Numerical Analysis for Mechanical Integrity of Oval Shape Wellbore in Creep Formation
Authors: Zhu Mingchi , Liu Nanxiang , Chen Xuyue , Zou Yiqi , Li Gengchen , Zhang Zilin , Wang Rong 
Published: August 2024

ABSTRACT:
Affected by the erosion and dissolution of drilling fluid, the salt gypsum layer is easy to form irregular wellbore, which is easy to cause casing damage. In this paper, the combined model of cement sheath casing is established, and the influence mechanism of elastic modulus and ovality of cement sheath on equivalent stress of casing is studied by finite element method. The results show that when the ovality is constant, the equivalent stress of the casing increases first and then decreases with the increase of the elastic modulus of the cement sheath. When the elastic modulus of cement sheath is less than that of formation, the equivalent stress of casing increases with the increase of ovality of cement sheath. When the elastic modulus of cement sheath is greater than the elastic modulus of formation, the equivalent stress of casing increases first and then decreases with the increase of ovality of cement sheath. Compared with the circular cement sheath, the oval cement sheath will lead to a significant increase in the maximum stress of the casing; when the elastic modulus of cement sheath exceeds 50GPa, the influence of ovality can be reduced.

1. INTRODUCTION
In the well section of salt gypsum layer, due to the erosion and dissolution of drilling fluid, it is easy to cause the enlargement of well diameter and the formation of irregular wellbore, which in turn changes the stress state of casing and causes casing damage. In this regard, domestic and foreign scholars have carried out a lot of research on the influence of wellbore shape on casing stress state. Guo et al. (2018) analyzed the stress and strain of casing in circular wellbore by numerical simulation method; Based on the theory of elastic mechanics, Shi et al (2015). derived a new analytical model of casing-cement sheath-formation system stress, and discussed the influence of initial stress state and inner casing pressure on stress distribution; Chen et al (2018). established a mechanical model of wellbore stability in regular wellbore considering thermal effect and porous elastic effect; Luo et al (2020). calculated the initial stress state of the casing-cement sheath-formation system by establishing a model, and reasonably explained the real situation of wellbore integrity; Liu et al (2015). established the casing-cement sheath-formation stress distribution model of directional wells under non-uniform in-situ stress through the undetermined coefficient method, and discussed the research method of cement sheath mechanical integrity of directional wells under non-uniform in-situ stress conditions. Based on the theory of elastic mechanics, Zhang et al (2013). used the finite element method to study the influence of the elastic constant of the cement sheath on the structural integrity of the consolidated body; Based on elastic mechanics, Wang et al (2014). studied the influence of cement sheath characteristics on the stress distribution of multi-layer casing assembly under non-uniform ground stress; Considering the thermo-solid coupling effect, Zhao et al (2017). studied the stress distribution and variation of multi-layer casing assembly under non-uniform in-situ stress; Wang et al (2018). have shown that during the drilling process, due to factors such as wellbore collapse and diameter expansion, elliptical wellbores are often formed; Firme et al (2016)., Mei et al (2014). studied the influence of borehole ovality on casing stress by finite element method; Fan et al (2019). and Yang et al (2021). used finite element and experimental methods to study the stress and displacement changes during the failure evolution of cement sheath during multi-stage fracturing; Based on the theory of elastic mechanics, (Wu et al 2022). obtained the semi-analytical solution of mechanical properties of elliptical borehole casing-cement sheath-formation under non-uniform ground stress.