Measuring and Predicting Dynamic Sag
- Robert Jerome Murphy (Halliburton) | Dale Eugene Jamison (Halliburton) | Terry Hemphill (Halliburton Energy Services Group) | Stephen A. Bell (International Specialty Products) | Carl Eduard Albrecht (Halliburton)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2008
- Document Type
- Journal Paper
- 142 - 149
- 2008. Society of Petroleum Engineers
- 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.3.4 Scale, 2 Well Completion, 1.6 Drilling Operations, 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 1.11 Drilling Fluids and Materials
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Weighting-material sag is a reoccurring problem with many oil-based drilling fluids. Attempts to correlate sag tendencies to various rheological properties commonly used to benchmark drilling fluids have had limited success in prevention and anticipation of sag problems in the field. This paper presents a new testing apparatus for dynamic and static settling-rate (sag) measurements, which has proved to provide a better understanding of the sag phenomena and a better means to characterize fluid performance. This apparatus greatly expands the precision of sag measurements over previous techniques and allows testing conditions similar to those experienced downhole. Good correlation has been found between settling-rate measurements and performance of drilling fluids in the field.
Sag is a variation in density of a drilling fluid caused by settling of suspended particles or weighting material in a wellbore. Laboratory and field experience suggests that sag is often worse in dynamic situations caused by pumping, pipe rotation, and tripping. However, sag can occur in either static or dynamic conditions. In the presented apparatus, measurements are performed at prescribed shear rates, elevated temperatures to 177°C (350°F), and pressures to 690 bar (10,000 psi). Additionally, the apparatus requires only a 50-cm3 sample for complete analysis. The settling-rate measurements obtained are useful in planning and as a diagnostic tool for sag performance in active drilling-fluid systems.
Preliminary Laboratory Studies
A typical way to control the shear of a non-Newtonian drilling fluid is to use a concentric-cylinder configuration with the sample fluid occupying the annulus. If either the outer or inner cylinder is rotated relative to the other, the annular fluid is subjected to an approximately uniform shear field that can be modeled easily. The configuration is comparable to the common oilfield viscometer and is commonly referred to as "Searle geometry" if the inner cylinder rotates relative to a stationary outer cylinder or as "Couette geometry" if the outer cylinder rotates relative to a stationary inner cylinder.
Cylinder rotation combined with axial flow of the annular fluid would more closely resemble the borehole configuration, but would greatly complicate the computational modeling and control. Flow loops usually expose the sample to a range of shear rates in contrast to the constant shear rates possible in the simpler system. A flow loop also would require a high-pressure pumping system, as well as added unnecessary bulk, sample volume, and system complexity.
A preliminary study apparatus was assembled (Fig. 1), which consisted of a clear-plastic outer cylinder approximately 2 m (6 ft) long and 7.62 cm (3 in.) internal diameter (ID), with sealing caps closing the ends. Bushings in the caps supported a rotatable concentric inner stainless-steel tube of 3.81-cm (1.5-in.) outside diameter. This gave a diameter ratio of 0.50. In later studies, another clear tube was centered in the original outer tube with an internal diameter of 5.08 cm (2 in.), giving a diameter ratio of 0.75. The narrower annular gap more closely approximates ideal Searle flow.
The entire apparatus was pivoted on a bench-mounted knife edge, near the center, and tilted at 45° from vertical. A pivoted strut from the top end of the tube rested on a electronic laboratory digital scale, setting the angle of tilt and allowing the measurement of the imbalance force.
A gear motor mounted on the upper end of the outer tube was arranged to belt drive the inner cylinder. The motor speed was adjustable by an electronic drive. A temperature-controlled bath was connected to the inner rotating tube in a way that allowed the tube to rotate while fluid from the temperature controlled bath circulated through it.
When the annulus of the tubes was filled with a sample of drilling fluid, changes in the center of gravity could be tracked by monitoring the scale readings. Sample taps at intervals along the bottom side of the sloped outer tube allowed measurements of the density of the fluid at that those points.
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