Oil well cement admixtures refer to the chemicals added in the oil well cement slurry, which can improve the performance or comprehensive performance of the cement slurry through physical or chemical action to ensure the smooth cementing operation and improve the cementing quality. From the development process of cementing technology, cementing slurry system has developed from pure cement at the beginning to various characteristic slurry systems. In order to meet the cementing needs of various complex wells, oil well cement admixtures have also developed very rapidly. Generation after generation of cement admixtures have come out one after another. In order to improve the performance of cement slurry, sometimes the cement itself is not as good as admixtures Use and status. In the 1940s, foreign oil well cement additives have become a very important part of cementing technology. They have begun to improve the cementing quality through the rheological design of cement slurry, forming the theory of cement slurry displacement efficiency. Compared with foreign countries, domestic oil well cement admixtures started relatively late. At first, most of the technologies were based on foreign theoretical research, but after decades of development, they have made relatively significant progress.

There are many kinds of oil well cement admixtures at home and abroad, which can be summarized as follows: early strength agent, retarder, fluid loss reducer, dispersant, defoamer, lightening agent, weighting agent, other special functional admixtures, etc. Fluid loss reducer, retarder and dispersant are the main additives in cementing. The following is a brief introduction of the above admixtures.

Early strength agent, also known as accelerator, is a kind of admixture which can accelerate the hydration of cement, shorten the thickening time of cement slurry and improve the early strength of cement paste. Early strength agent is mainly used to shorten the setting time of cement slurry system, and is often used in surface casing cementing, plugging, plugging and other operations. There are three types of early strength agents: inorganic salts, organic compounds and composite early strength agents.

Retarder is a kind of additive which can effectively prolong the setting time of cement slurry, prolong the safe pumping time of cement slurry and moderately reduce the consistency of cement slurry through physical and chemical action. Retarder is mainly used to increase the thickening time of cement slurry, reduce the viscosity of cement slurry, so that the cement slurry can be pumped to the predetermined position within a safe time range, so as to ensure the construction safety. The function of retarder is realized by the interaction between retarder and cement slurry.

Due to the different chemical and physical properties of each retarder, the interaction between retarder and cement slurry is also different. At present, the retarder theory can not fully explain the process of the interaction between retarder and cement, which needs further exploration. The more recognized ones are: adsorption theory, complexation theory, precipitation theory, nucleation theory, etc. At present, there are mainly the following types: hydroxy carboxylic acids, lignin and its derivatives, sugar compounds, organic phosphoric acids, cellulose and its derivatives, inorganic salts, synthetic polymer compounds.

When cementing, when there is a pressure difference between the inside and outside of the liquid column, the cement slurry will "Penetrate" into the easily permeable formation, and the liquid phase in the cement slurry will penetrate into the formation, which is called water loss. Fluid loss reducer can ensure the liquid content of cement slurry, make the cement slurry have a certain fluidity, so as to ensure the construction safety. The type and mechanism of fluid loss additive will be introduced in detail in the following article.

Dispersant is a kind of additive which can obviously improve the rheological properties of cement slurry. Dispersant can effectively reduce the liquid-solid ratio of cement slurry and improve the construction performance of cement slurry. Even in the case of very low displacement and pump pressure, the cement slurry can also be turbulent flow, so that the displacement efficiency of cement slurry can be improved and excellent cementing quality can be obtained. The main components of dispersants are generally surfactants, including cationic, anionic and non-ionic dispersants. The main mechanism of dispersants is adsorption and dispersion. Due to the structural differences of each dispersant, its micro mechanism is also different. The commonly used dispersants for oil well cement mainly include sulfonate, aldehyde ketone polycondensate, hydroxyl polysaccharide with low molecular weight, low molecular compound such as polyhydroxycarboxylic acid, etc. the first two kinds of dispersants are widely used in the field.

Defoamer is mainly used to eliminate the bubbles produced by many oil well cement admixtures in the process of slurry preparation, so as to avoid the serious consequences such as slurry density deviation, admixture dosage inconsistent with the design and so on. Defoamer can diffuse on the surface of the foam to form an unstable film, thereby playing a defoaming role. Defoamers are generally a kind of compounds with short branched chain, weak hydrophilic group polarity and high molecular activity in molecular structure. The common ones are siloxanes, phosphates, glycerin polyether, high molecular alcohol, long chain hydroxyl compounds, etc.

Lightening agent is a kind of additive used to reduce the density of cement slurry and static pressure of cement slurry column. Reducing the density of cement slurry can effectively reduce the hydrostatic column pressure, prevent the cement slurry from fracturing the formation and the occurrence of lost circulation accidents, and ensure the cementing quality; low density cement slurry system can also prevent the cement slurry from polluting the oil and gas reservoir and improve the oil and gas recovery; low density cement slurry system is also commonly used in the cementing operation of long cementing section to reduce the number of cementing operations and shorten the cementing operation cycle, So as to improve the economic benefits. According to the mechanism of reducing the density of cement slurry, lightening agents can be divided into the following categories: one is bentonite, which has strong adsorptivity and can absorb a large amount of free water to improve the slurry making rate. It can reduce the density by increasing the liquid-solid ratio of cement slurry; the other is low-density materials, which can significantly reduce the overall density of cement slurry because of its lighter density than cement, For example, floating beads, glass beads, etc., such materials are mostly inert materials, usually in the form of filling between cement particles and particles; three is gas foaming technology, it is through the cement slurry or chemical gas production method, the formation of foam cement slurry system to reduce the density of cement slurry, this kind of technology can usually get ultra-low density cement slurry.