Patrice Creux

Strong Specific Hydroxide Ion Binding at the Pristine Oil/Water and Air/Water Interfaces

Despite claims, based largely on molecular dynamics simulations, that the surface of water at the air/water interface is acidic, with a positive charge, there is compelling experimental evidence that it is in fact basic, with a negative charge due to the specific adsorption of hydroxide ions. The oil/water interface behaves similarly. The pH dependence of the zeta potentials of oil drops has been measured by two very different techniques: on a single drop in a rotating electrophoresis cell and on about 10(14) submicrometer drops in a 2 vol % emulsion by an electroacoustic method to give similar results with a sigmoidal pH dependence characterized by an isoelectric point at pH 2-3 and a half adsorption point about pH 5.5, or at 10(-8.5) M hydroxide ion. This indicates that hydroxide ion is absorbed much more strongly than other anions. The pH dependence of a single N(2) bubble has also been measured and has the same pH dependence, independently of whether HCl or HI is used to adjust the pH. These similarities between the pH dependences of the zeta potentials of air bubbles and oil drops, as well as those reported from streaming potentials on solid inert surfaces such as Teflon, indicate that water behaves similarly, with only subtle differences, at each of these low dielectric hydrophobic surfaces, with an isoelectric point of pH 2-4. In acidic solutions at pHs below the isoelectric point, the surface is indeed positive, consistent with spectroscopic observations of the adsorption of hydrogen ions.

Computational Permeability Determination from Pore-Scale Imaging: Sample Size, Mesh and Method Sensitivities

In this work, a complete work flow from pore-scale imaging to absolute permeability determination is described and discussed. Two specific points are tackled, concerning (1) the mesh refinement for a fixed image resolution and (2) the impact of the determination method used. A key point for this kind of approach is to work on enough large samples to check the representativity of the obtained evaluations, which requires efficient parallel capabilities. Image acquisition and processing are realized using a commercial micro-tomograph. The pore-scale flows are then evaluated using the finite volume method implemented in the open-source platform OpenFOAM®. For this numerical method, the influence of the different aspects mentioned above are studied. Moreover, the parallel efficiency is also tested and discussed. We observe that the level of mesh refinement has a non-negligible impact on permeability tensor. Moreover, increasing the refinement level tends to reduce the gap between the methods of computational measurements. The increase in computation time with the mesh is balanced with the good parallel efficiency of the platform.

Measurement of the Zeta Potential of Oil Drops with the Spinning Tube Zetameter

The spinning tube zetameter, an apparatus which was utilized up to now to keep a gas bubble from settling while electrophoretic measurement was carried out, can be used to measure the zeta potential of an oil drop as well. This piece of equipment is brought into play for estimating the influence of various formulation variables (pH, ionic strength, surfactant type or mixture) on the zeta potentials of oil drops. The results are compared to those reported in the literature.

Charge transfer between colliding hydrometeors: Role of surface tension gradients

A mechanism is postulated to account for the exchange of electrical charge that has been observed to take place when hydrometeors in a “cloud” created in the laboratory collide. A hydrodynamic flow induced by variation of the surface tension at an air/water interface is regarded as the main driving force. This flow created by forces acting within an interfacial region will transport the excess negative charge that normally resides at an air/water interface from one interfacial region to another, thereby yielding a separation of charge. An estimate of the quantity of charge exchanged by this process, using reasonable parametric values, is in accord with laboratory observations. In some interactions our proposed mechanism is only valid if a stable thick water film coats the surface of one of the colliding hydrometeors. Measurements are presented to show that the ice/water interface is negative, and this is a condition needed to confer stability of thick water films on ice.

Competitive adsorption of surfactants at air/water interfaces

The zeta potential of an air bubble suspended in an aqueous solution of mixed fluorocarbon and hydrocarbon anionic surfactants is studied over a wide range of concentrations and mixture compositions. The zeta potential is related to surfactant ion adsorption. The two surfactants, which exhibit an antipathy manifested by micellar demixing, compete for surface sites. The total surfactant adsorption is reduced when both surfactants are present. Adsorption phenomena are closely correlated to the micellar phase diagram.

Sodium Fluoride at the Air/Water Interface

The zeta potentials of a single N2 bubble in near neutral aqueous solutions of 10–5 to 10–2 M NaF have been measured with a rotating cell zetameter. The zeta potentials in the NaF solutions are slightly more negative than those of comparable NaI concentrations. A simple interpretation of this observation is that the small size of the fluoride ion leads to its stronger adsorption at the interface, despite its smaller dielectric decrement. This would not be consistent with independent electrospray results from 100 µM solutions that bromide ion is less strongly adsorbed than iodide ion, i.e. that surface affinity increases with ion size. An alternative interpretation of the fluoride experiment is that the fluoride surface affinity is the consequence of stronger ion pairing with the NaOH that spontaneously forms at the interface.

Non-classical Nucleation Model for Cold Production of Heavy Oil

We examine the gas bubble nucleation phenomenon encountered in extra heavy oil during cold production. The nucleation model described in this work is based on so-called non-classical nucleation. Using this method, we show mesoporous cavities could be at the origin of the nanobubble trapping mechanism. The results obtained show this physical approach tends to demonstrate the pre-existence of gas bubbles in these crevices (surface roughness).The physics of capillarity used here is based on traditional Laplaces law and an original disjoining pressure expression. We test for several wettabilities in our mathematical model. The first configuration envisaged is for oil-wet rocks, although the cavity is assumed to be gas-wet. Water wettability is considered a second time, taking into account a precursor water film between the rock and the entrapped bubbles. The mix of these two configurations could represent nucleation in a global mixed-wet porous media. However, we show in the first part of this article that water presence does not affect the initial bubble radius. Nevertheless, a bubble growth model developed in the second configuration shows that bubble confinement could play an important role on gas bubble nucleation and the early first steps of its development.

Micro-continuum Framework for Pore-Scale Multiphase Fluid Transport in Shale Formations

A micro-continuum simulation framework is proposed to study the complex pore-scale dynamics associated with hydrocarbon recovery from shale gas. The model accounts for the presence of immiscible fluid phases and for transport mechanisms in the nanoporous structures including slip flow, adsorption, surface and Knudsen diffusion. We employ the concept of sub-grid models to simulate the transport phenomena in shale gas. Specifically, we use high-resolution FIB–SEM images that provide information on the spatial distribution of the minerals, resolved pore space, and sub-resolution porous regions. The model is used to investigate several production scenarios at the pore-scale. In one setting, the organic matter is in direct contact with a micro-crack; in the other setting, clay regions are sandwiched between the organic matter and the “open” crack. The simulations show that it is important to account for the presence of multiple immiscible fluid phases because they can play a critical role in hydrocarbon production from shale-gas formations both in terms of production rate and in terms of residual mass of hydrocarbon. Moreover, we show that, because of wettability conditions, the rate of hydrocarbon recovery, as well as the ultimate recovery, depends strongly on the spatial distribution of the kerogen and clay in the vicinity of the micro-cracks.