Abstract
In the last decade drilling fluids have undergone a continuous evolution towards the challenges of new drilling technologies. This process forced cementing companies to improve the mud-cake cleaning systems efficiency in order to achieve successful cementing jobs.
A basic procedure to evaluate drilling fluids compatibility with spacers, preflushes and cement slurries is provided in API recommended practice 10 B. However, no quantitative methods to evaluate preflushes mud removal efficiency are proposed. On the other hand, several laboratory methods have been developed to analyze this issue, although most of them are based on experiments performed at unrealistic down-hole conditions.
This paper presents a new laboratory method to dynamically evaluate preflushes performance. The drilling fluid is circulated through a cell at a similar hydrodynamic flow regime expected in the well annulus. The mud cake is generated on four different filtering materials, applying differential pressure at the bottom hole circulating temperature.
The mud and filter-cake removal is evaluated by two different techniques, static gravimetric and dynamic impedance. This last technique provides a continuous indication of the cake thickness variation during the test. Therefore, the preflushes chemical, viscous and abrasive action can be evaluated while the test is conducted.
Three case studies, a low solids KCl, a PHPA and a plain water base drilling fluid, are reported were the performance of different preflushes systems are evaluated. The results of these studies were used to modify the preflushes pumping sequence and to adjust their volumes based on the information obtained from the impedance trends. The cleaning efficiency of viscous spacers used in-between chemical washes and the effect of mud filter cake leek off are discussed.
