Fluid Induced Particle Size Segregation in Sheared Granular Assemblies

We perform a two-dimensional molecular-dynamics study of a model for sheared bidisperse granular systems under conditions of simple shear and Poiseuille flow. We propose a mechanism for particle-size segregation based on the observation that segregation occurs if the viscous length scale introduced by a liquid in the system is smaller than of the order of the particle size. We show that the ratio of shear rate to viscosity must be small if one wants to find size segregation. In this case the particles in the system arrange themselves in bands of big and small particles oriented along the direction of the flow. Similarly, in Poiseuille flow we find the formation of particle bands. Here, in addition, the variety of time scales in the flow leads to an aggregation of particles in the zones of low shear rate and can suppress size segregation in these regions. The results have been verified against simulations using a full Navier-Stokes description for the liquid.Comment: 11 pages, REVTEX format, ps figures compressed uuencoded separately or by e-mail from [email protected]. A postscript version of the paper will be available from http://www.ica1.uni-stuttgart.de/local/WWW/papers/papers.htm

A quasi steady state method for solving transient Darcy flow in complex 3D fractured networks accounting for matrix to fracture flow

International audienceModeling natural Discrete Fracture Networks (DFN) receives more and more attention in applied geosciences, from oil and gas industry, geothermal recovery. The fractures may be either natural, or artificial in case of weil stimulation . Accounting for the flow inside the fracture network , and accounting for the transfers between the matrix and the fractures, with the same level of accuracy is an important issue for calibrating the wells architecture and for setting up optimal resources recovery strategies. Recently, we proposed an original method allowing to model transient pressure diffusion in the fracture network only. The matrix was assumed to be impervious. A systematic approximation scheme was built, allowing to model the initial DFN by a set of N unknowns located at the intersection between fractures. The higher N, the higher the accuracy of the model. The lowest order approximation N = 1 appears under the form of solving a transient problem in a resistorjcapacitor network, a so-called pipe network. Its topology is the same as the network of geometrical intersections between fractures. In this paper, we generalize this approach in order to account for flux es from matrix to fractures. We show that in the case of weil separated time scales between matrix and fractures, the preceding model need only to be slightly modified in order to incorporate these fluxes . The additional knowl-edge of the so called matrix to fracture transfer function allows to modify the mass matrix that becomes a time convolution operator. This is reminiscent of existing space averaged transient dual porosity models

Multiscale Description and Upscaling of Fluid Flow in Subsurface Reservoirs

Natural geological formations are complex objects, involving geological, mechanical, physico-chemical processes occurring over very wide length scales and time scales. Phenomena ranging from the molecular scale to several hundred of kilometers may influence the overall behaviour of fluid transport in a geological formation. For example, wettability properties, themselves due to molecular effects, have a very strong impact on the water/oil displacements in oil reservoirs. Analogously, reservoir heterogeneities that cover a large range of spatial scales play an essential role to channel fluid-flows, especially when they are coupled with non linearities inherent to fluid dynamics. In order to face this complexity, and to be able to hierarchize the influence of the various relevant geological and physico-chemical phenomena, it is thus essential to handle a multiscale description of fluid transport in these reservoirs. This is an essential tool to help reservoir engineers to focus on the crucial phenomena that control the flow. This helps them to integrate data, and this results in a lowering of the uncertainties of the reservoir description that enhances economical decisions. In this paper, we present classical upscaling approaches, as well as more recent multiscale concepts

Up-Scaling Two-Phase Flow in Heterogeneous Reservoirs: Current Trends

In this paper, we review the main upscaling techniques of two phase flow. Purely numerical fine to coarse gridding methods are investigated, as well as more physical approaches where the focus is on the algebraic form of the averaged transport equations. A detailed numerical and analytical study of the dynamics of the water-oil front shows that stochastic approaches cannot neglect the viscous coupling between the pressure and saturation. A very strong interaction exists between the heterogeneity and the stable or unstable character of the fluid flow displacement. This coupling is responsible of a qualitative and quantitative change of the form of the large scale equations, that must be accounted for by any upscaling procedure. Once this problem is solved, it is easier to devise optimal physically based discretisation procedures, that allows Monte-Carlo studies to be accelerated. Examples dealing with stratified or stochastic media will also be presented

Geostatistical Parameters Estimation Using Well Test Data

In this paper we describe a new method to obtain estimations of the geostatistical parameters (GPs) such as the correlation length, lc and the permeability variance sigma2ln from well test data. In practical studies, the GPs are estimated using geological and petrophysical data, but often, these data are too scarce to give precise results. The proposed method uses the Bayesian inversion theory, in conjunction with a fast evaluation of well tests that implies upscaling techniques. The method was tested using synthetic well-test data performed on some training images, and estimations of the underlying correlation length, lc and permeability variance, sigma2ln were recovered. These estimations give a correct order of magnitude of the actual values, but as noticed in similar methods, the uncertainties are high. Once the GPs are estimated, other well established techniques can be used to get well-test matched reservoir images consistent with the geostatistical model. We will see that excellent well test data are needed, and that the method could be improved using multiple well test data

Editorial

International audienceNo abstract availabl

Réalisation d'une cellule à cisaillement elliptique

We describe an experimental device which allows us to subject a fluid to an alternating shear motion (frequency 50 to 300 Hz, amplitude of displacement 2 to 20 μm) elliptically, circularly or linearly polarized (shear rate 10 to 1 000 s-1) and simultaneously, to perform optical observations (microscopy, diffraction) of the ordering induced by the shear, as well as viscosity estimation in the range few tens to few hundred poises. After having been developed to study convective instabilities in nematic liquid crystals, this set-up is currently used to study the ordering of sheared suspensions, and their viscosity.Nous décrivons un système expérimental permettant de soumettre un fluide à un mouvement de cisaillement alternatif (fréquence de 50 à 300 Hz, amplitude de déplacements de 2 à 20 μm) polarisé elliptiquement, circulairement ou rectilignement (taux de cisaillement variant d'environ 10 à 1 000 s -1) et de réaliser dans le même temps des observations optiques (microscopie, diffraction) des structures induites par le cisaillement ; nous pouvons également évaluer la viscosité du fluide dans ces conditions d'écoulement dans une gamme allant de quelques dizaines à quelques centaines de poises. Après avoir été développé pour étudier des instabilités convectives dans les cristaux liquides, ce système est actuellement utilisé pour observer des mises en structures de suspensions et évaluer leur viscosité