CO2 Diffusion Research Articles

Experimental study on CO2 flooding within a fractured low-permeability reservoir: Impact of high injection rate

The rapid decay of reservoir energy in the late development of fractured low-permeability reservoir leads to poor development benefit. Injecting CO2 into such reservoir at high pressure and rate can rapidly replenish reservoir energy and fully utilize the advantages of CO2 flooding, which is an efficient approach for CCUS-EOR. Previous laboratory studies have mainly been conducted under conventional low injection rate, with little discussion on the impact of high injection rate (above 20 times the conventional low injection rate) on CO2 flooding, especially regarding the limited reports on the effect of high injection rate on CO2 flooding within a fractured low-permeability reservoir. In this paper, microscopic visualization and fractured-core experiments of CO2 flooding at different injection rates were conducted. The variations in microscopic displacement front and sweep characteristics under different injection rates were clarified. The influence of CO2 injection rate on the macroscopic and pore scale oil production characteristics in fractured low-permeability cores under different injection modes were analyzed. The results indicate that injection rate significantly influences the microscopic displacement characteristics. The final sweep coefficient at 10 μL/min was 25.07 %, and the pre-breakthrough sweep coefficient accounted for approximately 90 %. While for 200 μL/min, the final sweep coefficient was 34.83 %, and the post-breakthrough sweep coefficient accounted for above 50 %. During CO2 flooding in fractured low-permeability core, the oil production before CO2 channeling dominates the final oil recovery. Compared to 0.1 mL·min−1, oil recovery increases by above 20 % at a CO2 injection rate of 4 mL·min−1 at 6 PV. The high-pressure gradient formed by high injection rate enhances CO2 diffusion, promoting the oil mobilization in near-fracture and deep matrix. And it results to a decrease in the minimum pore scale of oil mobilization. Soaking enhances the interaction between CO2 and oil as well as the fluid exchange between fracture and matrix. It further reduces the minimum pore scale of oil mobilization, and the enhancing effect of soaking gradually strengthens with increasing injection rate. The results can provide theoretical reference for application of CO2 flooding within a fractured low-permeability reservoir.

Key role played by mesophyll conductance in limiting carbon assimilation and transpiration of potato under soil water stress

IntroductionThe identification of the physiological processes limiting carbon assimilation under water stress is crucial for improving model predictions and selecting drought-tolerant varieties. However, the influence of soil water availability on photosynthesis-limiting processes is still not fully understood. This study aimed to investigate the origins of photosynthesis limitations on potato (Solanum tuberosum) during a field drought experiment.MethodsGas exchange and chlorophyll fluorescence measurements were performed at the leaf level to determine the response of photosynthesis-limiting factors to the decrease in the relative extractable water (REW) in the soil.ResultsDrought induced a two-stage response with first a restriction of CO2 diffusion to chloroplasts induced by stomatal closure and a decrease in mesophyll conductance, followed by a decrease in photosynthetic capacities under severe soil water restrictions. Limitation analysis equations were revisited and showed that mesophyll conductance was the most important constraint on carbon and water exchanges regardless of soil water conditions.DiscussionWe provide a calibration of the response of stomatal and non-stomatal factors to REW to improve the representation of drought effects in models. These results emphasize the need to revisit the partitioning methods to unravel the physiological controls on photosynthesis and stomatal conductance under water stress.

Experimental investigation on enhancing oil recovery of volcanic fractured oil reservoir by gas injection huff-n-puff considering gas diffusion

The pore structure of volcanic fractured oil reservoirs is complicated, and the traditional gas displacement and waterflooding are not ideal. In this paper, the pore structure characteristics of volcanic rock were analyzed. Then, the diffusion procedure of CO2 and natural gas in saturated oil porous media was analyzed by pressure drop method. Finally, huff-n-puff experiments of CO2 and natural gas were conducted to analyze factors on the recovery factor (RF), and the oil mobilization pattern was quantitatively characterized. By huff-n-puff, we mean injecting gas from one end and then soaking it for a period of time before producing oil, utilizing pressure-driven and physicochemical reactions to affect the oil. The results show that the pore size distribution range is large. The diffusion of CO2 and natural gas can be divided into three stages: rapid diffusion, slow diffusion and equilibrium, and the diffusion coefficient of CO2 is 10 times that of natural gas under same condition. Compared with vertical fractures and horizontal fractures, reticulate fractures have more influence on gas diffusion coefficient. After 6 cycles, the RF of CO2 is 15% higher than that of natural gas, and the contribution of the first 4 cycles to the RF is 91.38%. In addition, CO2 huff-n-puff primarily mobilizes oil in medium (0.1 μm < r < 1 μm) and large (r > 1 μm) pores. These research results can provide a good theoretical guidance for the efficient development of VFOR and CO2 sequestration.

High quality perovskite thin films synthesized by low-temperature supercritical carbon dioxide annealing method

Thermal annealing is a conventional method used to eliminate residual solvents in perovskite films, reducing internal and interfacial defects. However, due to the high saturation vapor pressure of the residual solvent dimethyl sulfoxide, it is difficult to quickly remove the residual solvent at low temperatures. In this paper, the high permeability and diffusion of supercritical CO2 (ScCO2) were used to quickly remove the residual solvent of film at low temperatures. The effects of ScCO2 annealing on the surface morphology, residual solvents, internal and interfacial defects of perovskite thin films were investigated. The experimental results indicate that ScCO2 annealing significantly improves the removal efficiency of residual solvents compared to low-temperature annealing, effectively minimizing internal and interfacial defects. Additionally, the interaction energy of CO2 with the perovskite crystal plane reduces the surface energy of the crystal, resulting in a significant increase in the average grain size of the crystal by about 91.2%. Notably, the average crystal size of the film prepared by chlorobenzene as an antisolvent increased from 209.35 nm to 704.57 nm. The high penetration of ScCO2 also promoted uniform crystal growth, thereby reducing crystal structure deformation and defects. The carrier lifetime of perovskite thin films after ScCO2 annealing increased by over 58.8% on average.

Ambient Pressure Carbonation Curing of Cold-bonded Fly Ash Lightweight Aggregate Using Coal-fired Flue Gas for Properties Enhancement and CO2 Sequestration

In the current CO2 curing process, pure CO2 gas with a concentration exceeding 99% is primarily used. However, flue gas, which typically contains 10-30% CO2, can also be utilized for carbonization. This study sought to explore the viability of employing flue gas for carbonation and assessed the impact of impurity gases such as SO2. Two typical industrial solid wastes (fly ash and coal gangue) were used to substitute a portion of the cement to prepare light aggregates, which were carbonized under varying concentrations of CO2 and SO2. The porosity and water absorption of the samples decreased after carbonation. A higher degree of carbonation was observed at increasing CO2 concentration. Aggregates carbonated with 15% CO2 improved the CO2 absorption by 48%. The actual CO2 uptake reached up to 58.3% of the theoretical value. The presence of SO2 has been found to impact the uptake of CO2. The CO2 uptake initially declined and then increased as the SO2 concentration increased. The existence of SO2 led to varied increases in the leaching concentrations of the aggregates following the process of carbonation, and some even exceed standard limits. In the presence of both CO2 and SO2, SO2 reacted with the aggregates, resulting in the creation of calcium sulfate. This reaction disrupted the structure of the aggregate, facilitating the diffusion of CO2 into the samples.

Enhanced thermochemical heat storage performance of CaO/CaCO3 via pyroligneous acid impregnation and Co/Mn co-doping

Calcium-based thermochemical heat storage (TCHS) is an economical and environmentally friendly technology for efficient heat storage. It can strengthen the flexibility in power systems of renewable energy and promote the recovery and storage of waste heat in energy-intensive industries. However, CaO/CaCO3 is extremely prone to sintering at operational temperatures. Consequently, a cost-effective approach was proposed to synthesize calcium-based TCHS materials modified by pyroligneous acid (PA) impregnation and co-doping with dual transition metals. The heat storage performance, physicochemical property, and anti-sintering mechanism of synthesized CaO were comparatively examined. The sample prepared with a molar ratio of Ca: Co: Mn = 100:3:7 (PACa-Co3Mn7) exhibited an initial energy storage density of 1.76 MJ/kg, which gradually rose to 1.88 MJ/kg. After 20 cycles, the value decreased by only 4.39 % from its peak, indicating outstanding TCHS performance. Characterization results indicated that the specific surface area and pore volume of PACa-Co3Mn7 were 10.48 m2/g and 0.035 cm3/g, which were 2.34 and 2.69 times those of CaO stemmed from the calcination of limestone, respectively. The fruitful pores provided pathways for CO2 diffusion within the CaO particles or CaCO3 product layer. The uniformly distributed Co and Mn elements combined to form Ca3CoMnO6. Ca3CoMnO6 could serve as the inert carrier that mitigated the grain-growth of CaO, thereby stabilizing the pore structure and resisting the sintering of PACa-Co3Mn7. Furthermore, density functional theory (DFT) calculations demonstrated that the free Mn and Co elements efficiently promoted the electrons transfer between CaO and CO2, which improved the reactivity of PACa-Co3Mn7. Therefore, CaO modified by PA impregnation and co-doping with dual transition metal is a promising candidate for the TCHS system.

Enhancing CO2 Foam Stability with Hexane Vapours: Mitigating Coarsening and Drainage Rates

CO2 foams often suffer from poor stability due to high coarsening and coalescence rates, limiting their effectiveness in various applications. While it is well-established that small amounts of insoluble vapours such as alkane or fluorocarbon vapours can impede coarsening and recent studies have demonstrated their impact on coalescence, their specific effect on the CO2 foams has not been studied. This research provides a comprehensive examination of stabilising effect of hexane on CO2 foams in the presence of sodium dodecylbenzenesulfonate (SDBS). We investigated the foam stability of CO2 foams, benchmarking them against N2 foams by analysing foam lifetime, liquid drainage rate, and the evolution of bubble size. Additionally, we quantified the stabilising impact of hexane by calculating the coarsening rate. To gain insights into the adsorption mechanism of surfactants in the presence of hexane, we conducted surface tension and interfacial dilational rheology measurements, which demonstrated an increased adsorption of surfactant molecules at the interface and increased dilational viscoelasticity of interface when n-hexane was present. The introduction of hexane significantly improved foam stability, reducing coarsening rates by more than an order of magnitude. This improvement in foam stability is attributed to inhibited CO2 diffusion from the bubbles, as well as enhanced surfactant adsorption and surface elasticity, resulting in an approximate 3.6-fold increase in foam half-life.

Synthesis of fluorinated ether free aromatic backbone copolymers containing trifluoromethyl and dimethylamino groups for CO2 separation: Investigation of pure and mixed gas performance under dry and humid conditions

A novel series of linear, high molecular weight aromatic copolymers containing trifluoromethyl and dimethylamino groups was synthesized via facile super-acid catalyzed polyhydroxyalkylation and investigated as CO2 gas separation membrane materials. The molar ratio of the two ketone comonomers used in copolymerization, trifluoroacetophenone (TFAp) and N,N-dimethylamino trifluoroacetophenone (dMATFAp), was systematically varied to produce five copolymers with different dMATFAp contents ranging from 12 to 73 %. The influence of dMATFAp content on physicochemical and mechanical properties as well as pure gas and water vapor separation performance was studied. All synthesized copolymers displayed excellent film forming ability, high thermal stability and high Tg values (Tgs > 380 °C). Pure gas permeability measurements revealed that the increase of dMATFAp molar content from 12 % to 34 % resulted in improved gas permeability, while, on the contrary, further increase of dMATFAp content from 34 to 73 % led to substantial gas permeability decrease ascribed to FFV decrease. In particular, among copolymers, the one with dMATFAp content of 34 % presented the highest CO2 permeability value of 290 Barrer due to its highest CO2 diffusion and CO2 solubility coefficients associated with its higher FFV value. The CO2/CH4 performance of synthesized copolymers fell very close to 2008 upper bound limit and exceeded most of the super-acid catalyzed copolymer membranes reported in the literature and commercial membranes. CO2/CH4 and CO2/N2 selectivity values for the different synthesized copolymers were in the range of 29.9–40 and 22.7–44, respectively. The study of the effect of dMΑTFA on water permeability unveiled that the membrane with the highest dMΑTFAp molar content (73 %) showed the highest water permeability value (23,100 Barrer) as well. This was attributed to the increased concentration of N-methyl substituent of dMΑTFAp which can interact with water molecules thereby increasing solubility. Under process gas stream conditions using a mixture of 3 % H2O-48.5 % CO2-48.5 % CH4, both CH4 and CO2 permeability were significantly reduced (by 35 % and 47 %, respectively) due to the preferential sorption of water vapor over other gases present in the mixture, highlighting the negative effect of water vapor on gas permeability. Accordingly, the CO2/CH4 separation factor was slightly increased from 23.8 to 29. Finally, stability test of the copolymer (BP-TFAp)66-(BP-dMATFAp)34 at 40 °C under humid mixed gas conditions showed that CO2 and CH4 permeability remained unchanged for one month after an initial condition stabilization that is required implying that these copolymer structures are promising materials for CO2 separation.

Exploring enthalpies of CO2 and N2 adsorption on Zn- and Co-based zeolitic frameworks at varying temperatures and pressures

This study investigates the thermodynamics of CO2 and N2 adsorption on zeolitic imidazolate frameworks (ZIFs), focusing on the effects of metal ions under varying temperatures and pressures. Zn- and Co-based ZIFs were synthesized using 2-methylimidazole and characterized by XRD, FT-IR, Raman spectroscopy, and BET isotherms, confirming high microporosity. Gas adsorption experiments were performed at pressures up to 3000 kPa and temperatures between 268 and 323 K. The CO2 adsorption capacities vary from 274 to 157 cm³/g, while N2 ranged from 67 to 29 cm³/g as temperature increased. The synergistic dynamics of temperature and pressure enhanced CO2 diffusion and induced pore structure changes via a gate-opening mechanism. Isosteric enthalpy (ΔHads) values, determined using Freundlich-Langmuir/Clausius-Clapeyron and virial fits, were -22 ± 5 kJ/mol for CO2 and -15 ± 2 kJ/mol for N2, confirming that physisorption is the dominant mechanism. Gibbs free energy (ΔGads) for CO2 ranged from -12 to -18 kJ/mol, and entropy (ΔSads) from -0.01 to -0.03 kJ/mol/K, suggesting spontaneous and thermodynamically favorable CO2 adsorption, though spontaneity decreased with rising temperature. ZIFs containing Zn exhibited high CO2 adsorption capacity, while those with Co showed better CO2 selectivity over N2, highlighting their potential for efficient CO2 capture under various temperature and pressure conditions.

CO2 adsorption on fluorinated covalent triazine frameworks (CTFs): Structure remodeling and performance enhancement

In consideration of the fact that covalent triazine frameworks (CTFs) have shown great potential and application prospects as CO2 adsorbent to alleviate the greenhouse effect and climate change by adsorption process for carbon emissions, a kind of promising activated fluorinated covalent triazine frameworks (referred to O-CTFs) was prepared as the adsorbent to enhance CO2 adsorption. The O-CTFs was prepared by structure remodeling of the fluorinated CTFs based on a series of ring-opening, substitution and polymerization reactions. The remodeling mechanism was revealed by theoretical calculations and those characterization methods of SEM, BET, EDS, FTIR, XPS, 13C-NMR and HRMS. Subsequently, the adsorption thermodynamics and kinetics of CO2 on the CTFs and O-CTFs were systematically investigated to reveal the influence of structure remodeling on CO2 adsorption behaviors. The results showed that the structure remodeling developed porous structure by forming more mesopores and micropores, then modified surface group distribution by forming new nitrogen- and oxygen-containing groups such as pyridine and pyrrole rings. The increase of pore volume, especially micropore volume, resulted in the dynamic capacity promotion by 217.5 % at 273 K. And the dual screening effects of surface group distribution and porous structure led to the CO2/N2 selectivity improvement by 202.4 %. Then the amount increase of the mesopores promoted the rate increase of CO2 adsorption by 59.9 % by means of accelerating the internal diffusion of CO2 in porous structure. In addition, the isosteric heat of adsorption decreased by 10.0 %, meaning that CO2 molecules were easily desorbed from the O-CTFs. These are beneficial for the preparation of promising CO2 adsorbent and extended application of CO2 adsorption processes.

Carbon dioxide and nitrate reduction reactions tailoring kinetics over Cu2O with mesoporous carbon channels for boosting electrocatalytic urea synthesis

The electrocatalytic synthesis of urea from CO2 and NO3-, powered by renewable electricity, constitutes a sustainable and attractive method, alternative to Haber-Bosch process. However, the much lower kinetic rate of carbon dioxide reduction reaction (CO2RR) compared to that of nitrate reduction reaction (NO3RR) on Cu-based catalyst leads to low urea yield and Faradaic efficiency. Herein, we report aerophilic nanocomposites of hydrophobic mesoporous carbon (CMK-3) and Cu2O nanocubes to tailor the microenvironment on the surface of Cu2O for matching the kinetics of both CO2 and NO3- reduction reactions and boosting urea yield. The electrochemical experiments showed that the Cu2O/CMK-3 with hydrophobic groups blocked the NO3RR, accelerated CO2 diffusion, and effectively balanced the reduction rates of CO2RR and NO3RR, resulting in a high urea yield of 46.9 mmol h−1 g−1, which was about twice of that over Cu2O. The operando ATR-SEIRAS spectroscopy results revealed that mesoporous carbon promoted the reduction of CO2 to *CO and optimized the amounts of *CO and *NH adsorbed on the surface of Cu2O, thus facilitating the C-N coupling reaction.

Dynamics of water, CO2, ethane and their mixtures in ZSM-22 zeolite: the role of polarity and hydrogen bonding

Abstract Understanding the interplay between confinement effects and intermolecular interactions is essential for predicting molecular diffusion in zeolites. In this study, we investigate the diffusion behavior of ethane, CO2, and water in ZSM-22 molecular sieves, focusing on the effects of mixing these fluids. Our results reveal that while CO2 has minimal impact on ethane diffusion, water significantly slows ethane’s motion by forming molecular bridges across the pore structure, reducing ethane’s diffusion by up to 30%. Ethane, in turn, restricts water’s mobility, and reduces the water–water coordination number from 2.22 to 0.73 depending on concentration. The diffusion of CO2 in mixtures shows a 40% increase in pure state under confinement. The role of polarity and hydrogen bonding is crucial, with water molecules exhibiting 1.2 hydrogen bonds in the confined state—much lower than the 3.4 bonds in bulk water. Molecular rotation in ZSM-22 of all fluids occurs at two distinct time scales: the short-time fast rotation dominated by molecular inertia and the long-time rotation hindered by fluid-zeolite interactions. For water, hydrogen bonding further restricts full rotational freedom. These findings provide a comprehensive understanding of how ethane, water, and CO2 interact and diffuse in nanoporous materials.

Construction of a sunflower-like S-scheme WO3/ZnIn2S4 heterojunction with spatial charge transfer for enhanced photocatalytic CO2 reduction

The construction of step-scheme (S-scheme) heterojunction has become an effective strategy for enhancing photocatalytic CO2 reduction. In this study, a sunflower-like S-scheme WO3/ZnIn2S4 heterostructure is fabricated through the in-situ growth of ZnIn2S4 nanosheets on WO3 hollow sphere, aimed at improving selective CO2 reduction. The constructed embedding configuration, featuring a WO3 hollow sphere core and an outer layer of ZnIn2S4 nanosheet, increases the contact interface and establishes a significant built-in electric field, thereby promoting the migration and separation of photogenerated carriers. The confined internal cavity of the three-dimensional (3D) sunflower-like architecture aids in limiting the free diffusion of CO2, and enriching CO2 molecules at the surface of the photocatalyst, consequently enhancing reaction kinetics.Moreover, S vacancies on ZnIn2S4 nanosheets promote the chemisorption and activation of CO2. As a result, the optimized evolution rates for CH4 and CO of 58.7μmol g–1h−1 and 51.3μmol g–1h−1, respectively, are achieved for WO3/ZnIn2S4 heterojunction. The corresponding electron selectivity for CH4 reaches 82.1 %, which is 13 and 1.5 times higher than those of bare WO3 and ZnIn2S4, respectively. This provides valuable insights into the design of muti-functionalized S-scheme heterojunction photocatalysts.

Experimental and molecular simulation of carbon dioxide solubility in hexadecane at varying pressures and temperatures

CO2-EOR can make underground storage efforts possible, addressing both energy demands and climate issues. This study establishes an in-situ method to measure the degree of CO2 solubility in hexadecane in a wide range of P-T conditions. CO2 and hexadecane with known volumes were mixed in silica capillaries and examined by Raman spectroscopy after reaching an equilibrium at certain P-T condition to establish a relationship between mole fraction of CO2 in hexadecane and ratio of Raman peak area. Results showed that solubility of CO2 decreases with increasing temperature and increases with pressure. Complementing the experimental data, hybrid grand canonical Monte Carlo/molecular dynamics (GCMC/MD) simulations were performed to study the swelling effect of CO2/hexadecane system and the diffusion of CO2 within hexadecane. Simulation results were validated against Raman spectroscopy and previously published CO2 solubility data, as well as a genetic algorithm-based (GA) predictive model, all matching with high accuracy and conforming each other. Based on molecular simulation results, the necessity of accounting for volumetric changes in solubility calculations to enhance the accuracy of predictive models in similar systems was revealed. Additionally, in contrast to temperature, the effect of pressure on the diffusion coefficient remains relatively minimal. Ultimately, this study provides solutions for in-situ probing techniques to determine the solubility of various fluids in a wide range of P-T conditions, processes supporting CO2-EOR and carbon storage operations underground.

Simulation Study on Diffusion and Local Structure of CH4, CO2, SO2, and H2O Mixtures into Double-Layers Graphene.

Graphene has been widely studied as an ideal material for the adsorption and separation. In this work, we used molecular dynamics simulations to investigate the evolution of diffusion and local structure of CH4, CO2, SO2, and H2O mixtures into double-layers graphene under seven different interlayer spacings and four different CO2 concentrations. The results showed that the adsorption of CH4 and CO2 molecules on the graphene surface weakened with increased interlayer spacing. The diffusion capacities of CH4 and CO2 in the mixed system were significantly improved by increasing the interlayer spacing. In interlayer spacings ranging from 5 to 10 nm, the diffusion capacities of each component varied significantly in the order CH4 > CO2 ≫ H2O > SO2. Compared with CH4 and CO2, the local structures of SO2 and H2O were more affected by the interlayer spacing. Larger interlayer spacings or higher CO2 concentrations were advantageous for the formation of stronger hydrogen bond structures between H2O molecules. When the CO2 concentrations were between 10% and 20% and the interlayer spacing of graphene was 8 nm, the graphene structure exhibited the best adsorption and separation effects on CH4 and other components.

Experimental and simulation study of single-matrix, all-polymeric thin-film composite membranes for CO2 capture: Block vs random copolymers

High-performance, additive-free, all-polymeric thin-film composite (TFC) membranes were developed for CO₂ capture, focusing on a comparison between block and random copolymers (referred to as PTF) composed of hydrophobic poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) and CO2-philic polar poly(oxyethylene methacrylate) (POEM) chains. The PTF random copolymer, synthesized via free-radical polymerization (FRP), exhibited a disordered morphology. In contrast, the PTF block copolymer, synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization, formed a well-ordered hexagonally packed cylindrical structure, creating an amphiphilic, microphase-separated nanostructure. Molecular dynamics (MD) simulations revealed that in both copolymers, there was minimal interaction between the gases (CO₂ and N₂) and the hydrophobic PTFEMA segments, while CO₂ showed strong affinity for the hydrophilic POEM segments. The block and random copolymers demonstrated similar CO₂ permeance, which can be attributed to their comparable CO₂ diffusivity and solubility. However, the block copolymer exhibited significantly lower N₂ permeance than the random copolymer, resulting in nearly quadruple the CO₂/N₂ selectivity. This increase in selectivity was supported by the lower N₂ mean squared displacement (indicating reduced diffusivity) observed in the block copolymer. The PTF block copolymer outperformed the commercial Pebax block copolymer, achieving CO₂ capture efficiencies that surpass industrial standards for CO₂ separation and capture. This positions the single-matrix PTF block copolymer as a promising alternative to mixed-matrix membranes for practical applications in gas separation technologies.

Quickly Calculating the Activity Coefficient of a NaCl Solution Based on Machine Learning Algorithms.

The activity coefficient represents the deviation between an actual solution and an ideal solution, influencing the solubility and diffusion of CO2 within a saltwater layer. Consequently, it serves as a crucial parameter for numerical simulations of CO2 storage in deep saltwater layers. However, in numerical simulations of CO2 geological storage, the majority of studies rely on the Helgeson-Kirkham-Flowers (HKF) equation to compute activity coefficients, which necessitates obtaining Debye-Hückel (DH) parameters. The conventional method calculates the DH parameters via an interpolation algorithm, which requires a long computation time during the numerical simulation. Therefore, developing a method to quickly and accurately calculate activity coefficients is vital for the overall model efficiency. This study employed machine learning algorithms to train DH parameters derived from the IAPWS-95 method. It could establish empirical formulas for DH parameters as functions of temperature and pressure, which were then substituted into the HKF equation to swiftly compute activity coefficients. The results demonstrate that the activity coefficients obtained using this method exhibit a small relative deviation from experimental values, with an average coefficient of determination of 0.9463 and an average relative error of 2.28%. Furthermore, the computational speed was improved by 48%. This approach reduces the calculation time for activity coefficients in geochemical reaction modeling, enabling DH parameters to be calculated solely based on temperature and pressure, which is easy to use and has high accuracy. It facilitates rapid calculation of activity coefficients for solutions within a temperature range of 0 to 300 °C and a pressure range of 0 to 200 MPa. Ultimately, this study holds significant importance for the numerical simulation of geochemical reactions.

Explicit and explainable artificial intelligent model for prediction of CO2 molecular diffusion coefficient in heavy crude oils and bitumen

Optimizing CO2 injection for enhanced oil recovery (EOR) requires a precise estimation of the CO2-diffusivity coefficient in porous media. This study developed a predictive model for the molecular diffusivity coefficient of CO2 in bitumen and heavy crude oils using a Bayesian regularized artificial neural network. The unique contributions of the developed model compared to existing models include: First, a simple and accurate mathematical correlation between inputs and CO2-diffusivity has been presented, making it easy to use particularly for individuals with limited understanding of machine learning. Secondly, the time it takes the model to make a prediction (its inference latency) has been established and the model has been physically validated using trend analysis. Fourthly, independent datasets were used to test for the model generalizability.The model was evaluated using 260 data points from the literature, with 70 % for training and 30 % for testing. Key performance metrics were calculated, including root mean square error (RMSE = 0.03), and coefficient of determination (R2 = 0.996). Furthermore, an outlier detection analysis using the statistical leverage approach demonstrated that the data used was of high quality. A relevancy factor analysis revealed that pressure had the greatest impact on CO2 diffusivity, followed by temperature, while CO2 mass fraction had the least impact. The developed model operates within a temperature range of 295 K – 363 K and a pressure range of 1 MPa – 8MPa. Overall, the outcomes of this study contribute to the efficient prediction of CO2 diffusivity in heavy crude oil and bitumen.

Unveil the controls on CO2 diffusivity in saline brines for geological carbon storage

Accurate estimation of the diffusivity of CO2 in formation brine, one of the dominant mass transfer mechanisms, is crucial for characterizing CO2 migration and distribution in Geological Carbon Storage (GCS). However, the effects of pressure, temperature, and the composition and concentration of ions on the CO2 diffusion coefficient in saline aquifers are complex. This study used in-house-designed equipment to determine the diffusion coefficient of supercritical CO2 in synthetic brines, such as deionized water and NaCl, CaCl2, and KCl solutions under different pressure and temperature conditions. First, CO2 solubilities at initial pressure and temperature were measured to provide the initial CO2 concentration at the gas-liquid interface. Then, pressure decay curves were collected for the selected brines and the diffusion coefficients of CO2 molecules were calculated based on Fick's second law. Experimental results show that CO2 solubility increases with increasing pressure and decreasing temperature and ionic strength. Moreover, CO2 solubilities at same pressure, temperature and ionic strength conditions vary in different synthetic brines. We also found that CO2 diffuses faster in brine at higher pressures and temperatures but slows down as ionic strength increases. The type of ions affects the diffusion differently, except at higher pressures, where their impacts balance out due to hydration effects. In addition, a good linear relationship between CO2 diffusion coefficient and solubility was observed, indicating the CO2 diffusion coefficient could be estimated by solubility. The findings in this study shed a light on the CO2 migration during commercial-scale geological storage and enhanced oil recovery processes.

Dynamics of impure CO2 and composite oil in mineral nanopores: Implications for shale oil recovery and gas storage performance

Evaluating the application effectiveness of impure CO2 in shale reservoirs is a prerequisite for reducing CO2 purification costs. Therefore, molecular dynamics simulations were applied to study the influential mechanisms of impurity gases, mineral types and oil components on impure CO2 enhanced oil recovery (impure CO2-EOR) and gas storage (GS). The orientation force between H2S and polar oil components and the strong adsorption of H2S on mineral walls improve the solubility and diffusivity of impure CO2 in shale oil. Moreover, the induction force between H2S and CO2 carry additional CO2 to walls, and the hydrogen bonds formed between H2S and walls further enhance oil–gas competitive adsorption, resulting in more adsorbed oil being stripped and more H2S/CO2 being sequestered. N2 hinders the dissolution of H2S/CO2 into shale oil and the diffusion of desorbed oil towards the center of nanopores. Therefore, the EOR and GS performances in nanopores are positively and negatively correlated with the proportion of H2S and N2 in impure CO2, respectively. Both EOR and GS performances of impure CO2 decrease sequentially in illite, quartz, and kerogen nanopores, because the strong adsorption of shale oil on kerogen walls causes oil molecules to arrange tightly and dissolve in kerogen. It weakens the solubility and diffusivity of impure CO2 in shale oil, and also increases the resistance of impure CO2 to displace adsorbed and dissolved oil. By contrast, illite walls have the weakest adsorption on shale oil and the largest number of hydrogen bonds formed with H2S. The increase of heavy component content further enhances the interaction between oil molecules and the adsorption of shale oil on wall, thereby weakening the effects of various impure CO2-EOR mechanisms.