Abstract
A film of drilling fluid used prior to cementing operation often remain due to wettability properties of the base mud. The remaining drilling mud might therefore contaminate the cement during its placement and be detrimental to zonal isolation of the well and its integrity. The goal of this experimental study is to provide results that improve understanding of dominating factor during cement contamination and bad placement. We conduct our experiments on a scaled-down cement plug set up, made of a 2” diameter steel casing and two main scenarios of the plug leakage test is assessed: (1) contaminated cement that had different concentration of oil based drilling mud mixed in it and pour into the plug, (2) neat cement pour into a casing with film of oil-based mud on the wall. The first tests are performed with different fractions of OBDM dispersed in a neat Portland G cement. The mechanical properties of contaminated cements are monitored under relevant downhole conditions, high pressure and high temperature from the slurry state to the hardened state. Decrease in the cement strength and increase to the viscosity were observed, indicating a deterioration of cement with the content of oil-based mud. Substantial increase of leakage in contaminated plug is also recorded after contamination. We then study a second configuration, where films of oil-based mud are put on the casing wall prior to cement placement to the cement setup, in a case of neat Portland G cement and expansive cement.