1. Setup of dynamic settling test (DST) and parts of apparatus are demonstrated in Figure 1:
Figure 1: DST test apparatus, designed to fit into standard HPHT consistometer with adjustable
motor speed control.
2. The standard cement slurry cup paddle is designed to maintain the entire mass of the
slurry agitated and homogeneous even if the fluid has settling tendencies. The paddle that
is used to detect dynamic settling likewise keeps the fluid stirred throughout the entire
cup, but allows solids settling at the low rpm used.
3. Several of the blades used in the standard paddle have been removed, but it still includes
the two vertical bars (the vertical bars are used to clean the slurry off the walls of the
slurry cup).
4. A round, flat plate was added to collect the solids settling dynamically from the slurry.
5. While bringing the fluid to downhole conditions of temperature and pressure, stirring at
150 rpm, the solids stay in suspension due to the level of energy imparted to the fluid by
the redesigned paddle at that speed.
6. Once the rotation speed is reduced to 25 rpm, the paddle still imparts enough agitation to
keep the slurry in motion, but at that speed, with this paddle, the agitation is not enough
to prevent the solids from settling on the bottom plate under dynamic conditions, if the
slurry has tendencies to settle.
7. After the motor speed is reduced to 25 rpm, the DST is allowed to continue for at least
30 min. Although 30 min is the minimum time, longer times can be simulated depending
on well conditions. After the 30 min period, the motor is turned off and the test is
reduced to 190 degF at bottomhole pressure as soon as possible. Then the pressure is
slowly bled off at 1,000 psi/min, followed by removing the cup from the HPHT
consistometer and oil is blotted from the top of the cup/diaphragm side prior to opening
the retaining ring assembly.
8. Once settling starts to take place, this is noticeable in the consistency chart of the
consistometer. The solids at the bottom of the cup start to load and grab the paddle and
the recorded consistency starts to increase or to look “jagged”.
9. The final confirmation of the existence or nonexistence of settling is after the test, when
the slurry cup is opened and the fluid examined for settling. Likewise, the contour of the
cone is evaluated for gelation behavior.
10. Thickened and/or gelled fluids pour out of the cup even if their consistency has increased
substantially.
11. Slurries that settle or gel leave a clearly distinguishable cone-shaped deposit of packed
solids on the bottom plate. Also, the contour of the cone indicates whether the slurry
demonstrates gelling/underdispersed, stable, or settling behavior (Appendix B, Figure
B1).
Dynamic Settling Test (DST) Outline of Procedure
1. DST procedure for detecting dynamic settling requires the use of a variable speed cement
consistometer equipped with a special slurry cup paddle that allows detection of dynamic
settling.
2. The fluid is brought to downhole conditions of temperature (BHCT) and pressure as is
normally done, using a thickening time schedule, stirring at 150 rpm (normal rotation
speed dictated by API RP10B-2).
3. Once the cement slurry is stabilized at BHCT and BHP, the rotation speed of the
consistometer is reduced to 25 rpm and continued for at least 30 min.
4. Rotation at the low rpm is continued for a minimum of 30 min and can be continued for a
longer period of time, depending on well placement conditions.
5. After that, rotation is stopped and the cup removed from the consistometer and opened
without turning it over, being careful to blot any remaining oil from the top of the
cup/diaphragm assembly
6. At the end of the test, samples of the slurry should be taken from the top third, middle
third and bottom third of the cup using a tared 10-mL syringe, and the densities measured
as for the static settling test. The syringe collection orientation should be at the midpoint
between the center and outside of the cup. Note: In some cases, there is a hard packed
cone at the bottom of the plate that may prevent density measurement from the bottom
third of the cup. In that case, results should be collected from top and middle thirds with
close observation of the cone measurements. If a density measurement is not possible
from the bottom third, more than likely a redesign of the slurry will be necessary.
7. After the slurry density measurements are completed, the paddle/plate assembly is gently
lifted from the cup. Note: In case of hard packed cone, this may be difficult. In that
case, the bottom of the cup with the pivot bearing/lid can be unscrewed and the
paddle/plate assembly can be pushed through the pivot bearing/lid side of the cup.
8. Observations need to be made as to the condition of the slurry: settling or segregation,
gelation, packing on the bottom or paddle, etc.
9. After the paddle/plate is pulled from the cup, a spatula or straight edge needs to cut the
cone along the center axis using the outside paddle blades as a cutting guide to create a
true cross-section of the cone. Then, the height of the cone of solids at the bottom the
paddle can be measured and recorded for the center (position 1 in Figure 2), middle
(position 2 in Figure 2) and outside of the paddle (position 3 in Figure 2).
1
2
3
Figure 2: Image of flat plate and sedimentation measurement positions, where the center of
the plate is position 1, middle is position 2, and the outside of the plate is position 3.
10. A true nonsettling fluid will not form a cone across the bottom plate of the paddle (zero
cone height) for positions 1, 2, and 3 as indicated in Figure 2.
Acceptance Benchmarks
11. Not more than 5% difference in density from top to bottom of the fluid column inside the
cup should be observed.
12. The maximum cone height allowed is ½ in for any position (positions 1, 2, or 3).
13. A cone height difference greater than 50% between the center (position 1) and outside
(position 3) indicates gelation and NOT sedimentation. Therefore, the contour of the
cone is just as important as the center cone height. The equation for determination of %
degree of dispersion:
ℎ𝑒𝑖𝑔ℎ𝑡𝑐𝑜𝑛𝑒_𝑐𝑒𝑛𝑡𝑒𝑟 −ℎ𝑒𝑖𝑔ℎ𝑡𝑐𝑜𝑛𝑒_𝑜𝑢𝑡𝑠𝑖𝑑𝑒
ℎ𝑒𝑖𝑔ℎ𝑡𝑐𝑜𝑛𝑒_𝑐𝑒𝑛𝑡𝑒𝑟
× 100% = % 𝑑𝑒𝑔𝑟𝑒𝑒 𝑜𝑓 𝑑𝑖𝑠𝑝𝑒𝑟𝑠𝑖𝑜𝑛
(1)
Validation, Summary, and Interpretation
1. An API static sedimentation test and GO/NO GO test have been performed on several
slurries that also underwent DST using the procedure outlined in the preceding section.
A typical GO/NO GO result is shown in Appendix 1.
2. In several cases, the slurry only showed minor static settling tendencies in the API static
sedimentation test. Frequently, some segregation of the solids within the slurry was
noticed toward the bottom at the end of the static test, but no bed of solids within the
slurry was noticed toward the bottom of the cup. However, during the dynamic stability
3.
4.
5.
6.
7.
testing, significant density trend (>5% density difference from top to bottom and
substantial cone (>1/2 inch across the bottom of the plate) were frequently observed.
Both GO/NO GO and API static sedimentation testing run in conjunction with the DST
suggest that it is possible for a slurry to show very little static sedimentation, but
demonstrate serious dynamic stability/settling behavior. A cone might even indicate
gelation problems, as demonstrated in Appendix B and explained more fully in Part 6.
One possible explanation for dynamic sedimentation is that statically, the fluid is allowed
to develop static gel strength which contributes to solids suspension.
Under dynamic conditions, the static gel strength cannot be developed, and if the fluid
does not have enough viscosity at downhole conditions, solids may settle out.
Another interesting phenomena noticed during the DST test is gelation behavior observed
during measurement of the cone height for the center, middle, and outside parts of the
bottom plate. Using equation 1, it can be determined whether the slurry is
underdispersed.
If the slurry meets Acceptance Benchmarks outlined on page 3, the slurry is said to be
optimally dispersed with acceptable dynamic sedimentation.
1. Thickening Time plot
2. Time for low shear interval at 25 rpm
3. Tared 10-mL syringe fluid densities (wt in grams/10 mL volume) x 8.33
lbm/gal = density (lbm/gal) from top third, middle third, and bottom
third of slurry cup.
4. Cone height from center, ½ radius, and radius of slurry cone on top of
bottom plate.
5. % degree of dispersion
Appendix A
40
100
540
36
90
480
32
80
420
28
70
360
24
60
300
20
Consistency (Bc)
600
Pressure (kpsig)
Temperature (°F)
Customer: Chesapeake
Instrument: Consistometer4 866
Comments:
File No: L10-544 Talbert 9-14-16H-1 Prod. Pilot TT6-2
Test Start: 11/12/2010 2:49:31 PM
Test Stop: 11/13/2010 12:31:36 AM
50
240
16
40
180
12
30
120
8
20
60
4
10
0
0
0
0:00
Schlumberger
Longview Lab
2:00
4:00
6:00
Time (HH:MM)
Test File Name: L10-544 Talbert 9-14-16H-1 Prod. Pilot TT6-2.tst
Printed: 11/13/2010 8:48:52 AM
8:00
10:00
12:00
Successful Go-No
Go Test – No
Change in
Consistency (BC)
after 20 min
shutoff and
motor turn-on.
30 Bc @ 9:34:51
50 Bc 9:35:18
70 Bc @ 9:35:45
100 Bc @ 9:41:39
Figure A1: Typical result for successful GO/NO GO test. In high temperature environments, slurries
typically pass GO/NO GO testing, demonstrate results similar to the slurry above but can show
significant sedimentation behavior, which can range from moderate to severe sedimentation (see yellow
to red panels in Figure A2).
Dynamic Stability Criteria based on cone height
Cone height>>0.5in –
Failure and Re-design
with more D154 or look
at dispersant conc.
Cone height~0.5 in –
Discuss with Line
Management before
acceptance.
Cone height<0.5 in –
System passes and is
validated for field
placement.
TYPICAL DST RESULT WITH POLYMERS ABOVE 300 oF
Figure A2: Examples of different levels of sedimentation observed after DST.
Appendix B
False DST Result – Sometimes a cone could indicate
gelation and NOT sedimentation
Potential Remedy – Adjusting D121 or D194 Conc.
Center Cone Height (in)
1.4
Center Cone Height
1.2
1
0.8
0.6
0.4
Over-dispersed
Under-dispersed
0.2
0
0
0.02
0.04
0.06
0.08
0.1
D194 Concentration
Figure B1: The DST can be used as a slurry optimization tool
for slurries at higher temperature.
If there is more than a 50% difference in cone height between
the center position measurement (position 1) and the outside
paddle cone measurement (position 3), then the slurry could be
underdispersed at bottom hole conditions. Equation 1 can be
used to determine whether the slurry may be underdispersed. One
way to verify if the slurry is underdispersed is to increase
high temperature dispersant/retarder aid (D121 or D194). If the
cone morphology/shape changes to a flatter profile, then the
slurry is approaching an optimal dispersion state (see Figure
B1). The cone typically reaches a minimum height, then
increases in height as more dispersant is added, but this time
with a flatter deposition profile. Note: It is absolutely
necessary to optimize the slurry in the DST before attempting
HPHT rheology measurements. There is no way to gain insight
into slurry stability by HPHT rheology measurements alone. At
low shear rates (3, 6, 30 rpm), the gel and/or sediment will
grab the rotor, giving erroneous readings at those shear rates.
For that reason, it is imperative to first optimize the slurry
using Figure B1 as a guide before performing HPHT rheological
measurements.