Plug and abandonment (P&A) of oil and gas wells is an essential process to prevent the oil and gas reservoir fluids migration over time and possibly contaminating other formations and also fresh water resources. In order to plug and abandon a well, a high quality cement plug placement is required. One of the most common methods of cement plug placement is the dump bailing method. In this method, a fixed volume of cement is dumped using a bailer on a mechanical plug in the wellbore. The cement slurry occupies the wellbore and also the annular region outside the dump bailer. In the processes of cement slurry placement, an extensive range of Newtonian or non-Newtonian fluids is used to remove the in-situ fluid (drilling fluid or water) in the wellbore. Based on the large number of parameters such as the density and viscosity differences between the fluids, the geometry type (pipe, annulus, etc.), the operation conditions (velocity, geometry inclination, dumping height), various kinds of placement and mixing flows can occur, and different flow regimes (e.g. inertial, viscous) can develop. In this paper, we experimentally study the placement of a heavy fluid to replace an in-situ light fluid in an inclined closed-end pipe (representative of the dump bailing method). The two fluids are Newtonian and miscible, and they have the same viscosity. We investigate the effects of some of the flow parameters such as the dumping height, the pipe inclination, and the inflow velocity of the heavy fluid on the degree of mixing and the placement quality and efficiency. Our results show that the the most efficient displacement happens with the shortest dumping height and at lower inclination from vertical. Also, a high inflow velocity displaces the light fluid promptly with more mixing in comparison with a low inflow rate. The results can help us to develop strategies for improving the dump bailing method in the P&A of the oil and gas wells.

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