Suggestion

NOTE: No cement slurry should ever be pumped without a laboratory test using the actual materials that will get mixed on the job. (Cement, additives and field water).

Perform compatibility tests between the cementing fluids and wellbore. Drilling or control fluids, formation fluids and any other fluid potentially in contact with the cement slurry.

a)    Thickening Time to 80 BC (Bearden units)

Thickening time (also called pumping time) testing should be performed at the estimated bottom hole or treatment depth temperature and pressure using HTHP consistometer or benchtop pressurized consistometer.

• Test temperature considerations
  • For primary cement jobs, casing/liner cementing and cement plugs , the highest circulating hole temperature should be used for the thickening time test. The preferred method to determine the BHCT is with computer simulation software (alternatively the API correlation/tables in the RP 10B-2 / ISO 10426-2 to be used when applicable).
  • For remedial/squeeze cementing , the test temperature can generally be determined using the API correlation/tables in the API RP 10B-2 / ISO 10426-2. However, anytime the configuration of pipe size, depth and displacement rate/time does not correspond to API schedules, job-specific customized test temperatures and schedules based on actual job parameters and thermal computer models should be applied.
    • When squeezing in production and injection wells, if accurate recent downhole temperature data is available.
  • For coiled tubing operations, hole static temperature at the treatment depth should be used.
• Thickening (pumping) time requirement considerations
  • For casing/liner cementing and plug cementing  the pumping time is estimated as the total operational time Plus a safety factor of no less than two  hours.
    • Thickening time tests for particular cases, such as liner cementing, cement plugs or two-stage cementing, should consider the need of a static time  to identify any gelling tendency in the cement slurry in actual conditions.

Thickening Time should be no more than is needed to place the slurry while allowing for contingencies safely. Shorter times increase risk. Avoid longer times because they are subject to increased measurement error and uncertainty.

• For remedial/squeeze cementing , pumping times should be long enough to allow the cement slurry to travel to the placement depth. Then the injection of the cement slurry to the target zone. Time is also allowed for repeated squeeze-hesitation cycles. Then we allow time to reverse or direct circulate any excess cement out of the well . The following should be considered:
  • A minimum safety margin of two hours is recommended.
  • When testing the slurry for a hesitation squeeze, it is recommended to simulate the shutdown times in the laboratory during testing of the cement slurry.
  • An additional safety factor should be considered for circulation squeeze. In these operations, the actual thickening time is typically reduced and the gel strength development considerably accelerated due to the loss of filtrate while the cement slurry flows under pressure behind the casing between the perforations.
• For coiled tubing cementing we recommend a minimum thickening time of 8 hours.
  • For laboratory testing, actual field mixing and pumping conditions should be reproduced by the cementing contractor considering the following:
    • Additional mixing energy imparted to the cement slurry in actual conditions (batch-mixing of a relatively small cement volume and the high friction pressure inside the coil) makes the API mixing procedure described in API RP 10B-2 / ISO 10426-2 section 5 insufficient.
    • The thickening time for a cement slurry mixed according to API mixing procedures may be reduced up to 75% in coiled tubing cementing.

b)    Free Water

• Free water should be measured following the API RP 10B-2 / ISO 10426-2.
• For the following applications, free water should be zero (0%) percent:
  • In primary cementing of casing/liner and in plug cementing, for cement slurries in front of permeable zones, reservoir zones and/or highly deviated sections, zero percent (0%) free water is mandatory. In highly deviated section (> 40o degrees deviation) the free water test should be conducted at the same angle.
  • Squeeze cementing.
  • Coiled tubing cementing.
  • For other applications, such as surface casing, the cementing contractor should provide a recommendation. In general, this property becomes secondary to the other cement slurry properties.

c)    Fluid Loss

• Fluid loss should be measured following the API RP 10B-2 / ISO 10426-2.
• For slurries in front permeable zones or reservoir section, < 30 – 100 ml/30 min is desirable.
  • Cement slurries employed in narrow annular gaps, < 3/4” clearance, might require lower fluid loss values.
• For squeeze cementing we define the fluid loss according to the job objectives, the formation permeability, and the fluid injection rate. The following table provides a reference to select the required fluid loss value:

• For coiled tubing cementing the fluid loss should be less than 100 ml/30min.

d)    Rheology

• Cementing fluids (spacer and cement slurry) rheology should be measured following the API RP 10B-2 / ISO 10426-2.
• Rheology measurements are needed to build the model for the job into the computer simulation software for the cement placement, u-tubing effect, dynamic downhole pressures and thermal computer models.
• For coiled tubing cementing, the rheology of the cement slurry should be the lowest possible (for the lowest possible friction pressure inside the coil) without compromising its stability, i.e., no sedimentation of solids and no free water.
  • The recommended rheological values for coiled tubing cementing:
    • Minimum possible yield point of 5 to 10 lbf/100 ft2.
    • A minimum possible plastic viscosity of less than 50 cP.

e)    Compressive strength test

• We measure the compressive strength values following methods and recommendations in API Recommended Practice 10B-2 / ISO 10426-2 section 7.
• Sonic Logs, like CBL/VDL, are set to identify the cement with certain Acoustic Impedance. This value is best estimated from the transit time in the Ultrasonic Compressive Strength Analyzer (UCA).
• Failure to use the correct temperature and heating schedule for the compressive strength test could cause impaired cement evaluation interpretation. Test temperatures references are presented below:
  • Recommendation for compressive strength test temperature:
    • Casing cementing
      • Lead cement slurry (if present) > Use static temperature at the top of the lead cement length. For long cement columns in casing cementing or when the bottom-hole circulating temperature (BHCT) is higher than the static temperature at the top of the cement column refer to API Recommended Practice 10B-2 / ISO 10426-2 section 7.
      • Tail cement slurry: > Bottom hole static temperature, BHST.
    • Cement plugs, placed with drill pipe or coiled tubing > Static temperature at the top of the cement plug.
    • Cement squeezes > Static temperature at the top perforation/injection point or top of cement inside the casing.
    • For liner cementing > Hole static temperature at the liner hanger depth.
    • Alternatively, we obtain a more precise test temperature from thermal computer models for any of the above cases.

f)     Static Gel Strength Development

• Where cross-flow or fluids migration is a risk, the transition from 100 to 500 lbf/100 ft2 should be in less than 45 min.

g)    Stability tests

• For all critical operations, including highly deviated liner cementing, jobs involving small annular gap, high temperature, and coiled tubing cementing, perform the tests as per API Recommended Practice 10B-2 / ISO 10426-2 section 15.