Optimal slug size for enhanced recovery by low-salinity waterflooding due to fines migration

The objective of this study is modelling of low-salinity water (LSW) slug injection followed by continuous highsalinity chase drive. We discuss the case of fines-assisted low-salinity waterflood, where low salt concentration of the injected water results in mobilisation and migration of the in-situ reservoir fines and the consequent formation damage. This diverges the injected water flux into low-permeability zones and enhances sweep efficiency. We extend the mathematical model for two-phase flow with varying salinity and fines migration for the case of over and undersaturated fines, where the detaching drag torque, exerting the fine particle, exceeds or is below the attaching electrostatic torque, respectively. The laboratory study has been performed to determine how movable clays and the induced permeability damage is distributed between the layers. We distinguish between two competitive physics effects of fines-assisted low-salinity: (i) plugging of the high-permeability layer that results in the flux diversion into unswept zones yielding enhanced oil recovery, (ii) induced formation damage behind the waterfront in low-permeable zones that reduces the recovery. These opposite effects indicate the existence of an optimal low-salinity slug size that results in maximum recovery. The modelling shows that for all simulated cases there does exist an optimal low-salinity slug size, which is the main result of the work. It was found that the volume of optimal low-salinity slug has an order of magnitude of the pore volume of the high-permeability layer.