Low-salinity waterflooding is a relatively new method for improved oil recovery that has generated much interest. It is generally believed that low-salinity brine alters the wettability of oil reservoir rocks towards a wetting state that is optimal for recovery. The mechanism(s) by which the wettability alteration occurs is currently an unsettled issue. This paper reviews recent studies on wettability alteration mechanisms that affect the interactions between the brine/oil and brine/rock interfaces of thin brine films that wet the surface of reservoir rocks. Of these mechanisms, we pay particular attention to double-layer expansion, which is closely tied to an increase in the thickness and stability of the thin brine films. Our review examines studies on both sandstones and carbonate rocks. We conclude that the thin-brine-film mechanisms provide a good qualitative, though incomplete, picture of this very complicated problem. We give suggestions for future studies that may help provide a more quantitative and complete understanding of low-salinity waterflooding.

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The fundamental study of phase transition kinetics has motivated experimental methods toward achieving the largest degree of undercooling possible, more recently culminating in the technique of rapid, quasi-isentropic compression. This approach has been demonstrated to freeze water into the high-pressure ice VII phase on nanosecond time scales, with some experiments undergoing heterogeneous nucleation while others, in apparent contradiction, suggesting a homogeneous nucleation mode. In this study, we show through a combination of theory, simulation, and analysis of experiments that these seemingly contradictory results are in agreement when viewed from the perspective of classical nucleation theory. We find that, perhaps surprisingly, classical nucleation theory is capable of accurately predicting the solidification kinetics of ice VII formation under an extremely high driving force (|∆µ/k B T | ≈ 1), but only if amended by two important considerations: 1) transient nucleation and 2) separate liquid and solid temperatures. This is the first demonstration of a model that is able to reproduce the experimentally observed rapid freezing kinetics.

We present thermodynamic models for the five most commonly studied phases of the energetic material octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX): liquid HMX and four solid polymorphs (α-, β-, γ-, and δ-HMX). We show results for the density, heat capacity, bulk modulus, and sound speed, as well as a phase diagram that illustrates the temperature and pressure regions over which the various HMX phases are most thermodynamically stable. The models are based on the same equation of state presented in our recently published paper [Myint et al., Ind. Eng. Chem. Res., 2016, 55, 2252 on another energetic material, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). We combine our HMX and RDX models together so that the equation of state can also be applied to liquid and solid mixtures of HMX/RDX. This allows us to generate an HMX/ RDX phase diagram and calculate the enthalpy change associated with a few different kinds of phase transitions that these mixtures may undergo. Our paper is the first to present a single equation of state that is capable of modeling both pure HMX and HMX/RDX mixtures. A distinct feature of HMX is the strongly metastable nature of its polymorphs. This has caused some ambiguity in the literature regarding the thermodynamic stability of α-HMX. By examining possible arrangements for the relative order of the six different solid−solid transition (α−β, α−γ, α−δ, β−γ, β−δ, and γ−δ) temperatures, we conclude that α-HMX must be thermodynamically stable so that the HMX phase diagram must have an α phase region.

We present equations of state relevant to conditions encountered in ramp and multiple-shock compression experiments of water. These experiments compress water from ambient conditions to pressures as high as about 14 GPa and temperatures of up to several hundreds of Kelvin. Water may freeze into ice VII during this process. Although there are several studies on the thermodynamic properties of ice VII, an accurate and analytic free energy model from which all other properties may be derived in a thermodynamically consistent manner has not been previously determined. We have developed such a free energy model for ice VII that is calibrated with pressure-volume-temperature measurements and melt curve data. Furthermore, we show that liquid water in the pressure and temperature range of interest is well-represented by a simple Mie-Grüneisen equation of state. Our liquid water and ice VII equations of state are validated by comparing to sound speed and Hugoniot data. Although they are targeted towards ramp and multiple-shock compression experiments, we demonstrate that our equations of state also behave reasonably well at pressures and temperatures that lie somewhat beyond those found in the experiments.

Energetic materials are substances that can undergo rapid, exothermic reactions when subjected to an external stimulus, such as heating. In this work, we show that the well-known Peng−Robinson equation of state can be applied to energetic materials, whether they are pure components, liquid mixtures, or solid mixtures. We are specifically interested in two energetic materials: hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 2,4,6-trinitrotoluene (TNT). We model RDX and TNT in both their liquid and solid phases, as well liquid and solid mixtures of the two compounds. Our work examines temperatures and pressures as high as about 500 K and 2500 bar, respectively. The Peng−Robinson equation of state provides a good representation of experimental volumetric (e.g., density and bulk modulus), thermal (heat capacity), and phase behavior (melting temperature and solubility) data. It can be applied to other energetic materialsranging in complexity from pure components to multiphase, multicomponent mixturesby adapting the procedures described in this study.

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The density increase from carbon dioxide (CO 2) dissolution in water or hydrocarbons creates buoyancy-driven instabilities that may lead to the onset of convection. The convection is important for both CO 2 sequestration in deep saline aquifers and CO 2 improved oil recovery from hydrocarbon reservoirs. We perform linear stability analyses to study the effect of fluid compressibility and interface movement on the onset of buoyancy-driven convection in porous media. Compressibility relates to a non-zero divergence of the velocity field. The interface between the CO 2 phase and the aqueous or hydrocarbon phase moves with time as a result of the volume change that occurs upon CO 2 dissolution. Previous stability analyses have neglected these two aspects by assuming that the aqueous or hydrocarbon phase is incompressible and that the interface remains fixed in position. The stability analyses are used to compute two key quantities: (1) the critical time and (2) the critical wavenumber. Our results indicate that compressibility has a negligible effect on the critical time and the critical wavenumber in CO 2-water mixtures. We use thermodynamics to derive an expression which shows that the two opposing physical processes which contribute to the divergence are comparable in magnitude and largely cancel each other. This result explains why compressibility does not significantly affect the onset, and it also demonstrates the link between compressibility and the volume change that causes movement of the interface. Compared to when the interface is fixed in position, a moving interface in CO 2-water mixtures may reduce the critical time by up to around 10%, which can be significant in low permeability formations. The decrease in the critical time due to interface movement may be much more pronounced in hydrocarbons than in water. This could have important implications for CO 2 improved oil recovery.