Modelling the Effect of Contamination on the Rheological Properties of Cement Slurry Using Oil-Based Mud as Contaminant, To Improve Cementing Job Design and Execution in the Oil & Gas Industry

Cementing job failure often occurs due to cement slurry contamination with drilling mud during primary cementing and remedial cementing jobs in the oil & gas industry. The enormous consequences of cement slurry contamination with drilling mud include drastic reduction in the thickening time of cement slurry and increased rheological properties of the cement slurry due to the much more viscous resultant cement slurry/mud mixture often leading to incomplete cement job, poor mud removal, poor cement bond, gas channeling, gas kick, blow out, downtime, revenue loss and remedial cementing. The rheological properties of cement slurry and drilling mud are strongly dependent on temperature. There are also emergency situations in the oilfield where bottomhole temperature changes to a higher temperature requiring adjustments in the original cementing job design to include the new bottomhole temperature when Laboratory services are not immediately available. The aforesaid and the associated cost of remedial operations prompted this work to investigate the effect of temperature and contamination on the rheological properties of cement slurry using oil-based mud as contaminant to invent a process to better avoid cement slurry contamination and to improve cementing job designs and execution in the oil & gas industry. The methods applied include Laboratory testing and mathematical modeling.  Mathematical models were developed using Microsoft Excel to predict the rheological properties of contaminated cement slurries as a function of temperature and contamination with oil-based mud as the contaminant. Results showed that the Plastic viscosity and yield stress of the contaminated cement slurries increased with temperature and extent of contamination with oil-based mud but the yield stress also decreased with extent of contamination at high temperatures. The very high regression coefficient obtained with each of the mathematical models showed that the mathematical models can be confidently applied to predict the rheological properties (Plastic viscosity and yield stress) of the contaminated cement slurries as a function of temperature and extent of contamination with the oil-based mud.