Seepage Behavior Research Articles

Modeling and simulation on heat-injection enhanced coal seam gas recovery with experimentally validated non-Darcy gas flow

Abstract Heat-injection into coal seam has gained great attention as a supplementary method for the improvement of unconventional gas recovery from deep reservoirs in recent years. However, it involves a lot of ambiguous interactions among the deformation of formation, the transport of gas and the diffusion of heat in the porous coal media, especially the property of gas. In this paper, the seepage behavior of coal seam gas (CSG) that obeys non-Darcy flow law was directly observed in a conducted laboratory experiments. Then, a general Forchheimer equation was confirmed and strictly validated by the experimental data to characterize the non-Darcy seepage phenomenon. Subsequently, a fully coupled thermo-hydro-mechanical model was mathematically established. Based on the finite element approach, a numerical simulation for heat-injection enhanced coal seam gas recovery was then implemented. The susceptibilities of coal permeability and gas production to the gas sorption, heat-mass transfer, and the porosity-seepage characteristics of coal were quantified by a series of numerical scenarios. Final modeling and simulation results reveal that: (1) coal seam gas flow obeys non-Darcy law when the gradient of gas pressure is higher, but trends to be laminar flow that obeys Darcy law when the gradient is lower, and (2) heat-injection to stimulate coal seam could improve the production efficiency, increasing the Langmuir volume sorption parameter, the temperature correction coefficient and the initial values of permeability can improve coal permeability and enhance gas production.

Multiscale simulation of ink seepage into paper: A mesoscopic variational model

This work focuses on the study of ink seepage and spreading into paper to ease the deciphering of digitized ancient documents and therefore make them legible and researchable. A three-dimensional model is proposed for the simulation of ink seepage into paper at the fiber level. The model takes into account a finite volume of ink and describes the system state through gravity, cohesion and adhesion force interactions. At the mesoscopic scale, the writing paper is modeled using a discretized fiber structure generated using a numerical deposition procedure. A modified Monte Carlo method is introduced for the simulation of the ink dynamics. Finally a multiphase lattice Boltzmann method is suggested to fine-tune the mesoscopic variational model parameters and it is shown that the ink seepage behaviors predicted by the proposed model can resemble those predicted by a method relying on first principles.

Particle flow code method-based seepage behavior analysis and control effect evaluation for soil levee

Considering the material non-uniformity and time-varying mechanics performance in seepage development of soil levee, the particle flow code (PFC) method and the fast Lagrangian analysis of continua (FLAC) approach are combined to implement the numerical analysis and evaluation for seepage behavior and its control effect in soil levee engineering. Based on the coupling characteristics of particle and water in soil levee, a PFC-based numerical simulation method of seepage development is introduced. To improve the accuracy and efficiency of above method, the soil particle generation technology according to soil grading curve is presented. An approach, which refers to geotechnical centrifuge test and similarity principle, is developed to build the scale model of prototype levee. The calculation restriction of PFC method, which is integrated into FLAC tool, is broken. Lastly, the numerical simulation for seepage behavior of one actual levee engineering with seepage control measures is implemented by the proposed method. The seepage control mechanisms of selected measures are analyzed. The macroscopic and mesoscopic effects of selected measures on different levee foundations are evaluated. It is indicated that the seepage control effect can be described more finely with the average coordination number and the vertical effective stress which are obtained by the proposed numerical method.

Analysis of Sluice Foundation Seepage Using Monitoring Data and Numerical Simulation

For sluices built on soil foundations, seepage safety of the foundation is one of the most concerns during operation of sluices. Monitoring data could reflect the real seepage behavior in the foundation, but of which the shortcoming is that generally only the local seepage states can be measured. The seepage field in the whole foundation can be analyzed by numerical simulation. The permeability coefficients of the foundation materials significantly affect the numerical simulation results; however, it is difficult to accurately determine the values of permeability coefficients. In this paper, an approach based on response surface method (RSM) for calibration of permeability coefficients was proposed, and the efficiency of parameter calibration is improved by constructing the response surface equation instead of time‐consuming finite element calculation of foundation seepage. The seepage in a sluice foundation was analyzed using monitoring data and numerical simulation. The monitoring data showed that the seepage pressure in the foundation periodically varies with high value in flood season and low value in dry season. After calibration of the permeability coefficients of the foundation materials using the measured seepage pressure, the seepage fields in the foundation for different water levels were numerically simulated to investigate the cause for the periodical variation of the seepage pressure and the seepage safety of the foundation was assessed with the calculated seepage gradients. The methods adopted in this study could be applied to seepage analysis for sluice foundations with similar geologic conditions and antiseepage measures.

Unsteady Seepage Behavior of an Earthfill Dam During Drought-Flood Cycles

Climate change with extreme hydrological conditions, such as drought and flood, bring new challenges to seepage behavior and the stability of earthfill dams. Taking a drought-stricken earthfill dam of China as an example, the influence of drought-flood cycles on dam seepage behavior is analyzed. This paper includes a clay sample laboratory experiment and an unsteady finite element method seepage simulation of the mentioned dam. Results show that severe drought causes cracks on the surface of the clay soil sample. Long-term drought causes deeper cracks and induces a sharp increase of suction pressure, indicating that the cracks would become channels for rain infiltration into the dam during subsequent rainfall, increasing the potential for internal erosion and decreasing dam stability. Measures to prevent infiltration on the dam slope surface are investigated, for the prevention of deep crack formation during long lasting droughts. Unsteady seepage indicators including instantaneous phreatic lines, equipotential lines and pore pressure gradient in the dam, are calculated and analyzed under two assumed conditions with different reservoir water level fluctuations. Results show that when the water level changes rapidly, the phreatic line is curved and constantly changing. As water level rises, equipotential lines shift upstream, and the pore pressure gradient in the dam’s main body is larger than that of steady seepage. Furthermore, the faster the water level rises, the larger the pore pressure gradient is. This may cause internal erosion. Furthermore, the case of a cracked upstream slope is modelled via an equivalent permeability coefficient, which shows that the pore pressure gradient in the zone beneath the cracks increases by 5.9% at the maximum water level; this could exacerbate internal erosion. In addition, results are in agreement with prior literature that rapid drawdown of the reservoir water level is detrimental to the stability of the upstream slope based on embankment slope stability as calculated by the Simplified Bishop Method. It is concluded that fluctuations of reservoir water level should be strictly controlled during drought-flood cycles; both the drawdown rate and the fill rate must be regulated to avoid the internal erosion of earthfill dams.

Impact of transient seepage on slope stability of earth-rock dams with geomembrane barrier defects

The water–air two-phase unsaturated seepage theory was employed to analyse the transient seepage field of an earth-rock dam with an impervious geomembrane barrier during rapid drawdown of the reservoir. Discussed is the resulting seepage behaviour of the dam after locations of defects in geomembranes and hydraulic conductivity of dam materials were changed. In addition, the unsaturated soil strength theory was used to calculate the anti-sliding safety factor of the upstream dam slope during drawdown. The uncertainty of effective stress parameters of dam materials was considered, and the Monte Carlo method was employed in the reliability analysis of the stability of the upstream dam slope. The results indicate that the location of defects has a significant influence on the initial phreatic surface distribution and the seepage flux mainly depends on the hydraulic conductivity of dam materials. Geomembranes with defects on the dam surface have an apparent hysteresis effect on the descending of the phreatic surface when the reservoir level drops, which causes a great decrease in the stability of the upstream dam slope. Matric suction significantly affects the stability of the upstream dam slope, while the air phase has little effect on slope stability when geomembranes are placed on the upstream surface of the dam.

The Permeable Character of CSG Dams and Their Seepage Fields

Current studies regarding the permeable properties, corrosion properties, and seepage characteristics of cemented sand and gravel (CSG) materials are based on laboratory tests. Thus, there is a lack of studies analyzing the permeable character of seepage fields based on monitoring data from real prototypes working under practical operating conditions. In this paper, on the basis of measured data from the Dahuaqiao cofferdam, we establish an inversion analysis method for unsteady seepage fields covering different time periods within a time sequence. The results indicate an effective dynamic change law for the material permeability coefficient and the real dynamic evolution characteristics of seepage fields. The permeability coefficient of CSG exhibits a “self-healing” phenomenon similar to concrete, with the seepage characteristics of a dam tending to become stable over time. Under the long-term action of water pressure, the seepage behavior of the dam body shows no obvious deterioration, suggesting that CSG can satisfy the required anticorrosion property expected of dam construction materials. Thus, abnormal CSG might serve as an effective antiseepage layer that can meet the running requirements of cofferdams. The results of this research can provide reference for further improvement in the CSG dam design theory.

Monitoring models for base flow effect and daily variation of dam seepage elements considering time lag effect

Affected by external environmental factors and evolution of dam performance, dam seepage behavior shows nonlinear time-varying characteristics. In this study, to predict and evaluate the long-term development trend and short-term fluctuation of the dam seepage behavior, two monitoring models were developed, one for the base flow effect and one for daily variation of dam seepage elements. In the first model, to avoid the influence of the time lag effect on the evaluation of seepage variation with the time effect component of seepage elements, the base values of the seepage element and the reservoir water level were extracted using the wavelet multi-resolution analysis method, and the time effect component was separated by the established base flow effect monitoring model. For the development of the daily variation monitoring model for dam seepage elements, all the previous factors, of which the measured time series prior to the dam seepage element monitoring time may have certain influence on the monitored results, were considered. Those factors that were positively correlated with the analyzed seepage element were initially considered to be the support vector machine (SVM) model input factors, and then the SVM kernel function-based sensitivity analysis was performed to optimize the input factor set and establish the optimized daily variation SVM model. The efficiency and rationality of the two models were verified by case studies of the water level of two piezometric tubes buried under the slope of a concrete gravity dam. Sensitivity analysis of the optimized SVM model shows that the influences of the daily variation of the upstream reservoir water level and rainfall on the daily variation of piezometric tube water level are processes subject to normal distribution.

Seepage problems on fractured rock accompanying with mass loss during excavation in coal mines with karst collapse columns

Under the background of China as a big coal-hungry country lasting till 2050, deep mining becomes more and more significant. However, deep coal seams usually have a very complex geological structure like Karst collapse columns, and excavation is often accompanied with water-inrush accidents, restricting the development of the coal industry. In order to study on seepage problems on fractured rock accompanying with mass loss during excavation in coal mines with karst collapse columns, researchers studied on water-inrush mechanism in karst collapse columns directly, researched on seepage behavior of fractured rock with pressure as the basis of studying on water-inrush mechanism indirectly, studied the fluid flow change in fractured rock regarding the change of liquid flow type in fractured rock as seepage instability, and observed and studied the phenomenon of mass loss during seepage in succession. In the following research, (1) the cementation, lithology, and match rate of testing samples, which are the foundation of the simulation study, need to be determined in more detail; (2) migrated particles in fractured rock should be distributed which are from migration, corrosion, erosion, and abrasion; (3) multi-permeate agents in the fractured geological structure, their diffusion and convection, and the related chemical reactions should be involved; (4) more and more contemporary mathematical methods will be introduced to help us to study the complex dynamic system; and (5) experimental equipment needs to be designed and improved. What we did before, do now, and will do later is to penetrate the mysteries of seepage problems on fractured rock accompanying with mass loss during excavation in coal mines with karst collapse columns.

Mathematical model of fractured horizontal well in shale gas reservoir with rectangular stimulated reservoir volume

Production performance analysis of shale gas development has posed a great challenge for scientific researchers owing to the complex percolation mechanisms and massive hydraulic fracturing. The unique accumulation and transporting mechanisms in multiscaled shale pores, such as adsorption, desorption, diffusion, and slippage, lead to gas seepage behaviors deviating from Darcy flow. Compared to previous studies, this paper presents a comprehensive mathematical model incorporating all the above percolation mechanisms for multistage fractured horizontal wells, in which rectangular stimulated reservoir volume (SRV) is taken into account. A numerical solution for such a complex model is obtained by extending the boundary element method to application. The verification of our method is confirmed by comparison to semi-analytical results of a simplified model. From the well testing type curves, nine flow regimes were identified, most of which are not observed in conventional linear flow models. The effects of sensitive parameters, such as fracture number, size and permeability of the SRV, and formation permeability, on pressure type curves were investigated. The comprehensive model in this paper provides another method to guide the development of shale gas reservoirs, including interpretation of formation properties, guiding fracturing design, and conducting post-fracture evaluation.

Integrated certainty and uncertainty evaluation approach for seepage control effectiveness of a gravity dam

• A convex set–based method identifying seepage behavior of a gravity dam is proposed. • An integrated approach is developed to analyze the certain and uncertain seepage fields. • The seepage control measures can be simulated and their influence ranges can be determined. The seepage problems of dam engineering with complex seepage control measures are usually investigated by numerical simulation methods such as the finite element method (FEM). In general, the material and geometrical parameters and the boundary conditions on dam seepage are regarded as definite values. The heterogeneous and uncertain changes of seepage parameters and boundary conditions, which are caused by seepage control measures and material aging, are not considered. To describe more comprehensively the seepage behavior of a gravity dam and to identify more objectively the seepage control effectiveness of a gravity dam, the conventional FEM implementing numerical analysis for dam seepage is combined with the nonprobabilistic convex set–based method. Firstly, the FEM is introduced to implement the detailed simulation of typical seepage control measures and the certain seepage field analysis. Then a nonprobabilistic convex set–based numerical simulation method, which is suitable for the description of the uncertain seepage field of a gravity dam, is presented. The influence range of seepage control measures can be determined with the proposed method without the probability distribution knowledge of uncertain parameters. Lastly, an actual gravity dam undergoing seepage reinforcement is taken as an example. The seepage control effectiveness of the gravity dam is assessed with the FEM-based calculation model of the seepage field and the analysis model of the uncertain seepage field before and after seepage control implementation. In addition, the effect of the seepage field on the stress field is considered to fulfill the further analysis for the process and mechanism of seepage control of typical measures.

A Strain-Based Percolation Model and Triaxial Tests to Investigate the Evolution of Permeability and Critical Dilatancy Behavior of Coal

Modeling the coupled evolution of strain and CH4 seepage under conventional triaxial compression is the key to understanding enhanced permeability in coal. An abrupt transition of gas-stress coupled behavior at the dilatancy boundary is studied by the strain-based percolation model. Based on orthogonal experiments of triaxial stress with CH4 seepage, a complete stress-strain relationship and the corresponding evolution of volumetric strain and permeability are obtained. At the dilatant boundary of volumetric strain, modeling of stress-dependent permeability is ineffective when considering the effective deviatoric stress influenced by confining pressure and pore pressure. The computed tomography (CT) analysis shows that coal can be a continuous medium of pore-based structure before the dilatant boundary, but a discontinuous medium of fracture-based structure. The multiscale pore structure geometry dominates the mechanical behavior transition and the sudden change in CH4 seepage. By the volume-covering method proposed, the linear relationship between the fractal dimension and porosity indicates that the multiscale network can be a fractal percolation structure. A percolation model of connectivity by the axial strain-permeability relationship is proposed to explain the transition behavior of volumetric strain and CH4 seepage. The volumetric strain on permeability is illustrated by axial strain controlling the trend of transition behavior and radical strain controlling the shift of behavior. A good correlation between the theoretical and experimental results shows that the strain-based percolation model is effective in describing the transition behavior of CH4 seepage in coal.

Numerical Simulation of Hydraulic Fracture Propagation in Coal Seams with Discontinuous Natural Fracture Networks

To investigate the mechanism of hydraulic fracture propagation in coal seams with discontinuous natural fractures, an innovative finite element meshing scheme for modeling hydraulic fracturing was proposed. Hydraulic fracture propagation and interaction with discontinuous natural fracture networks in coal seams were modeled based on the cohesive element method. The hydraulic fracture network characteristics, the growth process of the secondary hydraulic fractures, the pore pressure distribution and the variation of bottomhole pressure were analyzed. The improved cohesive element method, which considers the leak-off and seepage behaviors of fracturing liquid, is capable of modeling hydraulic fracturing in naturally fractured formations. The results indicate that under high stress difference conditions, the hydraulic fracture network is spindle-shaped, and shows a multi-level branch structure. The ratio of secondary fracture total length to main fracture total length was 2.11~3.62, suggesting that the secondary fractures are an important part of the hydraulic fracture network in coal seams. In deep coal seams, the break pressure of discontinuous natural fractures mainly depends on the in-situ stress field and the direction of natural fractures. The mechanism of hydraulic fracture propagation in deep coal seams is significantly different from that in hard and tight rock layers.

Experimental study on distributed optical fiber heated-based seepage behavior identification in hydraulic engineering

The theoretical analysis and physical experiment on seepage behavior identification with distributed optical fiber temperature sensor system (DTS) are implemented. The distributed optical fiber-based approaches monitoring the seepage velocity and phreatic line in embankment dam or dike engineering are proposed. Firstly, the physical heat transfer process between optical fiber and porous material is analyzed. The basic principle identifying seepage behavior according to optical fiber temperature is presented. Secondly, a DTS-based test platform identifying seepage behavior in embankment dam or dike engineering is developed. Thirdly, the heat transfer coefficient in heat transfer theory is adopted to describe the heat transfer between optical fiber and saturated porous material. The experimental analysis for the relationship between overall heat transfer coefficient (i.e., the ratio of heating power and temperature rise) and seepage velocity is implemented. The mathematical model representing above relationship is built and an empirical method is given to monitor the seepage velocity in hydraulic engineering. Fourthly, the effect of moisture content on optical fiber temperature rise is analyzed by the experiment. The optical fiber arrangement plan is designed to monitor the phreatic line in embankment dam or dike engineering, the method is proposed to determine the position of phreatic line.

Seepage Behavior of an Inclined Wall Earth Dam under Fluctuating Drought and Flood Conditions

Most dam seepage safety analyses evaluate a stable seepage field, infiltration line, and seepage flow. However, under fluctuating drought-flood conditions, the soil surface becomes cracked, making it too complicated to conduct a routine seepage analysis. In this paper, the seepage characteristics of an inclined wall dam under fluctuating drought-flood conditions were studied using a large-scale physical model. The results show that after drought, the permeability coefficient of clay with low crack development was smaller than that of the saturated permeability, but for cracked areas, with high permeability, permeability coefficients were much larger than those of the saturated permeability. As the water level rose, cracks could heal themselves, but this healing was superficial, and the soil could no longer return to its predrought state. After crack healing, the integrity of the dam slope was much less than its original state. In addition, the formation of cracks increased the porosity of the soil and changed the soil permeability, which permanently weakened the antiseepage performance of the clay soil. This study evaluated the seepage flow behavior of an inclined wall dam under fluctuating drought-flood conditions, and results can be used to monitor and protect the weak parts of seepage dams in practical engineering projects.

Determination of the optimum sealing method for Azad pumped storage dam considering seepage analysis

Seepage analysis of the upper reservoir of the Kurdistan Azad pumped storage dam with a volume of 3.8E+5 m3 is a key step for selection of the optimized sealing method. More than 60% of the Lugeon test results show very permeable behavior for the pit and abutments of the reservoir. In this study, regarding the permeability value of the reservoir abutments and pit obtained by means of Lugeon tests, seepage rate was computed using various analytical and numerical methods. Reservoir total seepage discharge was computed in steady-state regime by means of analytical and numerical methods (SEEP/W finite element software) as 1.14E+05 and 1.15E+05 m3/day, respectively. Distinct element method (UDEC software) showed variable behavior for the seepage flow. In the reservoir initial impounding stages, the amount of seepage was very high (1.70E+06 m3/day). Then, the seepage rate decreased gradually and reached to a constant value (1.12E+05 m3/day). For the reservoir 3D modeling, Seep3D commercial software was used and has shown water seepage discharge of about 1.18E+05 m3/day, means 3% of total reservoir volume. Based on the above-mentioned results and regarding behavior of seepage in the initial impounding stages, sealing element is necessary to prevent from seepage phenomenon. Clay blanket, concrete lining, asphaltic concrete, and geosynthetic are the proposed methods in a large water reservoir. After a feasibility study on various sealing methods, geomembrane was suggested as the best sealing method with the reasonable implementation cost.

The influence of different diffusion pattern to the sub- and super-critical fluid flow in brown coal

Sub- and super-critical CO2 flowing in nanoscale pores are recently becoming of great interest due to that it is closely related to many engineering applications, such as geological burial and sequestration of carbon dioxide, Enhanced Coal Bed Methane recovery ( ECBM), super-critical CO2 fracturing and so on. Gas flow in nanopores cannot be described simply by the Darcy equation. Different diffusion pattern such as Fick diffusion, Knudsen diffusion, transitional diffusion and slip flow at the solid matrix separate the seepage behaviour from Darcy-type flow. According to the principle of different diffusion pattern, the flow of sub- and super-critical CO2 in brown coal was simulated by numerical method, and the results were compared with the experimental results to explore the contribution of different diffusion pattern and swelling effect in sub- and super-critical CO2 flow in nanoscale pores.

Distributed optical fiber-based monitoring approach of spatial seepage behavior in dike engineering

The failure caused by seepage is the most common one in dike engineering. As to the characteristics of seepage in dike, such as longitudinal extension engineering, the randomness, strong concealment and small initial quantity order, by means of distributed fiber temperature sensor system (DTS), adopting an improved optical fiber layer layout scheme, the location of initial interpolation point of the saturation line is obtained. With the barycentric Lagrange interpolation collocation method (BLICM), the infiltrated surface of dike full-section is generated. Combined with linear optical fiber monitoring seepage method, BLICM is applied in an engineering case, which shows that a real-time seepage monitoring technique is presented in full-section of dike based on the combination method.

Numerical simulation of the free surface and water inflow of a slope, considering the nonlinear flow properties of gravel layers: a case study.

Groundwater is an important factor of slope stability, and 90% of slope failures are related to the influence of groundwater. In the past, free surface calculations and the prediction of water inflow were based on Darcy's law. However, Darcy's law for steady fluid flow is a special case of non-Darcy flow, and many types of non-Darcy flows occur in practical engineering applications. In this paper, based on the experimental results of laboratory water seepage tests, the seepage state of each soil layer in the open-pit slope of the Yanshan Iron Mine, China, were determined, and the seepage parameters were obtained. The seepage behaviour in the silt layer, fine sand layer, silty clay layer and gravelly clay layer followed the traditional Darcy law, while the gravel layers showed clear nonlinear characteristics. The permeability increases exponentially and the non-Darcy coefficient decreases exponentially with an increase in porosity, and the relation among the permeability, the porosity and the non-Darcy coefficient is investigated. A coupled mathematical model is established for two flow fields, on the basis of Darcy flow in the low-permeability layers and Forchheimer flow in the high-permeability layers. In addition, the effect of the seepage in the slope on the transition from Darcy flow to Forchheimer flow was considered. Then, a numerical simulation was conducted by using finite-element software (FELAC 2.2). The results indicate that the free surface calculated by the Darcy–Forchheimer model is in good agreement with the in situ measurements; however, there is an evident deviation of the simulation results from the measured data when the Darcy model is used. Through a parameter sensitivity analysis of the gravel layers, it can be found that the height of the overflow point and the water inflow calculated by the Darcy–Forchheimer model are consistently less than those of the Darcy model, and the discrepancy between these two models increases as the permeability increases. The necessity of adopting the Darcy–Forchheimer model was explained. The Darcy–Forchheimer model would be applicable in slope engineering applications with highly permeable rock.

Study on Fluid-solid Coupling Mathematical Models and Numerical Simulation of Coal Containing Gas

Based on coal seam gas migration theory under multi-physics field coupling effect, fluid-solid coupling model of coal seam gas was build using elastic mechanics, fluid mechanics in porous medium and effective stress principle. Gas seepage behavior under different original gas pressure was simulated. Results indicated that residual gas pressure, gas pressure gradient and gas low were bigger when original gas pressure was higher. Coal permeability distribution decreased exponentially when original gas pressure was lower than critical pressure. Coal permeability decreased rapidly first and then increased slowly when original pressure was higher than critical pressure.