Oil well cement is a kind of hydraulic cementitious material, which is mixed with water in a certain proportion to form cement slurry, and hardened intocement stone with a certain compressive strength and permeability in the well.

1)  Cement hydration process

The hydration of oil well cement is exothermic reaction. According to the formation of its hydration rate and structure, it can be roughly divided into four processes: initial stage, tardy stage, setting stage and hardening stage.

A. Starting period.

The hydration reaction occurred rapidly within a few minutes of mixing cement dry powder with water, and a large amount of hydration heat was generated, forming a layer of calcium silicate gel on the surface of cement mineral. Because this action was initially carried out on the surface of cement particles, only a part of water was consumed, and the rest of the water was filled with cement particles, and the cement slurry had flow ability.

B. Tardiness.

As the initial permeability of calcium silicate gel formed on the surface of cement mineral is very low, it prevents the further hydration of minerals and slows down the hydration rate. During this period, the flow properties of cement slurry are relatively stable, and the pumping wells of cement slurry should be completed during this period. This period can last from tens of minutes to hours.

C. Setting period.

As the hydration continues to develop to the depth of cement particles, calcium silicate gel on the mineral surface expands and the hydration process accelerates. The hydrates interact to form a network structure, which loses the flow performance and cement slurry enters the condensation stage. This time takes about tens of minutes.

D. Hardening period.

With the continuous deposition of hydrate, a large number of crystals are precipitated, the porosity and permeability of the system are gradually reduced, the strength is gradually increased, and the cement stone with microcrystalline structure is hardened.During this period, with the permeability of cement stone decreasing, the rate of hydration reaction gradually slows down, but it lasts for a long time, up to tens of days or even years. The obvious characteristic of hardening period is the increase of strength, which is also required by cementing.

2)  Common additives for oil well cement

Because the underground environment is much worse than the ground conditions, in order to make the cement slurry widely used in oil field drilling, completion, work over and other operations, the density, consistency, thickening time and compressive strength of the cement slurry have higher requirements, the use of pure cement is far from meeting the technical requirements, it must rely on additives to adjust its performance. At present, the types of commonly used admixtures mainly include: retarder, loss reducing agent, dispersant, defoamer, accelerator, high temperature strength stabilizer, lightening agent, etc,

Its main functions are as follows:

Retarder: mainly to extend the thickening time or setting time of cement slurry.

Accelerator: mainly to shorten the thickening time of cement paste and increase the early compressive strength of cement paste.

Fluid Loss Additives: mainly used to prevent rapid water loss of cement slurry and protect oil and gas reservoir.

Lightening agent: mainly to reduce the density of cement slurry and prevent the leakage of cement slurry in the low-pressure leakage layer.

Dispersant: used to improve the flow performance of cement slurry, which is conducive to the turbulent state of cement slurry under low pump speed and pump pressure.

Defoamer: prevent and avoid foaming of dissolved water and stabilize the density of cement slurry.

High temperature resistant strength stabilizer: under the condition of deep well and high temperature, silica powder is added to prevent the cement stone compressive strength from fading.