Distribution of multiphase fluids in porous media: comparison between lattice Boltzmann modeling and micro-x-ray tomography.

A parallel implementation of the three-dimensional Shan-and-Chen multicomponent, multiphase lattice Boltzmann method (LBM) was used to simulate the equilibrium distributions of two immiscible fluids in porous media. The simulations were successfully validated against cone-beam x-ray microtomographic data on the distribution of oil (decane), water, and air phases in a 5-mm cube of porous medium composed of packed quartz sand grains. The results confirm that LBM models allow for the straightforward incorporation of complex pore space geometry determined from x-ray microtomography measurements and that simulated wetting and nonwetting phase distributions are consistent with x-ray observations on both macroscopic and microscopic scales.

Role of grain-to-grain contacts on profiles of retained colloids in porous media in the presence of an energy barrier to deposition.

Deposition of 36-microm gold-coated hollow microspheres in two porous media (glass beads and quartz sand, 710-850 microm) was examined using X-ray microtomography (XMT) in the presence of an energy barrier to deposition under fluid velocity conditions representative of engineered filtration systems. XMT allowed examination of the deposition at different locations at the grain surfaces (deposition at grain-to-grain contacts versus single-contact deposition). We demonstrate that in the presence of an energy barrier to deposition, grain-to-grain contacts strongly influence colloid deposition and the spatial distribution of retained colloids in porous media. This result contrasts drastically with observations in the absence of an energy barrier to deposition, where consistency with filtration theory was observed. In the presence of an energy barrier, colloids were dominantly retained at grain-to-grain contacts, and the concentration of retained particles varied nonmonotonically with transport distance. It is proposed that the nonmonotonic profiles resulted from translation of surface-associated microspheres and subsequent immobilization at grain-to-grain contacts. This hypothesis is demonstrated using a conceptual model. The mutability and sensitivity of retained profiles to system conditions (from hyper-exponential to nonmonotonic) may reflect the interplay of different deposition mechanisms under different conditions.

Pore-scale observation of microsphere deposition at grain-to-grain contacts over assemblage-scale porous media domains using X-ray microtomography.

The prevalence of colloid deposition at grain-to-grain contacts in two porous media (spherical glass beads and angular quartz sand, 710-850 microm) was examined using X-ray microtomography (XMT) under conditions where the colloid-grain surface interaction was solely attractive (lacking an energy barrier to deposition), and under fluid velocity conditions representative of engineered filtration systems. XMT allows pore-scale observation of colloid deposition over assemblage-scale porous media domains. Colloids visible in reconstructed images were prepared by coating gold on hollow ceramic microspheres (36 microm in size) (to render densities only slightly higher than water). A significant fraction of the deposited microspheres were deposited at grain-to-grain contacts (about 20% in glass beads, 40% in quartz sand) under the conditions examined. The deposited microsphere concentrations decreased log-linearly with increasing transport distance regardless of the environment of deposition (grain-to-grain contact versus single-contact deposition). The profile shape was, therefore, consistent with filtration theory, and the observed deposition rate coefficients were also well predicted by filtration theory. The ability of filtration theory to predict the magnitude and spatial distribution of deposition demonstrates that filtration theory captures the essential elements of deposition in the absence of an energy barrier despite a lack of accounting for grain-to-grain contacts. The observed factor of 2 greater deposition at grain-to-grain contacts in quartz sand relative to equivalently sized glass beads is consistent with greater grain-to-grain contact lengths and greater fraction of small pores in the quartz sand relative to the glass beads, as determined via a pore structure analysis algorithm (medial axis algorithm).