Paul Carman

Engineering the Mud-Spacer-Cement Rheological Hierarchy Improves Wellbore Integrity

Maintaining the density hierarchy for wellbore fluids has long been an accepted engineering practice whereas the rheological hierarchy for mud, spacer and cement is sometimes not achieved due to tedious testing or limitations in the field. Establishing appropriate rheological and friction pressure hierarchies prevent fluid (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 microemulsion spacer formulation that improves well integrity. The 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 elevated bottomhole circulating temperatures. In addition, compatibility 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 maximizing displacement efficiency, wellbore cleanup, long-term effective zonal isolation and sustainable hydrocarbon production. It is not always possible to accomplish turbulent flow during cementing. 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. The 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.