Sealing Failure Mechanism and Control Method for Cement Sheath during Hydraulic Fracturing.

Area of Science:

• Petroleum Engineering
• Geomechanics
• Materials Science

Background:

• Cement sheath integrity is critical for wellbore stability and zonal isolation in hydraulic fracturing.
• Understanding failure mechanisms under cyclic loading is essential for preventing gas migration.

Purpose of the Study:

• To investigate the sealing failure mechanisms of cement sheaths during hydraulic fracturing.
• To propose effective control methods to prevent cement sheath failure.

Main Methods:

• Whole wellbore numerical modeling of casing-cement sheath-formation assembly.
• Analysis of cement sheath failure modes at various depths.
• Laboratory experiments simulating cyclic casing pressure on cement samples.

Main Results:

• Cement sheath failure, including tensile cracks and micro-annulus formation, occurs at different depths due to stress variations.
• Expansion cement slurry with high Young's modulus mitigates tensile stress above the intermediate casing shoe.
• Low residual strain cement slurry significantly enhances cement sheath integrity and durability under cyclic loading.

Conclusions:

• Cement sheath failure mechanisms are depth-dependent, with tensile failure above and micro-annulus formation below the intermediate casing shoe.
• High Young's modulus cement slurries are recommended above the intermediate casing shoe.
• Low residual strain cement slurries are recommended below the intermediate casing shoe to prevent micro-annulus formation and ensure sustained casing pressure integrity.