Frederico Wanderley Tavares

Specific ion adsorption and surface forces in colloid science.

Mean-field theories that include nonelectrostatic interactions acting on ions near interfaces have been found to accommodate many experimentally observed ion specific effects. However, it is clear that this approach does not fully account for the liquid molecular structure and hydration effects. This is now improved by using parametrized ionic potentials deduced from recent nonprimitive model molecular dynamics (MD) simulations in a generalized Poisson-Boltzmann equation. We investigate how ion distributions and double layer forces depend on the choice of background salt. There is a strong ion specific double layer force set up due to unequal ion specific short-range potentials acting between ions and surfaces.

Co-Ion and Ion Competition Effects: Ion Distributions Close to a Hydrophobic Solid Surface in Mixed Electrolyte Solutions

We consider within a modified Poisson-Boltzmann theory an electrolyte, with different mixtures of NaCl and NaI, near uncharged and charged solid hydrophobic surfaces. The parametrized potentials of mean force acting on Na+, Cl-, and I- near an uncharged self-assembled monolayer were deduced from molecular simulations with polarizable force fields. We study what happens when the surface presents negative charges. At moderately charged surfaces, we observe strong co-ion adsorption and clear specific ion effects at biological concentrations. At high surface charge densities, the co-ions are pushed away from the interface. We predict that Cl- ions can also be excluded from the surface by increasing the concentration of NaI. This ion competition effect (I- versus Cl-) may be relevant for ion-specific partitioning in multiphase systems where polarizable ions accumulate in phases with large surface areas.

Anion-Specific Partitioning in Two-Phase Finite Volume Systems: Possible Implications for Mechanisms of Ion Pumps

In two-phase finite volume systems of electroneutral phospholipids, the electrolyte concentration is different in the two phases. The partitioning is highly anion-specific, a phenomenon not accounted for by classical electrolyte theories. It is explained if ionic dispersion forces that lead to specific ion binding are taken into account. The mechanism provides a contribution to active ion pumps not previously considered.

Ion Specific Interactions Between Pairs of Nanometer Sized Particles in Aqueous Solutions

For the classical DLVO theory, which deals only with electrostatic forces acting between ions and colloids, all ions in solution with the same charge should result in the same force between colloids. Ion specificity does occur in the opposing attractive Lifshitz forces but only very weakly. Ion size parameters, inner and outer Helmholtz planes are used to fit the specificity but that do not work. At, and above, biological salt concentrations other, non electrostatic (NES) ion specific forces act that are ignored in such modeling. To exemplify the general ideas we use a system that corresponds to pairs of nanoparticles. We show that ion specific double layer forces can be understood once NES forces acting between ions and colloids are included consistently in non-linear theory and in Monte Carlo simulations.

Classic Examples From Literature

This chapter presents the application of this theory to examples of the literature, from very simple systems (binaries without gravitational field) to more complex ones, like actual reservoirs influenced by thermal diffusion and gravity simultaneously. Examples showing simulations of laboratory experiments splitting the components of simple mixtures by thermal diffusion are also discussed, testing some model parameters. The current practical limitations of such experiments, along with their importance in modeling more complex thermal separation phenomena, open a wide research line. Finally, both thermal diffusion models proposed by the research groups cited in Chapter 4, Irreversible Thermodynamics Applied to Reservoir Engineering, are also tested in case studies for which temperature gradient has contributed for attenuation or enhancement of the compositional grading predicted by isothermal hypothesis.

Irreversible Thermodynamics Applied to Reservoir Engineering

This chapter describes the microscopic equations of fluid transport, which support irreversible thermodynamics. Different contributions for internal entropy generation, such as heat conduction, molecular diffusion, and viscous dissipation are derived. Soret and Duffour effects are discussed and the methods for calculation of thermal diffusion parameters for compositional grading proposed by two main investigation groups in the literature will also be presented. The systems of equations proposed independently by these two groups to eliminate the diffusive fluxes at steady state are compared to those obtained by the constant-temperature hypothesis along reservoir depth. The latter characterizes the gravity-modified thermodynamic equilibrium state. Finally, the equations governing fluid moving by natural convection in 2D systems with lateral temperature gradients are also derived, including a numerical solution sketched specifically for 2D domains.

General Comments and Perspectives

The results presented along this textbook do not lead us to definitive conclusions. But do provide a basis for new discussions and elaboration of new hypothesis, premises, and ideas that would certainly be of help to increase the reliability of the compositional grading predictions. We reinforce that this discipline, although fascinating from the point of view of aggregating value from theory formulation to its final application, must be considered as one more important tool on predicting fluid properties and settling diagnostics of connection/disconnection in oil and gas reservoirs. We strongly recommend the interaction between fluid-modeling engineers with geologists, geochemists, and geophysicists that may bring up more information to corroborate the conclusions generated by grading calculations. In many occasions, the acquired information by these correlate-area professionals serve as inputs for the formulation or reformulation of problems. In this chapter, we enumerate some suggestions for future works that can help improve current predictions; some of them already going on in new joint venturing projects between academy and industry.

Reservoir Fluids and PVT Analysis

One classifies reservoir fluids (liquids or gases) as a function of the relative position of its actual temperature and pressure to the critical point conditions in its phase envelope. Then one proceeds to a description of the experimental procedures for determination of Pressure, Volume and Temperature (PVT) properties of a reservoir fluid. These properties represent the volumetric behavior of this complex hydrocarbon-plus-contaminant mixture as a function of temperature and pressure. Therefore, they can be simulated through thermodynamic equilibrium algorithms using equations of state, once these are duly calibrated to reproduce such experimental data.

Phase Equilibrium Under the Influence of the Gravitational Field

In this chapter, we study the influence of the gravitational field on the conditions necessary for the establishment of the thermodynamic equilibrium in systems with relatively considerable height. The derivation of the maximum-entropy condition (or minimum energy) remains on the specification of total volume and mass, but the total energy now contains a potential-energy term to be added to the internal-energy one. We will see that, although the temperature-equality condition remains the same, the hydrostatic charge generated by the gravitational field leads to equating the so-called piezometric pressure at each height level, introducing also an additional term in the condition of both chemical potential and fugacity equality. After a brief literature review, we discuss the application of this approach in the study of compositional grading in oil reservoirs considered as isothermal, revisiting some classical examples of the literature.

The Influence of Molecular Association

In this chapter, we discuss the influence of molecular association on compositional grading of oil and gas reservoirs. This phenomenon, of which the hydrogen bonding is one of the main examples, is characterized by strong short-range attractive interactions and is intimately related to the polarity of fluid molecules. The angular orientation of the polar sites in a group of molecules and the short distance between them are crucial for the attractive potential around these areas to allow its association, which presents characteristics very similar to a strong (but not permanent) chemical bond.