Experimental study of dynamic barite sag in oil-based drilling fluids using a modified rotational viscometer and a flow loop

During drilling operations, control of the sub-surface pressure is of utmost importance. High density minerals, such as barite and hematite, are used to increase the density of drilling fluids and thereby control these pressures. However, contributing factors, such as the gravitational force, cause the weighting material particles to settle out of the suspension. This is designated as “sag” within the drilling industry and can lead to a variety of major drilling problems, including lost circulation, well control difficulties, poor cement jobs, and stuck pipes. The study of this phenomenon, including ways to mitigate its effects, has long been of interest.In this paper several methods for evaluating dynamic barite sag in oil-based drilling fluids are examined in a flow loop with the use of a rotational viscometer modified by the addition of a sag shoe (MRV). Tests using the MRV in the range of 0–100 RPM were conducted, and the effects of rotation speed on sag were correlated with flow loop tests performed by varying the inner pipe rotation speed. The combined effects of eccentricity and pipe rotation on dynamic barite sag in oil-based drilling fluids are also described in this paper. Flow loop test results indicate that pipe rotation has a greater impact on reducing sag when the pipe is eccentric rather than concentric. Additionally, results in the MRV indicate a strong correlation between the test RPM and the degree of measured sag.

Research Highlights
► Based on the two fluids tested, the settling of the barite particles will not occur if the yield stress is higher than 12 lbf/100 ft2.
► The MRV does not correlate with flow loop sag tests that incorporate pipe rotation, pipe eccentricity, and annular velocity.
► An eccentric drill pipe induces more sag than a concentric one. However, pipe rotation has a greater impact on the reduction of sag in the eccentric case than in the concentric one.
► The combination of pipe rotation and fluid circulation significantly reduces sag.