Edo S. Boek

Lattice-Boltzmann studies of fluid flow in porous media with realistic rock geometries

We present results of lattice-Boltzmann simulations to calculate flow in realistic porous media. Two examples are given for lattice-Boltzmann simulations in two- and three-dimensional (2D and 3D) rock samples. First, we show lattice-Boltzmann simulation results of the flow in quasi-two-dimensional micromodels. The third dimension was taken into account using an effective viscous drag force. In this case, we consider a 2D micromodel of Berea sandstone. We calculate the flow field and permeability of the micromodel and find excellent agreement with Microparticle Image Velocimetry (@m-PIV) experiments. Then, we use a particle tracking algorithm to calculate the dispersion of tracer particles in the Berea geometry, using the lattice-Boltzmann flow field. Second, we use lattice-Boltzmann simulations to calculate the flow in Bentheimer sandstone. The data set used in this study was obtained using X-ray microtomography (XMT). First, we consider a single phase flow. We systematically study the effect of system size and validate Darcys law from the linear dependence of the flux on the body force exerted. We observe that the values of the permeability measurements as a function of porosity tend to concentrate in a narrower region of the porosity, as the system size of the computational sub-sample increases. Finally, we compute relative permeabilities for binary immiscible fluids in the XMT rock sample.

Computer simulation of rheological phenomena in dense colloidal suspensions with dissipative particle dynamics

The rheological properties of colloidal suspensions of spheres and rods have been studied using dissipative particle dynamics (DPD). We have measured the viscosity as a function of shear rate and volume fraction of the suspended particles. The viscosity of a 30 vol% suspension of spheres displays characteristic shear-thinning behaviour as a function of shear rate. The values for the low- and high-shear viscosity are in good agreement with experimental data. For higher particulate densities, good results are obtained for the high-shear viscosity, although the viscosity at low shear rates shows a dependence on the size of the suspended spheres. Dilute suspensions of rods show an intrinsic viscosity which is in excellent agreement with theoretical results. For concentrated rod suspensions, the viscosity increases with the third power of the volume fraction. We find the same scaling behaviour as Doi and Edwards for the semidilute regime, although the explanation is unclear. The DPD simulation technique therefore emerges as a useful tool for studying the rheology of particulate suspensions.

Spontaneous Imbibition in Nanopores of Different Roughness and Wettability

The spontaneous imbibition of liquid in nanopores of different roughness is investigated using coarse grain molecular dynamics (MD) simulation. The numerical model is presented and the simplifying assumptions are discussed in detail. The molecular-kinetic theory introduced by Blake is used to describe the effect of dynamic contact angle on fluid imbibition. The capillary roughness is modeled using a random distribution of coarse grained particles forming the wall. The Lucas-Washburn equation is used as a reference for analyzing the imbibition curves obtained by simulation. Due to the statistical nature of MD processing, a comprehensive approach was made to average and smooth the data to accurately define a contact angle. The results are discussed in terms of effective hydrodynamic and static capillary radii and their difference as a function of roughness and wettability.

LATTICE BOLTZMANN SIMULATION OF THE FLOW OF NON-NEWTONIAN FLUIDS IN POROUS MEDIA

We present a LB study of the flow of single-phase non-Newtonian fluids, using a power law relationship between the effective viscosity and the local shear rate. Channel flow experiments were carried out to measure the velocity profiles. The simulation results are found to be in good agreement with theory. We also report simulations of the flow of non-Newtonian fluids in a 2-D porous medium.

Lattice Boltzmann simulation of the flow of binary immiscible fluids with different viscosities using the Shan-Chen microscopic interaction model.

We present a lattice Boltzmann study of the flow of a binary fluid where the fluid components have different viscosities. For this purpose, a microscopic interaction model (due to Shan & Chen) is used. The model is validated for Poiseuille flow of layered immiscible binary fluids and the dispersion of a capillary wave. We then study the unstable displacement of a viscous fluid by a less viscous fluid in a two-dimensional channel. Although a finger-like structure was observed in many simulations, it is not clear if this structure was produced due to viscous fingering or due to other effects.

Structure and phase behavior of a model clay dispersion: A molecular-dynamics investigation

Reversible molecular-dynamics (MD) simulations have been carried out on simple models for dispersions of circular Laponite clay platelets to investigate the local structure on a mesoscopic scale. The platelets carry discrete charged sites interacting via a screened Coulomb potential. In model A all surface sites have identical negative charge, while model B also includes rim charges of opposite sign. These two models were used in a series of simulations in the semidilute regime, and for three values of the Debye screening length. The structure of the dispersions is characterized by translational and orientational pair distribution functions, and by the corresponding structure factors. Qualitative differences in the pair structure arising from variations in concentration and screening length lead to a tentative identification of sol, gel, and crystal phases. The rim charges have a dramatic effect on the local structure in the strong screening regime, leading to T-shaped pair configuration and clustering of...

Analysis of Morphology of Crystals Based on Identification of Interfacial Structure

A new theoretical approach for the prediction of the growth habit of crystals is presented. This approach is based on a newly derived relation between the growth rate of crystal surfaces and habit-controlling factors, and includes a key step: a so-called interface structure ~IS! analysis. This analysis is to formulate the influence of the fluid phase on the crystal morphology. The essential of the IS analysis is to identify the adsorbed growth units which is in dynamic equilibrium with solid units at the crystal surface, and to calculate their concentration. It follows that a key external habit-controlling factor, the so-called surface scaling factor, can be calculated from the analysis. Based on detailed molecular dynamic ~MD! simulation data, our formalism is applied to predict the morphology of urea crystals grown from aqueous solutions. Urea crystals grown from the solutions turn out to possess a needlelike shape, in excellent agreement with experiments. This is one of the first examples of the successful theoretical prediction of morphology of crystals, and will provide a new way of thinking and understanding of the influence of the mother phase on crystal habits.

From wave function to crystal morphology: application to urea and alpha-glycine

In this paper the relation between the molecular electron density distribution and the crystal growth morphology is investigated. Accurate charge densities derived from ab initio quantum chemical calculations were partitioned into multipole moments, to calculate the electrostatic contribution to the intermolecular interaction energy. For urea and alpha-glycine the F-faces or connected nets were determined according to the Hartman-Perdok PBC theory. From attachment energy and critical Ising temperature calculations, theoretical growth forms were constructed using different atom-atom potential models. These were compared to the Donnay-Harker model, equilibrium form and experimental growth forms. In the case of alpha-glycine, the theoretical growth forms are in good agreement with crystals grown from aqueous solution. Crystals obtained by sublimation seem to show some faces which are not F-faces sensu stricto.

Molecular dynamics simulations of aqueous urea solutions: Study of dimer stability and solution structure, and calculation of the total nitrogen radial distribution function GN(r)

Molecular dynamics simulations have been performed in order to study the structure of two molal urea solutions in D2O. Several initial dimer configurations were considered for an adequate sampling of phase space. Eventually all of them appeared to be unstable, when system size and periodic boundary conditions are chosen properly, even after a very careful equilibration. The total nitrogen scattering function GN(r), calculated from these simulations, is in good agreement with neutron scattering experiments when both intra- and intermolecular correlations are considered and the experimental truncation ripples are introduced by a Fourier transform of GN(r) back and forth. The simple pair potential model that we used gives results in good agreement with experiments and with a much more involved potential model, recently described in the literature [J. Chem. Phys. 95, 8419 (1991)].

Molecular-dynamics simulations of interfaces between water and crystalline urea

Molecular-dynamics simulations of several water-crystalline urea interfaces have been performed. The structure and dynamics of water close to the urea crystal surface are discussed in terms of density profiles, positional and orientational distribution functions, and diffusion coefficients. The water structure close to the interface is strongly determined by the structure of the crystal surface: the (001) and (111) interfaces reveal strong adsorption of water while the (110) and () interfaces do so to a lesser extent. Assuming that the growth rate of a specific crystal face decreases with increasing solvent adsorption, the appearance of only (111) on the urea growth form is predicted. We argue that on the other hand the dominance of (110) over (001) cannot be explained using a simple layer growth model.