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The movement of water from one aqueous system to another through a semipermeable membrane. Osmotic movement is driven by activity differences between the two systems and can be considered as a vapor-phase transfer. An oil mud acts as an osmotic system. Emulsion film surrounding each brine droplet in an oil mud acts as semipermeable osmotic membrane and allows water molecules to pass back and forth, but restricts ions and larger molecules. Clays in shale formations also are aqueous systems that interact by osmosis with oil-mud droplets.

See: activity of aqueous solutionsbalanced-activity oil mudcalcium chlorideChenevert methodhygrometeroil-mud emulsifierosmotic pressure