Solute transport in porous media studied by lattice Boltzmann simulations at pore scale and x-ray tomography experiments

With the aid of nondestructive microfocus x-ray computed tomography (CT), we performed three-dimensional (3D) tracer dispersion experiments on randomly unconsolidated packed beds. Plumes of nonreactive sodium iodide solution were point injected into a sodium chloride solvent as a tracer for the evaluation of the dispersion process. The asymptotic dispersion coefficient was obtainable within the experimental scale and was summarized over Péclet numbers from 11.7 to ∼860. Then, the lattice Boltzmann method and moment propagation method were used to elucidate the mechanisms embedded in the dispersion phenomenon. The methods were rigorously verified against the classical Taylor dispersion problem and extended to simulate fluid flow and tracer dispersion in high-resolution 3D digital porous structures from CT. The method of moments, Lagrangian velocity correction function, and dilution index were thoroughly analyzed to evaluate the dispersion behaviors. Numerical simulations revealed ballistic and superdiffusive regimes at the transient times, whereas asymptotic dispersion behaviors appear at longer characteristic times. Besides, the observed transient times unanimously persist over convective length scales of around 12 particles transversely and 16 particles longitudinally. The estimated dispersion coefficients from simulation are in consistence with the experimental result. Furthermore, the simulation also enabled the identification of regimes, including diffusive, power law, and mechanical dispersion. Thus, the proposed experimental and computational schemes are of practical means to study dispersion behaviors by direct pore scale imaging and modeling.

Micro-tomographic analyses of specific interfacial area inside unconsolidated porous media with differing particle characteristics from microscopic to macroscopic scale

HYPOTHESIS

In capillary trapping in an unconsolidated porous medium, the interphase area is influenced by the distribution of the trapped phase clusters. These attributes, in turn, are affected by particle characteristics, indicating that the interphase area is affected by the particle characteristics.

EXPERIMENTS

A micro-tomography technique was used to observe capillary trapping of a nitrogen-water system. The effect of particle characteristics on micro- to macroscopic properties was measured, respectively: pore size distribution (PSD) and porosity, bubble size distribution (BSD) and saturation, and bubble surface area and specific interfacial area. Capillary trapping experiments were carried out for media consisting of different particle shapes, range of sizes, and degrees of uniformity.

FINDINGS

Particle characteristics govern not only the PSD but also the pore shape. Then, the PSD and pore shape govern the BSD and bubble surface area, thus, affecting the specific interfacial area. The specific interfacial area of highly angular particles differs from that of highly spherical particles and natural sands. On the basis of these findings, a statistical model of PSD and a bubble morphology model are developed. Comparisons of specific interfacial area with uniform bubble distribution and thermodynamic filling assumption models show that those assumptions predict interfacial area less accurately.