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 induced fracture 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 non-Newtonian fluids. The impact of Newtonian fluids on the stability of nonaqueous fluid (NAF) systems and weighted spacers are often disregarded during cement job design and testing.
During cement placement, especially in extended reach 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 the mud interface increases significantly. Though preferred for cementing, Newtonian fluids have a detrimental effect on the stability of the fluid interface. Whenever the yield point of the interface 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 by formation fluid influx and a flow pathway created while cement is setting. This channel can become a flow path for hydrocarbons to the surface, leading to sustained casing pressure or a hydrocarbon seep to surface or seabed.
In the Caspian region, an engineering approach was implemented to manage cementing equivalent circulating densities (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 an evaluation of design considerations for cementing ERD wells.