Behavior of Microemulsions Under Compression

Compression to reservoir pressures can alter microemulsion phase behavior, on which the success of microemulsion-based phase behavior, on which the success of microemulsion-based EOR processes depends. For convenience, however, phase behavior studies usually are conducted at atmospheric pressure. Extrapolating phase behavior data from atmospheric to reservoir pressures requires the volumetric properties of each phase pressures requires the volumetric properties of each phase under compression, including the isothermal compressibility as a function of pressure and composition. Two topics are addressed here. First, compressibility data up to 16 MPa [2,300 psi] are presented for several systems of oil and water with surfactants and proto surfactants and the data are fit to simple models for pressure and composition dependence. The pressure dependence fits the Tait equation in this pressure range; selected high-pressure data indicate how the compressibility deviates at much higher pressures. To within the resolution of the data, the composition dependence is linear in volume-fraction composition; moreover, the effective compressibilities of oil and water in solution appear to be near their pure-component values. More data are needed to verify and determine the exact dependence. Second, a simple compressibility cell is described that allows both rapid determination of compressibility and direct determination of the phase boundary in pressure. Both phase separation and coalescence of distinct phases with increasing pressure was observed in microemulsion systems, although the phase boundaries in pressure for these systems were not mapped out systematically. The effect of dissolved gases such as methane, which can affect microemulsion phase behavior at high pressures, is not addressed here. Introduction The phase behavior of mixtures of oil and water with surfactant is crucial to the analysis and design of microemulsion flooding processes for EOR. Phase behavior governs the local compositions and saturations of the various fluid phases, which, through fractional flow relations, determine microscopic displacement. Moreover, phase behavior influences the fractional flow relations phase behavior influences the fractional flow relations themselves through its influence on interfacial tension, wettability, and fluid properties. The importance of phase behavior to process design has prompted a wide phase behavior to process design has prompted a wide range of laboratory studies of microemulsion phase behavior. However, compression to reservoir pressure distorts phase behavior from that observed in the laboratory. A phase behavior from that observed in the laboratory. A familiar example of the effect of pressure is described by the Clausius-Clapeyron equation for the change in freezing point of a pure liquid. More generally, in multicomponent point of a pure liquid. More generally, in multicomponent mixtures equilibrium conditions derive from the required equality of chemical potentials ...............(1) between phases A and B. Any intensive variable influences phase behavior through its effect on chemical potentials; that of pressure acts through the partial potentials; that of pressure acts through the partial molar volumes: .......(2) where is partial molar volume and p is a fixed reference pressure such as 100 kPa [14.5 psi]. Because for each component the molar volume can differ between phases, a change in pressure can upset the equality of phases, a change in pressure can upset the equality of chemical potentials unless phase compositions change to restore equilibrium. Thus the shift in phase boundaries under compression results from the dependence of chemical potentials on pressure and composition. Phase behavior in microemulsion systems is especially sensitive to intensive variables like temperature and salinity. Recent reports disagree on its sensitivity to pressure. O'Connell and Walker and Good found that pressure significantly altered the phase behavior of pressure significantly altered the phase behavior of microemulsion systems made with several synthetic oils. Nelson, on the other hand, observed a negligible pressure effect on two microemulsion systems, one made with a crude oil and one with a synthetic oil. Evidently the thermodynamic properties that govern the effect of pressure can differ among microemulsion systems. To extrapolate phase behavior from atmospheric to reservoir pressures by using Eq. 2 requires knowledge of volumetric properties of the phases involved; specifically, the partial molar volume, vi, of all components in each phase must be known as functions of pressure and phase must be known as functions of pressure and composition. These may be derived from the partial molar volumes at atmospheric pressure, more easily measured in the laboratory, and a correlation for the compressibility of the mixture as a function of pressure and composition, . An example of the correction of chemical potentials for pressure effects is given in Ref. 18. SPEJ p. 536