Most oil well cement slurries will be added to some additives in order to modify cement properties so cement operation can be properly performed. In this article, it will cover some of important cement additives in general terms so that it will help you understand the basic function of each additive.
Accelerators
Accelerators are added to shorten the time for cement to properly set and it will reduce rig time while waiting on the cement (WOC). Accelerators are crucial in shallow depths where bottom temperature is low. In the deeper section, it may not require an accelerator, because the well is hot enough and the thickening time will be normal. The WOC time is generally based on the time required to obtain 500 psi compressive strength of the cement.
Chemical used as accelerators are as follows;
- Calcium chloride 1.5 – 2.0 %
- Sodium chloride 2.0 – 2.5%
- Sea water
If the percentage of these additives is high, they will act as retarders instead of accelerators.
Retarders
Retarders have the opposite function as accelerators because they are used to prolong thickening time of cement in order to prevent premature setting of cement. Retarders are used in the deeper hole section where bottom hole pressure is high.
Chemical used as retarders are as follows;
- Calcium lignospulphanate
- Saturated salt solution
Light Weight Additives (extenders)
Lightweight additives are used to reduce slurry density. Some sections in the well may require a reduced cement weight because a formation fracture gradient is quite low. With normal cement density, it can cause cement losses and it will result in a bad cement job. Reducing the cement density will cause decreasing in compressive strength and increasing in thickening time.
Chemicals used as light weight additives are as follows;
- Bentonite (2%-16%)
- Pozzolan
- Diatomaceous earth (10%-40%)
Heavy Weight Additives
Heavy weight additives are added in order to increase slurry density. Some overpressures zones must be cemented with a high density cement.
Chemicals used as heavy weight additives are as follows;
- Barite
- Hematite
- Sand
Fluid Loss Additives
Fluid loss additives are added to prevent dehydration of cement; therefore, the cement will maintain proper water portion and the cement properties will not be destroyed. Squeeze cementing requires a low fluid loss because cement must be squeezed into intended zones before the filter cake builds up. Fluid loss for a primary cement job maybe as critical as a squeeze cement job.
Chemicals used as fluid loss additives are as follows;
- Organic polymers 0.5 – 1.5%
- Carboxymethyl hydroxyethyl cellulose 0.3% – 1.0%
Friction Reducing Additives
Friction reducing additives are used to improve the cement slurry flow properties. The additives will reduce the viscosity so that turbulence flow will happen with a low pumping pressure. With the help from friction reducing additive, the risk of fracturing formation from excessive pressure can be minimized.
Chemicals used as friction reducing additives are as follows;
- Polymers 0.3 – 0.5lb/sx
- Salt 1-6 lb./sx
- Calcium lignosulphanate 0.5 -1.5 lb./sx