Professional Term

Oil well cement additive and its application
Section 1  Overview

With the development of petroleum industry, the field of oil and gas exploration and development is expanding day by day, and the drilling technology is gradually improved. Deep wells, ultra deep wells and special wells come out frequently, which puts forward higher requirements for oil well technology. Especially in the case of complex geological structure and bad underground conditions, the use of pure cement has been far from meeting the requirements of the technology, so we must rely on additives to adjust its performance. For example: the filling agent can increase the slurry production, save cement and reduce the cost; the accelerator or retarder can adjust the thickening time, which can not only ensure the construction safety, but also meet the strength requirements of continuous operation within the specified time; the fluid loss additive can reduce the filtration effect of permeable formation on the water slurry, protect the oil and gas reservoir, and enhance oil recovery; Dispersant can improve the rheological properties of cement slurry, increase the displacement efficiency, improve the cementing quality and prolong the life of oil wells. Therefore, the research, production and application of admixtures have been developed rapidly at home and abroad.
Admixtures can change the performance of cement slurry without changing the basic composition. Its relationship with cement is complementary. Generally speaking, oil well cement additives can be divided into:

(1) The admixtures for adjusting the thickening time of cement slurry are: accelerator and retarder;

(2) The additives to change the water loss performance of cement slurry are: fluid loss additive;

(3) The additives to improve the rheological properties of cement slurry are dispersant;

(4) Cement slurry density regulator: weighting agent and hydroxyl reducing agent;

(5) The additives to reduce circulation loss: plugging agent;

(6) The admixture to improve the thermal stability of cement paste is quartz sand;

(7) Prepare saline mud;
Section 2  Accelerator

In shallow well or surface casing cementing, although the cement slurry can meet the requirements of pumping, there are often problems such as long thickening time and slow strength development, which seriously affects drilling footage and cementing quality. The performance of cement slurry can be changed by adding accelerator or early strength agent, which can not only meet the requirements of cementing operation, but also make the strength meet the requirements of continuous drilling as soon as possible. Of course, it is also used to alleviate the over retarding caused by the addition of other admixtures, such as dispersant and fluid loss additive, in order to accelerate the development of compressive strength and shorten the waiting time. Strictly speaking, there is a difference between the early strength and the early strength: the early strength refers to the obvious improvement of the early compressive strength, but there is no requirement for the setting time, such as the commonly used dispersant, which has a certain early strength effect; while the coagulant requires shortening the setting time, that is, accelerating the setting, regardless of the strength development speed. Both of them often have the same nature, but they are different.

Chloride accelerator
Chlorine free accelerator
Compound accelerator

Section 3  Retarder

The function of retarder is to effectively extend or maintain the time of cement slurry in liquid state and pumpability. The best retarder should have retarding effect in any temperature range, and the thickening time is directly proportional to its dosage. It has good adaptability to various oil well cement and has good compatibility with different types of other admixtures. In other words, it does not affect other properties of cement slurry, such as thickening, increasing static shear force, hindering strength development, etc. For the cement slurry with specified formula, its thickening time has good predictability and repeatability, and it should also be anti pollution, non-toxic, tasteless, non explosive, non combustible and non polluting to the environment.

Oil well cement retarders have the following types:

(1) Lignosulfonate and its isomers or derivatives;

(2) Tannins, sulfonated tannins and their derivatives;

(3) Carboxylic acids, hydroxy carboxylic acids, isomers or derivatives and their salts;

(4) Sodium or calcium salts of gluconic acid and its derivatives

(5) Cellulose and its derivatives with low molecular weight;

(6) Organic or inorganic phosphates,

(7) Boric acid and its salts.
Lignosulfonate, Tannic acid and its sulfomethyl salts
Hydroxycarboxylic acids and their salts

Carbohydrate compounds
Cellulose derivatives

inorganic compound
Section 4 Weighting Agent
The additive or additive that increases the density of cement slurry is called weighting agent. Its own density is higher than that of cement. Generally, the particle size distribution of weighting material should be equivalent to that of cement. If the particle size is too large, it is easy to settle out of the cement slurry, and if it is too small, it is easy to increase the consistency of cement slurry. The water consumption should be less, and the cement should be inert in the hydration process, and has good compatibility with other admixtures. Barite, pyrite and hematite are the most commonly used weighting agents for densification of cement slurry.
Section 5  Anti-gas Channeling Agent
In cement injection of oil and gas wells, especially in some high-pressure gas wells, the formation adjacent to the annulus contains a certain pressure of primary gas. This gas can enter the annulus after cement injection and cause gas leakage. Sometimes, it may reach the surface or other low-pressure layers. This is called gas channeling, also known as gas invasion. It is usually harmful and dangerous, and it needs to be corrected by adding cement. In order to prevent this phenomenon, compressible cement, impermeable cement or thixotropic cement are usually used. Anti gas channeling agent is used to prepare compressible cement, impermeable cement, thixotropic cement, zero free water cement and non cementitious cement. Generally speaking, the same anti gas channeling agent may have multiple functions, which can not only make the cement slurry thixotropic, but also make the cement slurry have no free water precipitation. However, no matter what kind of admixture is used, the physical properties such as water loss, thickening time and compressive strength must be controlled within the range required by cementing design.
Section 6   Fluid Loss Control Additives
The water loss control agent is used in cement slurry to keep the water / solid ratio unchanged. This is achieved by controlling the filtration rate of water in the slurry into the adjacent permeable zone.

The use of fluid loss reducer can help prevent:

- the cement slurry was dehydrated prematurely, resulting in the annulus clogging and unable to complete the injection operation.

- the fluidity of cement slurry is changed, which reduces the efficiency of replacing slurry.

- cement slurry filtrate damages the mining layer.
Water loss is usually adjusted according to the type of cementing operation. For a casing injection, the water loss of cement slurry should be less than 250 ml, while for cement squeezing operation, the control of water loss is more important, and the maximum water loss should not exceed 150 ml.

When all other conditions are the same, the water loss of cement slurry increases with the decrease of specific gravity, that is, the higher the water cement ratio is, the worse the effect of water loss reduction is. Because of this, it is difficult to control the water loss of filling cement slurry.

Another factor which has great influence on water loss is the dispersion degree of cement particles in cement slurry. Unless a dispersant is used, it will prove difficult to control the loss of water no matter how much water loss control additives are used. This is because dispersants, when dispersing cement particles, also allow them to gather more closely in the filter cake. In this way, the permeability of the filter cake to the water between the cement slurry gaps is reduced, and the filtration rate is also reduced according to Darcy’s law.

The action mechanism of fluid loss additive is as follows
(1) To improve the viscosity of cement slurry to prevent dehydration; (2) to increase the static shear force of cement slurry, once it is still, gelation will occur, neither static pressure nor external pressure will be transmitted; (3) granular materials with different particle size distribution will block the formation voids or micropores; (4) The water-soluble polymer can be adsorbed on the surface of cement particles to form an adsorption hydration layer, resulting in cement particles bridging and then forming a network structure, binding more free water, blocking the internal voids of cement and reducing the permeability of cement filter cake. According to this principle, there are only two kinds of fluid loss additive for oil well cement: solid particle material and water-soluble polymer.
Section 7  Plugging agent

During drilling or cementing, all or part of drilling fluid and cement slurry will be lost to high permeability formation, natural or induced fractured formation, pore or porous formation, resulting in serious consequences, which is lost circulation. In order to control the lost circulation of wells, although many conventional and special plugging materials have been researched and developed, there are not many plugging agents to deal with serious leakage, and all kinds of cement and a small part of chemical plugging agents and their mixtures are still the most commonly used, such as lost circulation materials, polymers and cementitious materials, bridge plug agents and dual-purpose plugging and water plugging agents.
Section 8 Surfactant and spacer

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