In the process of CO2 geological storage, residual water plays a crucial role in controlling the safe migration and storage of CO2 in deep saline aquifers. This study investigates the influence mechanisms of porosity (phi), pressure (P), and injection pressure difference (Delta P) on the formation of residual water using an orthogonal design experimental method and a core displacement system combined with Nuclear Magnetic Resonance (NMR) technology, with residual water saturation (SW) serving as the evaluation index. Using 3 artificial homogeneous sandstone cores (porosities: 5 %, 10 %, 15 %), a total of 11 core-flooding experiments (each lasting over 24 h) were conducted. The results indicate that the injection pressure difference (standardized coefficient:-0.573) has the greatest impact on residual water saturation, followed by porosity (0.572), while pressure (-0.380) has the least effect. Quantitatively, higher injection pressure difference and pressure reduce residual water, thereby increasing CO2 storage capacity and enhancing sequestration safety; conversely, increased porosity elevates residual water, potentially limiting storage efficiency. The prediction model for residual water saturation, derived from multiple linear regression analysis using SPSS software, is: SW = 0.782 + 1.206 phi-0.04P-0.202 Delta P. The application of NMR technology not only elucidates the influence mechanisms of the aforementioned 3 factors on residual water saturation from a microscopic perspective but also quantitatively assesses the water saturation of different pore sizes in the rock core and the distribution rate of residual water across various pore sizes. This orthogonal-NMR integration establishes a novel macro-micro investigative framework, providing both parametric control and mechanistic insights with broad applicability in subsurface flow studies.

CO2 geo-sequestration is widely recognized as an effective strategy for reducing and managing carbon emissions. During the CO2-water displacement process, the residual water left behind can reduce reservoir pore volume and cause pressure buildup, thereby decreasing the overall efficiency of CO2 storage. In this study, nine sets of displacement experiments were conducted using three natural rock cores, focusing on the impact of CO2 phase states and rock properties (mineral composition, permeability, and pore structure), as well as their coupled effects on the formation of residual water. Experimental results show that residual water saturation during CO2 displacement is synergistically regulated by CO2 phase states, rock properties and their coupling effects. From the perspective of CO2 phase states, residual water saturation follows the order of gas phase > supercritical phase > liquid phase. From the perspective of rock properties, quartz and feldspar content is positively correlated with residual water saturation by enhancing rock hydrophilicity; residual water saturation is positively correlated with permeability and porosity, yet negatively correlated with median/average pore throat radius. Building on previous work, we further elucidate the fundamental coupling mechanisms by which CO2 phase states and rock properties synergistically regulate residual-water formation. These findings consequently offering critical practical guidance for optimizing site selection optimization and enhancing storage efficiency in saline-aquifer CO2 sequestration projects.

Improving the efficiency of CO2 geological storage is crucial for mitigating climate change and ensuring energy security. However, there is still a lack of in-depth understanding of how banded heterogeneous reservoirs influence the CO2-water displacement process. To address this issue, this study utilized nuclear magnetic resonance (NMR) technology and fractal theory, conducting supercritical CO2 displacement experiments on five sets of water-saturated artificial sandstone cores, which were embedded with different lengths of low-/high-permeability bands. The results show that the length of the bands influences the variation in residual water saturation by regulating the heterogeneity of the rock core. Increasing the length of both low-/high-permeability bands significantly reduce residual water saturation. While low-permeability bands enhance the uniformity of CO2 distribution in different pore regions and weaken the capillary end effect to optimize the displacement path; high-permeability bands, act as low-resistance channels to improve CO2 flow connectivity, optimize the injection path, and enhance the displacement effect. Furthermore, the fractal dimensions of both the whole rock core and its macropores exhibit a positive correlation with the low-/high-permeability bands lengths. Longer band lengths lead to more complex pore structures, which in turn effectively improves CO2 flow connectivity and ultimately enhances displacement efficiency. This study reveals the spatiotemporal evolution of CO2-water displacement under varying band lengths, and elucidates the mechanisms by which these bands affect pore structure heterogeneity and fluid flow connectivity. This study establishes a theoretical basis for optimizing the design of CO2 geological storage schemes and enhancing their long-term controllability and stability.

ObjectiveCO2 breakthrough pressure, a core parameter for characterizing the sealing performance of saline aquifers as cap rocks, holds important scientific significance and application value for ensuring the safety of CO2 geological storage engineering and assessing its storage potential. However, there is a lack of systematic studies on the variation patterns and mechanisms of breakthrough pressure under different occurrence phases of CO2 (supercritical, liquid, and gaseous) currently. Accordingly, universal and reliable conclusions are yet to be reached. MethodsThis study conducted experiments on a natural low-permeability unsaturated sandstone core. Using the step-by-step method, 13 groups of breakthrough pressure experiments were carried out under different CO2 phases to deeply explore the variation patterns of CO2 breakthrough pressure under these phases. Results and Conclusions The results indicate that the CO2 phase is not directly correlated with the magnitude of the CO2 breakthrough pressure. Under the experimental conditions of this study, the CO2 breakthrough pressure at any phase transition point was higher than that under a single phase. Under the condition of a single CO2 phase, the CO2 breakthrough pressure decreased with an increase in pressure, exhibiting an exponential relationship with the latter. Pressure produced the greatest impact on the breakthrough pressure of supercritical CO2 but posed the smallest impact on that of gaseous CO2. The influence of pressure on CO2 breakthrough pressure is caused by the synergistic effect of the interfacial tension of the CO2-water system and the wettability (the contact angle) of the CO2-water-rock system, as well as the two-phase density difference and viscosity ratio. Sensitivity analysis results show that the viscosity ratio was a significant factor affecting the CO2 breakthrough pressure, with a total contribution rate of up to 39.9%. Analysis of the lg Ca-lg M displacement stability diagram further confirmed that viscosity acted as a dominant factor affecting the experimental results of this study. Comparison with the formation pressure data and practical application to engineering sites in basins reveal that the experimental conditions of this study are consistent with actual engineering. The results of this study serve as an important reference for accurately evaluating the storage capacity of saline aquifers and guiding target siting for CO2 geologic geological storage.

Residual water critically constrains CO2 geological storage efficiency, yet we discover that elevated CO2 concentration in gas mixtures systematically reduces its saturation. By revealing that N2 co-injection paradoxically enhances storage safety despite increasing residual water, this work provides transformative strategies for optimizing carbon sequestration with impurities. Specifically, in this study, nine sets of core-flooding experiments were conducted using three different ratios of a CO2/N2 gas mixture (50 % CO2 + 50 % N2, 75 % CO2 + 25 % N2, and 99.99 % CO2). The experimental results indicate that an increase in the ratio of CO2 within the gas mixture leads to a progressive reduction in residual water saturation, with the order of saturation being 50 % CO2 + 50 % N2 > 75 % CO2 + 25 % N2 > 99.99 % CO2. Furthermore, a detailed mathematical relationship that delineates the connection between residual water saturation and drainage duration is presented, with coefficients a and b examined thoroughly. The incorporation of N2 into the CO2 mixture raises residual water saturation, as lower concentrations of CO2 correlate with increased residual water saturation. However, the presence of N2 effectively extends the CO2 breakthrough time, making it more difficult for CO2 to penetrate the rocks and thereby enhancing storage safety. As the concentration of CO2 in the gas mixture increases, a reduction in contact angle values and interfacial tension (IFT) occurs. This variation results in a decrease in capillary pressure, which facilitates the displacement or migration of fluids within the core pore space. Concurrently, the viscosity ratio of the gas phase to the liquid phase reduces the viscous resistance in the pores and improves displacement efficiency. Furthermore, the experimental results are primarily influenced by capillary forces. This study enhances the theoretical understanding of CO2 storage, and provides valuable insights for evaluating the feasibility of CO2 storage projects that include impurities.

Gas breakthrough pressure is a pivotal parameter for assessing the sealing integrity of cap rocks, which is vital for the safety of CO2 geo-sequestration and underground gas storage. This study presents a systematic evaluation of the breakthrough pressures of CO2 and N-2 in an unsaturated sandstone of low permeability at various temperature and pressure conditions. The focus of this research is on the variations in breakthrough pressures for CO2 and N-2 under different experimental conditions, as well as a comparative analysis between the two gases. Meanwhile, the research examines the impact of two-phase fluid properties, including interfacial tension (IFT), viscosity ratio, and wettability, on breakthrough pressure. The findings demonstrate a positive correlation between the breakthrough pressures of CO2 and N-2 with temperature and pressure, respectively. Notably, the breakthrough pressure for N-2 is higher than that for CO2 under identical conditions. Furthermore, N-2 breakthrough pressure is more sensitive to changes in temperature or pressure compared to CO2. There is an inverse correlation between CO2/N-2 breakthrough pressure and effective permeability. CO2 demonstrates a greater ability to migrate through the rock core, facilitating easier breakthrough compared to N-2. Our analysis indicates that the viscosity ratio is the primary factor influencing CO2 and N-2 breakthrough pressures, whereas the effects of IFT and wettability are relatively minor. This study provides valuable insights into the factors that influence breakthrough pressure and aids in assessing the sealing ability of cap rock.

The evaluation of cap rock closure efficiency and the exploitation of natural gas significantly depend on the breakthrough pressure parameter. This experiment assesses the breakthrough pressures of CO2 and CH4 in partially saturated sandstone with low-permeability under various pressure and temperature conditions using a stepwise method. The novelty of this study lies in enhancing the understanding of the trends in CO2/CH4 breakthrough pressure under differing temperature and pressure conditions, as well as the comparative differences between them. Additionally, the study examines the effects of characteristics such as viscosity ratio, interfacial tension, and wettability on breakthrough pressure. The findings reveal a positive correlation between CO2 breakthrough pressure and both pressure and temperature, while CH4 breakthrough pressure exhibits a negative correlation with these variables. Under varying pressure and temperature scenarios, the breakthrough pressure of CO2 surpasses that of CH4, with both being more significantly influenced by the pressure conditions. Furthermore, CH4 breakthrough pressure is more sensitive to changes in pressure or temperature compared to CO2. Based on the displacement stabilization phase diagram, viscosity ratio and capillary force emerge as the dominant factors affecting the breakthrough processes of CO2 and CH4. This study provides valuable references for evaluating the sealing properties of caprock, offers guidance for CO2-EGR engineering, and contributes to the establishment of numerical models for CO2/CH4 mixed gases.

CO2 breakthrough pressure is a key indicator for studying CO2 percolation behavior and assessing the sealing capacity of caprocks, which is critical for ensuring the safety and effectiveness of CO2 geo-sequestration. However, existing research on how CO2 phase, initial pressure difference, and confining pressure influence CO2 breakthrough pressure is limited, with inconsistent and unreliable conclusions. To address this gap, this study investigated the mechanisms behind CO2 breakthrough pressure using natural low-permeability unsaturated sandstone as the experimental material. A total of 27 CO2 breakthrough pressure experiments were conducted under various CO2 phases, initial pressure difference, and confining pressures using a step-by-step methodology. The results showed that there was no fixed or predictable relationship between breakthrough pressure and CO2 phase. However, in the presence of a single CO2 phase, the breakthrough pressure declined exponentially as the pressure increases. The influence of pressure on breakthrough pressure was attributed to the combined effect of CO2-H2O interfacial tension (IFT), contact angle of CO2-H2O-rock system, and two-phase viscosity ratio. The study clarifies interactions influencing CO2 breakthrough pressure under varying conditions and elucidates the role of CO2 phase behavior. These findings provide experimental data to improve assessments of CO2 sequestration safety and caprock integrity, enabling more accurate evaluations of geological storage potential.

Gas breakthrough pressure is a key parameter affecting gas production and evaluation of tight reservoir sealing capabilities. This study aims to explore the impact of different injection methods on CH4 breakthrough pressure in unsaturated rocks. COMSOL Multiphysics was used to simulate the CH4 breakthrough process, and comparative analysis was conducted using step-by-step and continuous injection methods. The results show that the step-by-step method has higher measurement accuracy under low CH4 breakthrough pressure and is suitable for scenarios that require precise evaluation, whereas the continuous injection method is more efficient under high CH4 breakthrough pressure and is suitable for rapid evaluation needs. According to outcomes of simulation, this research suggested a numerical optimization framework aimed at forecasting the breakthrough pressure of CH4 and verified the accuracy and applicability of the model through linear fitting of experimental data and predicted values. In addition, the study also conducted a sensitivity analysis on the pore distribution index (m) and injection flow rate (uin) in the van Genuchten model. The results show that uin has a small impact on breakthrough pressure, whereas m has a considerable effect on breakthrough pressure. An increase in m leads to an increase in breakthrough pressure, thereby enhancing the sealing performance of rock core. This study reveals the applicability difference between the step-by-step method and the continuous injection method in predicting CH4 breakthrough pressure and proposes an effective prediction method based on numerical simulation, which provides valuable insights for selecting gas injection methods and predicting breakthrough pressure in rocks. (c) 2025 Society of Chemical Industry and John Wiley & Sons, Ltd.

To comprehensively assess the underground performance of compressed CO2 energy storage in aquifers (CCESA) and accurately capture the interactions of thermo-hydro-mechanical (THM) processes, a simulator integrating wellbore-reservoir coupling and THM coupling is developed by further considering nonlinear wellbore multiphase flow and geomechanical processes. The simulator is validated through geological CO2 sequestration (GCS) benchmark tests and the TOUGH-FLAC simulator. The THM response and the performance of CCESA, as well as the impact of temperature and geomechanical effects on system performance, are analyzed. Results show that, under THM simulation conditions, changes in effective stress and displacement show that the temperature effect predominantly governs the geomechanical processes. The energy round-trip efficiency reaches 100.31 % due to the geothermal supply. Comparative analyses show that mechanical deformation mitigates pressure and temperature fluctuations. While reservoir cooling reduces system pressure, it intensifies pressure fluctuations and enhances the productive CO2 mass rate. Mechanical deformation decreases the mass fraction of productive CO2 by 0.005 % but increases the energy rate by 0.8 x 105 W and energy round-trip efficiency by 0.06 %.

ObjectiveThe pore structures of clastics play a crucial role in assessing the potential of saline aquifers for geologic CO2 sequestration. Current studies on pore structures during CO2 sequestration focus primarily on artificial cores. However, compared to artificial cores, natural cores feature more complex pore size distributions and stronger heterogeneity. MethodsThis study examined a natural core from deep reservoirs in the Ordos Basin and two artificial cores with different porosities through CO2-water displacement experiments under water-saturated conditions. Using a nuclear magnetic resonance (NMR) analysis and imaging system for multiphase fluid displacement, this study analyzed the impacts of pore structures (pore heterogeneity) on CO2-water displacement in these cores. Results and ConclusionsThe artificial cores with porosities of 5% and 15% exhibited CO2-water displacement efficiencies of 67.09% and 46.71%, respectively, whereas the natural core with a porosity of 15% displayed a displacement efficiency of 37.67%. The NMR-derived transverse relaxation time (T2) spectra of the natural core showed a unimodal pattern, suggesting strong heterogeneity. In contrast, the T2 spectra of artificial cores manifested bimodal patterns, with uniform pore size distributions and low residual water saturation. Notably, a lower porosity corresponded to high pore connectivity, which allowed for sufficient contact between CO2 and the core. This characteristic effectively reduced interfacial tension and significantly enhanced CO2-water displacement efficiency. Compared to the artificial core with the same porosity, the natural counterpart showed significantly reduced CO2-water displacement efficiency due to its strong heterogeneity, with residual water occurring predominantly in small pores. Therefore, in the assessment of the potential of clastic reservoirs for CO2 sequestration, it is necessary to thoroughly consider the impact of core heterogeneity and preferentially utilize natural cores or heterogeneous artificial cores in CO2-water displacement experiments. This will help predict CO2 sequestration behavior more accurately and avoid overestimated CO2 sequestration efficiency parameters arising from the application of artificial cores made of homogeneous materials. The results of this study will provide a theoretical basis and experimental support for siting, potential assessment, and injection scheme design for geologic CO2 sequestration in saline aquifers.

The burgeoning expansion of urban rail transit has brought the safety of tunnel construction to the forefront. Accidents arising from mechanical failures in the surrounding rock and soil serve as substantial impediments to its progression. This research delves into the acoustic emission (AE) response characteristics and the detrimental effects of uniaxial loads on silty clay. To achieve this, an experimental system was devised to ascertain both mechanical properties and AE responses. A damage model, predicated on cumulative AE counts, was developed, and the principles governing damage evolution were distilled. Following this, the Particle Flow Code (PFC) was employed for numerical simulation. By manipulating mesoscopic parameters, we exerted control over the macroscopic mechanical attributes. This enabled a deep dive into the AE response and the energy shifts during the failure mechanism, offering a mesoscopic lens to understand deformation and failure. Our findings suggest: (1) The AE response during failure can be stratified into five distinct phases, with pronounced AE activity in the loading failure domain, aligning with established engineering practices. (2) The damage model, rooted in cumulative AE counts, adeptly captures the sequential damage evolution, closely mirroring the stress-strain dynamics. (3) PFC effectively simulates internal fractures and the AE dynamics during failure, pinpointing areas of susceptibility for targeted interventions. This research stands as a pivotal reference for engineering stability initiatives, augmenting our ability to foresee and preemptively address potential damages.

This study focused on the effects of the CO2 phase on the formation mechanisms of residual water. Based on nine groups of core-flooding experiments, the order of residual water saturation was gaseous CO2 > supercritical CO2 > liquid CO2, and a quantitative power function relationship between residual water saturation and displacement time for different CO2 phases was proposed. The experimental results show that when CO2 transitions from the gaseous phase to the supercritical phase and then to the liquid phase, the decrease in the interfacial tension and cosine value of the contact angle in two-phase flow can lead to a decrease in residual water saturation. Meanwhile, an increase in the viscosity ratio of two-phase flow weakens the viscous fingering phenomenon and can also lead to a decrease in residual water saturation. However, the logCa-logM stability phase diagram reveals that the viscous force is the primary factor influencing all core-flooding experiments.

Theoretical and experimental studies have demonstrated that splash erosion on agricultural land can affect agricultural production and the ecosystem. However, the splash erosion process mechanism of loess erosion under the effects of a single raindrop remains unclear. Therefore, this study investigates the effects of single-raindrop splash erosion on the cohesion and internal friction angle of compacted loess under multiple compaction, water contents, and single-raindrop kinetic energies. The results showed that the depth and diameter of the crater were linearly related to both the raindrop kinetic energy and the loess compaction. The variation of crater depth is always lower than the variation of crater diameter, and the effect of the amount of raindrop energy on crater splash erosion is higher than the effect of the variation of compaction on crater splash erosion. Raindrops with higher kinetic energy have higher erosiveness, and this can shorten the ponding point time. When the water content is constant, the fitting curve of cohesion and compaction is the exponential relationship. The increment of cohesion decreases with increasing water content when the water content is lower than the optimum water content (12.7%) and increases with increasing water content when the water content is higher than the optimum water content. However, the increment of internal friction angle consistently increases with increasing water content, which is not affected by the optimum water content. This study could be a reference for soil erosion control in the Loess region.

CO2在地下深部咸水含水层的封存过程中,岩石孔隙内形成的残余水对CO2可注入性、封存量及安全性都有负面影响,因此深入探究各因素影响下残余水的形成演化具有重要意义.文章利用取自鄂尔多斯盆地深部储层的天然岩心,在40℃和8 MPa的实验条件下利用3种纯度的CO2(99.999％CO2、75％CO2+25％N2以及50％CO2+25％N2)进行饱水岩心驱替实验,以探究CO2纯度对残余水的影响.实验表明,在3种CO2纯度下的岩心驱替实验中,达到突破点和排水终点所需时间排序均为:99.999％CO2<75％CO2+25％N2<50％CO2+50％N2;不可再降残余水饱和度大小排序为:99.999％CO2<75％CO2+25％N2<50％CO2+25％N2.分析发现,CO2纯度变化会导致界面张力、湿润性以及黏度比等两相流系统重要性质的变化.通过对LogCa-LogM驱替稳定性图进行分析,确定了毛细力是影响本实验结果的主导因素.研究结果对不同条件下残余水饱和度的预测以及CO2封存量的评价等方面有重要价值.

Gas breakthrough pressure is a key parameter to evaluate the sealing capacity of caprock,and it also plays important roles in safety and capacity of CO2 geological storage.Based on the published experimental results,we present numerical simulations on CO2 breakthrough pressure in unsaturated low-permeability rock under 9 multiple P-T conditions(which can keep CO2 in gaseous,liquid and supercritical states)and thus,a numerical method which can be used to accurately predict CO2 breakthrough pressure on rock-core scale is proposed.The simulation results show that CO2 breakthrough pressure and breakthrough time are exponential correlated with P-T conditions.Meanwhile,pressure has stronger effects on experimental results than that of temperature.Moreover,we performed sensitivity studies on the pore distribution index λ(0.6,0.7,0.8,and 0.9)in van Genuchten-Muale model.Results show that with the increase of λ,CO2 breakthrough pressure and breakthrough time both show decreasing trends.In other words,the larger the value of λ is,the better the permeability of the caprock is,and the worse the CO2 sealing capacity is.The numerical method established in this study can provide an important reference for the prediction of gas breakthrough pressure on rock-core scale and for related numerical studies.

The temporal and spatial resolutions of rainfall data directly affect the accuracy of hydrological simulation. Weather radar has been used in business in China, but the uncertainty of data is large. At present, research on radar data and fusion in small and medium-sized basins in China is very weak. In this paper, taking the Duanzhuang watershed as an example, based on station data, Shijiazhuang's radar data are preprocessed, optimized and fused. Eleven rainfall events are selected for fusion by three methods and quality evaluation, and three flood simulations are used to test their effect. The results show that preprocessing and initial optimization have poor effects on radar data improvement. The rainfall proportional coefficient fusion method performs best in rainfall spatial estimation, where the R2 values of the three inspection stations are increased to 0.51, 0.78 and 0.82. Three fusion datasets in the peak flow and flood volume of flood simulation perform better than station data. For example, in the No.20210721 flood, the NSE of the three fusion data increased by 39, 30 and 48%. This shows that the fusion method can effectively improve the data accuracy of radar and can obtain high temporal and spatial resolution rainfall data.

Breakthrough pressure of CH4 is an important parameter in shale gas exploitation and the evaluation of cap rocks. To date, many studies have been carried out to investigate CH4 breakthrough pressure based on experimental methods, meanwhile, some studies have also proposed numerical simulation theories on multiphase flow in porous media. However, there are few studies on numerical simulation methods for predicting CH4 breakthrough pressure on rock-core scale under various conditions. Therefore, in this study, we propose a numerical method that can be used to predict CH4 breakthrough pressure in partially saturated low-permeability rock sample under multiple P-T conditions based on our published experimental results. The simulative results show that both CH4 breakthrough pressure and breakthrough time are exponentially related to temperature and pressure conditions, moreover, CH4 breakthrough pressure and breakthrough time decrease with increasing temperature and pressure. Under the multiple P-T conditions in this study, the effect of pressure on numerical results is greater than that of temperature. By comparing the numerical results with the published experimental data, it has been validated that the numerical method proposed in this study is reasonable. Furthermore, we have parametrically swept 3 key properties in CH4-H2O-rock systems, namely, dynamic viscosity, density, and interfacial tension; the order of the effects of 3 properties on the CH4 breakthrough process is, dynamic viscosity > interfacial tension > density. The numerical method established in this study can provide an important reference for the prediction of gas breakthrough process in rock core and for related numerical studies and engineering practice.

Study on CO2 breakthrough pressures in caprocks under formation temperatures (T) and pressures (P), which can make CO2 in gaseous, liquid and supercritical states, are vital to the seal ability of caprocks for CO2 sequestration. Laboratory testing is necessary to obtain breakthrough pressures as there is no in situ method to accurately estimate the factors that can affect its variability. In this study, we use the step-by-step method to study CO2 breakthrough pressures in unsaturated low-permeability sandstone, and carried out 9 cases of CO2 breakthrough pressure experiments under multiple P-T conditions that can make CO2 in gaseous, liquid and supercritical states, respectively. The variation of CO2 breakthrough pressures (Pbt) were analyzed based on 4 properties of multiphase flow: CO2-H2O interfacial tension (gamma), CO2-H2O-rock wettability, CO2-H2O viscosity ratio (M), and CO2H2O density difference. The results show that CO2 breakthrough pressure and breakthrough time are positively correlated with experimental temperatures and pressures. The main property affecting CO2 breakthrough pressures is the CO2-H2O viscosity ratio, which is more affected by the P-T conditions (or CO2 phase). Moreover, the breakthrough pressures of CO2 under 9 groups of multiple P-T conditions (which can make CO2 in various phases) is Supercritical CO2 > Liquid CO2 > Gaseous CO2. This study has important guiding significance for the selection of target caprock for CO2 geo-sequestration.