Placing Off-Bottom Cement Plugs: The Influence of Contamination From Sub-Optimal Placement in Vertical Wells

Abstract In Canada, a common approach in well abandonment is the placement of cement plugs using the balanced plug method. This involves the injection of cement slurry through a concentric injector into a wellbore typically filled with wellbore fluid. We use numerical simulations to examine the effect of contamination of the cement slurry on the dynamics of the plug placement in vertical wells. The slurry is characterized across varying contamination levels using API RP 10B-2 standards, and its rheology is modeled using three models: (i) Bn Linear, where rheological properties follow a linear variation with contamination based on the Bingham model; (ii) HB Linear, which uses the Herschel-Bulkley model with linear rheological variations; and (iii) HB Exponential, which employs the Herschel-Bulkley model with exponential rheological variations. Results show that slurry rheology varies exponentially with contamination level. Rapid decrease in the yield stress and consistency of the contaminated slurry expedites the onset of shear instabilities and changes the mixing layer characteristics below injector. HB exponential model predicts less particle sedimentation compared to other models, which improves the plug placement. Overall, we highlight the influence of rheology in modelling cement plug placement processes, a key contributor to the success of well abandonment operations.