 Failure to achieve successful zonal isolation could cause the well to never reach its full potential. 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 jobs. These factors illustrate the importance of a successful primary cement job.
Primary cementing exposes Portland cement to conditions far more extreme than its original intended use. 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 condition 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 quickly. In most cases, too quickly to allow for 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. The set retarder must also 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.
Lignosulphonates have been used as oil well cement retarders for many years. Although the exact mechanism of lignosulphonates 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 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 lignosulphonate 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 lignosulphonates 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 lignosulphonates have been shown to adsorb very strongly to hydrated C3A. When this occurs the concentration of lignosulphonate left in solution drops dramatically thus, preventing the majority of the lignosulphonate from reaching the C3S surfaces reducing the effectiveness of the additive. For this reason, lignosulphonates are believed to perform best in cements with low C3A levels.
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