PSI - Issue 58

Heng Yang et al. / Procedia Structural Integrity 58 (2024) 144–149 H. Yang et al. / Structural Integrity Procedia 00 (2022) 000–000

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and Dabrowski et al., 2018; Firme and Roehl et al., 2019). However, owing to the creep of salt formation, the stress on the wellbore increases with time, which may cause the wellbore integrity failure (Yang and Bu et al., 2021). The forms of wellbore integrity failure mainly include casing failure (Guo and Blanford et al., 2015; Velilla and Fontoura et al., 2015) and integrity failure of cement sheath. The integrity failure of cement sheath includes shear failure, tensile failure, and debonding failure (De Andrade and Sangesland, 2016; Zhang and Shen et al., 2017; Liu and Gao et al., 2018; Zhao and Li et al., 2019; Bu and Ma et al., 2020; Bu and Tian et al., 2020; Bu and Yang et al., 2022; Zhao and Yang et al., 2023). Wellbore integrity failure will cause significant damage to the environment, health, and economy. Therefore, for the problem of wellbore integrity failure in the salt rock formation, it is meaningful to study the effects of creep of the salt rock formation on the integrity failure of cement sheath. In this study, firstly, the creep experiments of salt rock under different deviatoric stresses were carried out, and the creep law of the salt rock was obtained. Then, the casing-cement sheath-formation finite element method (FEM) model in salt rock formation was established. Then the influence of salt rock creep on the integrity failure of cement sheath under the condition of uniform in-situ stress was studied. Finally, the influence of elastic parameters of cement sheath on the integrity of cement sheath was studied, and measures to reduce the failure risk of cement sheath integrity were proposed. Nomenclature ε ��� steady state creep rate q Von Mises stress σ � maximum horizontal in-situ stress σ � minimum horizontal in-situ stress R 2 goodness of fit

2. Experiment and numerical model 2.1. Creep experiments of the salt rock

To study the influence of the creep of the salt rock on the integrity of the cement sheath, it is necessary to obtain the creep law of the salt rock. The salt rock samples with a length diameter ratio of 2:1 are subjected to creep tests under different deviatoric stresses. The creep curves of salt rock under different deviatoric stresses are shown in Fig. 1. It can be seen from Fig. 1 (b) that the steady state creep rate increases with Von Mises stress, and there is a good power law fitting relationship between the steady state creep rate and Von Mises stress.

0.06

1.80E-007

Creep experiments Fitting results

0.05

1.60E-007

deviatoric stress 5MPa deviatoric stress 10MPa deviatoric stress 15MPa deviatoric stress 20MPa deviatoric stress 26MPa

4.00E-008 Steady State Creep Rate(s -1 ) 6.00E-008 8.00E-008 1.00E-007 1.20E-007 1.40E-007

0.04

 .cr = 3.743e-10q 1.876 R 2 = 0.96

0.03

Axial Strain

0.02

0.01

2.00E-008

0.00E+000

0 10000 20000 30000 40000 50000 60000 70000 80000 0.00

5

10

15

20

25

30

Von Mises Stress(MPa)

time(s)

Fig. 1. Results of salt rock creep experiments under different deviatoric stresses: (a) creep curves under different deviatoric stresses; (b) fitting results of steady state creep rate.