We simulate fluid invasion into a gelled cement slurry using a scaled laboratory experiment. This process is relevant to the construction of oil and gas wells, in which a tall column of cement suspension must resist fluid invasion through a combination of static pressure, yield stress, and interfacial tension. The sufficiently over-pressured fluids may enter from the surrounding rocks, leading to failure of the well integrity. Here, we model the cement suspension using a Carbopol solution (yield stress fluid) and apply different over-pressured invading fluids through a centrally positioned hole at the bottom of the circular column. We study water, glycerin, silicon oil, and air as invading fluids, in order to delineate the effects of yield stress, interfacial tension, and column height on fluid invasion. We find that the invasion is easiest for miscible fluids that penetrate locally at significantly lower invasion pressures than immiscible fluids. Viscosity affects this process by retarding the initial diffusive mixing of the fluids, which tends to weaken the gel locally. More viscous invading fluids require larger invasion pressures and result in larger invasion domes. The silicon oil penetrated in the form of a slowly expanding dome, resisted at the walls of the column – effectively by a Poiseuille flow above it in the Carbopol. Invasion pressures were significantly larger than those for the glycerin solutions. The largest invasion pressures were, however, found for air, which is influenced approximately equally by interfacial tension and yield stress.

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