Mixing Behavior of Viscous Fluids Following Placement in Plug and Abandonment of Oil and Gas Wells

Abstract The final phase in the lifecycle of a hydrocarbon well is plug and abandonment (P&A), designed to prevent reservoir fluids from leaking or migrating to upper layers and the atmosphere. P&A operations involve placing cement plugs in the wellbore at specific target depths. Since P&A costs make up a substantial portion of total decommissioning expenses, careful planning and accurate execution are essential: a failed P&A operation can lead to costly, complex repairs and pose considerable environmental and financial risks. To minimize failure risks, it is crucial that the cement plug is placed in a stable position within the well. Various methods are used to position the cement slurry, with the “off-bottom” technique being common in Western Canada. This involves injecting cement slurry into the wellbore, where it rests atop another fluid within the well. The cement slurry is a dense high-viscosity fluid, while the wellbore fluid supporting the cement slurry is typically freshwater for P&A. Off-bottom plugs are also set during drilling operations, where the lower fluid can be the drilling fluid or a specially formulated viscous pill. Equally, a range of mechanical support devices can be used to prevent motion of the cement slurry. Inspired by the off-bottom technique, our study experimentally investigates the placement of a high-viscosity fluid on top of freshwater in an inclined pipe, representing the inside of the casing. The fluids involved are miscible with a constant density difference. Our study ignores the effects of mixing during pumping of the slurry into the well. Instead, we analyze the effects of the high-density slurry on the stability of the plug, that is placed on top of the in-place fluid. In particular, we focus on the influence of inclination angle on fluid placement dynamics. The results show that tilting the pipe reduces fluid mixing and enhances segregation, with the low-viscous fluid accumulating on the upper side of the pipe. Increasing the inclination angle enhances fluid stratification and separation while reducing exchange flow effectiveness. Additionally, the front velocity of the low-viscous fluid is significantly influenced by the inclination angle, exhibiting larger variations due to the combined effects of gravity and inertia. The findings from this study can ultimately be applied to improve cement plug placement using the off-bottom method during P&A operations in oil and gas wells.