Capillary Barrier Effects Research Articles

Characterisation of the Pinus banksiana root system on analogues of a cover with capillary barrier effects

ABSTRACT To control acid mine drainage, a cover with capillary barrier effects (CCBE) can be constructed over mine wastes. This cover relies on a fine-grained material (moisture retaining layer; MRL) that maintains a high degree of saturation (≥85%). However, the roots of deep-rooted plants on CCBE can penetrate the MRL and extract water, thus impacting the performance of the cover. Furthermore, as cover systems are expected to perform for hundreds of years, CCBE design must anticipate long-term ecosystem changes that will influence cover performance. However, data representative of the rooting of future ecosystems at reclaimed sites can be difficult to acquire by conventional methods. Using natural analogues of cover systems can help to obtain this information. In this paper, the root systems of nine Pinus banksiana (61 to 89 years old) were studied in sand over silt layers (used as CCBE analogues) in the boreal zone of Quebec, Canada. The maximum rooting depth (MRD), coarse root colonisation intensity along the soil profile, and root length densities (RLDs) were characterised in both material layers. The MRD ranged from 25 cm to 160 cm (depending on the distance of the trenches from the trees), with some roots able to penetrate 120 cm into the MRL. However, root colonisation of the MRL was mostly concentrated in the surface sand layer, with rooting occurrences and densities > 90% in the top 50 cm of sand. These data are critical to optimise the thickness of the coarse layer to protect the MRL from root intrusion and to predict the long-term performance of CCBEs. Abbreviations CCBE: cover with capillary barrier effects; CS: cover system; MRD, maximum rooting depth; MRL, moisture retaining layer; NA, natural analogue; Pb, Pinus banksiana; PL, protection layer; RS, root system. Core ideas A NA of a CCBE was used to provide rooting data representative of future mature ecosystems; Useful data were obtained to adjust the thickness of the PL over the MRL at the design stage; Data were obtained for modelling the long-term CCBE performance, including the effect of deep-rooted plants.

Drainage Characteristics, Capillary Barrier Effect, and Diversion Length of Flat Earthen Roof of Historical Kemaliye Houses

ABSTRACT Kemaliye houses stand out as remarkable examples of Anatolian architecture, renowned for their distinct mixed construction method and the utilization of diverse and sustainable materials. The flat earthen roof, constructed using local soils and traditional techniques, adds to the unique character of these houses. However, due to the need for roof maintenance, particularly in winter, residents have adopted modern roofing methods. In order to preserve the architectural integrity of historic houses, research into the mechanisms and performance of flat earthen roofs using local materials and original construction techniques is essential. To achieve this objective, original materials, and construction techniques were investigated with 1D infiltration laboratory tests and analyzed using numerical modelling. The results indicated the existence of a capillary barrier effect on the roofs. Additionally, water balance and diversion length of the roof is obtained through experimental study and numerical analysis. Furthermore, numerical analyses indicated that the capillary barrier effect may be reduced or weakened as a result of frost heave and the formation of desiccation cracks. Understanding these phenomena is a vital step in developing effective solutions for preserving and maintaining roofs in their original architectural form.

Modeling the influence of forest vegetation and climate change on the long-term performance of a cover with capillary barrier effects used to control acid mine drainage: the Lorraine case study

ABSTRACT The Lorraine mine site in Témiscamingue region (Québec, Canada) was reclaimed with a cover with capillary barrier effects (CCBE) to limit acid mine drainage generation. The main objective of this study is to predict the long-term hydrogeological behaviour of the CCBE at Lorraine site, including the influence of vegetation and climate change (CC). Numerical simulations showed that performance of the CCBE is: (1) not affected by forest vegetation and CC in the saturated zone in the north of the Lorraine site and (2) more sensitive to vegetation in the unsaturated zone in the south where the phreatic level is low.

Monitoring moisture dynamics in multi-layer cover systems for mine tailings reclamation using autonomous and remote time-lapse electrical resistivity tomography

The dynamics of moisture content in cover systems constructed on mining wastes were monitored at the pilot scale using 2D autonomous, remote, and non-invasive time-lapse electrical resistivity tomography combined with conventional point sensors. A methodology was proposed to process the daily hydrogeophysical datasets from 23 m-long instrumented sections of covers with capillary barrier effects (CCBEs) designed to act as oxygen barriers, and covers with low saturated hydraulic conductivity layers (LSHCCs) designed to limit the water infiltration rate. Hydrogeophysical monitoring suggested that CCBEs were able to maintain high degrees of saturation in the moisture-retaining layer throughout the one-year monitoring period, which would make it an efficient oxygen barrier. Larger spatio-temporal changes in moisture content were observed in LSHCCs and most of the low hydraulic conductivity layers remained below 85% saturation, which was attributed to the combined effect of low precipitation, rapid vegetation development, and water percolation through the cover. The methodology proposed in this pilot-scale "proof-of-concept" study allowed the hydrogeological behavior of the cover systems to be monitored in the 23 m-long instrumented sections using continuous geoelectrical data, which demonstrated that this innovative monitoring technique could be useful for geochemical and geotechnical monitoring programs in large-scale mining waste storage facilities.

Pore-scale investigation on the effect of capillary barrier on two-phase displacement in dual-structure porous media

Although immiscible fluid–fluid displacement in porous media has received extensive attention, understanding the dynamics behavior within complex structures remains elusive. This study utilizes the direct numerical simulation by solving the Navier–Stokes equations and coupling with the volume of fluid method to examine oil–water flow in porous media across various contact angles θ and capillary number Ca. Three kinds of artificial porous media were generated with designed opening angle β, including single-structure and dual-structure models. A theoretical analysis of the capillary barrier phenomenon, as well as its occurrence conditions, is identified under water-wet conditions. Generally, when θ + β < 90°, the capillary force consistently drives oil displacement from throats to pores. Conversely, if θ + β > 90°, the direction of the capillary force can move toward the water phase side and prevent the fluid interface from continuing to move. For a single-structure porous medium, the dynamics behavior of fluids is controlled by the capillarity, wettability, and geometric structures. The greatest efficiency occurs when the condition θ + β = 90° is met, particularly at an intermediate Ca. For a dual-structure porous medium with smaller opening angles inside, the water phase tends to infiltrate the embedded pore structure due to weaker capillary barrier effects. Conversely, larger opening angles within the embedded structure lead to stronger capillary barrier effects, hindering water entry into the interior porous medium. This obstruction forces the water phase to bypass and traverse longer flow paths, resulting in the formation of a large amount of residual oil.

Seasonal hydrology and gas transport in a composite cover on sulfide tailings

This study presents the field performance of a five-layer composite cover to mitigate acid mine drainage in legacy sulfide tailings in northern Ontario, Canada. Installed in 2008, this cover comprised sand, clay, geosynthetic clay liner, sand, and waste rock layers. To evaluate the effectiveness of the cover in reducing water and oxygen ingress, groundwater and vadose zone hydrological characterization, stable water isotope analysis, pore-gas measurements, oxygen flux calculations, and variably saturated flow modeling were conducted. The results indicate that the clay layer stayed nearly saturated in the spring, fall, and winter, but temporary desiccation occurred during the summer. Compared to uncovered tailings, the cover significantly lowered diffusive oxygen flux. In the summer, fall, and winter, the capillary barrier effect of the cover functioned effectively and inhibited percolation. Atmospheric pore-gas oxygen concentrations at one out of the three monitoring locations indicate potential cover imperfections that enabled oxygen transport into the tailings. In the spring and early summer, snowmelt infiltration resulted in percolation that compromised the capillary barrier effect, as well as lateral drainage. The resulting increase in water saturation in the cover limited oxygen transport. Despite potential cover imperfections, this composite cover reduced oxygen and water ingress a decade after installation.

An infiltration model for inclined covers with consideration of capillary barrier effect

Covers with capillary barrier effect (CCBEs) are widely used as surface cover systems in landfills, mine tailings and water-sensitive soil slopes to prevent rainwater from infiltrating into the protected materials below. In the engineering design, the water-prevention performance of the CCBEs must be quantificationally evaluated according to the local meteorological conditions. In this study, according to the moisture distribution during infiltration, the fine-grained soil layer was divided into saturated, transition and initial zones. Capillary barrier effects were analyzed by considering water migration and redistribution at the interface between fine and coarse-grained layers. Then, the total time, including the time to reach the interface (tHf), the time for overcoming the capillary barrier effect (tc) and the time for passing through the coarse-grained layer (tg), was determined to quantificationally evaluate the water-prevention capacity of the inclined CCBEs under different rainfall conditions. For the case studies selected from literature, the calculated results by the proposed model agree well with the measured data. Finally, some parameters and the factors influencing the capillary barrier effect were further analyzed.

Comparative study of performance of wicking and conventional railway geotextiles under the synergetic simulation of train loads and flooding

Geotextiles are used within railways and highways to reduce settlement and improve drainage characteristics. However, the drainage performance of conventional geotextiles under unsaturated conditions can be limited due to capillary barrier effects. Therefore, more recently, wicking geotextiles consisting of woven fibers with deep groves have been developed to improve drainage performance. Despite these potential performance benefits and many related highway studies, there has been minimal study into the performance of wicking geotextiles for railways. Therefore, this study performs large-scale laboratory tests on railway ballast-subgrade materials under three conditions: non-stabilized (NSS), conventional geotextile stabilized (CGSS), and wicking geotextile stabilized (WGSS). Each test sample is subject to 600,000 cyclic loading cycles over 3 phases. The first phase is a stationary phase for consolidation, the second is to simulate loading directly after rainfall, and the third is to simulate loading after flooding. The results are analyzed and four conclusions are drawn: 1) both geotextile types prevent rainfall water from infiltrating into subgrade soils, however the wicking shows elevated performance, 2) the wicking geotextile produces capillary suction, which compared to the conventional geotextile, significantly delays infiltration and helps to reduce the moisture content in subgrade soils, 3) both geotextiles are capable of separation and filtration during rainfall and flooding simulation, however the wicking offers additional performance, 4) the deformation of both geotextiles is similar.

Pore-Scale Oil-Water Flow Behaviors in Different Development Adjustment Schemes for Enhancing Oil Recovery in Waterflooding

Development mode adjustment is an important measure to further enhance oil recovery after primary waterflooding. Investigating the oil-water flow behaviors in the pores is significant to deepening the understanding of the macrobehavior of waterflooding and the designation of the reservoir development plan. Previous studies mainly focused on the change in recovery factor and macroflow characteristics, while less attention was paid to the causes of the change in recovery factor and the difference in macrocharacteristics. In this paper, the numerical simulation technology by coupling the Navier-Stokes equation with the method of volume of fluid is employed to investigate the dynamic formation mechanism of the remaining oil at pore-scale in the primary waterflooding, and then the pore-scale oil-water two-phase behaviors under three waterflooding development adjustment schemes: changing flooding direction, turning extraction well to injection well, and increasing injection rate are studied. The research shows that when the viscous resistance of the water-bearing channel is less than the sum of the capillary barrier and drainage capillary resistance (the resistance in the displacement of wetting phase displacement using the nonwetting phase), the remaining oil is formed. Water➔oil➔water➔oil displacement mode is formed in the process of changing flow direction, which makes the force of the phase interface tend to balance, reduces the capillary effect in waterflooding, and improves oil recovery. In the process of developing the scheme of turning extraction into an injection well, through multipoint injection, it advances from the central water-bearing area to the oil-bearing area on both sides in multiple paths, forming a larger spatial spread range than that in the change of flooding direction. Under the influence of the capillary barrier effect and drainage capillary resistance, when the injection rate is increased, the remaining oil can restart to move only when the flowing rate exceeds a certain value. The small viscous resistance in the water-bearing channel and the lateral resistance from the capillary barrier limiting the lateral sweep of water are the primary reasons for the insignificant improvement of oil recovery under the condition of a low liquid injection rate. The findings of this study can help for better understanding of pore-scale flow mechanism behaviors and their influences on the macroscopic development features in the waterflooding process.

Optimization of Soil-Sludge Mixtures by Compaction for Potential Use in Mine Site Reclamation

Studies have indicated the potential of mixtures of silty soil and sludge produced by active treatment of acid mine drainage for use in covers with capillary barrier effects for mine site reclamation. Very high water contents of sludge in the settling pond could negatively affect the required hydrogeotechnical properties of soil sludge mixtures with high sludge contents. The challenge is then to determine the optimum wet sludge content of soil-sludge mixtures with air entry values (AEV) and/or saturated hydraulic conductivity (ksat) required for use in mine site reclamation covers. This paper presents a method to determine the optimum wet sludge content βopt for obtaining the maximum dry density of compacted soil-sludge mixtures. Two types of soil (S1 and S2) and two types of sludge (A and W) were tested. It was observed that βopt can be determined when the initial water content of the soil used in the mixture is lower than the optimum water content determined from the Proctor curve of the soil alone (10 wt% and 6 wt% for soils S1 and S2, respectively) and that βopt does not change with increasing initial soil water content. Optimum wet sludge contents found were low (≈15 wt% and ≈7 wt% for mixtures containing soils S1 and S2, respectively) for the test conditions, indicting a limited quantity of reusable sludge in the mixtures. For all mixtures, the water content corresponding to βopt was close to the optimum water content of the soil alone. Results of soil water retention and saturated hydraulic conductivity (ksat) tests conducted on selected optimized mixtures indicated that the mixtures based on soils S1 and S2 have air entry values higher than 20 kPa and would be suitable for use in the moisture retention layer of covers with capillary barrier effects, while soil S2 and the derived mixtures exhibited ksat < 10−7 cm/s and would be potential materials for the low permeability layer in low saturated hydraulic conductivity covers.

Pore network modeling of capillary barrier effects: impact of pore sizes

Although the working principles of capillary barrier effects (CBEs) have been well explained based on unsaturated soil mechanics, the selection of materials for engineering structures with CBEs mostly relies on designers’ experience or previously reported data, largely due to the lack of understanding of the microscale behavior of CBEs. This study explores the impact of pore sizes on CBEs based on pore network modeling. The results indicate that the pore size variability in either the fine or coarse layer has a remarkable influence on CBEs, with the latter having a dominating influence. A larger coarse-to-fine mean pore size ratio results in more effective CBEs. In addition to selecting materials with more uniform pore sizes, the coarse-to-fine mean pore size ratio is recommended to be >6 (median particle size ratio of 30) to ensure the performance of CBEs.

Experimental modelling of hydraulic parameters for fluid flow in stratified porous media

From works in in-situ seepage through dams and laboratory experiments using Layered Heterogeneous Porous Media (LHPM), it has been noted that a refraction-like phenomenon, such as that experienced in light, affects fluids’ flowlines when crossing the contact interface of layers characterised by different porosity viz-a-viz permeability. This concept has many applications in fluid dynamics, such as the dispersion process in stratified media. Currently, no study exists that models and analyses the relationship between the porosities of two layers in contact and the resulting flowline refraction using validated LHPM data. Hence, this work aims to establish a relationship between the porosity ratio Φr of stratified media made up of two layers and the refraction angle θmax of the maximum volume flux qmax. Volume flux data from the flow of a single-phase fluid through five LHPMs with Φr ranging from 0.8325 to 0.9524 were used in the modelling. The flow was oriented from the lower to the higher porosity vis-à-vis permeability layer. It was found that θmax which is also the refraction angle of the peak solute plume flux, refracts away from the normal as Φr reduces. This indicates an increase in the dilution rate vis-à-vis spread of plumes with the reduction in homogeneity between the two layers. Also, θmax does not correlate with the stratification inclination α, but the qmax which is also the peak solute plume flux correlates with α. Furthermore, an efficient model, which is the best-unbiased estimator, with R2=0.99 was derived. Findings from this work can help better understand solute plume dispersion and the general fluid flow dynamics in stratified media such as capillary barrier effect covers for pollution control and hydrocarbon reservoirs.

Migration and redistribution of LNAPL in inclined stratified soil media

Leakage of light non-aqueous phase liquid (LNAPL) into soil can cause serious environmental issues. In this study, a two-dimensional device with adjustable dip angles was designed to investigate the migration and redistribution of LNAPL in natural inclined stratified soil media by the light transmission visualization (LTV) technology. The captured experimental images were processed to obtain the diesel distribution based on gray value which could represent the LNAPL saturation distribution. LNAPL may not be able to penetrate through the fine-coarse interface due to the capillary barrier effects. In this case, the vertical and horizontal migration distances (V and H), contaminated area (S), as well as deviation angle (γ) of centroid increased with the dip angle. Increasing the leakage amount to more than 30 mL would result in LNAPL breakthrough at the 10°-inclined interface, leading to much larger V, H, S, and γ than those at 10 mL, while 20-mL LNAPL failed to break through. In the latter case, a lower leakage rate than 10 mL/min would cause larger H and γ but similar V or S in the long term. This study could enrich the understanding of LNAPL contamination in vadose zone, providing reference for the prediction and treatment in realistic inclined contaminated sites.

Efficacy of Pond Ash as a Cover Material in Single and Dual Capillary Barriers

In this paper, numerical study results that highlight the capability of using pond ash as a cover material in covers with capillary barrier effects are succinctly summarized. A model was exclusively developed and calibrated using commercial software SEEP/W for this study. After calibration, the efficiency of pond ash as a coarse-grained layer (CGL), fine-grained layer (FGL), unsaturated drainage layer (UDL), and seepage control layer (SCL) was investigated using various combinations. The CCBE made of pond ash obtained from the output and input points of an ash pond as alternate fine- and coarse-grained layers were observed to perform well. The performance of pond ash as UDL and SCL was also observed to be good. The approach presented in this study is valuable in assessing likely slope failures of coal ash storage facilities that may be triggered by rainfall events. The study aids in significantly reducing rainfall infiltration, improving the overall stability of the slopes, and promoting sustainability by utilising the concept of “waste covering waste”. The approaches used in this study can be extended in the rational design of slopes to address the future challenges anticipated with unprecedent rainfall events and its negative impacts associated with climate change effects.

Capillary Barrier Effect in an Earthen Roof of Historical House

A capillary barrier is a widely used drainage system in recent years. It has been in practice since the end of the 1950s, and research over the past 60 years has provided valuable insight into its safe and economical design. Although capillary barrier application is not an old and traditional technique, it is possible to see its primitive application on earthen roofs of traditional buildings. One of these applications is in the earthen roofs of historical and protected houses in Kemaliye district in eastern Turkey. In this study, the capillary barrier behaviour of the earthen roof system of historical Kemaliye houses, which has existed for centuries, has been experimentally investigated. In this context, soil index properties and water characteristic curves were determined by conducting laboratory experiments on samples taken from the soil roof of a traditional Kemaliye house. In order to examine the capillary barrier behaviour of the roof in precipitation, 1D model infiltration experiments were carried out. In the light of the findings obtained from these experiments, it was concluded that the earthen roofs of traditional Kemaliye houses show varied capillary barrier behaviour depending on the initial conditions of the layers.

Preventing the secondary salt deterioration in the repaired area of basal erosion of earthen sites based on capillary barrier effect

Basal erosion is one of the most typical salt deteriorations of earthen sites in arid and semi-arid areas. Its formation is mainly related to the water and salt migration and enrichment at the bottom of earthen sites walls, and can cause collapse of site walls. Rammed roof-propping reinforcement is the main repair technology for the basal erosion of the earthen sites, but there is a risk of secondary salt deterioration in the repaired area due to the subsequent water and salt migration. Based on the capillary barrier theory, this study regulates the particle size composition of repair soil to explore repair materials that can effectively mitigate the secondary salt deterioration. Firstly, Arya-Paris model and Fredlund-Xing’s model were used to convert the particle size distribution curves (PSDC) of soil with different particle composition into soil–water characteristic curves (SWCC), to preliminarily judge the capillary barrier potential of different soil structures. Then carry out water-salt migration test and mercury intrusion porosimetry test to verify the predictions. The research shows that when repair soil is coarser than earthen site soil, it can form an effective capillary barrier to block the capillary rise of water and salt, thus reducing the secondary salt deterioration in the repaired area. When repair soil is finer than earthen site soil, it will promote the capillary rise of water and salt, thus aggravating the salt deterioration in the repaired area. It is proposed to use the repair material of adding 20% sand on the basis of the original earthen site soil particle composition for the rammed roof-propping reinforcement of the earthen sites.

Effects of flow rate and pore size variability on capillary barrier effects: a microfluidic investigation

Capillary barrier effects (CBEs) have been applied in capillary barrier systems as effective means of protecting underground regions from wetting. However, the microscale behavior of CBEs and related influencing factors are not well understood. This study utilized microfluidics to investigate effects of flow rate and pore size variability on CBEs at the microscale. Imbibition processes of water displacing air were imposed on three water-wet microfluidic chips with different degrees of pore size variability and injection rates. The obtained results demonstrated that the increasing injection rate changed water invasion pattern from finger growth to compact displacement. The CBEs could increase Swf (water saturation of fine sections at the onset of breakthrough) by up to 44%. The increase in Swf became smaller at a higher injection rate but was insensitive to the pore size variability within the investigation range. This study also elaborated the specific effect of inertia on pore body invasions under different pore characteristics. Among the materials with close average pore size, those with more uniform pore sizes are recommended for the construction of capillary barrier systems. For capillary barrier systems with robust CBEs, increasing the thicknesses of coarse layers has insignificant effect on reducing deep percolation.

Stability Analysis of the Inclined Capillary Barrier Covers under Rainfall Condition

Capillary barrier covers consist of fine-grained soil layer overlying coarse-grained soil layer, which are widely used as surface covers for mine tailings, solid waste landfills, and low-level radioactive waste repositories. On one hand, the capillary barrier covers can effectively prevent the rainfall water infiltrating into the toxic and hazardous materials below. On the other hand, the infiltrated water stores and diverts in the fine-grained soil layer, leading to a reduction in the stability of the capillary barrier covers. In this study, a stability analysis method for the capillary barrier covers was established based on the Green-Ampt model and the Janbu method. Firstly, the infiltration process of capillary barrier covers was analyzed and divided into four stages. The variation of the wetting front profile during infiltration, caused by the capillary barrier effect, was depicted based on the law of mass conservation. Next, the wetting front is assumed to be the potential sliding surface. As the infiltration goes on, the stability of capillary barrier covers in different stages was analyzed through the limit equilibrium method. Both the water redistribution and the influence of seepage force in the capillary barrier covers were considered in the proposed method. Finally, using the examples in the published articles, the availability and superiority of the proposed method was verified.

Field-scale compression of Sphagnum moss to improve water retention in a restored bog

The restoration and subsequent regeneration of Sphagnum moss on harvested peatlands can result in poor hydrological connectivity between the regenerated Sphagnum moss and the remnant cutover peat due to the formation of a capillary barrier that favours retention of water in the underlying cutover peat. In a restored cutover peatland in Québec, Canada, this resulted in a lower soil water content of the regenerated Sphagnum moss compared to a natural analogue, which may limit carbon sequestration potential. Previous work has shown it may take > 40 years for the regenerated moss layer to overcome this capillary barrier effect, however mechanical compression may provide a novel restoration technique to weaken the capillary barrier in restored peatlands. This study evaluated the effectiveness of field-based mechanical compression to ameliorate the capillary barrier effect and increase the moisture content in a restored cutover peatland. A section of restored Bois-des-Bel peatland was compressed using a John Deere 6430 series tractor in January 2016, followed by hydrological monitoring May-August. The Compressed site was compared to a nearby Uncompressed site, and a natural peatland ∼ 2 km away, to determine relative success of mechanical compression. The compression reduced moss height by 48%, increasing bulk density by 37% and reducing the proportion of macropores by 15% throughout the moss layer. Consequently, compression increased soil water retention and average soil moisture content at the Compressed site was consistently higher throughout the moss profile than in the Uncompressed site. This increased the resilience of the moss to drying, though the degree of increased resilience is unknown. Despite not having the same hydrophysical properties of the Natural site, the Compressed site was wetter than the Uncompressed site due to the lessening of the capillary barrier effect and moving the surface closer to the water table, thus decreasing the water stress of the regenerated Sphagnum carpet.

Investigation of the Effect of Capillary Barrier on Water–Oil Movement in Water Flooding

Water flooding technology is widely used to improve oil recovery efficiency in oilfields. The capillary barrier effect induced by the complex pore structures in the reservoir rocks is a crucial reason for the trapping of a great deal of residual oil in oil reservoirs after water flooding. However, the formation condition along with the effect on the recovery rate of the capillary barrier under different wettability conditions should be investigated further. To bridge the gap between the microscopic mechanism of the capillary barrier effect and the macroscopic mechanism of oil displacement efficiency, a simple conceptual capillary model is constructed to obtain the formation conditions of the capillary barrier using the analysis method, and its influence on macroscopic oil displacement efficiency in the porous media model with an opening angle of 45° is systematically investigated in this study using direct numerical simulations (DNS) coupled with the volume of fluid method. The results showed that the capillary barrier effect plays a significant role in the formation of the residual oil in the reservoir rock and the contact angle and the opening angle are the primary factors for the formation of the capillary barrier. The capillary force is the driving force when the oil–water interface advances in the throat channel under water-wet conditions, while the capillary force hinders the movement of oil–water movement when the liquid flows out of the throat channel and when θ + β > 90o. Furthermore, the highest oil displacement efficiency is achieved at the intermediate capillary number and in the case that the minimum conditions of occurrence of the capillary barrier phenomenon are satisfied. This is of great significance for controlling the optimized contact angle to further enhance the oil recovery rate of current oil reservoirs using waterflooding technology.