Cement slurry contamination with oil-based drilling fluids

Primary cementing constitutes the displacement of a cement slurry in an annular oil well. In the event of an unwanted mixture of drilling fluid and cement slurry during this operation, cement properties may be compromised. The aim of this study was to investigate the contamination in cement slurries caused by oil base drilling fluids, focusing on the major changes of its technological properties. To assess these changes, compatibility testing was used according to the American Petroleum Institute Recommended Practice for Testing Well Cements (API RP 10B). The changes in rheology, compressive strength, slurry sedimentation testing and thickening time of the obtained mixture were determined, and the formed hydration compounds were evaluated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influence of this wetting agent in the drilling fluid on the zeta potential of the cement particles was also determined. The results confirmed an increase in the plastic viscosity and yield point of the cement slurry, which could impair the continuity of the well cementing. These changes in the behavior of cement slurry may cause serious problems during the operational sequence of cementation, compromising the life of the oil well, which may cause accidents on drilling rigs, leading to loss of life and damage to the environment.