Behavior of Microemulsions Under Compression

Compression to reservoir pressures can alter microemulsion 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 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 protosur-factants 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.