Abstract Widely used numerical models of solute transport processes in subsurface aquifers are limited to nonlocally refined rectangular, or logically rectangular, structured grids. This presents an unsuitable option to efficient numerical simulations maintaining an acceptable level of accuracy. Optimal selection of locally refined cells for efficient solute transport models is challenging to the current generation of numerical models. We present a novel and relatively simple to implement algorithm addressing these shortcomings. This method operates in four steps involving travel times simulations, a grid coarsening stage followed by a selective local grid refinement based on a cell‐wise indicator, and a final postprocessing step. The refinement index is the sum of weighted logarithmic distributions of scaled forward and backward travel times. We calculate representative flow and transport properties at the two scales of the composite grid with a flow‐based upscaling technique. We present two test problems to demonstrate the performances of this new gridding algorithm. We obtain the most important speedups for composite grids generated with the highest indicator thresholds. When hydrodynamic dispersion effects increase, we obtain less important speedups. An important outcome of this work is that grid design depends on nature and strength of the underlying flow and solute transport processes. Therefore, we suggest developing solute transport workflows integrating this grid generation algorithm as an integral component to build comprehensive and efficient groundwater models.
Summary Quantifying productivity/injectivity performance in highly heterogeneous carbonate reservoirs remains a challenge because of the interaction of many interdependent processes belonging to different scales. In particular, dynamics of particulate suspensions at the Darcy scale are intimately related to the competing mechanisms of hydrodynamic/colloidal release from pore surfaces, and blocking in pore bodies/throats. Additionally, within immiscible two-phase flows additional processes occur such as interphase mass transfer.
L'etat de charge d'une membrane poreuse peut avoir une influence considerable sur ses performances de filtration. La caracterisation des proprietes electriques de surface desmembranes de filtration constitue donc une etape primordiale pour comprendre et controler leur selectivite. Dans ce travail, les proprietes electriques de membranes d'ultrafiltration (UF) on ete etudiees. Quatre methodes dites transmembranaires - potentiel de membrane, conductivite de pores, potentiel d'ecoulement et effet electro-visqueux- ont ete utilisees. L'interpretation des donnees en terme de potentiel zeta a ete realisee a l'aide d'un modele de charge d'espace. La structure multicouche ainsi que le caractere selectif de la couche active d'une membrane de basse UF rendent difficile l'interpretation des donnees experimentales fournies par les techniques de caracterisation transmembranaires. Afin d'eviter ces difficultes, une methode de potentiel d'ecoulement tangentiel (TSP) a ete utilisee. La conversion des donnees de TSP en potentiel zeta est generalement effectuee a l'aide de l'equation d'Helmholtz-Smoluchowski (ou d'une equation similaire tenant compte de la conductance de surface). Il a ete montre dans cette etude que cette procedure est incorrecte dans le cas de membranes poreuses car elle neglige le phenomene de conduction dans le corps poreux de la membrane. Ce phenomene supplementaire a ete introduit dans la theorie de l'electrocinetique afin de permettre le calcul de la vraie valeur du potentiel zeta. Une nouvelle installation electrocinetique a egalement ete developpee afin de determiner le potentiel zeta de membranes tubulaires a partir de mesures de TSP et de resistance electrique.
This paper presents results of hydrogeological investigations and groundwater salinization in the Martil aquifer system based on the measurements achieved during 1996 and 1997. Piezometric maps and profiles have been established to characterize the global and local flow directions, distribution of hydraulic heads and temporal variation of groundwater level Groundwater flow is directed W. -E., and the hydraulic gradient is decreasing in the flow direction. Rainfall infiltration and river interaction are the main recharge of the aquifer system The groundwater-table is very close to the soil surface, especially in the centre of the plain (0.5-5 m). The Martil River, which crosses the plain, is subject to industrial and domestic waste-water inflows that pollute the river water leading to the degradation of its quality Indeed, the Tetouan City discharges into the river 58830 m 3 /day of waste water, and measurements downstream of the discharge sectors show that the electrical conductivity (EC) reaches high values. Without treatment of waste water in the river, the groundwater is also contaminated The groundwater salinization increases from upstream to downstream following the flow direction. Salinization is more affected by evaporation of groundwater in the centre of the plain where EC values are important, and by the waste-water recharge from the Martil river. This salinization is also important near the coast due salt-water intrusion indicated by some observation wells.
Cette étude présente une stratégie antibiofilm appliquée à une membrane de dialyse, l’AN69; il s’agit de réaliser une modification initiale de la surface de la membrane par un polyélectrolyte cationique, le poly(diallyldiméthyl-ammonium), noté PDADMA, selon un protocole de type « layer-by-layer ». Les caractéristiques physico-chimiques des deux membranes, l’AN69 et l’AN69 modifiée par le PDADMA, sont suivies par la détermination de la modification de la charge de la membrane en mettant en oeuvre des mesures de potentiels d’écoulement et de nombres de transports de Li + , de perméabilité hydraulique et des analyses morphologiques et topographiques effectuées par les microscopies électroniques à balayage et à force atomique, respectivement avant et après exposition à E. coli et à un biofilm marin. Nos résultats montrent tout d’abord un rôle majeur joué par l’attraction électrostatique entre les microorganismes et l’AN69 modifiée par le PDADMA à l’origine d’une adhésion forte des bactéries. Par ailleurs, nous avons mis au point un protocole original d’élimination du biofilm marin. Ce protocole consiste à immerger la membrane modifiée par le PDADMA et encrassée, dans une solution de chlorure de sodium 2M afin d’écranter les charges électrostatiques à l’origine de l’accroche du polyélectrolyte et permettre le décrochage du PDADMA qui entraîne avec lui l’encrassement (constitué de bactéries et autres résidus de biofilm). La recharge d’un film frais de PDADMA permet alors une réutilisation à l’infini de la membrane AN69. La simplicité de ce protocole « de régénération » ouvre la possibilité d’une modification non permanente des membranes de dialyse, dans le but de limiter les problèmes récurrents de biocolmatage et d’augmenter les durées de vie des membranes en milieu marin.
Abstract This work introduces a new unstructured gridding approach relying on feedback from a previous groundwater flow model. All cells in a relatively coarse model using a rectilinear grid are recursively subdivided following a cell wise specific discharge‐based indicator to generate quadtree, octree or Voronoï grids. This technique leverages the full potential of the latest MODFLOW engines. The suitability of this approach is demonstrated on challenging single and multilayered heterogeneous formations. The proposed method is straightforward to implement in existing software packages. It supports iterative updating of groundwater flow models from the legacy rectilinear to unstructured grids.
Prediction of CO2 injection performance in deep subsurface aquifers and reservoirs rely on the well ability to maintain high flow rates of carbon dioxide during several decades without significantly impairing the host formation. Dynamics of solid particulate suspensions in permeable media are recognized as one major factor leading to injection well plugging in sandstones. The invading supercritical liquid-like fluid can contain variable concentrations of exogenous fine suspensions or endogenous particles generated in-situ by colloidal of hydrodynamic release mechanisms. Suspended solids can plug the pores leading to possible formation damage and permeability reduction in the vicinity of the injector. As such, models which can predict well injectivity decline are useful in the operations of planning, design, and maintenance, related to carbon dioxide injection. In this study we developed a finite element based simulator to predict the injectivity decline nearby CO2 injection wells and also for production wells in the context of EOR. The numerical model solves a system of two coupled sets of finite element equations corresponding to the pressure-saturation two-phase flow, then a system of solute and particles convection-diffusion equations. Particle equations are subject to mechanistic rate laws of colloidal, hydrodynamic release from pore bodies, blocking in pore bodies and pore throats, and interphase particles transfer. The model was validated against available laboratory experiments at the core scale. At the field scale, challenges still exist for an accurate assessment of the permeability change due to limited current knowledge of supercritical CO2 and water phase micro-interactions at pore surfaces, but also to the multiscale nature of the numerical problem. Numerical demonstration examples in a saline sandstone aquifer reveal that formation damage during CO2 injection will primarily depend on the injected particles wettability, the injection flow rate, and the medium tortuosity. Other simulation examples are provided for a CO2 injection in a five spots pattern EOR oil field for performance assessment of the production with occurrence of in-situ sanding in a poorly consolidated sandstone reservoir.
