The Influence of Elliptical-Geometry Wellbore on Zonal Isolation
Danzhu Zheng, Z. Miska, Evren Ozbayoglu, Yusheng Zhang
Abstract
ABSTRACT: Zonal isolation is key to the safety of drilling, injection, and production. If zonal isolation fails, significant financial losses or environmental damage can occur. Cement integrity is critical to maintain zonal isolation. The objective of this paper is to study the influence of wellbore shape on cement sheath integrity and zonal isolation. To diagnose cement sheath integrity, cylindrical wellbore models which contain circular-shape casing, cement, and formation layers have been built. However, the wellbore geometry in real drilling process is elliptical instead of circular. In this study, we built an elliptical-geometry Casing-Cement -Formation System (CCFS) analytical model by applying complex variable method. Two case studies were performed with the elliptical-geometry CCFS model and circular-shape CCFS model that does not consider the wellbore shape factor. 1. INTRODUCTION Quality of cement is an integral part of well integrity. The change of downhole condition and post well site operation may lead to change of the stress-strain state of casingcement- formation system (CCFS) [1]. Cement sheath failure is manifested by inter zonal annular fluid movement and abnormally high annular pressure at some point behind casing up to and at the surface [2]. The detrimental effects of cement sheath failure are numerous and may cause lost revenue from lost production, potentially hazardous rig operations and producing operations. Losing zonal isolation may lead to contamination of freshwater table [3]. To understand the failure behavior of cement is significant for maintaining zonal isolation. Previous research has built analytical/numerical models for predicting the stress distribution around the circular wellbore [4] [5]. However, wellbore geometry in real drilling process is elliptical instead of circular. The geometrical irregularities can result from a combination of geology and/or drilling/completion operations [6]. Two reasons that may cause elliptical geometry were listed here: 1) Tilt angle is the angle between the centerline of the hole and the centerline of the bit. Figure 1 is a simple sketch of tilt angle. The red line shows the plane that is perpendicular to the hole axis. The geometry for this plane is elliptical because of tilt angle.