Simulation Investigation of Casing Eccentricity Estimation for Different Inclination Angles and Tensile Forces Using Finite Element Method

Abstract

Abstract Casing eccentricity has been considered as a problem during design and calculations of primary cementing, particularly in inclined and horizontal wellbores. Engineering calculations are usually based on the assumption of concentric annuls, which usually leads to erroneous or at least inaccurate results. One of the major problems of primary cementing is improper displacement of cement slurry around casing in the annular space. This often leads to gas channeling I gas wells, which consequently necessitates expensive squeeze job. Therefore, it is important for accurate engineering designs to use actual eccentricity values and not a concentric casing assumption in the derivations and application of used equations. This requires accurate eccentricity estimation. The ambitious objectives of this study are to estimate the casing eccentricity at inclined section of a directional well as a function of hole inclination angle, applied tension, and measured depth. To achieve these goals, a two-dimensional finite element method (FEM) was used. Furthermore, the influences of inclination angle, casing length, and tension force on the calculated values of casing eccentricity are investigated. Casing lengths of 102, 204, and 300 ft from the bottom of the hole with different inclination angles of 15°, 30°, 45°, 60°, 70°, and 90° were modeled under different tensile forces i.e., 0, 20000, 40000, and 60000 lbf., respectively. The results indicate that increase of inclination angle increases the casing eccentricity. The inclination angle has a major effect on the casing eccentricity when it is between 0° (vertical hole) and 30° (inclined), and change in effect when it is between 45° (inclined) and 90°(horizontal). The results also showed that as the applied tension force increases, the casing eccentricity decreases. Furthermore, it is concluded that the casing length has no effect on the calculated values of casing eccentricity. The attained results of this study provide more understanding of the role of casing eccentricity with respect to hole inclination and tensile force. Furthermore, it may have a real impact on well borehole cleaning, casing design, and cementing hydraulics of inclined/horizontal wellbore holes. Introduction and Literature Review Actual well bore annulus is rarely a concentric and uniform through its whole path but it is often an eccentric one, especially in deviated well holes. The casing eccentricity varies along the well borehole depending upon the hole depth and the inclination angle from the vertical direction. Stifferman (1974) studied experimentally the transport of drill cutting. He concluded that the hole eccentricity has a minor influence on cutting transport ratio in vertical annulus. Zeidler (1974) derived an equation to describe transport of mud in vertical annulus. Thomas (1978) studied the effect of eccentricity on carrying capacity of mud and also tested the applicability of Zeidler's transport model. He concluded that the casing eccentricity does affect the carrying capacity of drilling fluid and pointed out that Zeidler's model has some severe limitations. Iyoho (1980) also developed a semi-empirical model describing transport characteristics in directional holes. He indicated that the eccentricity is an important factor to achieve an efficient cleaning.