Seepage Behavior Research Articles

Experimental investigation on low-velocity seepage characteristics and influencing factors in a shale oil reservoir

The seepage behavior in shale oil reservoir is very complicated. In this study, the characteristics and influencing factors of low-velocity seepage in brittle shale oil reservoirs were investigated by high precision experimental methods. The velocity used in this study was as low as 10−4 ml/min, and at least eight different flow rates were measured for each experiment. During the operation, the pressure difference between the inlet and outlet was controlled within 0.35 MPa to eliminate the influence of stress sensitivity. The effects of core permeability, fluid viscosity, pore pressure, and propped artificial fracture on the seepage behavior were analyzed. The results show that the seepage flow rate and pressure gradient in shale core samples presents a convex relationship, which is entirely different from the results of a concave relationship in tight sandstone oil reservoirs. The slippage effect and boundary layer effect in the flow channels were investigated to explain the convex flow regime theoretically. The convex shape flow regime is caused by the heterogeneous of shale core samples associated with the completed developed micro-fractures in this area. The seepage flow divides into two stages: the non-linear stage when the applied pressure gradient is lower than 2 MPa/m and the linear stage when it is higher than 2 MPa/m. The non-linear seepage characteristics become more obvious with a higher viscosity crude oil. With the increase of pore pressure, micro-fractures in shale core samples gradually close, leading to the reduction of flowing channel and the significant decrease of apparent permeability. The seepage characteristics are similar to those of tight sandstone oil, and the flow rate and pressure gradient curve presents a concave shape. With a propped artificial fracture, the seepage flow behavior follows Darcy's law. This work provides a feasible and reliable method and valuable experiment data for the study of low-velocity seepage in shale reservoirs.

Study on the reservoir dam slope stability considering the effect of seepage

Water conservancy dam plays a significant role in flood control, power generation, shipping, irrigation, aquaculture, tourism, urban water supply and so on. The stability of the dam is extremely important in the engineering design and research processes. Based on the basic principles of finite difference and strength reduction methods, the software of FLAC3D is adopted to analyze the stability of the dam, considering the seepage state, the reservoir water level height and the seepage coefficient of the dam body. The results show that with the increase of the reservoir water level, the seepage behavior of the dam body is more obvious, and the horizontal displacement of the dam increases, while the dam slip safety factor is reduced. With the increase of the seepage coefficient of the dam body, the seepage discharge at the dam foot increases linearly, and the stability of the dam body decreases obviously. The seepage of dam foundation and dam slope stability is evaluated in this research. It is found that the reservoir water level and the dam body permeability are the important factors affecting the dam slope stability, so it is necessary to enhance the anti-seepage measures during the dam design and construction.

Pore-scale morphology and wettability characteristics of xenon hydrate in sand matrix - Laboratory visualization with micro-CT

The properties of gas hydrate bearing sediments (GHBS) depend on their changing morphology and wettability characteristics, which are in turn important for efficiency and safety issues during production. We present the results of xenon hydrate formation experiments using quartz sand packs at different salinities. Direct 3D visualization of hydrate distribution in the pore space was obtained with the help of high-resolution micro-computed tomography (CT). Four phases, xenon hydrate, xenon gas, brine and quartz sands were segmented and labelled in the raw micro-CT scanned images. A 3D reconstruction reveals typical pore-scale patterns of hydrate occurrence (pore-filling, grain-coating) and shows that hydrate preferentially aggregates at the gas-water interface, reflecting greater local access to xenon gas molecules. The hydrate cluster shape factor ranges from 2 to 10, indicating that xenon hydrate forms with a complex and irregular microstructure, consistent with random growth nature of hydrate. The hydrate saturation is found to decrease with salinity under excess-gas condition, which we interpret in terms of the hydrate formation kinetics to indicate that gas hydrate prefers to grow into the gas phase at the gas-water interface. A total of 270 local contact angles of water at the gas and hydrate interfaces in pores were measured with a micro-CT image analysis method to obtain information on the wettability variation of hydrate surfaces. The average was calculated to be 47.6°, indicating that xenon hydrate surface is hydrophilic. Our analysis of the xenon hydrate morphology provides a new method for revealing hydrate growth behavior at pore-scale. The wettability characterization offers key parameters for accurate numerical simulation of multiphase flow prediction and seepage behavior in GHBS.

Effect of damage on gas seepage behavior of sandstone specimens

In underground engineering, such as geological CO2 sequestration, unconventional oil and gas exploration, and radioactive waste storage, permeability of rock is important to evaluate the potential CO2 storage capacity, improve oil and gas production, and prevent leakage of radioactive waste. In this study, hydrostatic stress tests and triaxial compression tests with gas permeability measurements were carried out on intact and damaged sandstone specimens. Three series of experiment were designed to evaluate the permeability evolution laws of sandstone under different testing conditions. They included triaxial seepage tests on intact specimens under different confining pressures, triaxial seepage tests on damaged specimens with different extents of damage, and hydrostatic seepage tests on damaged specimens under increasing and decreasing gas pressures. Based on the experimental results, the effects of effective confining pressure, extent of damage and increasing and decreasing gas pressure on permeability of sandstone were investigated. It shows that the permeability of the intact sandstone specimens first decreased and then increased, followed by a constant value with increase in axial strain. The permeability of the sandstone specimens was observed to decrease with increase in effective confining pressure. The extent of damage affects the permeability evolution, but does not influence the failure patterns of damaged sandstone. As the gas pressure increased, the permeability of the damaged sandstone specimen increased. Under the same gas pressure condition, the permeability during the decreasing process is generally higher than that during the increasing process. These experiments are expected to enhance our understanding of seepage behavior in underground rock masses.

Effect of Wetting-Drying Cycle on the Deformation and Seepage Behaviors of Rock Masses around a Tunnel

Water inrush caused by the wetting-drying cycle is a difficult problem in tunnel excavation. To investigate the effect of the wetting-drying cycle on the stability of the tunnel surrounding rock, physical experiments and numerical simulations regarding the process of tunnel excavation with different wetting-drying cycle numbers were performed in this study. The evolutions of stress, displacement, and pore water pressure were analyzed. With the increase in cycle number, the pore water pressure, vertical stress, and top-bottom approach of the tunnel surrounding rock increase gradually. And the increasing process could be divided into three stages: slightly increasing stage, slowly increasing stage, and sharply increasing stage, respectively. The failure process of the surrounding rock under the wetting-drying cycle gradually occurs from the roof to side wall, while the baseplate changes slightly. The simulation results showed that the maximum principal stress in the surrounding rock mass of the tunnel increases, while the minimum principal stress decreases. Furthermore, the displacement of the rock mass decreases gradually with the increasing distance from the tunnel surface. By comparing the simulation results with the experimental results, well consistency is shown. The results in this study can provide helpful references for the safe excavation and scientific design of a tunnel under the wetting-drying cycle.

Experimental evaluation of physical, mechanical, and permeability parameters of key aquiclude strata in a typical mining area of China

Coal production safety and protection of water resources in coal mines located under aquifers requires an in-depth analysis of key aquiclude strata (KAS), including their physical, mechanical, and seepage characteristics. Based on hydrogeological conditions of thickness and buried depth of coal seam, thickness and water abundance of the aquifer, KAS thickness, etc., the experimental comprehensive testing method of mechanical and permeability parameters of KAS in the Wugou Coal Mine, China, are proposed. This paper applies state-of-the-art X-ray diffraction (XRD), scanning electron microscopy (SEM), and mechanical test methods to study mineral composition, microstructure, mechanical properties, water absorption, and softening coefficient of KAS rock samples collected from the mine under study. The comprehensive analysis of the results obtained made it possible to clarify the particular water-inrush mechanism and stability control of KAS using the revealed stress-strain-permeability dependencies of composite KAS and non-Darcy seepage behavior of post-peak fractured KAS. The model of KAS damage status with different backfill effects is established and successfully applied to the case study. Findings of this study are considered instrumental in the KAS-based prevention of water-inrush hazards and more rational exploitation of coal resources.

Seepage Analysis in Short Embankments Using Developing a Metaheuristic Method Based on Governing Equations

Seepage is one of the most challenging issues in some procedures such as design, construction, and operation of embankment or earth fill dams. The purpose of this research is to develop a new solution based on governing equations to solve the seepage problem in an effective way. Therefore, by implementing the equations in the programming environment, more than 24,000 models were designed to be applicable to different conditions. Input data included different parameters such as slopes in upstream and downstream, embankment width, soil permeability coefficient, height, and freeboard. With the use of this big data, a new process was developed to provide simple mathematical models for the seepage rate analysis. The study first used intelligent models to simulate the seepage behavior. Finally, the accuracy of the models was optimized using a new metaheuristic algorithm. This led to the ultimate flexibility of the final model presented as a new solution capable of evaluating different conditions. Finally, using the best model, new mathematical relationships were developed based on this methodology. This new solution can be used as a proper alternative to the governing equations of seepage rate estimation. Another advantage of the proposed model is its high flexibility that can be well applied to engineering design in this field, which was not possible using the initial equations.

Numerical study on permeability characteristics of fractal porous media**Project supported by the National Natural Science Foundation of China (Grant Nos. 51776037 and 51806147) and Natural Science Foundation of Jiangsu Province, China (Grant No. BK20170082).

The fractal Brownian motion is utilized to describe pore structures in porous media. A numerical model of laminar flow in porous media is developed, and the flow characteristics are comprehensively analyzed and compared with those of homogeneous porous media. Moreover, the roles of the fractal dimension and porosity in permeability are quantitatively described. The results indicate that the pore structures of porous media significantly affect their seepage behaviors. The distributions of pressure and velocity in fractal porous media are both non-uniform; the streamline is no longer straight but tortuous. When Reynolds number Re < 1, the dimensionless permeability is independent of Reynolds number, but its further increase will lead to a smaller permeability. Moreover, due to the higher connectivity and enlarged equivalent aperture of internal channel network, the augment in porosity leads to the permeability enhancement, while it is small and insensitive to porosity variation when ε < 0.6. Fractal dimension also plays a significant role in the permeability of porous media. The increase in fractal dimension leads to the enhancement in pore connectivity and a decrease in channel tortuosity, which reduces the flow resistance and improves the transport capacity of porous media.

A Preliminary Comparative Study of Seepage Behaviour Behind Retaining Walls with Crumb Rubber and Geocomposite

A retaining wall is built to provide support to the soil when there is a change in elevation of the ground. Weep holes present in the retaining wall help water to seep through it. Filter protection should be made behind the weep holes to prevent soil erosion around the weep holes. The classic filter material that is widely used is gravel, which is packed according to Hudson’s law. Laboratory experiments were conducted to understand the seepage function of alternative material such as crumb rubber and geocomposite (fabricated) in a homogenous sand layer and in-situ soil. The time taken by the water to reach the weep holes was calculated and compared. From the results, it is suggested to use crumb rubber as an alternative packing material behind the weep hole.

Hydraulic conductivity characteristics of carbonated reactive magnesia-treated silt

Carbonation of reactive magnesia (MgO) has recently received increasing attention in the area of soil stabilization and ground improvement. However, as a critical parameter in terms of long-term seepage behavior in the geotechnical analysis, the hydraulic conductivity of carbonated reactive MgO-stabilized silt has not been fully studied. In this context, the effect of water-MgO ratio (ratio of initial water content to MgO content, w0/c) and carbonation time on hydraulic conductivity (or permeability) characteristics was systematically investigated. Serial microstructural tests including mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) analyses were applied to elucidate the intrinsic mechanisms. The obtained results indicate that as the initial water-MgO ratio decreases, the void ratio gradually decreases and the reduction of hydraulic conductivity becomes less prominent because of the little presence of flow paths. The hydraulic conductivity of carbonated MgO-admixed silt similar to that of PC-treated silt is mainly governed by the porosity, and its correlation with void ratio is proposed in the article. The variations of permeability with void ratio are consistent with those of the cumulative pore volume from MIP results in general, and the medium pores (3–30 μm) are substantiated to be the primary contributor in controlling the permeability. SEM and DSC analyses reveal that the cementation of soil particles and filling of hydrated magnesium carbonates marginally reduce the voids and permeability. The reasons for changes of permeability behaviors have been confirmed by the pore-size distribution and microstructure characteristics.

Modelling of Seepage Behaviour Using Available Gaps at Signalized Intersection

Seepage behaviour is identified by the movement of two-wheelers from back of the queue in a signalized intersection to the front through small gaps between other vehicles. In mixed traffic, this is a common phenomenon when the signal turns red. This study attempts to model seepage using Cellular Automata (CA) based simulation. These interactions between the vehicles affect the safety and capacity of a facility. There are very few studies that include this behaviour in the CA based models. This study further calculates lateral and longitudinal gaps available between adjacent front and side vehicles to identify the forward movement possibilities. The study is compared with the simulation software VISSIM. It is evident that the software is unable to replicate the seepage behaviour, which is reflected in the trajectories.

Geogrid Reinforced Soil Walls with Marginal Backfills Subjected to Rainfall: Numerical Study

Altering natural landforms without impacting stability has become quintessential due to unprecedented infrastructure development. Geosynthetic reinforced soil wall (GRSW) as a retaining structure is a congenial option addressing land scarcity and right of way challenges. However, with dearth of good-quality backfill soil and prohibitive transportation costs, engineers are being forced to use native low-permeable marginal soils for constructing geosynthetic reinforced soil walls. Adverse climatic changes leading to erratic rainfall have triggered numerous failures in such soils due to their inefficiency to dissipate excess pore water pressure. The objective of the present paper is to evaluate applicability of different backfill soils for GRSW. Seepage and stability analyses were conducted to appraise the behaviour of a geogrid reinforced soil wall with rigid facing of height 9.6 m having 6° batter during rainfall. The effects of backfill soil types with fines content of 0%, 20% and 40% on seepage behaviour and stability were assimilated under moderate rainfall. It was ascertained that failure occurred faster when the backfill soil with 20% fines was considered. For backfill soil with 40% fines, a significant portion of rainfall was observed to simply disappear as runoff. In the section with sand backfill, failure did not occur as water drained off faster than rainfall infiltration, restraining mounting of water table.

Impact of double defects on seepage behavior of geomembrane lined earth-rock dams

In this study, the unsaturated seepage theory was employed to analyze the effect of different elevations and separations of double defects in geomembrane barrier on the seepage behavior of geomembrane lined earth-rock dams. The results indicated that the separation between the defects has a significant influence on the defect-induced leakage. The increasing separation results in the increase of defect-induced leakage. When the separation reaches 3 m, the defect-induced leakage tends to decrease. The defect-induced leakage increases again when the separation is greater than 3.5 m. And defect-induced leakage increases gradually with decreasing elevation of defect, but is still in the same order of magnitude. In addition, the wetted area near the defects expands with the increasing separation between double defects. The wetted areas can be delineated by a circle, by an ellipse or by two discrete circles depending on the separation and elevation of double defects.

A FRACTAL PERMEABILITY MODEL FOR POROUS–FRACTURE MEDIA WITH THE TRANSFER OF FLUIDS FROM POROUS MATRIX TO FRACTURE

The transfer of fluids from porous matrix to fracture is a key issue to accurately predict the fluid flow behavior in porous–fracture media. In this work, to take into account the transfer of fluids, the analytical model of dimensionless permeability is proposed based on the fractal geometry theory for porous media. The proposed model is expressed as a function of microstructural parameters of the porous matrix and fracture, such as the pore area fractal dimension [Formula: see text], fractal dimension [Formula: see text] for tortuosity of tortuous capillaries, the ratio [Formula: see text] of the maximum pore size in porous matrix to fracture aperture, as well as the ratio [Formula: see text] of the pressure difference along the fracture to that along the porous matrix layers. The model reveals that the ratios [Formula: see text] and [Formula: see text] have significant influences on the permeability contribution from the porous matrix to the seepage behavior of the fracture. While the contribution of porosity of leak-wall porous surface of the fracture to the permeability is less than 10%. The present results may provide an important theoretical foundation for exploration of petroleum, gas and geothermal energy extraction systems.

Simulations on the micro-seepage rules of gas and water based on micro-CT/CFD and the related contrastive analysis

This study first employed micro-computed tomography technique for scanning the gas-fat coal sample; then, using Avizo software, coal’s micro-pore structure model was established; finally, seepage behaviors of gas and water in the coal under a pressure of 3 MPa were simulated. Through comparison, it can be concluded that both gas pressure and water pressure decreased gradually along the seepage direction; at a same seepage length, water’s maximum seepage pressure significantly exceeded that of gas; gas’s velocity flow lines were overall fuller than water’s flow lines; using a same pore model, gas’s migration velocity far exceeded that of water; In terms of mass flow rate, water exceeded gas. The effluent mass flow rate at the outlet was smaller than the influent mass flow rate at the inlet. Overall, with the increase in the seepage length, gas and water’s mean seepage pressures and mass flow rates all decreased gradually, while their mean seepage velocities fluctuated greatly. Since the pores at the different coal cross-sections were developed to different degrees, the heterogeneity of the coal structure meant that the cross-sectional area of the pore channel varied irregularly along the seepage direction. The fitting curve can be reduced to three classes: logistic linear regression curve, Boltzmann function curve, Exp Dec curve, or poly curve.

Coupled Seepage–Stress Analysis of Selected Roller-Compacted Concrete Gravity Dam with Upstream Impervious Layer Considering the Seepage Behavior of Interface Thickness

The characteristic of seepage through the roller-compacted concrete gravity dams (RCCDs) is inhomogeneous, anisotropic and significantly different from the seepage through conventional gravity dams. This is due to the internal interfaces formed by thin rolling of layers which are weak links and the main channels for seepage. The provision of an upstream impervious layer (UIL) or less pervious upstream layer may efficiently reduce the excessive seepage in RCCDs. Several works such as those of Li et al. (Water Sci Eng 8(4):326–334, 2015) considered the upstream less pervious layer constructed with different concrete materials; however, the properties of interface thickness have not been included in their analysis. The present study attempts to carry out a seepage analysis of RCCDs with an upstream less pervious layer of different types of concrete materials including three-graded RCC, two-graded RCC, conventional vibrate concrete and grout-enriched vibrated RCC with a thickness of 2–3 m considering the significant properties of interface layers, viz. thickness and permeability. The results of the analysis emphasize the effect of less pervious upstream layer or UIL thickness on controlling seepage and also facilitate in understanding its effect on stresses. The property of RCC considered as inhomogeneous multi-laminated medium can be described by tangential and normal major coefficients of the permeability of the layer which depend on the coefficient of permeability and the thickness of the layer following cubic law. These properties are expected to simulate the seepage behavior of RCCD, and also the changes in the mechanical properties such as the elastic modulus with time and temperature are considered to predict the stress behavior of the structure. RCC dams are more pervious than conventional concrete dams and seepage through the dam exerts pore pressure throughout the body of the dam. From the point of its significance, considering the effect of seepage as body force is more realistic than considering it as an external force, viz. uplift pressure at the base of the dam. This method of analysis also facilitates in carrying out stress analysis of dam and foundation together. Hence, a coupled seepage–stress interaction is carried out while analyzing the selected section of RCCD with and without UIL using ABAQUS software. The results provide essential parameters such as pore pressure, stresses required to select the thickness and material of anti-seepage structure.

Efficiency analysis of seepage of Baz Ali small dam, Kurram Agency using clay blanket and cut-off wall with sand filter

Seepage always eventuates through the foundation of all dams. The flow of water through the dam body generate seepage forces and endanger the dam stability due to piping. Controlling seepage after construction is quite difficult and an expensive job, hence proper practices should be adopted for seepage remediation in the designing stage. In order to investigate the more effective techniques among downstream (D/S) sand filter with upstream (U/S) clay blanket and cut-off wall, the Baz Ali small dam was analyzed using finite element approach SEEP/W 2D. The seepage behaviour through the dam was intuited by employing 1 m thick clay blankets on the upstream side with extending lengths of 50 m, 100 m, 150 m, 200 m, 250 m and 300 m. Furthermore, 5 m, 7.5 m and 10 m deep cut-off walls having 0.5 m and 1 m thickness were embedded for seepage mitigation. The seepage values obtained from SEEP/W 2D models were compared with each other. The SEEP/W 2D results and cost analysis show that clay blanket is more effective and an economical technique than a cut-off wall for tackling seepage issue. Hence, an upstream blanket of 100 m length is more efficient to diminish the seepage up to 58.65% in contrast to the base case without the remedial system.

The Mining-Induced Seepage Effect and Reconstruction of Key Aquiclude Strata During Backfill Mining

When mining coal below aquifers, the height of the water-conducting zone above the mine panel is widely used as one of the main criteria in assessing mine safety. However, its direct application to backfill mining is quite problematic, and alternative approaches have to be used to more accurately reflect the relevant processes. The compaction characteristics and seepage behavior in crushed key aquiclude strata were experimentally investigated by analyzing the hydrogeological characters of aquiclude strata and aquifers in the Wugou coal mine. The results explained the mechanism of the backfill mining-induced seepage and indicated the feasibility of reconstructing the key aquiclude strata to protect regional water resources. Based on the regional geology and hydrogeological characteristics of the aquiclude strata, various reconstruction forms were analyzed, with the approbation of structural and seepage stability criteria of aquiclude strata. The research results were corroborated by field measurements and provide new theoretical guidance for protecting water resources during mining in China.

Study on pore size effect of low permeability clay seepage

Low-permeability clays are characterized by a large amount of clay minerals and a special distribution of pore sizes, so their seepage behavior shows an obvious pore size effect. Therefore, conventional seepage theories show deviations in describing the seepage behavior of low-permeability clays. This paper analyzes two reasons for the pore size effect in low-permeability clay seepage: the pore distribution property and the electric double layer effect of the clay surface. Considering these two factors, the seepage theory of the pore size effect based on the microscale seepage of the circular tube model is proposed. The rationality of this theory is tested by the seepage experiments of natural undisturbed clay and artificial clay, and from these experiments, three beneficial conclusions are drawn: (1) The results of the pore size effect seepage theory are in good agreement with the experimental results and are better than the modified Kozeny-Carman equation; (2) The properties of the electric double layer have a certain regular influence on the seepage behavior of the clay; (3) Considering the properties of the soil pore size distribution, the results obtained by using the pore size effect theory are much closer to the experimental results than the results calculated using the average pore diameter of the clay.

Effects of Pore Structure on Stress-Dependent Fluid Flow in Synthetic Porous Rocks Using Microfocus X-ray Computed Tomography

Based on microfocus X-ray computed tomography analyses, the relationship between the characteristics of pore structure and fluid flow behavior was investigated through the single-phase flow experiment. The macroscopic bulk fluid seepage behavior was fundamentally explained by the microscale flow mechanism considering all the porous microstructure analysis of artificial cores. The results indicate that the pore size and connectivity have significant effects on the initial permeability of the rock cores. The permeabilities obtained from different methods have a linear law relationship with porosities in tested artificial cores. The results also suggest that the permeability of core decreases exponentially with the increase in effective stress. The inner different pore structures have an important influence on the stress-dependent fluid flow in synthetic porous rocks. The polynomial equation yields well fittings of the artificial core which has the poor pore sorting characteristic. The permeabilities of the artificial core are more significantly affected by changes in the low effective stress range. The larger the pore channel of the artificial core is, the greater influence on permeability the pressure will have. The stress sensitivity of artificial core increases as the grain diameter decreases. The heterogeneous coefficient and the stress sensitivity of permeability are positively correlated.