DYNAMIC SETTLING TEST OVERVIEW

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).

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 Benchmarks11. 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:

heightconecenter−height coneoutsideheightconecenter

×100 %=%degreeof dispersion

Validation, Summary, and Interpretation1. 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

1 2 3

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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.

3. 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.

4. One possible explanation for dynamic sedimentation is that statically, the fluid is allowed to develop static gel strength which contributes to solids suspension.

5. 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.

6. 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.

7. If the slurry meets Acceptance Benchmarks outlined on page 3, the slurry is said to be optimally dispersed with acceptable dynamic sedimentation.

THINGS TO REPORT:1. Thickening Time plot2. Time for low shear interval at 25 rpm3. 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

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.

Customer: Chesapeake File No: L10-544 Talbert 9-14-16H-1 Prod. Pilot TT6-2Instrument: Consistometer4 866 Test Start: 11/12/2010 2:49:31 PM Test Stop: 11/13/2010 12:31:36 AMComments:

Schlumberger Test File Name: L10-544 Talbert 9-14-16H-1 Prod. Pilot TT6-2.tst 30 Bc @ 9:34:5150 Bc 9:35:18

Longview Lab Printed: 11/13/2010 8:48:52 AM 70 Bc @ 9:35:45100 Bc @ 9:41:39

0:00 2:00 4:00 6:00 8:00 10:00 12:00Tim e (HH :MM )

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Successful Go-No Go Test – No Change in Consistency (BC) after 20 min shutoff and motor turn-on.

Appendix B

False DST Result – Sometimes a cone could indicate gelation and NOT sedimentation

Potential Remedy – Adjusting D121 or D194 Conc.

00.20.40.60.8

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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.