Primary cementing in oilfield applications involves the placement of cement between the subterranean formation and the well casing. The goal of primary cementing is to achieve zonal isolation. Zonal isolation is defined as the exclusion of substances such as water or gas in one zone from oil in another zone. Successful zonal isolation involves the creation of a hydraulic barrier between the casing and the cement and between the cement and the adjacent ground formation. At the same time, fluid channels within the cement sheath must also be avoided.

Failure to achieve successful zonal isolation could cause the well to never reach full productivity. Another important consideration, is that work done to try and repair a faulty cement job may do irreparable damage to the producing formation resulting in the possibility of lost reserves, lower production rates and start-up delays. Problems may also arise during stimulation and tertiary recovery. These factors illustrate the importance of a successful primary cement job.

Primary cementing exposes Portland cement to conditions far more extreme than encountered in typical uses. Oil-well cements must withstand pumping conditions ranging from below freezing in some offshore, deep-water wells to temperatures in excess of 1,000 °F in some thermal recovery wells.

These variations in well conditions have a major effect on the time required for the cement to harden. The elevated temperatures and pressures of deep oil wells cause the cement to harden too quickly to permit proper placement of the cement column. In these cases a set retarder is used to prevent the cement from hardening before it is placed in the desired zone. Performance of the set retarder must be predictable enough to allow for minimization of the cement set time after it has been placed. This so-called "waiting on cement (WOC)" time can prove costly, given the cost of rig time and that further work on the well must be stopped until the cement hardens.

Lignosulfonates have been used as oil well cement retarders for many years. Although the exact mechanism of lignosulfonates retarding the set of Portland cement is not well understood, it has been postulated that the mechanism is a combination of adsorption and nucleation. Studies have shown sulfonate and hydroxyl groups adsorb onto the C-S-H (Calcium oxide-silicon dioxide-water)gel layer of the hydrating cement. This fact has lead to the hypothesis that the sulfonate and hydroxyl groups present in lignosulfonates allow them to adsorb onto and consequently, incorporate into the C-S-H gel layer. This incorporation causes a change in the morphology of the C-S-H gel leading to a more impermeable structure. This causes a type of waterproofing effect slowing further hydration.

While most of the lignosulfonate adsorbs onto the C-S-H gel, some remains in solution. This dissolved portion is believed to interact with Ca ions in solution causing inhibition of nucleation and thus slowing the crystallization of the cement.

It has also been well established that lignosulfonates predominantly effect the hydration kinetics of the C3S (silicate) phase of Portland cement. However, the effect of lignosulfonates on the C3A (aluminate) hydration kinetics has also been studied and should not be discounted, as lignosulfonates have been shown to adsorb very strongly to hydrated C3A. When this occurs, the concentration of lignosulfonate left in solution drops dramatically thus, preventing the majority of the lignosulfonate from reaching the C3S surfaces and reduces the effectiveness of the additive. For this reason, lignosulfonates are believed to perform best in cements with low C3A levels.

Lignosulfonates are generally used in amounts ranging from 0.1% to 3.0% by weight of cement. The retardation performance is dependent upon the chemical composition of the lignosulfonate. Recently, lignosulfonates have been used at bottom-hole circulating temperatures as high as 400 °F.

Recent environmental regulations have made lignosulfonates an even more attractive option as an oil-well cement retarder. Some companies have begun looking into replacing some of their synthetic cement set retarders with lignosulfonate-based retarders.

References

Chilingarian, G.V., and Nelson, E.B.: " Developments in Petroleum Science: Well Cementing," vol. 28, Elsevier Science Publishers B.V., Amsterdam, Netherlands (1990)