Evaluating the Mechanical and Rheological Behavior of Mud-contaminated Well Cements

Abstract

Oil and gas well cementing is the process of filling the gap between wellbore and casing with cement to support casing and prevent communication between formations behind casing. A good and consistent cement slurry is critical for cement job success and, consequently, very important for well integrity and performance throughout the life of the well.
When cement slurry is pumped down the casing in a cementing job, it comes in a direct contact with drilling fluids used to drill the uncased section. These fluids, if mixed with cement slurries, can act as contaminants causing an alteration in the properties of cement. It is, therefore, critical to evaluate the extent of cement properties alteration due to contamination by drilling and completion fluids.
In this work, an experimental study was carried out to investigate the effect of contamination by drilling fluids; water based, oil based (50% Diesel, 50% water) and oil based (70% diesel, 30% water) on cement physical, mechanical and rheological properties.
Cement slurries were prepared according to the API procedures and were mixed with different concentration of contaminants and cured at ambient temperature for 1, 2, 3 and 21 days. Physical, rheological, and mechanical properties were measured using API recommended practices and apparatus.
It is observed and as expected that regardless of mud base fluid, as contaminant percentage increases, density of cement slurry decreases due to dilution of cement by water in contaminant. For oil-based invert emulations (50/50 and 70/30), as contamination percentage increased plastic viscosity increased, but yield stress decreased. It was also found that clean and less contaminated cement samples develop higher compressive strength in shorter times than mud contaminated samples.