Multi-scale simulation of asphaltene aggregation and deposition in capillary flow

Asphaltenes are known as the 'cholesterol' of crude oil. They form nano-aggregates, precipitate, adhere to surfaces, block rock pores and may alter the wetting characteristics of mineral surfaces within the reservoir, hindering oil recovery efficiency. Despite a significant research effort, the structure, aggregation and deposition of asphaltenes under flowing conditions remain poorly understood. For this reason, we have investigated asphaltenes, their aggregation and their deposition in capillary flow using multi-scale simulations and experiments. At the colloid scale, we use a hybrid simulation approach: for the solvent, we used the stochastic rotation dynamics (also known as multi particle collision dynamics) simulation method, which provides both hydrodynamics and Brownian motion. This is coupled to a coarse-grained MD approach for the asphaltene colloids. The colloids interact through a screened Coulomb potential with varying well depth e. We tune the flow rate to obtain Peflow » 1 (hydrodynamic interactions dominate) and Re « 1 (Stokes flow). Imposing a constant pressure drop over the capillary length, we observe that the transient solvent flow rate decreases with increasing well depth e. The interactions between the mesoscopic asphaltene colloids can be related to atomistic MD simulations. Molecular structures for the atomistic calculations were obtained using the quantitative molecular representation approach. Using these structures, we calculate the potential of mean force (PMF) between pairs of asphaltene molecules in an explicit solvent. We obtain a reasonable fit using a -1/r2 attraction for the attractive tail of the PMF at intermediate distances. We speculate that this is due to the two-dimensional nature of the asphaltene molecules. Finally, we discuss how we can relate this interaction to the mesoscopic colloid aggregate interaction. We assume that the colloidal aggregates consist of nano-aggregates. Taking into account observed solvent entrainment effects, we deduct the presence of lubrication layers between the nano-aggregates, which leads to a significant screening of the direct asphaltene-asphaltene interactions. © 2009 The Royal Society of Chemistry

Hydrodynamic Rayleigh-Taylor-like instabilities in sedimenting colloidal mixtures

We study the sedimentation of an initially inhomogeneous distribution of binary colloidal mixtures confined to a slit using a coarse-grained hybrid molecular dynamics and stochastic rotation dynamics simulation technique. This technique allows us to take into account the Brownian motion and hydrodynamic interactions between colloidal particles in suspensions. The sedimentation of such systems results in the formation of Rayleigh-Taylor-like hydrodynamic instabilities, and here we examine both the process of the formation and the evolution of the instability, as well as the structural organization of the colloids, depending on the properties of the binary mixture. We find that the structural properties of the swirls that form as a consequence of the instability depend greatly on the relative magnitudes of the Peclet numbers, and much less on the composition of the mixture. We also calculate the spatial colloid velocity correlation functions which allow us to follow the time evolution of the instability and the time dependence of the characteristic correlation length. Finally, we calculate the growth rates of the unstable modes both directly from our simulation data, and also using a theoretical approach, finding good agreement

Choice of no-slip curved boundary condition for lattice Boltzmann simulations of high-Reynolds-number flows

\u3cp\u3eVarious curved no-slip boundary conditions available in literature improve the accuracy of lattice Boltzmann simulations compared to the traditional staircase approximation of curved geometries. Usually, the required unknown distribution functions emerging from the solid nodes are computed based on the known distribution functions using interpolation or extrapolation schemes. On using such curved boundary schemes, there will be mass loss or gain at each time step during the simulations, especially apparent at high Reynolds numbers, which is called mass leakage. Such an issue becomes severe in periodic flows, where the mass leakage accumulation would affect the computed flow fields over time. In this paper, we examine mass leakage of the most well-known curved boundary treatments for high-Reynolds-number flows. Apart from the existing schemes, we also test different forced mass conservation schemes and a constant density scheme. The capability of each scheme is investigated and, finally, recommendations for choosing a proper boundary condition scheme are given for stable and accurate simulations.\u3c/p\u3

Prediction of asphaltene deposition in porous media by systematic upscaling from a colloidal pore scale model to a deep bed filtration model

We have successfully validated the Asphaltene Option in ECLIPSE, using an experimental data set from the literature. We compare our results with a previous Deep Bed Filtration simulation model of the same data. We find that the experimental data can be reproduced by using the surface deposition parameter ¿ only. This provides an important simplification of the Asphaltene Option in ECLIPSE. Moreover, the values of ¿ can be predicted from a pore scale colloid dynamics simulation model using the Stochastic Rotation Dynamics (SRD) technique. It turns out that the values for ¿ obtained in the literature (Wang, 2001) and our own ECLIPSE results are consistent with the predictions from the SRD simulation model. This may give an advantage over other asphaltene deposition models available in the literatur

Drag, lift and torque correlations for non-spherical particles from Stokes limit to high Reynolds numbers

\u3cp\u3eAccurate direct numerical simulations are performed to determine the drag, lift and torque coefficients of non-spherical particles. The numerical simulations are performed using the lattice Boltzmann method with multi-relaxation time. The motivation for this work is the need for accurate drag, lift and torque correlations for high Re regimes, which are encountered in Euler-Lagrangian simulations of fluidization and pneumatic conveying of larger non-spherical particles. The simulations are performed in the Reynolds number range 0.1 ≤ Re ≤ 2000 for different incident angles ϕ. Different tests are performed to analyse the influence of grid resolution and confinement effects for different Re. The measured drag, lift and torque coefficients are utilized to derive accurate correlations for specific non-spherical particle shapes, which can be used in unresolved simulations. The functional forms for the correlations are chosen to agree with the expected physics at Stokes flow as well as the observed leveling off of the drag coefficient at high Re flows. Therefore the fits can be extended to regimes outside the Re regimes simulated. We observe sine-squared scaling of the drag coefficient for the particles tested even at Re=2000 with C\u3csub\u3eD,ϕ\u3c/sub\u3e=C\u3csub\u3eD,ϕ=0\u3csup\u3e∘\u3c/sup\u3e \u3c/sub\u3e+(C\u3csub\u3eD,ϕ=90\u3csup\u3e∘\u3c/sup\u3e \u3c/sub\u3e−C\u3csub\u3eD,ϕ=0\u3csup\u3e∘\u3c/sup\u3e \u3c/sub\u3e)sin\u3csup\u3e2\u3c/sup\u3eϕ. Furthermore, we also observe that the lift coefficient approximately scales as C\u3csub\u3eL,ϕ\u3c/sub\u3e=(C\u3csub\u3eD,ϕ=90\u3csup\u3e∘\u3c/sup\u3e \u3c/sub\u3e−C\u3csub\u3eD,ϕ=0\u3csup\u3e∘\u3c/sup\u3e \u3c/sub\u3e)sinϕcosϕ for the elongated particles. The current work would greatly improve the accuracy of Euler-Lagrangian simulations of larger non-spherical particles considering the existing literature is mainly limited to steady flow regimes and lower Re.\u3c/p\u3

Partial slip boundary conditions for collisional granular flows at flat frictional walls

We derive new boundary conditions (BCs) for collisional granular flows of spheres at flat frictional walls. A new theory is proposed for the solids stress tensor, translational and rotational energy dissipation rate per unit area and fluxes of translational and rotational fluctuation energy. In the theory we distinguish between sliding and sticking collisions and include particle rotation. The predictions are compared with literature results obtained from a discrete particle model evaluated at a given ratio of rotational to translational granular temperature. We find that the new theory is in better agreement with the observed stress ratios and heat fluxes than previous kinetic theory predictions. Finally, we carry out two fluid model simulations of a bubbling fluidized bed with the new BCs, and compare the simulation results with those obtained from discrete particle simulations. The comparison reveals that the new BCs are better capable of predicting solids axial velocity profiles, solids distribution near the walls and granular temperatures

Modification of kinetic theory for frictional spheres part II:Model validation

The hydrodynamics of a dense solid-gas fluidized bed is studied, using a two fluid model (TFM) based on our newly developed kinetic theory of granular flow (KTGF) for rotating rough particles. The TFM simulations are validated by comparing with PIV-DIA experimental data (Buist et al., 2014) and results obtained from discrete particle model (DPM) simulations of the bubbling fluidized bed. The TFM model predictions agree well with the experimental results for the time-averaged particle axial velocity and solids volume fraction. The predicted levels of the translational granular temperatures and solids circulation patterns compare reasonably well with the results obtained from the DPM simulations. The predicted rotational granular temperature in our TFM simulations shows an almost uniform distribution in the bed as a result of the assumptions that both the local mean rotational velocity and the gradient of the rotational granular temperature at the wall are zero, indicating directions for future improvement. A comparison between TFM simulations using the present KTGF model, and a more simple kinetic theory for rapid flow of slightly frictional, nearly elastic spheres derived by Jenkins and Zhang (2002), is carried out to investigate the influence of particle friction in the fluidized bed. The present KTGF model leads to better agreement with DPM simulations and experimental results for the axial particle velocity profiles and solids volume fraction distribution

Investigation of collisional parameters for rough spheres in fluidized beds

\u3cp\u3eThe effect of normal restitution coefficient and friction coefficient on the hydrodynamics of a dense bubbling solid-gas fluidized bed is investigated using a two fluid model (TFM) based on our kinetic theory of granular flow (KTGF) for rotating frictional particles. A comparison between TFM simulations using the present KTGF model, and a simpler KTGF model for rapid flows of slightly frictional, nearly elastic spheres derived by Jenkins and Zhang [1], is carried out. The simulation results reveal that both the coefficient of normal restitution and friction coefficient play an important role in the homogeneity of the bubbling bed. The particle friction has a strong effect on the solids flow patterns and distribution, while the normal restitution coefficient has a relatively small effect on both. The present model also predicts a larger amount of energy dissipation caused by the inclusion of particle friction. The present KTGF model leads to better agreement with detailed discrete particle model (DPM) simulation results for the axial particle velocity profiles and solids volume fraction distribution.\u3c/p\u3