Experimental quantification of the effect of oil based drilling fluid contamination on properties of wellbore cement

Portland cement hydration is a chemically sensitive process impacted by temperature, amount of water and the presence of any potential contaminants. Drilling fluids contaminated cement hydration products will therefore have altered microstructures, impacting porosity, permeability, and the overall mechanical properties of the cement sheath. The objective of this study is to quantify the impact of drilling fluid contamination on the ability of cement to provide zonal isolation. Class-H Portland cement-based slurries were prepared according to the API 10B RP. The cement slurry was contaminated with oil based drilling fluid, at 5%, 10%, and 30% by volume of cement casted into core and cured at 95 degrees C and 95% RH for a minimum of 30 days. The microstructural characterization, porosity, permeability, and unconfined compressive strength (UCS) were obtained at ambient conditions, unless specified otherwise. The results provide a striking demonstration of the degradation of wellbore cement performance imposed by oil based drilling fluid contamination. The overall degradation of cement cores is shown to be minimal for contamination up to 5%; to be considerable for 10% contamination, and to be detrimental for 30% contamination and above. Microstructural observations show that the drilling fluid forms relatively non-reactive, soft inclusions within hydrated cement matrix. The interfaces between cement and the fluid inclusions potentially contribute to porosity increase due to the presence of microfractures that emerge at the boundaries. The permeability increase with 10% contamination is roughly a factor of 6 and with 30% contamination is a factor over 130. The permeability is shown to decrease substantially with increasing confining stress for neat cement, but this stress-dependent permeability reduction is much less pronounced for contaminated cement. The samples contaminated by 30% had UCS reduction by a factor 3-4 at ambient temperature.