High-salinity diffusion

High-Salinity Diffusion Phenomena. Just as at intermediate salinities, the TRS system exhibited extensive convection at high salinities. The rate of phase equilibration was extremely rapid. Typically, the interfaces in this system moved further in a few hours than those in the PDM system moved in two weeks. 

Figure 17. Diagram of high-salinity diffusion phenomena in the PDM system.

FIGURE 9.12 High-salinity diffusion path for contact of composition D with oil (O) indicating an intermediate brine phase (b) and spontaneous emulsification in the oil phase (s.e.). Ic and w/o denote the lamellar liquid crystalline phase and a water in oil microemulsion, respectively. S/A denotes the surfactant/alcohol mixture in this pseudoternary diagram. (From Raney, K.H. and Miller, C.A., AIChE J., 33, 1791, 1987. With permission.)... 

Highly saline water of distinctive isotopic composition is often found in environments of such depth and low permeability that flow rates must be extremely low or zero. These waters are often characterized by Ca " -Cl compositions and stable isotope composition above the meteoric waterline (see Chapter 5.17). They apparently result from water-rock equihbration over very long periods of time. Geochemically, they have little influence on waters in active circulation systems, but mobile isotopes of the noble gases diffusing upward from this environment can be a powerful tool for understanding the flow systems into which they move. The noble-gas isotopes can also provide clues to the histories of these nearly static waters. 

The transition that occurs in diffusion paths near the high-salinity end of the three-phase regime is not as complex as that at the low-salinity end. As the triangle shrinks, passage through the... 

Calculated diffusion paths also successfully predicted the occurrence of spontaneous emulsification in the systems. Near optimum salinity where this phenomenon first appeared, brine drops spontaneously emulsified in the oil but were isolated from the bulk brine phase by a microemulsion. At high salinities, a more common type of spontaneous emulsification was seen with brine emulsifying in the oil directly above a brine layer. 

Figure 23. Calculated diffusion path at high salinity.

Decreasing spontaneous curvature further to negative values (high salinity) leads to a progressive lowering of the surfactant self-diffusion coefficient to the value of water and to that of the droplets in the W/0 microemulsions. (This decrease is not pronounced for nonionic surfactants because of the high solubility in oil and the concomitant contribution from single-molecule diffusion.)... 

Various experiments indicate that properties of the microemulsion phase change continuously with increasing salinity as inversion from a water-continuous to an oil-continuous microstrucmre occurs. For instance, electrical conductivity decreases continuously with increasing salinity (Bennett et al., 1982). In addition, the self-diffusion coefficient of oil as measured by nuclear magnetic resonance (NMR) techniques increases from small values at low salinities where oil is the dispersed phase to a value comparable to that of the bulk oil phase near and above the optimal sahnity. The self-diffusion coefficient of water, in contrast, decreases from a value comparable to that in pnre NaCl brine below and near the optimal sahnity to mnch smaller values at high salinities where water is the dispersed phase (Olsson et al., 1986). Thus the surfactant phase is bicontinuous near the optimal sahnity, as originahy proposed by Scriven (1976) and subsequently confirmed by electron microscopy (Jahn and Strey, 1988). 

In the tertiary mode floods. Figures 8 and 9, an oil bank is formed which is preceded by a sharp rise in both the pressure drop and the pH. The sharp pressure peaks in Figures 8 and 9 demons-strate that the oil phase was mobilized by locally high gradients in pressure whereas the diffuse pressure profile of the tertiary high pH/high salinity alkaline flood appears to be caused by the formation of swollen ganglia which restrict aqueous flow and which increase the macroscopic displacement efficiencies. 

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