The Influence of Casing Eccentricity on Zonal Isolation
Authors: Zheng Danzhu , Ozbayoglu Evren , Miska Stefan , Silvio Baldino , Liu Yaxin , Wang Junzhe 
Published: September 2023

Eccentricity of the casing frequently exists in actual drilling process. This is one of the main reasons responsible for poor mud and cement displacement. In such cases, zonal isolation problems occur more commonly when compared with casing aligned in a concentric alignment. Previous researchers have studied the cement displacement process caused by casing eccentricity. However, the wellbore stress distribution that caused by casing eccentricity is seldom studied. In this paper, we used an equivalent perturbed domain solution to calculate the stress on the casing-cement interface. Failure of cement sheath for eccentric casing condition is investigated.

For moderate eccentricity of the casing, it is possible to analyze the problem by means of perturbation techniques. Based on a higher-order boundary perturbation method developed by Parnes and Beltzer (1986), we extend the solution to an eccentric thick wall problem with inner and outer pressure, with the method of equivalent perturbed domains and apply it using a second-order scheme. The stress fields with respect to the original polar coordinate system are then calculated for various eccentricities and ratios of inner to outer radius and are presented in graphical form. Mohr-Coulomb failure criterion is used for cement failure prediction.

This paper intends to provide an analytical model to predict cement sheath failure for eccentric wellbore geometry. The result will benefit the cement system design and zonal isolation strategy for casing eccentric wells.

INTRODUCTION

Geometrical irregularities can result from a combination of geology and/or drilling/completion operations [1] [2]. Eccentricity of the casing normally exists in real drilling process, especially in horizontal wells. This is one of the main reasons responsible for poor mud removal and displacement, which means micro-annulus, voids, and defects may exist in casing eccentric wells more regularly than the other wells. Once these voids, cracks, and defects are connected, they can become channels that potentially provide pathways for migration and communication of hydrocarbons between formations, thus compromising zonal isolation [3-6]. Cement failure can lead to environmental problems such as contamination of fresh water table, sustained annular pressure buildup (SAP), leakage in aged wells [7], and the need for extensive/expensive remedial work. The detrimental effects of cement sheath failure could negatively affect hydrocarbon production and the productive life of the well. For example, for production wells, the presence of channels in the cement sheath can result in non-productive communication pathways, causing stimulation and completion inefficiency, thus negatively affecting completion effectiveness and well productivity [8]. For abandoned wells, the failure of cemented barriers and plugs can lead to uncontrolled leakage of formation fluids and gas, with vent flows to surface. In some extreme cases, the failure of cemented sheath can even result in well control accidents that could result in loss of life, economic loss, and environmental damage [9].