In Canada, the Alberta Energy Regulator’s (AER) liability report, issued in 2018, predicted that the number of inactive wells in the province will double by 2030. Despite the increase in the number of inactive wells, there is a need to close them properly to avoid hazards escape. Various aspects of well plug technologies in the Canadian abandoning industry are empirical. Many plugs are formed by injecting cement slurry into wells that are otherwise filled with fresh water for the slurry to build up on top of a water layer at a desired location. However, cement is heavier than water. Thus, successful plug placement following this methodology is questionable from the hydrodynamics perspective.

The present study aims to identify features of successful processes for placement of off-bottom plugs. To that end, we investigate mixing of fluids of different densities as the denser fluid is injected into the lighter fluid. Cement slurry is modeled as a viscoplastic fluid. The fluid motion is governed by hydrodynamic models, and the two fluids (i.e. cement and water) are considered to be miscible and are allowed to mix. Systematic numerical simulations aim to reveal how the characteristics of cement and the well configuration affect the placement process.

We show that successful plug placement depends on the formation of a mixed layer, of the wellbore fluid and cement slurry, below the injection site. We identify and provide representative cases of the processes promoting the formation of the mixed layer: high diffusion and growing instabilities.

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