Rheology Modification for Safe Cementing of Low-ECD Zones

Abstract

Long-term zonal isolation requires effective mud displacement. One best practice for achieving this is to use the highest pumping rates allowable during cementing operations. However, in zones with a narrow pressure window, fracture pressure sometimes does not support higher flow rates. This is because high flow rates result in high friction pressures, which can exceed the fracture pressure of the formation. This is an issue particularly in long horizontal wells. In such cases, fluid rheology is important for achieving better displacement efficiency without inducing fractures. The current work discusses a procedure in which the rheology of cement slurry can be tuned such that good displacement efficiency is attained at sufficiently low flow rates. The tuning should be such that the shear stress is almost invariant for varying shear rates. This reduces the friction pressures considerably during placement. A hydraulics model was run on an example well configuration with a narrow margin situation. Pressure response and displacement characteristics of two types of slurries were analyzed. These include conventional and modified slurry, the latter of which was tuned for better mud displacement at low flow rates. The modified slurry showed an increase in yield point with a shear stress profile that was almost invariant for the range of shear rates analyzed. A decrease in equivalent circulating density (ECD) occurred as a result of rheology modification, and this resulted in avoiding ECDs exceeding the fracture gradient at critical locations. Displacement efficiency increased by 10% when using the modified slurry at the same pump rates as those used for the conventional slurry. On the other hand, when targeting the same displacement efficiency as the conventional slurry, the required pump rates for the modified slurry were lower. This study indicates that better displacement can be attained through rheology modification, even by using relatively low flow rates, thereby maintaining low ECDs to help ensure effective cementing operations.

Fundamentals of Plug Placement

“…But when the fluid is under constant shear rate, the viscosity is decreased over time until it reaches an equilibrium condition. A thixotropic slurry can create a plug flow regime during placement and improve the mud displacement efficiency [41]. However, thixotropic behavior may challenge the plug stability when cement slurry is placed on a pill and while pulling the work string out of the plug.…”