-performance soft materials used in extreme environments.

Partially hydrolyzed polyacrylamide (HPAM) hydrogels are widely used as fracturing fluids for extracting underground hydrocarbon resources. Weighting salts are incorporated to enhance hydrogel density to address the extreme pressure encountered in ultra-deep reservoirs. These salts exert significant effects on performance; however, the underlying mechanisms remain poorly understood, and the development of high-performance fracturing fluids remains a significant challenge. This study systematically examines the influence of potassium formate (HCOOK) and sodium bromide (NaBr) as weighting agents on the viscoelasticity of high-density HPAM hydrogels. The findings indicate that HCOOK exerts a more pronounced adverse impact on the viscoelasticity of HPAM hydrogels compared to NaBr. Molecular dynamics simulations reveal that the inferior viscoelasticity of the HCOOK-based polymer hydrogels is attributed to stronger supramolecular interactions between the crosslinker and HCOOK. This study develops an enhanced-performance high-density fracturing fluid with a density of up to 1.40 g/cm3, a maximum drag reduction rate of 75.1 %, and temperature tolerance up to 160 degrees C by integrating ultra-high molecular weight HPAM, a dual crosslinking strategy, and NaBr as weighting salt. This research not only provides fundamental insights into the complex interactions among weighting agents, crosslinkers, and polymers but also presents an enhanced-performance high-density fracturing fluid for ultradeep hydrocarbon reservoirs.

Elevated temperatures in shale reservoirs are unavoidable during deep shale gas extraction. Thermal conductivity is an essential determinant influencing numerous heat-related activities. Nonetheless, comprehending the thermal conductivity of shale is difficult due to its varied mineral compositions, intricate microstructural characteristics, and the absence of direct measurement techniques. This study addresses these problems by utilizing Time-Domain Thermoreflectance (TDTR) technology, which offers accurate measurements of the thermal properties of mineral phases (at micrometer scale) in shale, such as clay and quartz. The test findings indicate that the clay matrix demonstrates a thermal conductivity of 1.98 W/(m & sdot;K) parallel to the bedding and 1.65 W/(m & sdot;K) perpendicular to it, whereas quartz displays an isotropic value of 6.94 W/(m & sdot;K) in both directions. A two-step homogenization methodology has been developed that accurately represents the layering distribution of grains and accounts for the anisotropic behavior of the clay matrix, integrating the Mori-Tanaka method within lamina and series-parallel models within lamina. The precision of this homogenization process is confirmed by further macroscopic measurements utilizing the laser flash technique, exhibiting an error margin of 5 %. Additionally, quantitative study evaluates the influence of four variables on the anisotropy of thermal conductivity by ANOVA. The findings indicate that the anisotropy of thermal conductivity is predominantly influenced by the orientation of the clay matrix.

HYPOTHESIS:The subzero solubility behavior of ultra-long-chain (≥ C18) polyoxyethylene nonionic surfactants remains uncharted. Correlating structural parameters with solubility metrics may reveal how molecular architecture and extreme conditions influence solubility beyond the traditional cloud point (CP) paradigm. EXPERIMENTS:Three series of ultra-long-chain polyoxyethylene nonionic surfactants, UnCL-M¯MPEG, were synthesized, incorporating methoxy polyethylene glycol (MPEG) headgroups of varying molecular weights (M¯MPEG, = 0.35, 0.50, 0.75, and 1.00 kg⋅mol-1), alkyl tails of different lengths (CL, L = 18, 20, 22, and 24), and degrees of unsaturation (Un, n = 0, 1, 2, and 3). Their structures were characterized using 1H NMR and mass spectrometry, while their solubility in propylene glycol (PG)/water mixtures was evaluated via a UV-vis spectrophotometer. And low-temperatures phase behavior was investigated using DSC, cryo-TEM, polarizing optical microscope (POM), and small-angle X-ray scattering (SAXS). FINDINGS:For the first time, three quantitative relationships were established inPG/water mixtures: PTT1 = 8.7MPEG - 5.6, CP = -3.6 L + 149.3, and CP = -10.7 n + 80.4 (where PTT1 denotes phase transition temperature 1). The surfactants U1C22-M¯MPEG, U1C24-0.35, and U0C22-0.35 demonstrated a PTT1 below the CP or at subzero temperatures, forming a mixed phase of liquid crystal and surfactant solid or hexagonal phase liquid crystal. Elevated molecular weight of headgroups (M¯MPEG) and extended tail length (L) reduced cryogenic solubility, whereas increased unsaturation (n) improved solubility under subzero conditions. These insights advance the fundamental understanding of how surfactant structure governs solubility and offer a strategic framework for developing ultra-long-chain surfactants tailored for subzero applications.

In unconventional reservoirs, the water film at the entrance of the dead-end nanopore brings obstacles to CO2 injection for displacing the residual oil. Thus, the structure and stability of the water film become key factors during oil displacement. Herein, we carry out molecular dynamics (MD) simulations to explore the effects of adding methanol to the CO2 injection flow on the interfacial properties of the water film. In the two-phase system, our results reveal that methanol would affect the surrounding water molecules at the interfacial region significantly, leading to a reduction in interfacial tension and a decrease in the interface stability. In the dead-end pore system, methanol is crucial for disrupting the hydrogen bond (H-bond) network and changing the deformation pattern of the interface, which leads to the earlier rupture of the water film. This work provides molecular insights into the water film rupture and offers a theoretical approach to carbon dioxide enhanced oil recovery (CO2-EOR) in unconventional reservoirs in the high water-cut stage.

The requirement for cryogenic supramolecular self-assembly of amphiphiles in subzero environments is a challenging topic. Here, the self-assembly of lamellar lyotropic liquid crystals (LLCs) are presented to a subzero temperature of -70 °C. These lamellar nanostructures are assembled from specifically tailored ultra-long-chain surfactant stearyl diethanolamine (SDA) in water/glycerol binary solvent. As the temperature falls below zero, LLCs with a liquid-crystalline Lα phase, a tilted Lβ phase, and a new folded configuration are obtained consecutively. A comprehensive experimental and computational study is performed to uncover the precise microstructure and formation mechanism. Both the ultra-long alkyl chain and head group of SDA play a crucial role in the formation of lamellar nanostructures. SDA head group is prone to forming hydrogen bonds with water, rather than glycerol. Glycerol cannot penetrate the lipid layer, which mixes with water arranging outside of the lipid bilayer, providing an ideal anti-freezing environment for SDA self-assembly. Based on these nanostructures and the ultra-low freezing point of the system, a series of novel cryogenic materials are created with potential applications in extremely cold environments. These findings would contribute to enriching the theory and research methodology of supramolecular self-assembly in extreme conditions and to developing novel anti-freezing materials.

The classical lyotropic liquid crystals (LLCs) prepared in pure water are not adapted to subzero temperatures. However, the fabrication of cryogenic LLCs is highly significant for gaining profound insights into self-assembly at extremely low temperatures. A proficient solution strategy, namely introducing alcohol antifreeze to water, faces a challenge in balancing frozen resistance and efficient self-assembly. Herein, we address this unmet challenge by selecting four alcohols with comparable structure to fabricate cryogenic LLCs system through the supramolecular self-assembly of a long-chain non-ionic surfactant. The freezing point, surfactant solubility, and self-assembly behavior at subzero temperatures were investigated. The alcohol antifreeze plays a significant role in depressing water freezing point and governing assembled microstructures. Both experimental and computational results revealed that alcohols bearing more hydroxyl groups are efficient to lower water freezing point while remain the capability to induce surfactant self-assembly. They can realize solvophobic balance and strong hydrogen-bond interactions. A simple approach based on Gordon parameter was established to evaluate the suitability as an anti-freezing medium for amphiphile self-assembly. Strikingly, a new LLCs system capable of withstanding the extremely low temperature of -80 degrees C was developed, which shows interestingly continuous phase transitions as temperature lowers to subzero. Overall, this study provides valuable insights into cryogenic supramolecular self-assembly, and offers a significant foundation and evaluation method for the selection of appropriate alcohol antifreeze for cryogenic self-assembly.

气体动理论是根据气体分子微观运动规律研究其宏观物理行为的理论,在航空航天、微机电系统、能源化工等领域有广泛应用. Boltzmann方程没有考虑分子自身体积并忽略除碰撞以外的其他分子间作用力,只适用于稀疏理想气体.本文从Boltzmann方程出发,层层递进,依次介绍稠密气体、真实气体以及纳米空间受限气体的动理学理论和建模方法及相关边界条件.相比于其他模型,纳米空间受限气体动理学模型能够准确描述分子间相互作用,在非平衡效应、高压真实气体效应以及空间受限效应可忽略时能够恢复至连续流体力学理论,是连接微观-介观-宏观等不同尺度的有效理论方法.结合数值模拟结果,本文着重介绍纳米空间受限气体输运中涉及的滑移现象和高压真实气体效应,并就纳米孔内页岩气输运机理进行分析.最后,探讨纳米空间受限气体动理学建模方面的局限性以及可能的发展方向.

The gas kinetic theory explains macroscopic fluid properties based on microscopic molecular interactions, and has wideapplications in aerospace technologies, microelectromechanical systems, and industrial processes. The Boltzmann equa-tion only applies to dilute (ideal) gases as it ignores the gas molecule size and molecular interactions other than binarycollision. Starting from the Boltzmann equation, the transport mechanisms of dilute, dense, real, and nanoconfined gasesare discussed along with their boundary conditions. Compared with other gas kinetic models, the nanoconfined gas kineticmodel produces results consistent with those of the molecular dynamics simulations at the microscopic level and recoversthe continuum fluid dynamics when the nonequilibrium, real gas, and confinement effects are negligible. Slip dynamics,real gas effects, and transport mechanisms of nanoconfined gases are numerically analysed. Finally, the limitations andpossible directions of nanoconfined gas kinetic modelling are briefly discussed.

To respond to the challenges posed by the intermittent nature of renewable energy sources, salt caverns are considered as ideal storage sites for energy such as hydrogen, compressed air, etc., as well as various other gases such as methane, helium, CO2, etc., due to their unique properties. However, the microfractures of salt rock are characterized by multiscale features and the microscopic flow properties of different gases in them are not yet clear. Here, we combine experiments and molecular dynamics simulations to investigate the multiscale flow of the above gases in salt rocks. First, the permeability of six gases has been evaluated to elucidate the microscopic mechanisms underlying the Klinkenberg effect. Second, based on the adsorption and flow characteristics of the gas, a multiscale permeability curve (ranging from 10(-9)similar to 10(- 3) m) was obtained for the salt rock slit. Furthermore, a fast method for predicting the permeability of salt rock samples was proposed, with predictions in the same order of magnitude as the experimental results. Finally, the storage requirements for different gases in salt caverns were discussed. This work provides multiscale insights into gas storage in salt caverns, which can guide the construction of salt cavern gas storage reservoirs and the assessment of leakage risk.

Rapid declines in unconventional shale production arise from the poorly understood interplay between gas transport and adsorption processes in microporous organic rock. Here, we use high-fidelity molecular dynamics (MD) simulations to resolve the time-varying adsorption of methane gas in realistic organic rock samples, known as kerogen. The kerogen samples derive from various geological shale fields with porosities ranging between 20% and 50%. We propose a kinetics sorption model based on a generalized solution of diffusive transport inside a nanopore to describe the adsorption kinetics in kerogen, which gives excellent fits with all our MD results, and we demonstrate it scales with the square of the length of kerogen. The MD adsorption time constants for all samples are compared with a simplified theoretical model, which we derive from the Langmuir isotherm for adsorption capacitance and the free-volume theory for steady, highly confined bulk transport. While the agreement with the MD results is qualitatively very good, it reveals that, in the limit of low porosity, the diffusive transport term dominates the characteristic time scale of adsorption, while the adsorption capacitance becomes important for higher pressures. This work provides the first data set for adsorption kinetics of methane in kerogen, a validated model to accurately describe this process, and a qualitative model that links adsorption capacitance and transport with the adsorption kinetics. Furthermore, this work paves the way to upscale interfacial adsorption processes to the next scale of gas transport simulations in mesopores and macropores of shale reservoirs.

The fracture process and behavior of shale play a key role in the success of hydraulic fracturing stimulation in shale reservoirs. In this study, the central straight cracked Brazilian disc (CSCBD) splitting test in three fracture directions (arrester, divider, and short transverse) is conducted to reveal the propagation behavior and mechanism of closed symmetric fractures in laminated shales, with a focus on the fracture process zone (FPZ). The developmental characteristics and anisotropy of the FPZ near the crack tip of Mode I fracture of shale are identified and quantified based on the digital image correlation (DIC) method. By monitoring the crack tip opening displacement (CTOD) during the whole process, the critical CTOD, as one of the pivotal cohesive parameters, is obtained, and the development of the FPZ can be phased into three stages by CTOD evolution patterns. Based on double-K fracture criterion, the initiation fracture toughness and unstable fracture toughness of shale fractures at different laminar configurations are also analyzed. Additionally, the cohesive fracture model in the shale FPZ is derived based on the power function form using the energy release rate method.

Nanopores are widely found in unconventional reservoirs, and the effect of nanoconfinement on CO2 enhanced oil recovery is still not fully understood. In this work, we have studied the kinetics of alkane replacement using CO2 in micropores (<2 nm) using molecular dynamics simulations. We found that the microstructures (number and density of adsorption layers) of n-decane in micropores changes dramatically with the pore size, leading to oscillations in the density and diffusivity of n-decane in the pores. A diffusion model was proposed to describe the replacement process of CO2 and analyze the different replacement mechanisms under different pore sizes. The structure change pattern and replacement efficiency were examined to show the superiority of microporosity in mining. This work paves a new way to explore the replacement mechanism in micropores, demonstrating the importance of micropores in CO2 enhanced oil recovery.

An accurate understanding of nanoscale gas transport mechanism is a fundamental research concern in many engineering applications, which remains as a research challenge currently. Two particle-actuation modes, namely, force-driven and pressure-driven methods, are evaluated and compared by molecular dynamics simulations of flows in nano-channels focusing on the characteristics of gas adsorption and slip velocity behaviors. The force-driven method may lead to unphysical properties when fluid inhomogeneities are significant since all fluid molecules are subjected to a same external force. By contrast, fluid molecules move forwards through the central part of the flow domain as a predominate pathway in a pressure-driven method. Results show that there is a significant difference between the two methods at smooth or small rough wall conditions, while the results tend to be consistent as roughness increases. The density distribution is uniform along the flow direction in force-driven cases, while adsorbed gas density increases in pressure-driven cases, leading to a smaller slip velocity near the outlet region. The effects of fluid wettability strength on solid surfaces and system temperature on gas adsorption/flow behaviors are also investigated and analyzed. This study is helpful for better understanding nanoscale gas dynamics and has many practical implications, such as the shale gas production.

One major challenge for a continuum model to describe nanoscale confined fluid flows is the lack of a boundary condition that can capture molecular-scale slip behaviours. In this work, we propose a molecular-kinetic boundary condition to model the fluid–surface and fluid–fluid molecular interactions using the Lennard–Jones type potentials, and add a mean-field force to the momentum equation. This new boundary condition is then applied to investigate the nanoscale Couette and Poiseuille flows using the generalised hydrodynamic model developed by Guo et al. (Phys. Fluids, volume 18, issue 6, 2006a, 067107). The accuracy of our model is validated by molecular dynamics simulations and other models for a broad range of parameters including density, shear rate, wettability and channel width. Our simulation results reveal some unexpected and unintuitive slip behaviours at the nanoscale, including the epitaxial layering structure of fluids and the slip length minimum. The slip length minimum, which is analogous to the Knudsen minimum, can be explained by competing fluid–solid and fluid–fluid molecular interactions as density varies. A new scaling law is proposed for the slip length to account for not only the competing effect between the fluid–solid and fluid–fluid molecular interactions, but also many other physical mechanisms including the competition between the fluid internal potential energy and kinetic energy, and the confinement effect. While the slip length is nearly constant at the low shear rates, it increases rapidly at the high shear rates due to friction reduction. These molecular-scale slip behaviours are caused by energy corrugations at the fluid–solid interface where strong fluid–solid and fluid–fluid molecular interactions interplay.