Thermodynamic Modeling of Pseudoternary Phase Behavior

Abstract

Abstract Phase behavior is the fulcrum at which the chemistry and physics of surfactant and solvent systems govern the engineering and economics of chemical flooding. Salient behavior is represented by pseudoternary diagrams that account for polar, nonpolar, and amphiphilic components. The common 2, 3, 2, phase-split progression--induced, for example, by salinity change imicroemulsion systems--is required by thermodynamic principles. Although such progressions can be simulated semiempirically, modeling them with a suitable free- energy function or an equation of mixture state is more reliable for interpolating and extrapolating limited data on phase splits and coexisting phase compositions for use in mechanism-based computer simulation of laboratory experiments and field applications. Simple equations of mixture state prove inadequate but lead to the promising new linearly screened Flory- Huggins (LSFH) equation, which accounts for simultaneous association of amphiphile with oil and water and aggregation of surfactant amphiphile into curved sheetlike structures that separate water-rich from oil-rich regions. From this equation for a ternary mixture are calculated representative sets of diagrams with continuous progressions of tielines and binodals, plait points, tie-triangles, and three-phase regions with their critical endpoints. Several overlapping regions of metastable one- and two-phase equilibria are identified. Free-energy surfaces are pictured, and the free-energy factor that jointly controls interfacial tension (IFT) is computed. Ultralow tensions are favored by low-relief free-energy surfaces; so also are long-lived metastable states. The exponentially screened Flory-Huggins (ESFH) equation, superior in some ways to the linearly screened version, also is discussed briefly. Computational methods are described for fitting the six parameters of the ternary equation to data as well as for predicting phase behavior from given parameters. Introduction Petroleum most often is recovered by causing a second fluid phase to displace liquid-phase petroleum from the pore space of reservoir rock. The exception would be a totally miscible process, which is generally impracticable. The split of gross local composition into immiscible phases and the equilibrium partitioning of components among the phases is called phase behavior. Phase behavior governs the economics and engineering of chemical flood process design. Such chemical flooding processes are of two types: those that rely on altering phase relations per se, as in the solubilization and swelling mechanisms, and those that alter fractional flow relations, as in the ultralow tension mechanism, where the target is the capillary forces that conspire with pore space to entrap residual oil and hold it immobilized. That the IFT behind capillary forces correlates with the pattern of phase behavior as a "field" variable--i.e., a variable that is equal among phases in equilibrium, or one that is nearly so, like salinity, is varied--has been recognized for some time. The recently developed mean field theory of multicomponent IFT's confirms that the same free-energy relations (most easily pictured as a mathematical surface) that dictate phase behavior also rule jointly the IFT's between phases. This is especially true near plait points, consolute points, and other critical points where universal dimensionless relations called scaling laws govern both the way that phases approach each other in composition and the way that IFT vanishes as any critical point is approached. SPEJ P. 945^