Advanced Casing Design with Finite-Element Model of Effective Dogleg Severity, Radial Displacements and Bending Loads

Abstract

A purpose-built finite-element model (FEM) is applied to simulate radial displacement of a casing string constrained within an outer wellbore. The FEM represents a fully stiff-string model wherein the casing is approximated by general beam elements with 6 degrees of freedom at each node to account for all possible physical displacements and rotations. Results predicted include deflection of the casing centerline from the wellbore centerline, effective dogleg curvature, bending deformation, wall contact forces, and bending stress magnification. In critical well casing design, accurate assumptions regarding bending stiffness may be necessary to avoid overly-conservative as well as non-conservative analysis. Challenging HPHT and extreme temperature wells are opportunities where increased design efficiency can be crucial. Alternatively, design for extreme loads such as overpull loads in long deviated wells may be non-conservative if severe bending stresses are not considered. A realistic case study is presented which demonstrates the possibility to achieve cost efficiency by means of optimized casing design. Also a case study is presented where a non-conservative design may result if severe bending loads are not modeled. The purpose-built FEM code is in many ways preferable to use of commercial FEA packages because of the timeconsuming effort required to build up the detailed model. In typical casing and tubular stress design, a "soft-string" model assumes casing strings are coincident with the wellbore centerline. The known or assumed wellbore curvature is applied directly to the casing string. Any effect of casing string stiffness and allowable radial displacement within the outer wellbore is ignored. In many cases this results in an overlyconservative analysis. Likewise the impact of bending stress magnification is typically ignored along with the effects of centralizer placement. This may also be non-conservative for critical overpull situations such as in ERD and horizontal wells.

Study on Dynamic Response of Downhole Tools under Perforation Impact Load

“…However, it is only in recent years that the research on the mechanics of the perforated tube string has been carried out [4][5][6][7][8][9][10][11]. Using the gap element theory of finite element method (FEM), a three-dimensional static analysis model of a perforated tube string was established by Zhang et al [5].…”

The Determination of Ultimate Releasable Hook Load in Casing Running Through an Air-filled Wellbore

“…Stefan Miska et al [12] take the effect of wellbore torsion into consideration, and developed a three-dimensional model to calculate the axial force and the contact force along the casing. McSpadden et al [13] discussed the axial force, shear moment and stress along the length of the casing string with stiff string model, and demonstrated that optimized casing-design efficiency may be achieved by avoiding overly conservative and costly design for HP/HT and extreme-temperature wells.…”

Forceful interaction of the casing string with the walls of a curvilinear well

“…A purpose-built fi nite-element model is applied by Mc-Spadden, Coker, et al [1] to simulate radial displace ment of a casing string constrained within an outer well bore. This represents a fully stiff -string model wherein the casing is approximated by general beam elements with 6 degrees of freedom at each node to account for all possible physical displacements and rotations.…”