1.2 reasons for failure of cement ring sealing 

The main reasons for the failure of the annulus sealing in oil and gas wells and gas storage wells are as follows:

(1) The deformation capacity of cement stone is insufficient (elasticity and plasticity).

(2) Conventional cement stone is a brittle material, which is easy to break.

(3) There are defects in the conventional hardened cement stone.

>The deformation capacity of cement stone is insufficient:

After the cement slurry solidifies in the well, the deformation capacity of casing, cement ring and well wall is different due to the complicated and changeable downhole conditions, the cement ring has poor capacity of change, and its deformation capacity is insufficient. The integrity of cement ring is easy to be damaged, which leads to failure of interlayer sealing and even collapse of casing.

>Brittle cement stone is easy to break:

Because of the characteristics of the hard brittleness of cement stone itself, the cement ring of oil well is complex in the subsequent construction and later production of cementing. When the cement ring force exceeds the yield limit of cement stone, if the perforation is impacted by the instantaneous high-energy jet, the brittle fracture occurs when the hard brittle cement stone can not enter the plastic deformation due to the short time of the force, which causes the brittle fracture around the hole The large area of cement ring breakage will lead to the failure of oil, gas and water efficiency between layers between thin cement rings, which directly affects the normal exploitation of oil and gas wells and the life of oil and gas wells.

>Defects in hardened cement stone

On the one hand, the hardened cement stone contains some cement particles, pores and mud cake which are not hydrated, which objectively creates conditions for the formation of microcracks.

Therefore, even if the cement stone formed by good cementing is a "inborn" brittle material with a large number of microcracks and defects, the defects will develop with the increase of strain rate or stress level.

On the other hand, because of the existence of internal defects, when cement ring is subjected to impact load or alternating load, a high stress concentration will be formed at the crack tip of initial defect. According to the fracture mechanics theory, with the development of stress level, once the fracture strength factor is larger than the fracture toughness of the material, the crack will expand rapidly, thus producing macro cracks, which will cause great difficulties in the channeling between the thin interlayer sections of oil and gas, the development after perforation, the effective reconstruction of reservoir and the gas storage.

>Deformation and failure of cement stone:

Impact or alternating force acts on the composite composed of casing cement ring formation to form impact pressure stress or alternating stress.

Due to the action of stress wave, the high tensile stress region will be formed in the local area of cement ring, and macro cracks will be generated, until the failure of cement stone under impact load or alternating load, the deformation and failure process can be divided into three stages: elastic deformation, plastic deformation and fracture failure. Therefore, the existing cement stone must be modified to reduce the damage of the cement ring caused by the impact load such as perforation and the alternating load of gas storage.