On shale swelling: the unrecognized role of diffusion osmosis

Currently, drilling operators focus on chemical osmosis as a mechanism to extract water out of shale in order to reduce its pore pressure and increase its strength, however, in doing so, the flux of hydrated ions into shale by means of diffusion osmosis and ionic diffusion is increased. Such practice may lead to an increase in shale's pore pressure and strength reduction which will definitely negate the beneficial impact of chemical osmosis in combatting wellbore stability problems. For the first time, this work investigated the separate impact of diffusion osmosis on shale's swelling and subsequent wellbore stability issues. The experimental set-up enabled us to isolate chemical osmosis forces which made diffusion osmosis the only force governing water exchange. Making the water activity of shale equals to that of the aqueous solution disabled chemical osmosis forces and activated ionic diffusion and diffusion osmosis owing to the imposed ionic concentration during the interaction. It was found that diffusion osmosis accompanies ionic diffusion since ions travel with their respective water clouds, also known as associated water. The transport of associated water has caused shale's swelling in the absence of a water activity gradient (chemical osmosis). This was confirmed through gravimetric water and ion measurements conducted at the end of the linear swelling tests. Results showed that the amount of water exchanged by diffusion osmosis was immense, thus one should not ignore it when designing water-based muds for drilling highly sensitive clays. Results also showed that the amount of water exchanged correlated well with the hydrated diameter of the invading ion. Moreover, results confirmed that the swelling magnitude may be related to the mineralogical composition and cation exchange capacity of shale.