Investigating the behaviour of NaCl brines and hydrocarbons in porous alumina using low-field NMR relaxation and diffusion methods.

The behaviour of multiple fluid phases within a porous medium is hard to predict. NMR measurements offer an excellent tool to probe such systems in a fast and non-invasive way. Such systems can be relevant to hydrocarbon recovery, catalysis, and CO2 and H2 geo-storage, among others. Since electrolyte solutions are always present in subsurface reservoirs, understanding their behaviour within porous media is highly important. In this study, we use NMR relaxation and diffusion methods to investigate the diffusion coefficients and strength of interactions between alumina surfaces and brines at various NaCl concentrations, focusing on the effect of salt concentration on transport and interactions within the porous structure. Furthermore, we study the spontaneous displacement of dodecane, a model hydrocarbon, from the same alumina pellets using the same brine solutions. Results show that brines of lower salinity consistently displace more dodecane in total, after soaking dodecane-saturated pellets in a brine solution for several days. This indicates that increased salt concentrations can reduce wettability towards the aqueous phase in simple metal oxide surfaces and highlights the capabilities of NMR to efficiently study such systems.

Mesoscale Simulations Reveal How Salt Influences Clay Particles Agglomeration in Aqueous Dispersions.

The aggregation of clay particles is an everyday phenomenon of scientific and industrial relevance. However, it is a complex multiscale process that depends delicately on the nature of the particle-particle and particle-solvent interactions. Toward understanding how to control such phenomena, a multiscale computational approach is developed, building from molecular simulations conducted at atomic resolution to calculate the potential of mean force (PMF) profiles in both pure and saline water environments. We document how it is possible to use such a model to develop a fundamental understanding concerning the mechanism of particle aggregation. For example, using molecular dynamics simulations conducted at the mesoscale in implicit solvents, it is possible to quantify the size and shape of clay aggregates as a function of system conditions. The approach is used to emphasize the role of salt concentration, which directly affects the potentials of the mean forces between kaolinite particles. While particle agglomeration in pure water yields large aggregates, the presence of sodium chloride in the aqueous brine leads instead to a large number of small aggregates. These results are consistent with macroscopic experimental observations, suggesting that the simulation protocol developed could be relevant for preventing pore blocking in heterogeneous porous matrixes.

Long-Range Ionic and Short-Range Hydration Effects Govern Strongly Anisotropic Clay Nanoparticle Interactions.

The aggregation of clay particles in aqueous solution is a ubiquitous everyday process of broad environmental and technological importance. However, it is poorly understood at the all-important atomistic level since it depends on a complex and dynamic interplay of solvent-mediated electrostatic, hydrogen bonding, and dispersion interactions. With this in mind, we have performed an extensive set of classical molecular dynamics simulations (included enhanced sampling simulations) on the interactions between model kaolinite nanoparticles in pure and salty water. Our simulations reveal highly anisotropic behavior, in which the interaction between the nanoparticles varies from attractive to repulsive depending on the relative orientation of the nanoparticles. Detailed analysis reveals that at large separation (>1.5 nm), this interaction is dominated by electrostatic effects, whereas at smaller separations, the nature of the water hydration structure becomes critical. This study highlights an incredible richness in how clay nanoparticles interact, which should be accounted for in, for example, coarse-grained models of clay nanoparticle aggregation.