Additives play an important role in the cementing of oil well. There are several types of cement additives that have been developed to allow cement to be used in many different oil and gas wells. To formulate the right cement mortar for any cementing job, the right additive must be chosen and the right amount added. Additives have different functions and are broadly categorized as accelerators, inhibitors, expanders, fluid loss agents, dispersants and many others. Each broad classification includes different classes of additives designed to perform approximately the same function in cement mortar design. However, there are some additives under each major type that are commonly used in the design of cement slurries for cementation operations in oil and gas wells. When cementing wells, Portland cement systems are designed for temperatures ranging from below freezing in permafrost regions to 350°C (662°F) in thermal reclamation and geothermal wells. Additives are available to improve the properties of the oil well slurry and achieve successful placement between the casing and the geological formation, developing rapid compressive strength and good drilling insulation over the life of the well. According to Cowan and Eoff, additives are commonly added to cement formulations to disperse cement particles, adjust curing time under conditions of temperature and pressure in the well, control liquid leaching losses from cement slurries during and after placement, and compensate for shrinkage as the cement hardens and hardens, improves the interface between the cement and casing and controls the flow and migration of formation fluids in the cement column during curing. 
Several types of cement additives have been developed to allow Portland cement to be used in many different oil and gas wells. Each main type contains different types of additives designed to perform approximately the same function in cement mortar design. However, there are some additives under each major type that are commonly used in the design of cement slurries for cementation operations in oil and gas wells. This document provides an overview of the broad classification of oil well additives, focusing on the additives commonly used in the cementation of oil and gas wells. Additives and some common types The degree of hydration when mixing water with cement can be changed with the help of chemical additives. Additives are chemicals and materials mixed in a base cement mortar to change the performance of cement. Due to the inherent nature of the base cement and the demands placed on the cement casing throughout the life of the well, the performance characteristics of the cement slurry are modified to address the specific and unique conditions of each well. Many of the additives used today are organic polymers formulated specifically for use in well cementation. Typical chemical additives for oils and cement wells include accelerators, retarders, expanders, fluid loss and circulation loss additives, dispersants and many more. 
- Accelerators
The accelerator is a chemical additive used to accelerate the natural reaction rate between cement and water, which reduces the cement thickening time, increases the early strength of the cement and saves the expensive drilling time. Cement slurries used in shallow wells where temperatures are low require accelerators to shorten the "wait for cement (WOC)" time before resuming drilling. In deep wells, higher temperatures improve the curing process and accelerators may not be necessary. Accelerators do not increase the maximum compressive strength of the cement, but rather promote the rapid development of strength.
- Retarders
These are chemical additives used to reduce the hydration rate of cement. Concrete commonly used in well applications does not have a sufficient fluid life for use at bottom hole circulation temperatures (BHCTs) above 100 F (38 C). To extend the thickening time, additives known as inhibitors are needed. Inhibitors prevent wetting and delay curing, allowing plenty of time for the slurry to set in the deep, hot pit. That is, it extends the thickening times to pump the cement into place. The retardation process is not fully understood, but it is known that the inhibitors bind to calcium ions and are able to prevent the growth of eringite crystals. Inhibitor effects depend on a number of factors, including additive dosage, processing conditions, and others. 
Therefore, the bottom cycle temperature (BHCT) must be carefully anticipated so that the correct damper concentration is used to avoid flashing or excessive curing time due to excessive residual cement slurry. Retarders are used at higher temperatures to allow time for the cement slurry to mix and set and also were speeding up the action of another additive can significantly shorten the time available to pump the cement.
- Extenders
Many formations will not support high cement columns of high-density slurries, these slurry weights must be reduced to protect low slope formations from fracture or for economic purposes. To reduce the weight of the cement mortar, expanders are used. Dilators are also known as water absorbers or lightweight inert materials. I t is a broad class of materials used to reduce mortar density and increase cement mortar production. 
Reducing slurry density reduces hydrostatic pressure during cementation of weak, brittle formations or empty reservoirs.
- Bentonite
Bentonite is the most common type of additive used to reduce the density of cement mortar. Bentonite is used to prevent solids separation, reduce free water, reduce fluid loss and increase slurry yield. Bentonite can be added to any of the API grades of cement and is often used with other expanders. This type of mud (sodium montmorillonite) is the same additive commonly used to build the viscosity of drilling fluid. However, it should be noted that bentonite marketed for use in a drilling fluid formulation is sometimes treated with an undesirable organic polymer to form a cement slurry as this tends to increase the viscosity of the slurry. The addition of bentonite reduces the density of the slurry due to its lower specific gravity and because the wetting ability allows the use of much higher water concentrations.