Plug and abandonment (P&A) of oil and gas wells is receiving an increased attention. The P&A operation is performed by placing a barrier, such as a cement plug to avoid reservoir fluids migration toward aquifers. To fulfill these requirements, the desired cement plug should be placed in the wellbore with minimum mixing with the in-situ fluid. A rigless way for placing cement slurry in the wellbore is through the dump bailing method, in which a relatively small amount of cement slurry is injected on a mechanical barrier inside the well to replace the in-situ wellbore fluids (mostly fresh water). The dynamics of the fluid placement is governed by several parameters, such as the flow and geometry parameters, and the fluid properties.

In this study, we analyze the fluid mechanics of the dump bailing method, via experimentally investigating the effects of the viscosity ratio between the replacing and replaced fluids in the process. The viscosities of the fluids involved have significant effects on the mixing and placement flow quality. In our experiments, the fluid placement is carried out in a near-vertical closed-end pipe (i.e. representative of the well casing) to replace an in-situ light fluid. The two fluids are considered to be miscible, and they have a fixed density difference. Our results show that the most efficient placement happens with the injection of the higher viscous fluid. The outcomes of this study can be used for improving the cementing processes in the dump-bailing method of P&A operations.

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