In adjusting the correct thickening time of a cement slurry for reliable placement into a wellbore at given bottomhole circulation temperature (BHCT), an engineer must select a suitable retarder with optimum dosage and compatibility for the cementing system. The BHCT for each drilled section of a wellbore can vary, in extreme cases, from below freezing temperatures to 500 °F or even higher. In this case, each of several cementing jobs in a well might require a different retarder, which has drawbacks for logistics, operations, and laboratory testing. Therefore it would be advantageous to identify one cement retarder that is effective over a broad temperature range.
The performance of acrylic acid-co-polyethylene glycol allyl ether (AA-co-PEGAE) as a retarder was evaluated in API cement slurries for various loadings, temperatures, and densities. A model of the working mechanism of AA-co-PEGAE as a cement retarder is proposed. Cement testing procedures, lab test results, logistical and operational aspects, and case histories from East Texas and the Gulf of Mexico are presented and discussed.
The study demonstrates that AA-co-PEGAE is applicable as retarder in API cement slurries from ambient temperatures to at least 550 °F without negatively affecting fluid loss, free fluid control, right-angle setting, or hard set of cement. Slight deviations in AA-co-PEGAE dosages did not cause tremendous changes in thickening times, and the retarder response was fairly predictable. The response of AA-co-PEGAE was even found to be linear and insensitive to BHCT up to 200 °F. Chelating of calcium ions seems to be the primary retardation mechanism of AA-co-PEGAE in cement slurries.
The findings serve to improve understanding the mode of action of cement retarders with the goal to optimize slurry designs and admixture compatibility. The robust and predictable characteristics of AA-co-PEGAE over a broad temperature range contribute to more reliable and safe placement of cement slurries.