Chapter 2 Cement Slurry Properties & Lab Testing

Question 1

What are the 10 slurry testing?

Answer 1

Slurry preparation for lab testing
Density
Rheology
TT
Compressive strength of set cement
Free water/fluid
FL
Settling cement in slurry
Gas Tightness/Anti-gas migration
Capability

Question 2

Explain
(1) Slurry Preparation for Lab Testing

Answer 2

-600mL slurry is prepared
Mixer operated at 4000rpm/15sec (add all solids & additives)
Followed by 12000rpm/35sec (to mix)

-Add all liquid first then solids

-If add Extenders (microspheres) - can only rotate @ 4000rpm/30sec followed by 4000rpm/3000sec

Question 3

Explain
(2) Slurry Density

Answer 3

• using Pressurized Fluid Density Balance

-slurry poured into cup, screw on pressure cap
-pressurizing plunger filled with slurry is attached to the cap
-pressure is applied to collapse bubbles
-place on fulcrum
-adjust sliding weight until balance

Question 4

Explain
(3) Slurry Rheology

Answer 4

-uses Rotating Sleeve Viscometer
-to measure the apparent viscosity of fluids at various shear rates & give indication of the mixability of the slurry at location
-limited to 180degF (82degC)
- use 300ml fresh cement & test immediately to avoid blooming effect
-if at 300rpm reading >275 - slurry too thick

Question 5

Explain
(4) Thickening Time

Answer 5

What is TT?
-time required for a slurry to no longer be pump able at a particular T & P

-Uses Pressurized Consistometer - to measure the consistency of a cement slurry
Unit: Bearden Unit of Consistency Bc

-In PCSB operations, for determining total pumpable time available on-location to execute job safely 40 Bc is taken as TT

If higher force is required (>40Bc), it is set very hard, not pump able

Question 6

Describe Thickening Time Test

Answer 6

-dynamic test = slurry is moved/stirred for the duration of the test
-informative needed: Depth, BHST, BHCT, csg size, pump rate

-performed on a consistometer
-a stainless steel slurry cup placed in an oil bath
-sample is heated to T & P as per well conditions while continuously stirring
-outer portion of the cup turns, and as the slurry begins to thicken or set, maintaining the paddle in a stationary position requires increasing force
- this force is measured as a deflection of the spring and converted to Bc

Question 7

During the Thickening Time Test, why do we turn off the motor for 10/30mins after the cement reaches test conditions?

Answer 7

To stimulate the dropping of the plug/liner setting and can indicate if there is gellation problem with the slurry

Question 8

What is the function of Compressive Strength Test?

Answer 8

To determine the force required to break an unconfined cement sample after it has been cured at T for a given period of time

Question 9

What is the device used to measure Compressive Strength Test?

Answer 9

Ultrasonic Cement Analyzer (UCA)

Question 10

Describe the Compressive Strength Test Procedure & Outcome using Non Destructive Sonic Test

Answer 10

The test cell contains an ultrasonic transducer and receiver. The transducer sends a sonic signal through the cement and the time required for the signal to travel through the sample is measured. Based on a set of computer algorithms, an equivalent CS is calculated.

-The UCA will give the user a plot of strength versus time, useful for WOC .

• The UCA depends on the computer algorithms to calculate CS based on the transition time of the sonic signal similar to used for the ultrasonic cement evaluation logs, like the CAST V, USIT, . The algorithms change with cement density & sometimes additives .

Question 11

Describe the Compressive Strength Test using the Destructive Test procedure

Answer 11

It is performed at a curing pressure of 3,000 psi.
The conventional compressive strength test is a destructive test where a 2”x2”x2” cube is prepared by pouring a slurry into a mould.
-It is placed under water in an autoclave and heated to the test temperature in four hours. The sample remains at the temperature and pressure for a time period, typically 8, 12, or 24 hours.

• The sample is removed from the mould and crushed on a hydraulic press. The force to crush the sample is divided by the cross sectional area of the sample and the result reported as the compressive strength in psi. This test is a destructive test and requires multiple samples of data in more than one test period.

Question 12

Explain the overview of CS Strength Testing

Answer 12

Strength testing is a static test performed at a single temperature and pressure. The curing pressure commonly used for the test is 3,000 psi.
• Temperature is the most important variable in determining CS development, It is recommended that tail cement slurries be tested at 85% of the BHST as it will not see the BHST of the well initial 24 hrs thus test results, if performed at BHST, will optimistic .
• CS over 500 psi is adequate for continuing any well operation such as drilling out the casing shoe, casing testing, etc. and @ 2000 psi for taking CBL & perforation .

Question 13

What is Free Water/ Fluid Test?

Answer 13

The test is an indication of water separation from a slurry.

Question 14

At what wells does Free water/Fluid is important?

Answer 14

-particularly important for higher angle wells where gas migration is may be encountered.

-The free water test will give an indication of the slurry stability under static conditions.

Question 15

Explain the procedure of Free Water Testing

Answer 15

-The test consists of taking a cement slurry that has been conditioned on a consistometer to BHCT

-placing into a 250 mL graduated cylinder for two hours.
-The fluid separated from the cement slurry at the end of the two-hour period is measured and reported as percent free water.
-It should be performed at either a 0° or 45° angle. The test is performed at atmospheric pressure and temperature of 180°F max .

For deviated wells, standard test angle is 45°. Slurries may show free water when tested at 45°, even if the 0° test showed no free water development due to an effect called the “Boycott Effect” showing solids settle faster at angle 45° being the most severe angle.
• For any slurry across a productive interval or any gas zone, the free water at angle should be zero (0).
• However, As practiced by PCSB , with the newer additives and more engineered slurries these days, it is encouraged to have NIL free water at all in any cement slurry

Question 16

What is Fluid loss Test?

Answer 16

measures the amount of fluid (mL) that can be forced out of a cement slurry at 1,000 psi differential pressure across a 325-mesh screen at the BHCT of the well

Question 17

Explain the Fluid loss Test procedure

Answer 17

After conditioning the slurry at BHCT for at least 20 minutes, the slurry is placed in the fluid loss cell and 1,000 psi differential pressure is applied across the 325-mesh screen. The test is begun and the filtrate from the slurry collected for 30 minutes or until the cement “blows dry.”
• As the surface area of the test cell is half (1/2) of a standard mud fluid loss test , the volume of filtrate is doubled to give the API fluid loss. Use of a stirred fluid loss cell is recommended for tests above 180°F (82°C).

Question 18

What is the importance of Fluid Loss?

Answer 18

FL is important for gas migration control, squeeze cementing, and narrow annular gaps.

Question 19

What is Gas Tightness/Anti migration Properties?

Answer 19

The transition time test is commonly used to identify slurries that have increased gas migration control. It determines the amount of time for a cement to go from 100 - 500 lbs /100 ft2 gel strength when measured at very slow shear rates (< 0.005 sec-1).
• Shorter the transition time, the less likely gas will migrate through the cement. For quality slurries it will be less than 45 minutes & preferably less than 30 minutes. The transition time of a slurry is not the only parameter to measure for gas tightness, but can give an indication of the quality of the slurry.

Question 20

What is Compatibility test?

Answer 20

evaluates the rheological compatibility of two or more fluids.

Question 21

Describe the Compatibility Test

Answer 21

the test is run on combinations of:
-spacer + mud
-spacer + cement
-cement + mud, and all three fluid mixtures.

Indications of incompatibility are fluid mixtures that show large viscosity increases, tendencies to gel, clabbering, etc. This test should be run whenever a non-aqueous mud is being used. SBM muds tend to clabber with water-based spacers.
• It is recommended that when dealing with a non-aqueous fluid at high temperature, a thickening time test of a 50:50 mixture of the mud and spacer be performed.