On the hydrodynamics of off-bottom plug placement: A 2D model problem

Abstract

Economic and environmental costs have renewed public and industrial interest in safe and long-term well abandonment. Balanced plug is a commonly used plug placement technique that relies on the injection of cement slurry in a well otherwise filled with lower density liquids. In the absence of a mechanical bridge, buoyancy-driven displacement flows may lead to the contamination of the plug or the failure of the process. To develop a mechanistic perspective of the processes that may prevent the buoyancy-driven failure of the placement process, we investigate the injection of a viscoplastic fluid, through a concentric injector, into a vertical channel filled with a lower density Newtonian fluid. The fluids are considered to be miscible and the effect of diffusion on flow development is characterized through exploring a wide range of Peclet numbers, 28≤Pe<∞. We identify two representative flow regimes dominated by either diffusive or advective transport. In the diffusive regime, once the injection starts, a layer of mixed fluids develops below the injector that slowly expands by diffusion. In the advective regime, a finger of the injected fluid invades the channel at the onset of injection. This viscoplastic finger breaks up subsequently, leading to the formation of a layer of mixed fluids below the injector. We provide a mechanistic description of the observed dynamics and show that the formation of a density-stratified layer of mixed fluids is essential for the accumulation of the injected fluid.