CO2 flow blockage in the near-wellbore zone: Fresh water stimulation against the salting-out effect

Abstract

We consider the injection of CO${}_2$ into a saline aquifer through a vertical well. We assume that the well is stimulated by the fresh water treatment to reduce the salting-out effect. We aim at an in-depth investigation of the situations when such a treatment performed prior to the injection of CO${}_2$ can substantially improve the injectivity. By using a radial reservoir model, we simulate the increase in the skin factor caused by the salt precipitation and deposition in the near-wellbore zone. By analyzing the simulation results, we demonstrate that the capillary-driven backflow can magnify the skin factor by orders of magnitude as compared to the case with zero capillary pressure. It can eventually lead to a complete clogging of the porous medium. We introduce the capillary number, which characterizes the influence of the capillary pressure and the intensity of halite deposition. We demonstrate that there exists the critical capillary number separating two qualitatively different scenarios of CO${}_2$ injection presented here for the first time. At the supercritical numbers, the injection cannot be blocked by the salt deposition, although the skin factor monotonically increases with the volume of injected CO${}_2$. The flow blockage cannot occur in such cases. At the subcritical numbers, the conditions of the complete clogging and zero well injectivity are always reached at a finite time. The fresh water stimulation can only postpone the flow blockage in time, but it cannot exclude such a negative manifestation. The derived estimates for the critical capillary number can be useful for predicting the evolution of the CO${}_2$ well injectivity and mitigating the development of the situations with the complete flow blockage.