2. Toughening principle of cement paste

2.1 the anti damage effect of elasticity and plasticity of cement paste.

From the deformation and failure of cement stone, we can see that:

The elasticity and plasticity of materials can reduce the stress concentration. Under short-term load.

The energy absorption of elastic-plastic body under load is mainly due to its plastic deformation.

Therefore, there are two aspects to improve the anti damage ability of cement paste.

Increase the plasticity of cement stone, improve the ability of anti impact damage.

Increase the elasticity of cement stone, improve the ability of resisting alternating load.

2.2 influence of elastic constant of cement paste on anti failure

A. Elastic modulus E and transverse deformation coefficient u (Poisson's ratio) are elastic constants of materials.

Property constant (representing the resistance of a material to deformation in the elastic range)

Degree).

B. The lower the elastic modulus is, the higher the deformability is

The greater the ability of recovery deformation.

Therefore, there are:

A. Reducing the elastic modulus: it is beneficial to increase the deformation of oil well cement stone within the elastic limit, so as to improve the deformation ability of cement sheath under complex working conditions (alternating load) and ensure the integrity of cement sheath.

B. Increasing the plastic deformation of oil well cement paste is helpful to reduce the stress concentration and increase the impact resistance of oil well cement paste.

2.3 toughening classification of cement paste

(1) Fiber toughening;

(2) polymer emulsion toughening;

(3) It is toughened by inert elastic materials;

(4) Composite (binary or multivariate) toughening.

2.4 toughening principle of cement paste

(1) Fiber toughening

Basic principles

Adding a certain amount of evenly dispersed fiber material into the cement slurry can reduce the stress concentration at the end of the original defects in hardened cement paste and delay the expansion of defects.

That is, crack inhibition effect (fiber is called crack inhibitor)

Fiber materials used in oil and gas well cementing must meet the following requirements.

(a) The fiber has good compatibility with cement admixture system to ensure the comprehensive engineering performance of cement slurry.

(b) Reasonable aspect ratio is the main index of fiber increasing the plasticity of cement paste. Under the same volume addition, the greater the aspect ratio, the stronger the plasticity of cement paste, but at the same time, the aspect ratio also affects the fluidity of cement paste.

(c) In the cement slurry, the largest size fiber should be able to pass through the cement slurry mixing system and casing substructure smoothly.

(2) polymer emulsion toughening

Polymer emulsion is evenly dispersed in cement slurry. In general, the emulsion and cement slurry do not have chemical action. The hydration of cement can absorb moisture in the emulsion particles. The emulsion particles after water loss can be condensed into film in the whole cement paste body, and these filamentary layers span the defects and cracks in the cement paste.

Due to its high tensile strength and deformation ability, the filamentous film can absorb the fracture energy of further expansion of defects and microcracks, thus effectively delaying the expansion rate of defects and microcracks, and providing certain tensile strength for brittle phase with more cracks.

(3) Toughening of insoluble inert elastic particles

Basic principle:

When the inert elastic particles are added into the cement slurry, they have high compressibility and elastoplasticity under high pressure. During the setting process of the cement slurry, with the shrinkage of the cement hydration, due to their elastoplastic characteristics, they will gradually release part of the compression energy and fill the pores, so as to improve the internal micro interface condition of the cement stone, form the structural deformation center that can absorb the strain energy, and can absorb the vibration energy It can restrain the generation and expansion of microcracks and improve the impact fracture resistance of oil well cement.Reasonable dosage: 

The amount of water-insoluble inert material has a great influence on the performance of cement system, and generally affects the development of compressive strength and flexural strength of cement paste.

In fact, the appropriate amount of elastic material is related to the type and fineness of elastic material, which should be determined by test.

It is generally believed that when less than 10%, it is usually about 3 ~ 6%.

(4) Composite (binary or multivariate) toughening

Basic principle:

Comprehensive use of fiber toughening reduces stress concentration and delays the crack propagation effect of defect expansion; polymer film toughened filamentous film can absorb defects and further expand the fracture energy of micro cracks: the absorbable vibration energy of water free elastic particles and the role of confining the production and expansion of micro cracks.