Abstract:

The shallow CO₂ geological storage in block A of the East China Sea faces challenges such as large difference in horizontal in-situ stress and significant influence of cyclic stress. However, existing cement sheath integrity models have not effectively established the coupling mechanism between non-uniform in-situ stress field and residual strain, making it difficult to effectively evaluate the sealing failure behavior of cement sheath in shallow low-pressure reservoirs of block A of the East China Sea. Therefore, aiming at the performance design and integrity evaluation of cement sheath under intermittent cyclic loading-unloading conditions of CCS pilot test well in block A of East China Sea with high in-situ stress difference, and considering the influence of non-uniform in-situ stress and residual strain of cement sheath, a casing-cement sheath-formation stress calculation model was established based on the evaluation criteria of shear failure and tensile failure safety factors of the cement sheath. Using the geological engineering parameters of CCS pilot test well in block A of East China Sea, mechanical stress analysis and integrity evaluation of cement sheath were conducted, and influencing factors such as in-situ stress difference, residual strain, and elastic modulus of cement sheath were analyzed. The results showed that the casing-cement bonding interface along the direction of minimum horizontal in-situ stress was the weak point for tensile failure and shear failure of cement sheath. When the wellhead injection pressure increased, the shear failure safety factor of cement sheath increased exponentially while the tensile failure safety factor increased linearly, and the cement sheath tended to undergo plastic shear failure first. In addition, the in-situ stress difference helped increase the margin of cement sheath failure safety factor, while the increase of cement sheath residual strain caused the increase of cement sheath failure safety factor. The shear failure safety factor of cement sheath first increased and then decreased with the increase of elastic modulus. Therefore, it is necessary to establish a dynamic control mechanism of injection pressure and optimize the cyclic loading path to suppress the cumulative effects of residual strain. To reduce the high cumulative risk of residual strain under conditions of high elastic modulus of cement, and to avoid the problem of low shear failure safety factor margin under relatively low elastic modulus conditions, it is necessary to determine the threshold of elastic modulus according to the injection pressure conditions in engineering practice, ensuring that the stress concentration effects are reduced while avoiding the local extreme value region of the shear failure safety factor.

Key words: carbon storage, well cementing, cement sheath, stress, safety factor, integrity evaluation