Cement retarders are chemical additives added to cement slurry to extend the time it would have taken for the cement to thicken, thereby keeping the slurry pumpable for as long as needed. Cement slurry retarders are especially needed for deeper wells having higher temperatures above 38°C (100°F).
This is because higher temperatures (common with deeper wells) will increase cement slurry’s rate of reaction and quicken the hardening or setting time. In other words, without using retarders for deeper wells, the cement slurry could harden along the way, making it difficult or impossible to pump them to the target zones.
Lignosulfonate as a Cement Retarder
It is important for cement slurry to be properly placed in the openhole before it hardens. API class A, C, G and H cements, commonly used in oilfield cementing operations, will harden quickly as temperature increases. The most widely used cement retarder in oilfield applications are the lignosulfonates.
Lignosulfonates are obtained during the processing of wood waste. They have the ability to slow down the hydration/hardening of cement slurries even up to temperatures as much as 200°F and above. Examples of lignosulfonates used are sodium and calcium lignosulfonates.
Lignosulfonates are available in 3 grades; filtered, purified and modified with the modified grade being able to keep cement slurry pumpable above 200°F. The purified grade can keep cement slurry pumpable at a much higher temperature than the filtered grade and the modified grade is used when downhole temperature is expected to be really high (above 200°F).
Choosing a Cement Retarder
Apart from lignosulfonates, other retarders are hydrocarboxylic acids, organophosphates and even synthetically derived retarders. When making a choice of a cement retarder, it is important to also consider the effect other additives in the cement slurry will have on the performance of the cement retarder.
Some cement retarders may not function as expected in the presence of some other additives; so the chemical interaction between cement slurry additives should be considered. Another chemical additive mixed with cement slurries are accelerators. Accelerators perform the opposite function of a retarder. An accelerator will quicken the rate of reaction and setting time of cement slurry.