An Examination of the Ternary Methane + Carbon Dioxide + Water Phase Diagram using the SAFT-VR Approach

Phase diagrams for hydrates beyond incipient condition — Complex behavior in methane/propane and carbon dioxide/iso-butane hydrates

Polyelectrolyte complexes (PECs) offer enormous material tunability and desirable functionalities, and consequently have found broad utility in biomedical and materials industries. Poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) are one of the most commonly used pairings to form PECs. However, various aspects of the phase behavior of PAA-PAH complexes have not been sufficiently quantified. We present a comprehensive experimental study depicting the binodal phase boundaries for the PAA-PAH complexes prepared in acidic, neutral and basic conditions using thermogravimetric analysis, turbidimetry and optical microscopy. In neutral and basic conditions, phase behaviors of the complexes were largely similar to each other and followed general expectations of PEC phase behavior, except for unusually high salt resistance with stable complexes observed up to 4 M NaCl concentrations. In acidic conditions, a remarkably different phase behavior of the PAA-PAH complexes was observed. The polymer content in the complex phase increased initially followed by an expected decrease as salt was added to the complexes. This behavior may result from a combination of associative phase separation of PAA and PAH chains, influenced by electrostatic interactions, and segregative phase separation which can be ascribed to the influence of a combination of the hydrophobic interactions of the aliphatic polymer backbone and the interpolymer hydrogen bonding of un- ionized acrylic monomer units. Our systematic investigations detailing these discrepancies in the PAA-PAH phase behavior are expected to clarify the inconsistencies among the reports in the literature and inform the materials design strategies for practical use of the PAA-PAH complexes and multilayer assemblies.

The high-pressure phase diagrams of the tetrahydrofuran(1) + carbon dioxide(2), + methane(2), and + water(2) mixtures are examined using the SAFT-VR approach. Carbon dioxide molecule is modeled as two spherical segments tangentially bonded, water is modeled as a spherical segment with four associating sites to represent the hydrogen bonding, methane is represented as an isolated sphere, and tetrahydrofuran is represented as a chain of m tangentially bonded spherical segments. Dispersive interactions are modeled using the square-well intermolecular potential. In addition, two different molecular model mixtures are developed to take into account the subtle balance between water-tetrahydrofuran hydrogen-bonding interactions. The polar and quadrupolar interactions present in water, tetrahydrofuran, and carbon dioxide are treated in an effective way via square-well potentials of variable range. The optimized intermolecular parameters are taken from the works of Giner et al. (Fluid Phase Equil. 2007, 255, 200), Galindo and Blas (J. Phys. Chem. B 2002, 106, 4503), Patel et al. (Ind. Eng. Chem. Res. 2003, 42, 3809), and Clark et al. (Mol. Phys. 2006, 104, 3561) for tetrahydrofuran, carbon dioxide, methane, and water, respectively. The phase diagrams of the binary mixtures exhibit different types of phase behavior according to the classification of van Konynenburg and Scott, ranging from types I, III, and VI phase behavior for the tetrahydrofuran(1) + carbon dioxide(2), + methane(2), and + water(2) binary mixtures, respectively. This last type is characterized by the presence of a Bancroft point, positive azeotropy, and the so-called closed-loop curves that represent regions of liquid-liquid immiscibility in the phase diagram. The system exhibits lower critical solution temperatures (LCSTs), which denote the lower limit of immiscibility together with upper critical solution temperatures (UCSTs). This behavior is explained in terms of competition between the incompatibility with the alkyl parts of the tetrahydrofuran ring chain and the hydrogen bonding between water and the ether group. A minimum number of unlike interaction parameters are fitted to give the optimal representation of the most representative features of the binary phase diagrams. In the particular case of tetrahydrofuran(1) + water(2), two sets of intermolecular potential model parameters are proposed to describe accurately either the hypercritical point associated with the closed-loop liquid-liquid immiscibility region or the location of the mixture lower- and upper-critical end-points. The theory is not only able to predict the type of phase behavior of each mixture, but also provides a reasonably good description of the global phase behavior whenever experimental data are available.

Effects of Non-Electrostatic Intermolecular Interactions on the Phase Behavior of pH-Sensitive Polyelectrolyte Complexes

The results of experimental study of oil extraction by supercritical carbon dioxide in a low-permeability reservoir are presented. As an object of study, we selected core samples from a low-permeability oil-saturated reservoir of one of the fields in Western Siberia, which is currently being developed in the regime of depletion of reservoir energy. The contact time of supercritical carbon dioxide with composite core models in three experiments was 8, 24, and 72 hours, respectively. Based on the results of laboratory experiments, the dynamics of the penetration of carbon dioxide along the depth of the composite core model was established. The value of the oil recovery factor and it’s distribution along the length of the core model in time is given. Keywords: carbon dioxide; low-permeability reservoir; mnimum miscibility pressure; slim-tube; extraction; oil recovery.

Modelling the phase behaviour and excess properties of alkane + perfluoroalkane binary mixtures with the SAFT–VR approach

The high-pressure phase diagram and other thermodynamic properties of the water + carbon dioxide binary mixture are examined using the SAFT-VR approach. The carbon dioxide molecule is modeled as two spherical segments tangentially bonded. The water molecule is modeled as a spherical segment with four associating sites to represent the hydrogen bonding. Dispersive interactions are modeled using the square-well intermolecular potential. The polar and quadrupolar interactions present in water and carbon dioxide are treated in an effective way via square-well potentials of variable range. The optimized intermolecular parameters are taken from the works of Galindo and Blas (Fluid Phase Equilib. 2002, 194−197, 502; J. Phys. Chem. B 2002, 106, 4503) and Clark et al. (Mol. Phys. 2006, 22−24, 3561) for carbon dioxide and water, respectively. The phase diagram of the mixture exhibits a number of interesting features: type-III phase behavior according to the classification of Scott and Konynenburg, three-phase behavior at low temperatures with its corresponding upper critical end point, a gas−liquid critical line at high temperatures and pressures that continuously changes from gas−liquid to liquid−liquid as the pressure is increased and gas−gas immiscibility of second kind. Only one unlike interaction parameter is fitted to give the best possible representation of the temperature minimum of the gas−liquid critical line of the mixture. This unlike parameter is then used in a transferable manner to study the complete pressure−temperature−composition phase diagram. The phase diagram calculated with SAFT-VR is in excellent agreement with the experimental data taken from the literature in a wide range of thermodynamic conditions. The theory is also able to predict a good qualitative description of the excess molar volume and enthalpy of the mixture as well as the most important features of the Henry's constants at different temperatures.

Spontaneous imbibition of surfactants could efficiently enhance oil recovery in low permeability sandstone reservoirs. The majority of studies have considered the application of individual surfactants to alter wettability and reduce interfacial tension (IFT). However, a significant synergistic effect has been reported between different types of surfactants and between salts and surfactants. Therefore, this study systematically studied the capability of a binary surfactant mixture (anionic/nonionic) and a ternary surfactant mixture (anionic/nonionic/strong base-weak acid salt) in imbibition enhanced oil recovery (IEOR). The interfacial properties and the cores' wettability were explored by IFT and contact angle measurements, respectively. Subsequently, the imbibition performances of different types of surfactant solutions were discussed. The results suggested that the surfactants' potential to enhance oil recovery followed the order of ternary surfactant mixture > binary surfactant mixture > anionic > nonionic > amphoteric > polymer. The ternary surfactant mixture exhibited strong capacity to reverse the rock surface from oil-wet (125°) to strongly water-wet (3°), which was more significant than both binary surfactant mixtures and individual surfactants. In addition, the ternary surfactant mixture led to an ultralow IFT value of 0.0015 mN/m, achieving the highest imbibition efficiency (45% OOIP). This research puts forward some new ideas on the application of the synergistic effects of surfactants in IEOR from low-permeability sandstone reservoirs.

The confinement of colloidal particles at liquid interfaces offers many opportunities for materials design. Adsorption is driven by a reduction of the total free energy as the contact area between the two liquids is partially replaced by the particle. From an application point of view, particle-stabilized interfaces form emulsions and foams with superior stability. Liquid interfaces also effectively confine colloidal particles in two dimensions and therefore provide ideal model systems to fundamentally study particle interactions, dynamics, and self-assembly. With progress in the synthesis of nanomaterials, more and more complex and functional particles are available for such studies. In this Account, we focus on poly(N-isopropylacrylamide) nanogels and microgels. These are cross-linked polymeric particles that swell and soften by uptaking large amounts of water. The incorporated water can be partially expelled, causing a volume phase transition into a collapsed state when the temperature is increased above approximately 32 °C. Soft microgels adsorbed to liquid interfaces significantly deform under the influence of interfacial tension and assume cross sections exceeding their bulk dimensions. In particular, a pronounced corona forms around the microgel core, consisting of dangling chains at the microgel periphery. These polymer chains expand at the interface and strongly affect the interparticle interactions. The particle deformability therefore leads to a significantly more complex interfacial phase behavior that provides a rich playground to explore structure formation processes. We first discuss the characteristic "fried-egg" or core-corona morphology of individual microgels adsorbed to a liquid interface and comment on the dependence of this interfacial morphology on their physicochemical properties. We introduce different theoretical models to describe their interfacial morphology. In a second part, we introduce how ensembles of microgels interact and self-assemble at liquid interfaces. The core-corona morphology and the possibility to force these elements into overlap upon compression results in a complex phase behavior with a phase transition between microgels with extended and collapsed coronae. We discuss the influence of the internal particle architecture, also including core-shell microgels with rigid cores, on the phase behavior. Finally, we present new routes for the realization of more complex structures, resulting from multiple deposition protocols and from engineering the interaction potential of the individual particles.

This paper presents a detailed fluid characterization study for a group of compartmentalized gas condensate & volatile oil reservoirs making up the Pauto Complex. Fluid oil-gas ratios range from 30 to 350 bbl/MMscf, producing from depths of 12,000–16,000 ft, with estimated IGIP>1 Tcf and significant oil/condensate resources. A single equation of state (EOS) was developed to describe the complex phase and volumetric behaviour of eight reservoir fluids undergoing depletion. The EOS also describes accurately the vaporization of retrograde condensate from one of the richer reservoir fluids when contacted by a leaner hydrocarbon gas. Years of experience in this thrust-belt basin have consistently demonstrated that the quality of fluids samples, PVT test design, lab-data QC, EOS tuning, and fluid initialization are essential to understand the hydrocarbon systems, reservoir compartmentalization, and recovery mechanisms. We show how integration of fluid characterization assists in the evaluation of depletion versus gas-injection strategies, development decisions, and ongoing reservoir management. The methodology used in this study has general application to PVT data acquisition and developing fluid characterizations exhibiting complex phase behaviour. We also provide standardized methods to QC the thermodynamic consistency of an EOS model, particularly when binary interaction parameters (BIPs) are used extensively. A single EOS model was developed successfully for describing the phase and volumetric behaviour of a wide range of fluids undergoing depletion and vaporization. This was achieved by modifying heavy-component properties, but more importantly, modifying BIPs extensively; these BIP modifications were possible only because considerable equilibrium compositional data was available. We believe this paper is significant as a proven case history. With the PVT data provided in our paper, the same problem can be used to test other fluid characterization methodologies and serve as a complex phase behaviour benchmark.

Polyelectrolyte complexes have a bright prospect for fabricating 3D periodic structures by direct ink writing. The phase behavior of complexes containing poly(acrylic acid) and poly(ethylenimine) and rheological behavior of Al 2 O 3 colloidal suspensions are characterized. The results reveal that the pH value of solution takes an important role on the phase behavior of polyelectrolyte complexes. When the [ COOH ]:[ NH x ] ratio is higher than or lower than the critical value of 0.6, the pH range of turbid complex solutions narrows down and meanwhile moves to acidic or alkaline region, respectively. The addition of pH regulators prompts polyelectrolyte exchange reaction and soluble complexes are suitable for preparation of cera‐mic suspensions. The polyelectrolyte suspensions with linear viscoelasticity at lower shear stress and good fluidity at higher shear stress are identified for direct ink writing of 3D structures with microsized feature.

The solubility of pure carbon dioxide in Athabasca bitumen was measured and compared with the literature data. Multiple liquid phases were observed at carbon dioxide contents above approximately 12 wt%. A correlation based on Henry's law was found to fit the saturation pressures at carbon dioxide contents below 12 wt%. The saturation pressure and solubility of carbon dioxide and propane in Athabasca bitumen, as well as the liquid phase densities and viscosities, were measured for three ternary mixtures at temperatures from 10 to 25 °C. Two liquid phases (carbon dioxide-rich and bitumen-rich) were observed at 13 wt% carbon dioxide and 19 wt% propane. Only liquid and vapour-liquid regions were observed for the other two mixtures (13.5 wt% propane and 11.0 wt% carbon dioxide; 24.0 wt% propane and 6.2 wt% carbon dioxide). The saturation pressures for the latter mixtures were predicted using the correlation for the carbon dioxide partial pressure and a previously developed correlation for the propane partial pressure. The mixture viscosities were predicted with the Lobe mixing rule. Introduction In Part I of this work(1), mixtures of carbon dioxide and propane were identified as a potential solvent for the VAPEX process. At typical heavy oil reservoir conditions (pressure of ~1.2 MPa and temperature of ~10 °C), propane and butane have sufficient solubility to reduce the oil viscosity to a level where gravity drainage can occur in an economic time scale. However, propane and butane are expensive solvents and the success of the process depends on how much solvent can be recovered. As well, the VAPEX process operates below the saturation pressure of the solvent and, therefore, propane and butane cannot be used at higher reservoir pressures where they exist only in the liquid phase. Methane can be added to achieve the desired pressures(2). However, carbon dioxide may also be a better VAPEX solvent than methane because it is more soluble in heavy oil and significantly reduces the viscosity(3). Mixtures of carbon dioxide and propane may achieve the desired reduction in viscosity while minimizing the required propane volumes. Hence, there is an incentive to evaluate mixtures of carbon dioxide and propane as a VAPEX solvent. VAPEX performance depends on the viscosity and density of the liquid phase that forms at the edge of the vapour chamber. In order to design and optimize VAPEX and other solvent-based processes, it is critical to be able to determine the diffusivity of the solvent in the heavy oil, identify the phases that form in the solvent and heavy oil mixtures at various temperatures and pressures, and determine the density and viscosity of the liquid phase. Other solvent-based processes (steam and solvent injection for heavy oil recovery and solvent extraction of oil sands) require similar data. In Part I of this work(1), saturation pressures and liquid phase densities and viscosities were measured for propane and Athabasca bitumen. There are also considerable data in the literature for mixtures of carbon dioxide and crude oils. Simon and Graue(4) measured the solubility, swelling and viscosity of mixtures of carbon dioxide and nine different oils.

Mixtures of hydrophobically graft-modified cellulose derivatives and their nonmodified analogues have been studied in aqueous solution. A qualitatively similar behavior was found in the phase behavior of nonionic as well as of cationic polymer systems. Over a large range of total polymer concentrations and mixture ratios the solutions phase separated into two phases of similar polymer concentration, with one of the phases enriched in the hydrophobically modified polymer. From the manufacturing process the cellulose derivatives investigated are likely to contain polymer chains with a rather continuous distribution in degrees of substitution and, possibly, substitution patterns. This causes a complex phase behavior that cannot be adequately described by a ternary representation. The multicomponent nature became apparent from composition analyses of the phases in equilibrium. It may thus be more appropriate to view the phase separation as a fractionation. A phase of small relative volume with a highly enhanced hydrophobe content (compared to the original hydrophobically modified polymer sample) was created. This was particularly obvious in more dilute solutions. Sometimes the phase separation was difficult to observe because the phases in equilibrium had similar polymer concentrations and, therefore, similar refractive indices. The observations presented here call for the attention of producers and users of these types of polymers.

Discussed herein are three aspects of fluid phase equilibrium as they relate to enhanced oil recovery processes and to the simulation of such processes. First, experimental phase equilibria measurements show that the addition of n-butane can lower saturation pressures relative to those of base oil + CO2 mixtures. Enrichment of the injection gas with only 10 mol·% n-butane reduces the saturation pressure at the L-V critical point by 1400 psia [9.65 MPa] and by nearly 2250 psia [17.51 MPa] for a 20 mol·% addition. Likewise, addition of 6 mol·% n-butane reduces the slim tube minimum miscibility pressure by approximately 550 psia [3·79 MPa]. Thus, a n-butane enriched CO2 injection gas can achieve multiple contact miscible displacements in lower pressure reservoirs. It would not be possible to miscibly flood those low pressure reservoirs with pure CO2 injection. Second, phase distribution curves from constant composition volumetric expansion experiments illustrate the solvent capacity of the injection gas for the base oil. Third, a set of equation of state parameters describing the fluid PVT and phase behavior is presented and used to represent the suite of measured properties. To mimic miscibl·e-like behavior during multiple contacting, severe compromises are required in the representation of base oil and first contact base oil + injection gas mixture properties.

2-Hydroxyacids display complex monolayer phase behavior due to the additional hydrogen bonding afforded by the presence of the second hydroxy group. The placement of this group at the position α to the carboxylic acid functionality also introduces the possibility of chelation, a utility important in crystallization including biomineralization. Biomineralization, like many biological processes, is inherently a nonequilibrium process. The nonequilibrium monolayer phase behavior of 2-hydroxyoctadecanoic acid was investigated on each of pure water, calcium chloride, sodium bicarbonate and calcium carbonate crystallizing subphases as a precursor study to a model calcium carbonate biomineralizing system, each at a pH of ∼6. The role of the bicarbonate co-ion in manipulating the monolayer structure was determined by comparison with monolayer phase behavior on a sodium chloride subphase. Monolayer phase behavior was probed using surface pressure/area isotherms, surface potential, Brewster angle microscopy, and synchrotron-based grazing incidence X-ray diffraction and X-ray reflectivity. Complex phase behavior was observed for all but the sodium chloride subphase with hydrogen bonding, electrostatic and steric effects defining the symmetry of the monolayer. On a pure water subphase hydrogen bonding dominates with three phases coexisting at low pressures. Introduction of calcium ions into the aqueous subphase ensures strong cation binding to the surfactant head groups through chelation. The monolayer becomes very unstable in the presence of bicarbonate ions within the subphase due to short-range hydrogen bonding interactions between the monolayer and bicarbonate ions facilitated by the sodium cation enhancing surfactant solubility. The combined effects of electrostatics and hydrogen bonding are observed on the calcium carbonate crystallizing subphase.