Abstract

In this thesis, we aim to unravel hydrodynamic challenges of common plug placement processes during the abandonment of oil and gas wells. Placing plugs in wells often starts with injecting cement slurry in wellbores filled with lower-density liquids (e.g. water). We develop a 2D model to investigate the flow dynamics during this process.
We first consider the ideal case where the injector and the wellbore are concentric. We illustrate that the injected fluid develops a finger below the injector. The finger breaks after an instability develops at the interface, leading to enhanced mixing below the injector. A mixed layer thus forms and prevents the downward displacement of the injected fluid and the placement failure.
The injection configuration is often far from ideal in field operations. We investigate the effect of the injector size and position in the wellbore. We illustrate, through different mechanisms, that the downward flow of the injected fluid is disrupted for a wide range of injector sizes and positions. However, the water content of the accumulated fluid varies notably with both geometric factors.
Exploring the range of density differences relevant to the placement operations, we characterize the changes in the flow dynamics and identify two displacement flow regimes where the front velocity either remains constant or accelerates after a quasi-steady velocity. Regardless, the displacement flow is unstable and is followed by the accumulation of the injected fluid.
To investigate the effect of the domain geometry on the predicted flow features, we conduct a brief comparison of the flow dynamics in 2D and 3D domains with and without the assumption of flow symmetry. We show that the 2D model is more effective in predicting the flow features compared to models which presume flow symmetry, when the flow is not known to be axisymmetric priori.
Placing the plugs often ends with pulling the injector out of the wellbore. Using a similar 2D model, we illustrate that the density unstable interface of the plug is affected by the pull-out only when the interface is very close to the injector. Pulling at lower velocities seem to facilitate the mixing at the interface.