Pore-Scale Analysis of the Permeability Damage and Recovery during Cyclic Freshwater and Brine Injection in Porous Media Containing Non-Swelling Clays

Abstract

Permeability damage in subsurface porous media caused by clay mobilization is encounteredin many engineering applications, such as geothermal energy, water disposal, oil recovery,and underground CO2 storage. During the freshwater injection into rocks containing brine, thesudden decrease in salinity causes native clay fines to detach and clog pore throats, leading to asignificant decline in permeability. The clay fines detach due to weakened net-attractive forcesbinding them to each other and the grain. Past experiments link this permeability damage on theimmediate history of the salinity and the direction of flow. To better understand this phenomenon, weconducted pore-scale simulations of cyclic injection of freshwater and brine into sandstone containingKaolinite clay. Our simulations establish a link between the clay-fine trajectory and the permeabilitytrend observed by Khilar and Fogler (1983). For a uniform clay size of 3 microns, we observe apermeability decline by two orders of magnitude during freshwater injection with respect to brineinjection. Increasing salinity and simultaneously reversing flow direction restores the permeability.The permeability restoration upon reversing the brine flow direction is attributed to the unblockingof pore throats in the reverse direction by the movement of the clay particles along the grain surfacesby the hydrodynamic force and the strong net-attractive force under high salinity.