Cementing operation is the most important part of the whole construction of a well, and no factor has a greater impact on the productivity of oil (gas) formation than the cementing quality.

Particularity of cementing engineering:

(1) It is a one-time project. If the quality is not good, it is generally difficult to remedy;

(2) It is a hidden project. The main process is underground and cannot be observed directly during construction. Quality control often depends on the accuracy of design and the quality of preparation, and is affected by a variety of factors;

(3) Affect the progress of subsequent works;

(4) It is a costly project;

The construction time is short, the process content is many, and the work volume is large. It is a project with strong technology.

Difficulties in cementing engineering:

(1) The requirements for cementing quality are high, especially for gas well cementing construction, which generally requires that the cement return to the ground.

(2) The one-time continuous sealing section is long and the construction pressure is high.

(3) Low pressure leakage formation cement slurry leakage and low return;

The well diameter is irregular, and the displacement effect of cementing construction is poor.

Common low density cement slurry system

The commonly used low-density cement systems at home and abroad mainly include: floating beads, bentonite, fly ash, diatomite and other low-density cement systems.

The minimum density limit of low density of floating beads is 1.38g/cm ³

The minimum density limit of bentonite system is 1.60g/cm ³

The density limit of low-density fly ash system is 1.55g/cm ³

The density limit of diatomite low-density system is 1.50g/cm ³

Foam cement can make the density of cement slurry lower than 1.0g/cm ³

HGS glass microsphere cement slurry system is less than 1.0g/cm ³

Problems of ultra low density cement slurry system

Due to the large amount of admixtures, the density of ultra-low density cement slurry decreases and the performance of cement slurry conflicts, which is highlighted in the following aspects:

1. The stability of cement slurry system is poor, and the system is separated by layers;

2. The water loss of cement slurry is difficult to control;

3. The rheological property of mud is poor and pumping is difficult;

4. The strength of cement paste develops slowly, the strength is low, and the permeability of cement paste is high, which is easy to cause the corrosion of corrosive media.

5. With the increase of pressure, foam cement slurry and floating bead cement slurry are easy to break, and the density increases rapidly, so the actual density of cement slurry increases under the bottom hole condition.

Technical requirements for ultra low density cement slurry system

1. The settlement stability of cement slurry is good, it is not easy to layer, and the density difference between the upper and lower is not more than 0.05g/cm ³。

2. It has good rheological properties, with n value greater than 0.70 and K value less than 0.20.

3. The compressive strength of cement paste of HgS hollow glass microsphere cement slurry system is almost twice that of cement slurry system with ordinary floating beads.

4. The water loss is small. The API water loss is controlled within 100ml, and some gas wells are even controlled below 30ml.

5. The thickening time is suitable for the construction requirements.

6. Short transition time and good gas channeling prevention performance.

Particle grading principle / close packing theory

(1) Low density technology mainly applies the principle of particle grading;

(2) Optimize the particle size distribution between cement and low-density filling materials to maximize the stacking ratio between materials.

(3) The amount of hydrated water can be reduced by using appropriate drag reducer in cement slurry.

(4) Reduce the filling water and improve the bulk density of the system.

(5) Three or more particles of different sizes are used for grading to increase compressive strength and reduce porosity and permeability.

Key technologies:

1. Design of ultra-low density reinforced materials close packing theory

2. The filling mineral particles have good filling property, relatively low specific surface area, smooth and dense surface and high chemical activity.