The Effect of wettability on relative permeability and imbibition in porous media
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AbstractThis study dealt with the effect of wettability on immiscible displacement in porous media. Relationships between the contact angle, measured external to the porous media, and two phase relative permeability-saturation properties for three consolidated intergranular porous media of widely differing porosity and permeability were deter mined for a range of wetting conditions. Some spontaneous imbibition and capillary pressure measurements were also made. Wettability control was achieved in these tests by using synthetic porous media composed of polytetrafluoroethylene (PTFE) and various pure fluid pairs which dis played essentially uniform wetting behavior with the solid. The PTFE media provided systems of fixed pore geometry which were ideal for systematic studies of the wettability variable. In situations where comparisons were possible with published work, the results of this study suggested that the three PTFE cores were representative models of naturally-occurring intergranular porous media. Spontaneous imbibition occurred when the system contact angle (θ) through the displacing phase was less than 60°, if the porous media contained an initial irreducible saturation of this phase. Only fluids with θ < 45° imbibed into cores initially saturated with the second phase. This result, which was independent of the porosity or permeability of the porous media studied, was explicable in terms of a composite surface effect. A correlation was suggested between contact angle and the imbibition rate for a given core. Once imbibition occurred, the ultimate amount of fluid imbibed was generally independent of θ and the imbibition rate. Steady-state relative permeability test results indicated that the most efficient innniscible displacement was obtained with fluids capable of imbibing into the porous media. Relative permeability saturation relations were independent of 8 for displacements with imbibing fluids. As θ was increased through values for which irnbibition did not occur, a consistent shift in the relative permeability properties of both phases was found when the media contained an initial irreducible saturation of the displacing phase. For forced displacements from a core initially saturated with one fluid, relative permeability curves were little affected when the system contact angle through the displacing phase was varied over the range of 180° - 90°. This study showed how the saturation history affected the influence of the contact angle on immiscible displacement. Nevertheless, reasons were given for maintaining the contact angle as the primary quantitative measure of wettability and its effects in porous media. Capillary pressures were determined concurrently with Penn State relative permeability measurements. The proposed method for evaluating the capillary pressure did not require the use of semi-permeable membranes for sensing pressures in the preferentially wetting phase. When wettability changes, as defined by contact angles, caused significant shifts in relative permeability relations, the capillary pressure properties of the system usually also underwent large changes.
Bibliography: p. 238-246.