1. Low Density Cementing Technology

Low density cement cementing can effectively reduce the cementing construction pressure, prevent well leakage and make the return depth of cement slurry reach the design well depth. The use of low-density cement can reduce the liquid column pressure outside the casing, so as to reduce the difference between the cement slurry column pressure and the formation pore pressure, implement reasonable differential pressure cementing, reduce the invasion of cement slurry filtrate and solid particles into the oil and gas reservoir, reduce the damage to the oil and gas reservoir, and help protect the oil and gas reservoir.

The density of low-density cement slurry depends on the type of lightening agent. Due to the different types of lightening agent added, the degree of density reduction is also different.

Generally, there are three types of cement slurry and low density materials:

(1) By adding bentonite, diatomite, hard asphalt, fly ash, expanded perlite, pozzolanic ash and glass, the density of this kind of cement slurry can be controlled at 1.40 ~ 1.60g/cm³range, called ordinary low density cement.

(2) By adding hollow beads, the density of cement slurry can be controlled in the range of 1.20 ~ 1.40g/day, which is called low density cement.

(3) using foam cement slurry, cement slurry density can be controlled at 0.90~1.20g/cm³ The scope is called foam cement, which is ultra low density cement.

According to the type of oil and gas reservoir and different downhole conditions, cementing can be selected from the above three types of low-density cement. All three types of low-density cement should meet the requirements of small water loss, good fluidity and high cement strength. The following describes the characteristics and application of three low-density cement pastes.

A. Ordinary low density cement

(1) Density range of cement slurry: 1.40 ~ 1.60g/cm³.

(2) Lightening agents used: bentonite, diatomite, hard asphalt, fly ash, expanded perlite, etc.a. Diatomite.

Diatomite is a kind of biological sedimentary rock formed by ancient microorganisms absorbing silicate solution dissolved in water to form a siliceous skeleton. It has pozzolanic activity.

The density of diatomite cement is 219g / cm³.  According to GB207-63 cement specific surface area, the specific surface area of diatom is 6750cm²/ g

b. Lightening material

"Fly ash" is a kind of ultra-fine pearl particles and amorphous material with high SIQ content, which has the excellent characteristics of pozzolanic materials. The specific surface area is 5690cm² / g and the density of "fly ash" is 231g / cm according to the "determination method of specific surface area of cement" in GB207-63 standard. After adding the above materials into the cement, due to its low density and large specific surface area, it has strong water absorption and water retention, so that the cement has a large water demand, which can reduce the density of the cement slurry, and the density value will not change due to the increase of pressure. Because of their small particles and strong water absorption, the cement slurry has the characteristics of colloid, so the initial consistency is large and the water separation and loss are small. Adding bentonite, expanded perlite powder and other cement stones has high strength, good corrosion resistance and compactness.