Zonal Isolation in narrow pressure windows has traditionally been challenging. This is due to cement slurry losses or potential fluid flow after cement placement. Accurate pressure data are essential for well control and successful primary cementing. Higher fluid densities and pumping rates can lead to fractured formations and lost circulation. Often, to mitigate the potential for dynamic losses during cement placement, low-density Newtonian fluids (preflush) are pumped ahead of weighted spacers. It is recognized in the industry that Newtonian fluids achieve a turbulent flow regime much easier than do non-Newtonian fluids. The impact of Newtonian fluids on the stability of nonaqueous fluid (NAF) systems and weighted spacers is often disregarded during cement job design.

During cement placement, especially in extended reach drilling (ERD) wells, depending on the volume of preflush needed to maintain adequate wellbore security, a long column of preflush/mud interface may be created. With the Newtonian phase in turbulent flow, the erodability of mud interface increases significantly. Though preferred for cementing, Newtonian fluids have a detrimental effect on the stability of the fluids mixture. Wheneverthe yield point of mixture falls below the critical value for solids suspension or the slip velocity, weighting material sags out of the mud. This results in hydrostatic imbalance in the fluid column. When the overall hydrostatic pressure falls below the formation pressure at any point in the wellbore, the cement slurry will be invaded and a flow pathway created while cement is setting. This channel can become a flow path for hydrocarbons to the surface.

In the Caspian region, an engineering approach was implemented to manage cementing equivalent circulating density (ECD) when designing cement jobs using lighter and Newtonian fluids ahead of weighted spacers. It has led to successful zonal isolation for recent wells and evaluation of the same shall be discussed.