Effects of oil-based drilling fluid on cement slurry performance and its mechanisms

Oil-based drilling fluid has been widely applied to wells with special geological or technological requirements, but contact contamination occurs while drilling fluid and cement slurry are mixed due to their incompatibility. However, researches on the effect of oil-based drilling fluid on the performances of cement slurry and its affecting mechanism are relatively lagged. Therefore, tests were conducted on the effect of oil-based drilling fluids on cement slurry performances such as mobility, thickening time, compressive strength, bonding strength, porosity and permeability through the change of blending ratio. In addition, preliminary investigation was performed on the influencing mechanism of oil-based drilling fluid blending on cement slurry performances by means of ultrasonic compressive strength, XRD and TGA analysis. It is shown that oil-based drilling fluid blending does not significantly change the thickening time of cement slurry, but results in the formation of some flocculation structures which will reduce mobility and replacement efficiency of cement slurry. The isolating capacity of the mixed cement sheath drops greatly due to the decrease of compressive strength and bonding strength and the increase of porosity and the permeability. To sum up, the influencing mechanism of oil-based drilling fluid blending on cement slurry performances is as follows. An emulsion structure is formed after oil-based drilling fluid and cement slurry are mixed. Then the mobility of cement slurry drops because its free water is constrained by the emulsion structure. Strength reduction and porosity and permeability increase of set cement is mainly caused by lubrication effect of oil phase and by pores which are formed in set cement by unconsolidated oil-based drilling fluids. ©, 2015, Natural Gas Industry Journal Agency. All right reserved.