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 six 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 bendingstress magnification. These results will provide for a more-accurate assessment of well integrity in terms of casing-stress safety factors and centralization before cementing, as well as more accurate prediction of running loads during the drilling phase.In critical-well-casing design, accurate assumptions regarding bending stiffness may be necessary to avoid overly conservative as well as nonconservative analysis. Challenging finite high-pressure/ high-temperature (HP/HT) and extreme-temperature wells are opportunities for increased design efficiency by avoiding overly conservative and costly designs, which can be crucial. Alternatively, design for extreme loads such as overpull loads in long deviated wells may be nonconservative if severe bending stresses are not considered.A realistic case study is presented that demonstrates the possibility to achieve cost efficiency by means of optimized casing design. A case study also is presented in which a nonconservative design may result if severe bending loads are not modeled. The purpose-built FEM code is in many ways preferable to the use of commercial finite-element-analysis (FEA) packages because of the time-consuming 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 overly conservative analysis. Likewise, the impact of bending-stress magnification is typically ignored, along with the effects of centralizer placement. This may also be nonconservative for critical overpull situations, such as in extended-reach-drilling (ERD) and horizontal wells.
For burst design, engineers routinely assume the casing annular space is filled by a fluid equivalent. This assumption ignores mechanical resistance provided by solid cement. Some studies addressed this shortcoming by modeling the cement sheath as a solid with elastic failure criteria. Prior work used cement elastic modulus and Poisson ratio to classify cement as 'ductile' (soft) or 'brittle' (hard).In the current study, numerical results from finite element analysis (FEA) indicate that casing burst resistance is increased by the presence of the cement sheath. This study focuses solely on improvement offered by the cement sheath to casing burst resistance and ignores consequences of cement failure on overall well integrity.Comparisons are provided for casing burst resistance assuming various backup profiles. These include fluid hydrostatics, solid cement matrix (both elastic and plastic response) and cement as 'loose' particles. The fluid hydrostatics include: a) mud weight in hole; b) cement slurry density; c) mixed-water density; d) normal pressure (salt-water column); and e) actual pore pressure. Calculations show that these fluid profiles are conservative when used as burst-resistance backup. Original cement slurry density is least conservative.Since well designers are familiar with fluid profile backup assumptions in casing burst design, recommendations are provided to approximate cement behavior as particles with a fluid profile. This allows ease of calculation and is consistent with current practice. Guidelines are provided to explicitly calculate the enhanced casing burst resistance due to the particulate cement.
Estimation of drilling and cementing temperatures using standard models and simulation tools is essential for robust casing design of critical HPHT wells. Standard casing and tubular analysis defines loads in terms of changes in thermal, pressure and mechanical conditions from an initial installed state. Often, the casing initial temperature conditions are assumed to be prevailing geothermal undisturbed temperatures (UDT). This is typically considered a conservative assumption which simplifies the design process and avoids definition or verification of the relevant sequence of drilling and cementing operations. However, for critical HPHT wells where design margins can be narrow, it may be necessary to describe the initial physical conditions as accurately as possible and to incorporate them into the casing design analysis. In this paper, the general methodology of engineering based casing design is reviewed and practical guidelines are presented to suggest when extra effort to accurately model initial casing temperatures may be critical and why conventional assumptions may be nonconservative. Because the current industry environment is placing significant demands on operator organizations and engineering staff, the time and effort required to gather data or to determine proper assumptions for detailed well design has to be justified. All other things being equal, simple worst-case assumptions which facilitate quick analysis and decision processes are favored over more detailed modeling. However, with the wells being constructed today increasingly classified as critical HPHT wells, the need for thorough and realistic model-based casing design must be identified when it is required. The case studies considered in this work result from a review of a wide range of critical HPHT well designs. It is significant that some combinations and load conditions indicate that assuming undisturbed geothermal temperatures is not necessarily conservative. This can be related to constrained thermal expansion in cemented zones. 1. Introduction Standard industry simulation tools which model and predict wellbore temperatures during drilling and cementing operations as well as production operations are widely available. Prediction of initial and subsequent wellbore temperatures feeds directly into rigorous tubular stress analysis which is often critical for robust casing and tubing design for HPHT wells. Mitchell and Wedelich (1989) describe in detail a comprehensive wellbore simulator with coupled thermal-hydraulic effects and discuss its application to optimal wellbore design. Goodman and Halal (1993) describe application of a model to predict of thermal and trapped annular pressure loads. The challenges associated with HP/HT wells underscore the importance of including state of the art thermal simulation in the well design process (Hahn et. al., 2000, 2003). The current industry environment presents many challenges to the effective use of the available simulation tools. Operator and engineering services organizations are all confronted with a shortage of experienced personnel. At the same time, challenging wells which require substantial design effort are becoming more prevalent. As with any detailed modeling technique, a great variety of input parameters must be accurately determined before thermal simulation and stress analysis can be effectively and competently evaluated. The identification, estimation and collection of correct input data can be a significant organizational cost in and of itself. Hence, the time and effort required to gather data or to determine proper assumptions for detailed well design has to be justified. In this environment, any simplifying assumptions which can streamline the design process or by-pass intensive modeling effort are quickly adopted. All other things being equal, simple worst-case assumptions which facilitate quick analysis and decision processes are favored over more detailed modeling. More rigorous, detailed modeling will be conducted on a selective basis only where the costs can be justified.
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.
Summary For burst design, engineers routinely assume that the casing annular space is filled by a fluid equivalent. This assumption ignores mechanical resistance provided by solid cement. Some studies addressed this shortcoming by modeling the cement sheath as a solid with elastic failure criteria. Prior work used cement elastic modulus and Poisson's ratio to classify cement as "ductile" (soft) or "brittle" (hard). In the current study, numerical results from finite-element analysis (FEA) indicate that casing burst resistance is increased by the presence of the cement sheath. This study focuses solely on improvement offered by the cement sheath to casing burst resistance and ignores consequences of cement failure on overall well integrity. Comparisons are provided for casing burst resistance, assuming various backup profiles. These include fluid hydrostatics, solid cement matrix (both elastic and plastic response), and cement as "loose" particles. The fluid hydrostatics include mud weight in hole, cement-slurry density, mixed-water density; normal pressure (saltwater column), and actual pore pressure. Calculations show that these fluid profiles are conservative when used as burst-resistance backup. Original cement-slurry density is least conservative. Because well designers are familiar with fluid profile backup assumptions in casing burst design, recommendations are provided to approximate cement behavior as particles with a fluid profile. This allows ease of calculation and is consistent with current practice. Guidelines are provided to explicitly calculate the enhanced casing burst resistance caused by the particulate cement.
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