Summary
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.
Introduction
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.
© 2008. Society of Petroleum Engineers
View full textPDF
(
3,452 KB
)
History
- Original manuscript received:
28 June 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
9 November 2007
- Manuscript approved:
10 November 2007
- Version of record:
20 June 2008
View Cart
