Integrity failure behavior and mechanism of cement sheath under cyclic differential pressure: application to underground energy storage wells

This study established an integrated cement sheath integrity experimental device and proposed a strain equivalence method to quantitatively reproduce the cement sheath deformation under downhole conditions in the laboratory to evaluate the effect of cyclic differential pressure (CDP) in underground energy storage wells (UESWs) on cement sheath integrity (CSI). The results indicate that when the cement sheath used for UESWs was cured for 7 and 28 d at 90 °C and 21 MPa, its elasticity moduli were 7.31 GPa and 7.38 GPa and its maximum deviatoric stresses were 35.2 MPa and 39.2 MPa, respectively. Combining the above mechanical parameters, the casing–cement sheath–formation mechanical models were used to calculate the stress and strain of the cement sheath. It can be found that at a differential pressure of 18 MPa, the cement sheath under downhole conditions was undergoing elastic deformation, and its first interface strain reached 0.0378%. In addition, when the cement sheath under experimental conditions reached the same strain, the equivalent differential pressure was 12.2 MPa. Furthermore, the CSI was measured. When the cement sheath was cured for 7, 14, and 21 days, an equivalent CDP of 12.2 MPa was applied to the cement sheath for a total of 90 cycles, and the cement sheath maintained good integrity. On the other hand, when the cement sheath was continuously cured for 28 d, the equivalent CDP was applied only 5 times (for a total of 95 cycles), gas channeling was recorded, and its integrity failed. Comparing the calculated and experimental results, under CDP, the cement sheath did not undergo real elastic deformation; instead, CDP caused strain accumulation and interfacial bonding degradation in the cement sheath with a skeleton pore structure, rather than elastic deformation.