Continuing with this series of posts covering some of the cementing techniques applicable in workover operations, I want to talk further about plug cementing.
In previous posts, I have covered extensively plug cementing. This time I am providing information regarding laboratory testing and some considerations when cement plugs are intended for well abandonment.
Cement Slurry Properties for plug cementing
Thickening Time to 70 BC (Bearden units)
a) Test temperature considerations
We shall use the highest bottom hole circulating temperature (BHCT) for cement plugs 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 can be used when applicable).
b) Thickening (pumping) time requirements considerations
For plug cementing, the pumping time is estimated as the total operational time (including mixing and pumping cement, displacement, static periods or pulling out the cementing string, circulation of excess cement, etc.) plus a safety factor.
Thickening time tests shall consider the need for a static time (no rotation in the consistometer for a short period) to identify any gelling tendency in the cement slurry in actual conditions, like before/during POOH drill pipe or before circulating excess cement out.
For plug cementing (not included sidetrack cement plugs and cement plugs intended to control losses), free water should be zero percent (0%).
Fluid loss control on cement slurries for cement plugs becomes an essential property if the cement plug is set in an open hole or in front of permeable zones (perforations). In this case, the recommendation is < 50 – 150 ml/30 min. For cement plugs inside a casing, fluid loss control is a secondary characteristic and values between 200 to 500 ml/30min are ordinarily acceptable.
Cementing fluids (spacer and cement slurry) rheology shall 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/or thermal computer models.
Compressive strength test
The compressive strength values are measured with the methods and recommendations in API Recommended Practice 10B-2 / ISO 10426-2 section 7. The reference test temperature for plug cementing is the static hole temperature at the top of the cement plug.
Some Cement plug requirements for well abandonment
a) Plug length at least 10 – 30m above the permeable or abandonment zone.
b) Weight and pressure (positive and/or negative) tested is usually required to ensure isolation.
c) Acid-soluble cement (cement/CaCO3 blend) can be used as part of the well suspension methodology as it can be removed quickly at a later date.
e) The preferred method is batch mixing the cement slurry volume
f) Cement shall have a compressive strength of 1,000 psi
For any additional information about workover and abandonment cementing techniques, or anything else, please let me know.