Study on the Influence Mechanism of the Absent Cement Sheath in Salt-Gypsum Layer on Casing

In the field cementing process of salt-gypsum layer, it often occurs that the casing and formation are not tightly sealed due to poor cementing quality, resulting in the lack of cement sheath in this layer, and then the casing damage accident occu rs. In order to solve this problem, a symmetrical finite element model of casing-missing cement sheath-formation is established in this paper. The finite element method is used to study the influence of cement sheath ring missing and partial missing on casing s tress. The results show that the maximum stress appears at the cement sheath defect junction, and the lack of cement sheath is in a stable high stress state. In addition, the higher the internal pressure, the greater the maximum stress of the casing in the annular missing section. Increasing the elastic modulus of cement sheath by adjusting the mud composition is a reasonable method to reduce the stress of wellbore and protect the integrity of wellbore. The influence of external pressure and cement sheath thickness on cementing quality is significant. In addition, increasing the internal pressure of casing and adopting high steel grade and high wall thickness casing are also effective design control methods. 1. INTRODUCTION Cementing sheath is one of the key factors to ensure the integrity of oil and gas wells. The stress and integrity of cement sheath directly affect the stress of casing. The existence of cement sheath can protect and support casing, seal formation, and prevent oil, gas and water channeling during drilling (Song et al. 2024 and Lian et al. 2017). In recent years, scholars have carried out a lot of related research. Cao et al. (2011) used numerical simulation method to study the casing damage caused by steam stimulation in thermal recovery wells under different degrees of circumferential loss of cement sheath. The research shows that with the increase of the degree of circumferential loss of cement sheath, the peak value of equivalent stress of casing decreases, and it is inferred that there is no thermal stress effect on casing when it is completely missing; Huang et al. (2012) studied the influence of the elastic modulus of cement sheath on casing stress. The research shows that when the elastic modulus of cement sheath reaches a certain intermediate value, the casing stress reaches the peak value. When the elastic modulus of cement sheath is greater than this value, the casing stress shows a downward trend; Jandhyala (2013) studied the effect of formation creep on cement sheath. The results show that the cement with high elasticity has strong bearing capacity; Li et al. (2014) studied the influence of the elastic modulus of the cement sheath and the loss of the cement sheath on the stress of the casing. It is concluded that the influence of the circumferential loss of the cement sheath and the radial loss of the cement sheath on the stress of the inner wall of the casing is basically similar. The loss of the cement sheath will cause stress concentration damage to the casing; Melo et al. (2015), Velilla et al. (2015) and Yu et al. (2017) studied the effect of salt rock creep on casing. The results show that the creep of salt rock will cause the increase of casing stress, which may lead to the yield failure of casing; Mai et al (2019) proposed a horizontal interval casing for shale gas wells; Dou et al (2019) analyzed the first cementation plane and the second cementation plane; Song et al (2021) studied the influence of cement sheath parameters on casing strength; Zheng et al (2022). proposed to use artificial neural network (ANN) to predict the fatigue failure of cement sheath; Zheng et al (2022). proposed the use of support vector machine (SVM) model to predict the fatigue failure of cement sheath; Jing et al (2023). studied the influence of cement sheath missing on casing stress under special working conditions; Zheng et al (2023) studied the failure of cement sheath under eccentric casing conditions; Zheng et al (2024). presented a general risk assessment workflow for oil well integrity, which can be applied to abandoned oil wells in any area.