Coupled osmotic flows have been studied as a means of stabilising shales exposed to water-based muds. The prime factor that governs the magnitude of chemical osmotic flow, i.e. the shale-fluid membrane efficiency, was investigated in detail. Its dependence on shale parameters, fluid parameters and external conditions was quantified. Membrane efficiency was found to increase with an increase in (hydrated) solute-to-pore-size ratio, with an increase in the shale`s high-surface area clay content and with a decrease shale permeability when increasing effective confining stress. Moreover, new drilling fluid chemistries for improving the efficiencies of low- and non-selective shale-fluid systems were identified. Induced osmotic flow with optimised shale-fluid membrane efficiencies in water-based environments is presented as a new strategy for improving wellbore stability in shales.