Maintaining the density hierarchy for wellbore fluids has been a routine while achieving a proper rheological hierarchy for mud, spacer and cement could have been compromised due to tedious testing and sometimes limitations in the field. Establishing appropriate rheological and friction pressure hierarchy prevent fluids (mud-spacer-cement slurry) intermixing especially in deviated and horizontal wells. The objective of this paper is to present a spacer rheological properties model along with a new micro-emulsion spacer formulation which improves well integrity. This water-based spacer system, with densities ranging from 8.5 to 16 ppg, was modeled to temperatures up to 325°F and provided proper suspension properties, confirming stability at bottomhole circulating elevated temperatures. In addition, ccompatibility of this spacer package with various synthetic based muds, oil based muds and cement slurries, designed for Gulf of Mexico, the US land, North Sea and the Middle East, plays a significant role in achieving great displacement efficiency, wellbore clean up, long term effective zonal isolation and sustainable hydrocarbon production.

It is not always possible to accomplish the turbulent flow. Therefore, a rheological model was developed to accomplish the ideal viscosity hierarchy by optimizing the spacer formulation design. Optimum rheological hierarchy occurs where the viscosity profile of a spacer system is higher than the viscosity profile of drilling fluid and lower than the cement slurry. Model's predictions have been validated by one atmospheric and two industry-known HPHT rheometers. The model predictions show that the rheological profiles of the spacer fluid, for all the main standard shear rates, are between the mud and cement profiles. Data obtained from field case histories show the improvements and added values such as ideal fluid compatibility, better displacement efficiency, friction pressure hierarchy and effective zonal isolation,

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