Methods including the step of producing a bulk fluid from a subterranean formation, the bulk fluid comprising at least water and a hydrocarbon. The bulk fluid is then sampled to form at least one sampled fluid. Next, constituent parameters of the sampled fluid are determined using the hydrophilic-lipophilic deviation (HLD) model. The constituent parameters include the salinity (S) of the sampled fluid, the salinity constant (b); the equivalent alkane carbon number for the hydrocarbon in the sampled fluid (EACN); T is temperature of the sampled fluid; the characteristic curvature for an ionic surfactant composition (cc) or for a nonionic surfactant composition (ccn); the surfactant temperature constant for the ionic surfactant composition (αT) or for a nonionic surfactant composition (cT). Also determining an optimal surfactant or optimal surfactant blend to achieve an oil-water separation morphological phase distribution of the sampled fluid.