Drainage Simulations Research Articles

Molecular dynamics of wetting layer formation and forced water invasion in angular nanopores with mixed wettability

The depletion of conventional hydrocarbon reservoirs has prompted the oil and gas industry to search for unconventional resources such as shale gas/oil reservoirs. In shale rocks, considerable amounts of hydrocarbon reside in nanoscale pore spaces. As a result, understanding the multiphase flow of wetting and non-wetting phases in nanopores is important to improve oil and gas recovery from these formations. This study was designed to investigate the threshold capillary pressure of oil and water displacements in a capillary dominated regime inside nanoscale pores using nonequilibrium molecular dynamics (NEMD) simulations. The pores have the same cross-sectional area and volume but different cross-sectional shapes. Oil and water particles were represented with a coarse grained model and the NEMD simulations were conducted by assigning external pressure on an impermeable piston. Threshold capillary pressures were determined for the drainage process (water replaced by oil) in different pores. The molecular dynamics results are in close agreements with calculations using the Mayer-Stowe-Princen (MS-P) method which has been developed on the premise of energy balance in thermodynamic equilibrium. After the drainage simulations, a change in wall particles' wettability from water-wet to oil-wet was implemented based on the final configuration of oil and water inside the pore. Waterflooding simulations were then carried out at the threshold capillary pressure. The results show that the oil layer formed between water in the corner and in the center of the pore is not stable and collapses as the simulation continues. This is in line with the predictions from the MS-P method.

Soil water balance: Comparing two simulation models of different levels of complexity with lysimeter observations

The simulation of the water balance in cropping systems is a useful tool to study how water can be used efficiently. However, this requires that models simulate water balance accurately. Beyond the typical comparison of model outputs with field observations, in this study we present the inter-comparison of models of different complexity with the same field dataset as a powerful method to assess model performance. The compared models were DSSAT (Decision Support System for Agrotechnology Transfer) and WAVE (Water and Agrochemicals in soil, crop and Vadose Environment), both describing one dimensional water transport. The soil water balance in DSSAT uses a simpler “tipping bucket” approach, while the more mechanistic WAVE integrates Richard's equation. The soil parameters were calibrated by using the Simulated Annealing (SA) global optimizing method. A continuous weighing lysimeter in a bare fallow provided the observed values of drainage and evapotranspiration (ET) while soil water content (SW) was supplied by capacitance sensors. An automated weather station recorded the weather data. After optimizing soil parameters with SA, both models performed well simulating the soil water balance components for the calibrated period. The use of cumulative values for ET and drainage in the optimization was more effective than using their daily values. For the validation period, the models predicted well soil evaporation over time but there were differences between models in the soil water and drainage simulations. In particular, WAVE predicted drainage well while DSSAT presented larger errors in the cumulative values. That could be due to the mechanistic nature of WAVE against the more functional nature of DSSAT. Further studies should be conducted to improve the quality of DSSAT drainage simulations. The good results from WAVE indicate that, after soil calibration, it could be used as a reasonable substitute for other models for periods when no drainage field measurements are available.

Gravitational drainage of compressible organic materials

AbstractA model was developed to simulate drainage of compressible particle suspensions, and study how cake compression and volumetric load influence the process. The input parameters were settling velocity, cake resistance and compressibility. These parameters were found using a new experimental method. Dextran‐MnO2 particle suspensions were drained as these resemble organic waste slurries with respect to settling and compressibility. It was demonstrated that cake compressibility must be taken into account to obtain adequate simulations. This implies that pressurized filtration resistances cannot be used for drainage simulations. In the filtration step, a distinct increase of dry matter from top to bottom of the cake was observed. During the subsequent consolidation, the cake compressed and a uniform dry matter profile was found. The final dry matter content of the cake increased with feed concentration and volumetric load. The drainage time increased proportionally with feed concentration and, more importantly, proportionally with squared volumetric load. © 2010 American Institute of Chemical Engineers AIChE J, 2010

A universal agro-hydrological model for water and nitrogen cycles in the soil–crop system SMCR_N: Critical update and further validation

Agro-hydrological models have widely been used for optimizing resources use and minimizing environmental consequences in agriculture. SMCR_N is a recently developed sophisticated model which simulates crop response to nitrogen fertilizer for a wide range of crops, and the associated leaching of nitrate from arable soils. In this paper, we describe the improvements of this model by replacing the existing approximate hydrological cascade algorithm with a new simple and explicit algorithm for the basic soil water flow equation, which not only enhanced the model performance in hydrological simulation, but also was essential to extend the model application to the situations where the capillary flow is important. As a result, the updated SMCR_N model could be used for more accurate study of water dynamics in the soil–crop system. The success of the model update was demonstrated by the simulated results that the updated model consistently out-performed the original model in drainage simulations and in predicting time course soil water content in different layers in the soil–wheat system. Tests of the updated SMCR_N model against data from 4 field crop experiments showed that crop nitrogen offtakes and soil mineral nitrogen in the top 90 cm were in a good agreement with the measured values, indicating that the model could make more reliable predictions of nitrogen fate in the crop–soil system, and thus provides a useful platform to assess the impacts of nitrogen fertilizer on crop yield and nitrogen leaching from different production systems.

A Modified LBM Model for Simulating Gas Seepage in Fissured Coal Considering Klinkenberg Effects and Adsorbability-Desorbability

A modified Lattice–Boltzmann method is proposed by considering the Klinkenberg effect and adsorbability-desorbability for the purpose of simulating methane gas seepage in fissured coal. The results show that the Klinkenberg effect has a little influence on methane gas seepage in fissured coal, so it can be neglected in engineering computations for simplicity. If both the Klinkenberg effect and the adsorbability-desorbability are considered, the Klinkenberg influence on gas pressure decreases as the Darcy coefficient increases. It is found by gas drainage simulations that near a drainage hole, the effect of adsorption and desorption cannot be neglected, and the location of the drainage hole has a great influence on drainage efficient λ when the hole is just located at the mid-zone of the coal seam, λ is 0.691808; when the hole is excursion down to 1.0 m from the mid-zone of coal seam, λ decreases to 0.668631; when the hole is excursion up or down to 2.0 m from the mid-zone of coal seam, λ decreases to 0.632917. The simulations supply an effective approach for optimizing the gas drainage hole location.

Comment on “Power law catchment‐scale recessions arising from heterogeneous linear small‐scale dynamics” by C. J. Harman, M. Sivapalan, and P. Kumar

Comment on “Power law catchment‐scale recessions arising from heterogeneous linear small‐scale dynamics” by C. J. Harman, M. Sivapalan, and P. Kumar

Applying climate model precipitation scenarios for urban hydrological assessment: A case study in Kalmar City, Sweden

There is growing interest in the impact of climate change on urban hydrological processes. Such assessment may be based on the precipitation output from climate models. To date, the model resolution in both time and space has been too low for proper assessment, but at least in time the resolution of available model output is approaching urban scales. In this paper, 30-min precipitation from a model grid box covering Kalmar City, Sweden, is compared with high-resolution (tipping-bucket) observations from a gauge in Kalmar. The model is found to overestimate the frequency of low rainfall intensities, and therefore the total volume, but reasonably well reproduce the highest intensities. Adapting climate model data to urban drainage applications can be done in several ways but a popular way is the so-called Delta Change (DC) method. In this method, relative changes in rainfall characteristics estimated from climate model output are transferred to an observed rainfall time series, generally by multiplicative factors. In this paper, a version of the method is proposed in which these DC factors (DCFs) are related to the rainfall intensity level. This is achieved by calculating changes in the probability distribution of rainfall intensities and modelling the DCFs as a function of percentile. Applying this method in Kalmar indicated that in summer and autumn, high intensities will increase by 20–60% by year 2100, whereas low intensities remain stable or decrease. In winter and spring, generally all intensity levels increase similarly. The results were transferred to the observed time series by varying the volume of the tipping bucket to reflect the estimated intensity changes on a 30-min time scale. In an evaluation of the transformed data at a higher 5-min resolution, effects on the intensity distribution as well as single precipitation events were demonstrated. In particular, qualitatively different changes in peak intensity and total volume are attainable, which is required in light of expected future changes of the precipitation process and a step forward as compared with simpler DC approaches. Using the DC transformed data as input in urban drainage simulations for a catchment in Kalmar indicated an increase of the number of surface floods by 20–45% during this century.

The impacts of climate change and urbanisation on drainage in Helsingborg, Sweden: Suburban stormwater

Assessment of the potential impact of climate change on water systems has been an essential part of hydrological research over the last couple of decades. However, the notion that such assessments should also include other changes is relatively recent. In this study, the potential impacts of climate change and continued urbanisation on stormwater flows to a suburban stream, Helsingborg, south Sweden, have been assessed using drainage simulations for present conditions as well as two climate (medium and high gas emission) and two water management storylines for subdivision. Climate change was simulated by altering a high-resolution rainfall record according to the climate-change signal derived from a regional climate model. Subdivision and urbanisation was simulated by altering model parameters chosen to reflect current trends in demographics and water management. The simulations were run for a 15 month period. It was found that city growth and projected increases in heavy rainfalls, both together and alone, are set to raise peak flow volumes and increase flood risk. Conversely, installation of a sustainable urban drainage system (SUDS) has a positive effect on the urban environment in general and can largely allay the adverse impacts of change road. The relative impacts of changes in climate and urbanisation are assessed for the combined sewer system which serves central Helsingborg in Semadeni-Davies et al. [Semadeni-Davies A., Hernebring, C., Svensson, G., Gustafsson, L.G., 2008. The impacts of climate change and urbanisation on drainage in Helsingborg, Sweden: combined sewer system. J. Hydrol. 350 (1-2), 100-113.

Modeling of Two-Phase Behavior in the Gas Diffusion Medium of PEFCs via Full Morphology Approach

A full morphology (FM) model has been developed for studying the two-phase characteristics of the gas diffusion medium in a polymer electrolyte fuel cell (PEFC). The three-dimensional (3D) fibrous microstructure for the nonwoven gas diffusion layer (GDL) microstructure has been reconstructed using a stochastic technique for Toray090 and SGL10BA carbon papers. The FM model directly solves for the capillary pressure-saturation relations on the detailed morphology of the reconstructed GDL from drainage simulations. The estimated capillary pressure-saturation curves can be used as valuable inputs to macroscopic two-phase models. Additionally, 3D visualization of the water distribution in the gas diffusion medium suggests that only a small number of pores are occupied by liquid water at breakthrough. Based on a reduced compression model, the two-phase behavior of the GDL under mechanical load is also investigated and the capillary pressure-saturation relations are evaluated for different compression levels.

Dual models of pore spaces

We present a model for pore spaces that consists of two parts related by duality: (1) a decomposition of an open polyhedral pore space into open contractible pore bodies separated by relatively open interfaces and (2) a pore network that is homotopy equivalent to the pore space. The dual model is unique and free of parameters, but it relies on regularity conditions for the pore space. We show how to approximate any pore space by the interior of a polyhedral complex such that the regularity conditions are fulfilled. Thus, we are able to calculate the dual model from synthetic porous media and images of real porous media. The pore bodies are unions of relatively open Delaunay cells with respect to the corners of the pore boundary, and the pore network consists of certain at most two-dimensional (2D) Voronoi cells with respect to the corners of the pore boundary. The pore network describes the neighborhood relations between the pore bodies. In particular, any relatively open 2D Delaunay face f separating two pore bodies has a unique (relatively open) dual network edge. In our model, f is a pore throat only if it is hit by its dual network edge. Thus, as opposed to widespread intuition, any pore throat is convex, and adjacent pore bodies are not necessarily separated by pore throats. Due to the duality between the pore network and the decomposition of the pore space into pore bodies it is straightforward to store the geometrical properties of the pore bodies [pore throats] as attributes of the dual network vertices [edges]. Such an attributed network is used to perform 2D drainage simulations. The results agree very well with those from a pore-morphology based modeling approach performed directly on the digital image of a porous medium. Contractibility of the pore bodies and homotopy equivalence of the pore space and the pore network is proven using discrete Morse theory and the nerve theorem from combinatorial topology.

Selection of regional historical rainfall time series as input to urban drainage simulations at ungauged locations

It has become established practice during the past 20 years to use high-resolution historical rainfall time series as input to hydrological model packages for detailed simulation of urban drainage systems. However, sufficiently long rain series are rarely available from the exact catchment in question and simulations are hence often based on available rain series from other locations. Extreme rainfall properties of importance to the performance of urban storm drainage systems vary significantly even in regions with only minor physiographic differences. Part of this variation can be explained by regional variations of the mean annual rainfall and the remaining statistical residue can be interpreted as statistical uncertainty. In Denmark, more than 75 high-resolution rain gauges are installed across a total area of 43,000 m. About 40 gauges had sufficiently long records to be included in a comprehensive national investigation where newly developed statistical regionalisation procedures were used to model the regional variation of extreme rainfalls. On this basis, a spreadsheet model was made available for estimation of extreme design rainfalls and the associated uncertainty at any location in the country. Statistics were furthermore computed to classify historical rainfall time series according to the developed regional model, and this makes it possible to assess the uncertainty related with using different historical rain series for simulations at ungauged locations. This research indicates that use of historical point rainfall data at ungauged locations introduces a significant uncertainty that is largely overlooked in today's practice. The engineering recommendation is to select historical rain series based on an evaluation of the local physiographic characteristics (e.g., the mean annual rainfall) and a (pre-defined) desired safety level of the simulations.

Quasi-one-dimensional and two-dimensional drainage of foam

Foam drainage is considered in a froth flotation tank containing a flowing froth. A simplified theory (the quasi-one-dimensional theory) exists in which horizontal variations across the tank of the foam’s typical Plateau border area can be neglected. Moreover, drainage is shown to be gravity dominated over most of the froth. For weakly decelerated foam flows, the gravity dominated drainage equation admits constant Plateau border area solutions, both horizontally and vertically. For strongly decelerated flows there are two solution branches: one with constant Plateau border area, and another branch with kinked solutions. The kinked branch is the relevant one in the case of a foam with non-uniform bubble sizes. The gravity dominated solutions are required to match onto boundary layer solutions at the base of the foam. Capillary suction becomes important in this layer, and the solutions have a well-known structure: that of a soliton on an already wetted foam. The simple quasi-one-dimensional solutions are shown to have remarkable agreement with full two-dimensional drainage simulations.

Wetting phase connectivity and irreducible saturation in simple granular media

We present drainage simulations that allow trapping of wetting phase in a simple but nontrivial granular medium, a dense random packing of equal spheres. The basis for the simulations is a network model derived directly from the known locations and dimensions of pore space features. This provides a means of evaluating the morphology of trapped wetting phase. The possible morphologies depend on the assumed connectivity of the wetting phase. At one extreme, we assume that the entire wetting phase except for pendular rings is connected. At the opposite extreme, we illustrate a low level of connectivity by assuming that pendular rings are trapped as soon as the pores surrounding them are drained; any wetting phase not yet drained from pore throats connecting these pores is also assumed to be trapped. Finally we consider a set of criteria involving larger neighborhoods within the network, which allows trapping in individual pores. Irreducible wetting phase saturations obtained in the latter case agree with experimental data. The numbers of pendular rings and liquid bridges are also consistent with observations. Because the agreement does not involve adjustable parameters, we conclude that a relatively simple, local evaluation of trapping criteria can yield physically representative wetting phase configurations.

Reduced methane emissions from large‐scale changes in water management of China's rice paddies during 1980–2000

Decreased methane emissions from paddy rice may have contributed to the decline in the rate of increase of global atmospheric methane (CH4) concentration over the last 20 years. In China, midseason paddy drainage, which reduces growing season CH4 fluxes, was first implemented in the early 1980s, and has gradually replaced continuous flooding in much of the paddy area. We constructed a regional prediction for China's rice paddy methane emissions using the DNDC biogeochemical model. Results of continuous flooding and midseason drainage simulations for all paddy fields in China were combined with regional scenarios for the timing of the transition from continuous flooding to predominantly mid‐season drainage to generate estimates of total methane flux for 1980–2000. CH4 emissions from China's paddy fields were reduced over that period by ∼5 Tg CH4 yr−1.

Two-dimensional hydrodynamic lattice-gas simulations of binary immiscible and ternary amphiphilic fluid flow through porous media

The behavior of two-dimensional binary and ternary amphiphilic fluids under flow conditions is investigated using a hydrodynamic lattice-gas model. After the validation of the model in simple cases (Poiseuille flow, Darcy's law for single component fluids), attention is focused on the properties of binary immiscible fluids in porous media. An extension of Darcy's law which explicitly admits a viscous coupling between the fluids is verified, and evidence of capillary effects is described. The influence of a third component, namely, surfactant, is studied in the same context. Invasion simulations have also been performed. The effect of the applied force on the invasion process is reported. As the forcing level increases, the invasion process becomes faster and the residual oil saturation decreases. The introduction of surfactant in the invading phase during imbibition produces new phenomena, including emulsification and micellization. At very low fluid forcing levels, this leads to the production of a low-resistance gel, which then slows down the progress of the invading fluid. At long times (beyond the water percolation threshold), the concentration of remaining oil within the porous medium is lowered by the action of surfactant, thus enhancing oil recovery. On the other hand, the introduction of surfactant in the invading phase during drainage simulations slows down the invasion process-the invading fluid takes a more tortuous path to invade the porous medium-and reduces the oil recovery (the residual oil saturation increases).

Drainage flow and solid transport simulation in defective building drainage networks

The flow conditions within multi-storey building drainage networks may be characterised as unsteady free surface flows and as such may be analysed and simulated by the use of the method of characteristics to solve numerically the governing equations of continuity and momentum. However the application of such simulations must take into account the possibility of defective system installation and must be able to predict the wave attenuation following appliance discharge and the consequent effects upon solid transport due to system defects caused by faulty installation or the development of obstructions to the flow. This paper presents both a summary of a detailed survey of defects in installed building drainage networks and the development of appropriate boundary conditions that allow the method of characteristics based computer simulation to accurately predict the effect of defects on system performance. The introduction of drainage simulations capable of providing design advice under the real conditions represented by this study is particularly timely in view of governmental pressure for water conservation and the introduction in 1998 of the new Water Regulations to replace the Water Bye-Laws that will introduce reduced and dual flush w.c. operation, together with reduced shower and other appliance flows.

A Computational Investigation of Some Effects of Temperature on Soil Moisture

A pore‐scale lattice model for fluid displacement in porous media is used to investigate the effect of temperature on (1) soil drainage and (2) soil freezing. The model is calibrated for a given soil, that is, input parameters for the lattice model are determined, using a single soil drainage curve measured at a given temperature. The calibrated model may be used to simulate drainage at different temperatures (1) using adjusted fluid parameters or (2) freezing at subzero temperatures. Drainage simulations are used to generate capillary pressure‐saturation or soil water characteristic function curves, while freezing simulations give rise to temperature‐liquid water content or soil freezing characteristic function curves. These curves are compared to experimental data to test the validity of assumptions regarding the pore‐scale mechanisms and processes underlying these continuum‐scale phenomena.

Gas Storage Reservoir Performance Optimization Through The Application Of Drainage And Imbibition Relative Permeability Data

Abstract Gas storage reservoirs are utilized in many locations to seasonally store gas in summer months for periods of high demand in the winter. This generally involves the cyclic pressurization and depressurization of the reservoir on an annual basis. If the storage reservoir overlies an aquifer or residual oil leg, this may involve the cyclic motion of the gas-liquid contact and gas zone. This paper documents an extensive series of reservoir condition relative permeability experiments conducted on core material from a gas storage reservoir in southern Ontario to investigate the effects of cyclical pressurization and depressurization on reservoir performance. The tests illustrate how the cycling of applied net overburden pressure and the presence of trapped gas and water saturations in the dynamic gas-liquid transition zone can affect the production and injection of gas in the storage reservoir. Applications of the data, and extensions to field performance are also presented. Introduction For many years, natural underground geologic traps have been utilized for storing gas and liquid hydrocarbons; these reservoirs are usually developed from known hydrocarbon reservoirs which have since been abandoned. The primary application of storage reservoirs is in the accumulation and storage of natural gas during periods of low demand in the summer months so that a large localized source of gas is readily available for periods of increased demand in the winter. For this reason, the reservoir is subjected to annual cyclic pressurization and depressurization as the gas is stored and removed from the reservoir. When an active aquifer or residual oil leg is present in the reservoir, these cyclic pressure fluctuations can result in an annual transgression and regression of the gas liquid contact. Encroachment of the liquid phase into the previously uninvaded portion of the reservoir is of interest to the operators of these fields because the residual saturation of the encroached fluid can have a profound effect on the deliverability and general performance of the contacted pore space. Experimentation and investigation of this phenomenon is possible through imbibition and drainage simulations to quantify the effects caused by two phase relative permeability relationships. An additional phenomenon associated with reservoir pressure cycling is that of rock compressibility effects. The gross overburden of the field remains constant as the internal pore pressure of the system oscillates. This yields a variation in the magnitude of the rock compressibility while the net overburden pressure changes as a function of pore pressure. The complexity of this scenario is further complicated by the compression and expansion of the trapped gas phase in the liquid filled pores. Hycal Energy Research Laboratories Ltd., in conjunction with Union Gas Limited, applied recent advancements in reservoir condition relative permeability techniques to conduct an extensive series of experiments to investigate the effects of these phenomena in the gas storage environment. Applications of the experimental data to field scenarios is also discussed.

Surface and subsurface drainage simulations for a claypan soil

A water management simulation model, DRAINMOD, was used to investigate the suitability of drainage systems for maize production on the claypan soils that occur widely in the midwest of the United States of America. These soils have a very slowly permeable shallow subsoil which severely limits root development and reduces crop yield. Climatological, crop and soil data, collected on a claypan soil, over several years, were used in this study. Various subsurface drain spacings and combinations of surface and subsurface drainage systems on a claypan soil were simulated. Results indicated that supplemental irrigation water is required for optimum maize production on these soils. With supplemental irrigation, a subsurface drain spacing of 12 m under good surface drainage, and one of 10 m under poor surface drainage, are required.