MANUAL (SAMPLE)

CASING DESIGN GUIDE

PTS 40.018DECEMBER 1992

PREFACE

PETRONAS Technical Standards (PTS) publications reflect the views, at the time of publication,of PETRONAS OPUs/Divisions.

They are based on the experience acquired during the involvement with the design, construction,operation and maintenance of processing units and facilities. Where appropriate they are basedon, or reference is made to, national and international standards and codes of practice.

The objective is to set the recommended standard for good technical practice to be applied byPETRONAS' OPUs in oil and gas production facilities, refineries, gas processing plants, chemicalplants, marketing facilities or any other such facility, and thereby to achieve maximum technicaland economic benefit from standardisation.

The information set forth in these publications is provided to users for their consideration anddecision to implement. This is of particular importance where PTS may not cover everyrequirement or diversity of condition at each locality. The system of PTS is expected to besufficiently flexible to allow individual operating units to adapt the information set forth in PTS totheir own environment and requirements.

When Contractors or Manufacturers/Suppliers use PTS they shall be solely responsible for thequality of work and the attainment of the required design and engineering standards. Inparticular, for those requirements not specifically covered, the Principal will expect them to followthose design and engineering practices which will achieve the same level of integrity as reflectedin the PTS. If in doubt, the Contractor or Manufacturer/Supplier shall, without detracting from hisown responsibility, consult the Principal or its technical advisor.

The right to use PTS rests with three categories of users :

1) PETRONAS and its affiliates.2) Other parties who are authorised to use PTS subject to appropriate contractual

arrangements.3) Contractors/subcontractors and Manufacturers/Suppliers under a contract with

users referred to under 1) and 2) which requires that tenders for projects,materials supplied or - generally - work performed on behalf of the said userscomply with the relevant standards.

Subject to any particular terms and conditions as may be set forth in specific agreements withusers, PETRONAS disclaims any liability of whatsoever nature for any damage (including injuryor death) suffered by any company or person whomsoever as a result of or in connection with theuse, application or implementation of any PTS, combination of PTS or any part thereof. Thebenefit of this disclaimer shall inure in all respects to PETRONAS and/or any company affiliatedto PETRONAS that may issue PTS or require the use of PTS.

Without prejudice to any specific terms in respect of confidentiality under relevant contractualarrangements, PTS shall not, without the prior written consent of PETRONAS, be disclosed byusers to any company or person whomsoever and the PTS shall be used exclusively for thepurpose they have been provided to the user. They shall be returned after use, including anycopies which shall only be made by users with the express prior written consent of PETRONAS.The copyright of PTS vests in PETRONAS. Users shall arrange for PTS to be held in safecustody and PETRONAS may at any time require information satisfactory to PETRONAS in orderto ascertain how users implement this requirement.

CASING DESIGN GUIDE

GENERAL

Contents

A.0 Overview

A.1 Foreword

A.2 Acknowledgements

A.3 Change control form

1.0 Introduction

1.1 Introduction

1.2 Purpose of casing

1.3 Casing types and functions.

1.3.1 Stove pipe, marine conductor or foundation pile.

1.3.2 Conductor string

1.3.3 Surface string

1.3.4 Intermediate string

1.3.5 Production string

1.3.6 Liner

1.4. The design process

1.4.1 Preliminary design

1.4.1.1 Data collection

1.4.1.2 Casing scheme selection

1.4.2 Detailed design

1.4.2.1 Selection of relevant load case

1.4.2.2 Uniaxial design

1.4.2.3 Triaxial design

1.4.2.4 Further design considerations

1.5 References

1.6 Appendix 1: International standards for tubular goods

1.6.1 Introduction

1.6.2 American Petroleum Institute (API)

1.6.2.1 API Committee 5 - tubular goods specifications and publications.

1.6.2.2 API : Committee 5 documents

1.6.2.3 Items under review

1.6.2.4 Shortcomings of API standards

1.6.3 International Standardisation Organisation (ISO)

1.6.3.1 ISO Technical Committee 67 (ISO/TC 67) oil industry matters.

1.6.4 Committee for European Normalisation (CEN)

1.6.5 Cooperation between ISO, CEN and API

1.7 References.

2.0 Introduction

3.0 Design parameters

3.1 Introduction

3.2 Lithological column

3.3 Formation-strength profile

3.3.1 Introduction

3.3.2 Borehole failure

3.3.3 Formation-strength gradient and equivalent mud weight

3.3.4 Measuring the formation strength

3.3.4.1 Introduction

3.3.4.2 Available measurement methods

3.3.4.3 Choosing the right method

3.4 Pore-pressure profile

3.5 Temperature profile

3.6 Hydrocarbon properties

3.7 H2S, CO2 and non-hydrocarbon formation fluid composition

3.8 References

4.0 Casing-scheme selection

4.1 Introduction

4.2 Minimum casing diameter

4.2.1 Design criterion

4.2.2 Well configuration and minimum casing diameter

4.2.2.1 Exploration and appraisal wells

4.2.2.2 Development wells

4.3 Minimum casing-shoe setting depth

4.3.1 Design criterion

4.3.2 Determination of wellbore pressure load

4.3.2.1 Pressure loading during drilling, mud circulation and tripping

4.3.2.2 Pressure loading during well control

4.3.3 Determination of wellbore strength

4.4 References

4.5 Appendix 2 : Well information forms

4.5.1 Exploration drilling information summary

4.5.2 Well summary

4.5.3 Well summary prognosis and results

4.6 Appendix 3 : Basic aspects of rock mechanics

4.6.1 Introduction

4.6.2 State of stress

4.6.2.1 Definitions, conventions

4.6.2.2 In situ-stress state

4.6.2.3 Pore pressure

4.6.3 Borehole failure - rock mechanics

4.6.3.1 Rock tensile strength

4.6.3.2 Theoretical relationship : wellbore strength - state of stress..

4.6.3.3 Fracture propagation

4.6.3.4 Wellbore strength in fractured formation

4.6.4 Other effects

4.6.4.1 Healing

4.6.4.2 Borehole fluid penetration

4.6.4.3 Depletion

4.6.4.4 Borehole shape

4.6.4.5 Chemical interaction

4.7 References

4.8 Appendix 4 : Procedures for leak-off and limit tests

4.8.1 Introduction

4.8.2 Testing procedure

4.8.2.1 Planning the test

4.8.2.2 Execution

4.8.2.3 Interpretation of the Leak-off graph

4.8.2.4 Formation breakdown, fracture re-opening

4.8.2.5 Reporting

4.8.2.6 Repeating a test

4.9 Appendix 5 : Specimen calculation of formation strength

4.9.1 Exploration well - example calculation

4.9.2 Appraisal well - example calculation

4.9.3 Development well - example calculation

5.0 Introduction

6.0 Load cases

6.1. Introduction

6.2. Pressure loads

6.2.1 Introduction

6.2.2 Collapse loads

6.2.2.1 Evacuation during drillinga) Internal pressure profile

b) External pressure profilec) Special cases

Air, foam or aerated drillingSalt loading

Formation compactionBlowout

6.2.2.2 Evacuation during production

a) Internal pressure profileb) External pressure profile

c) Special casesArtificial-lift wellsSalt loading

Formation compactionBlowout

6.2.3 Burst loads

6.2.3.1 Burst during drilling

a) Internal pressure profileb) External pressure profilec) Special cases

Over-pressured aquifer in borehole below casingSalt loading

6.2.3.2 Burst during production

a) Internal pressure profileb) External pressure profilec) Special cases

Gas-lift wellsSalt loadingGas-lift pressure on intermediate casing

6.3 Installation loads

6.3.1 Introduction

6.3.2 Dynamic loads

6.3.3 Static loads

6.4 Service loads

6.4.1 Introduction

6.4.2 Pressure loads

6.4.2.1 Actual axial forces

6.4.2.2 Collapse and burst loads

6.4.2.3 Reduced axial forces

6.4.3 Temperature loads

6.4.3.1 Actual axial forces

6.4.3.2 Collapse and burst loads

6.4.3.3 Reduced axial forces

6.4.4 Point loads

6.4.4.1 Production packer

6.4.4.2 Retrievable packer

6.4.4.3 Conductor casing

6.4.4.4 Reduced axial forces

6.5 Reference

7.0 Load determination

7.1 Introduction

7.2 Pressure loads on casing

7.2.1 Collapse load

7.2.2 Burst load

7.2.3 Formation load

7.3 Installation loads

7.3.1 Self-weight (in air)

7.3.2 Pressure (buoyancy)

7.3.3 Bending load

7.3.4 Dynamic drag

7.3.5 Shock load

7.3.6 Point load

7.3.7 Static drag

7.3.8 Temperature load

7.3.9 Maximum installation load

7.4 Service loads

7.4.1 Changes in tangential stress

7.4.2 Changes in radial stress

7.4.3 Changes in axial stress

7.4.3.1 Fundamental equation

7.4.3.2 Increase in internal pressure with fluid density and/or surface pressure

7.4.3.3 Reduction in internal pressure due to (partial) evacuation or reduced fluiddensity

7.4.3.4 Increase in external pressure with annulus pressure

7.4.3.5 Reduction in external pressure with annulus fluid level or fluid density

7.4.3 6 Increased internal pressure due to pressure test with retrievable packer

7.4.3.7 Temperature induced change in axial stress

7.4.3.8 Point-load-induced changes in axial stress

7.5 Load on stove pipes foundation piles, marine and conductor strings

7.5.1 Introduction

7.5.2 Stove-pipe, foundation-pile or marine-conductor design

7.5.3 Axial load and strain in conductor casing

7.5.3.1 Land wells or wells with subsea wellheads

7.5.3.2 Offshore wells with surface wellheads

a) Casing hangers at surfaceb) Casing hangers at seabed

7.5.4 Thermal growth of wellhead

7.6 References

8.0 Load-bearing capacity

8.1 Determination of the different types of casing strength

8.1.1 Collapse strength

8.1.2 Burst strength

8.1.3 Axial strength

8.1.4 Triaxial strength

8.2 References

9.0 Corrosion, wear and fatigue

9.1 Influence of corrosion on casing strength

9.1.1 Introduction

9.1.1.1 Site of downhole casing corrosion

9.1.2 Casing materials

9.1.3 Common types of corrosion

9.1.3.1 General corrosion

9.1.3.2 Galvanic corrosion

9.1.3.3 Pitting

9.1.3.4 Differential-aeration corrosion

9.1.3.5 Carbon-dioxide corrosion

9.1.3.6 Hydrogen-sulphide corrosion

9.1.3.7 Chloride-stress-corrosion cracking

9.1.3.8 Bacterial corrosion

9.1.3.9 Erosion/corrosion

9.1.3.10 Intergranular corrosion

9.1.4 Prevention and control of casing corrosion

9.1.4.1 Internal corrosion due to reservoir fluids

9.1.4.2 Internal and external corrosion due to drilling workover and completion fluids

9.1.4.3 External corrosion due to reservoir fluids, formation. fluids and surface water

9.1.4.4 All-round corrosion

9.1.4.5 Special forms of corrosion

9.1.5 New developments

9.2 Influence of wear on casing strength

9.2.1 Introduction

9.2.2 Site and timing of casing wear

9.2.3 Effect of wear on different types of casing strength

9.2.3.1 Collapse strength

9.2.3.2 Burst strength

9.2.3.3 Axial strength

9.2.3.4 Strength of connections

9.2.4 Wear mechanisms

9.2.4.1 Two-body adhesive wear

9.2.4.2 Two-body abrasive wear

9.2.4.3 Three-body abrasive wear

9.2.5 Modelling the wear process

9.2.5.1 Contact pressure

9.2.5.2 Contact surfaces

9.2.5.3 Relative velocity and contact time of mating surfaces..

9.2.5.4 Drilling-fluid composition

9.2.5.5 DRAGTORQ wear model

9.2.6 Controlling casing wear

9.2.6.1 Contact load

9.2.6.2 Hardfacing of tool joints

9.2.6.3 Drilling fluids

9.2.6.4 Wear-track length (WTL)

9.2.7 Designing for wear

9.2.8 Wear monitoring programme

9.2.9 New developments

9.3 Influence of fatigue on casing strength

9.3.1 Introduction

9.3.2 Fatigue failure parameters

9.3.2.1 Number of cycles to failure

9.3.2.2 Stress history

9.3.2.3 Stress concentrations

9.3.2.4 Residual stress

9.3.2.5 Range of stress

9.3.2.6 Loading method and sample size

9.3.2.7 Combined stress

9.3.2.8 Surface conditions

9.3.2.9 Corrosion fatigue

9.3.3 Specific issues

9.3.3.1 Externally generated loads

9.3.3.2 Internally generated loads

9.4 References

Click to jump to Chapter 10 - Chapter 21

10.0 Buckling

10.1 Introduction

10.2 Fundamental equation for reduced axial force

10.3 Resistance to buckling

10.3.1 Introduction

10.3.2 Vertical wellbore sections

10.3.3 Inclined straight wellbore sections

10.3.4 Curved wellbore sections

10.3.5 Use of top of cement to prevent buckling

10.3.6 Use of centraliser spacing

10.3.7 Use of surface force to prevent buckling

10.4 Post-buckling analyses

10.4.1 Introduction

10.4.2 Helical buckling

10.5 References

11.0 Design factors

11.1 Introduction

11.2 Collapse design factor

11.3 Burst design factor

11.4 Tension design factor

11.5 Compression design factor

11.6 Triaxial design factor

11.7 Summary

11.8 References

12.0 Connections

12.1 Introduction

12.2 Connection types

12.2.1 General remarks

12.2.2 Integral connection

12.2.3 Threaded and coupled connection

12.2.4 Comparison of integral and threaded/coupled connections

12.2.5 Thread forms

12.3 Connection sealing

12.3.1 Tapered interference-fit thread seal

12.3.2 Metal-to-metal seal

12.3.3 Resilient seal

12.4 Thread compounds

12.4.1 General remarks

12.4.2 Lubricating and sealing properties

12.4.3 Environmental aspects

12.4.4 Recommended thread compounds

12.5 Surface treatments

12.5.1 Process descriptions

12.5.2 Effect on galling resistance

12.5.3 Effect on sealing capability

12.5.4 Effect on corrosion resistance

12.6 Realiability and structural integrity of connections

12.6.1 Imposed loads

12.6.2 Structural integrity

12.6.3 Sealing capacity

12.6.4 Effect of bending loads

12.6.5 Failure mechanisms

12.7 Testing and qualification

12.7.1 Qualification tests

12.7.2 Other evaluation techniques

12.7.3 SIPM database

12.8 Thread protectors

12.8.1 General remarks

12.8.2 Performance criteria

12.9 Selection and ordering

12.10 References

13.0 Detailed casing design example

13.1 Introduction

13.2 Casing scheme and design parameters

13.3 Intermediate/production casing

13.3.1a Pressure loads - drilling phase

13.3.1b Pressure loads - production phase

13.4 Production liner

13.4.1 Pressure loads - production phase

13.4.2 Installation loads

13.4.2.1 Axial loads

13.4.2.2 Pressure loads

13.4.3 Service loads

13.4.3.1 Pressure loads

13.4.3.2 Temperature loads

13.4.3.3 Point loads

13.5 Intermediate/production casing

13.5.1 Pressure loads

1.3.5.2.1 Axial loads

13.5.2.2 Pressure loads

13.5.3 Service loads

13.5.3.1 Pressure loads

13.5.3.2 Temperature loads

13.5.3.3 Point loads

13.6 Surface casing

13.6.1 Pressure loads - drilling phase

13.6.2 Installation loads

13.6.2.1 Axial loads

13.6.2.2 Pressure loads

13.6.3 Service loads

13.6.3.1 Pressure loads

13.6.3.2 Temperature loads

13.6.3.3 Point loads

13.7 Conductor casing

13.7.1 Pressure loads -drilling phase

13.7.2 Installation loads

13.7.2.1 Axial loads

13.7.2.2 Pressure loads

13.7.3 Service loads

13.7.3.1 Pressure loads

13.7.3.2 Temperature loads

13.7.3.3 Point loads

14.0 Appendix 6 : Theories and definitions

14.1 Introduction

14.2 Definitions

14.3 Stress analysis theories

14.3.1 Introduction

14.3.2 Sign conventions

14.3.3 Lam equations

14.3.4 The axial stress equation

14.3.5 The shear stress equation

14.3.6 Hooke's Law

14.3.7 The principle of superposition

14.4 Failure theory

14.5 Buoyancy theory

14.5.1 Introduction

14.5.2 Pressure (buoyancy) load

14.5.3 Buoyancy factor

14.5.4 Neutral point for actual axial force (Fa = 0)

14.6 Simple stress analysis example

14.7 Buckling theory

14.7.1 Introduction

14.7.2 Buckling potential of pipe in air

14.7.3 Buckling potential of pipe in fluids

14.7.4 Neutral point for reduced axial force (Fa* = 0)

14.8 References

15.0 Appendix 7 : Calculation of axial and normal forces

15.1 Introduction

15.2 Straight inclined casing

15.3 Curved casing

16.0 Appendix 8 : Shock loads in casing

16.1 Introduction

16.2 Shock-load quantification

16.3 Concurrent drag and shock loads

16.4 References

17.0 Appendix 9 : Pressure build-up in heated sealed annuli

17.1 Introduction

17.2 Basic model for the annular pressure increase

17.3 Thermal expansion of the casing steel

17.4 Hydraulic expansion of the casing steel

17.5 Application of the models

17.6 Shortcomings of the models

17.7 References

18.0 Casing design in special cases

18.1 Introduction

18.2 High-pressure/high-temperature well

18.2.1 References

18.3 Squeezing salt well

18.3.1 References

18.4. Steam well

18.4.1 References

18.5 Horizontal well

18.5.1 References

18.6 Slimhole well

18.6.1 References

18.7 Permafrost well

18.7.1 References

18.8 Gravity structure

18.8.1 References

18.9 Reservoir compaction environment

18.9.1 References

18.10 Deep-water well

18.10.1 References

18.11 Gas-lift well

18.11.1 References

19.0 Operational aspects

19.1 Introduction

19.2 Ordering casing

19.3 Storage, handling and transport

19.4 Preparation for running

19.5. Running and testing

19.6 Monitoring the condition of installed casing

19.7 Equipment Newsletters on issues relating to tubular goods

19.8 References

20.0 List of symbols used in text

21.0 List of abbreviations used in text

A.0 Overview

A.1 ForewordCasing design is an integral part of the effort required to design, build and operate QualityWells, contributing monetary value over their entire life cycle, without compromisingsafety and environmental standards.

Effective casing design is aimed at:

Optimisation of the technical integrity of the Quality Well during:

a) the drilling phase, to cope with anticipated pressures and

b) the total life cycle (usually equal to the field life), to minimise intervention.

Time related aspects such as wear, corrosion and fatigue, which influence the load bearingcapacity of the casing strings, require particular attention.

Extremely important is also that good documented information on the casing design intentis known at the wellsite, in order to ensure that the operating envelope remains at all timeswithin the design criteria.

Optimisation of the commercial aspects, i.e. ensuring fit for purpose, cost effective designsand standardisation. In 1991, some $350 million was spent on casing/tubing (16% of theGroup's drilling expenditure), hence a determined effort will lead to considerable savings.

Early involvement of the Operations disciplines in greenfield exploration ventures and fielddevelopment plans is regarded as the prime vehicle for the preliminary casing schemeoptimisation. Computing tools now available will speed up the subsequent detailed designcalculations, allowing the casing designer to concentrate on high value input and alternativedesign options. They also support triaxial stress analysis which will permit further optimisation.

The material presented in this Guide is aimed at the Drilling Engineer with a knowledge of casingdesign equivalent to that provided in Round II. It is recommended that a Casing Design focalpoint be established in each Opco to collate relevant local expertise, develop it further whererequired, and address more complex issues.

This Guide interfaces with other SIPM supported documents, to which reference is made whereappropriate. Due attention has been paid to relevant international standards. Local Opco staffmay depart from the advice given in this Guide, provided the proper control procedures arefollowed and documented.

This Casing Design Guide is one of the functional documents issued by and with the authority ofEPO/51, the Head of Drilling Engineering. Any comments or observations for subsequentrevisions are to be documented on the enclosed "change control form" and forwarded to SIPM.This Guide replaces the Casing Design Manual, report EP-50600 of May 1980, which hasbecome obsolete.

A.2 Acknowledgements

The author wishes to thank all staff in SIPM, KSEPL and Opcos, who have contributed to thecompilation of this Guide.

He would like to extent this especially to KSEPL staff, who have contributed to the writing of therelevant status documents. In particular are mentioned:

D.J.M. Bax, RR/62, on connections

G.M. Bol, RR/53, on drilling fluids

P.J. Bontenbal, RR/62, on connections

F.J. Klever, RR/63, on structural engineering

P.J.M. Marchina, RR/55, on rock mechanics

R.J. Ooms, RR/63, on structural engineering

P. Oudeman, RR/57, on thermodynamics

J.H.G. Surewaard, RR/52, on gas kick modelling

J.P.M. van Vliet, RR/53, on drilling fluids

J.A. Wind, RR/52, on drilling engineering

H.W.M. Witlox, RR/63, on structural engineering

Special thanks goes to M. Wilcox, RR/52, for coordinating the efforts at KSEPL.

Review of the presented material has been conducted by several SIPM staff of whom thefollowing are mentioned for their contributions, constructive comments and remarks:

A.L. Carmona da Mota, EPO/51, on drilling engineering

T.S. Collard, EPO/53, on production operations

J.L. Beijering, EPO/51, on drilling engineering

R.G. Dodsworth, EPO/51, on transport and storage

R.A.W. Dubbers, EPD/52, on structural engineering

H.A. van den Hoven van Genderen, EPD/41, on production technology

P.J.P.A. Menger, MAIP/12, on materials procurement

D.E. Milliams, EPD/63, on corrosion and materials

N.E. Shuttleworth, EPO/51, on drilling engineering

Finally, special thanks are due to the sections of R.M. Holsnijders, EPF/54, and J.W. Burggraaff,EPX/39, who prepared the text and supporting figures.

P.J.J. Vullinghs

The Hague, December 1992

CHANGE CONTROL FORM CASING DESIGN GUIDE, EP 92-2000

1.0 Introduction

1.1 Introduction

Field experience and the results of research carried out both within and outside the Groupindicate that casing costs for both exploration and development wells can be cut withoutcompromising safety, and without adverse effects on the environment over the entire life cycle ofthe well, if an approach to "fit-for-purpose" casing design embodying the following features isadopted:

- early collection of all the relevant data by a multi-disciplinary team [1,2];

- selection of the casing scheme which is most cost-effective over the entire life cycle of thewell [2];

- accurate definition of the various load cases to which each casing string is likely to besubjected [3,4];

- accurate evaluation of the ability of the casing string to withstand the applied loads, using:

conventional uniaxial design methods to determine the overall resistance to internal andexternal pressure loads, and to the axial loads encountered during installation of thecasing, and

triaxial design methods involving detailed calculation of the radial, tangential and axialstresses on each volume element of the casing to determine the resistance to the actualservice loads experienced after the casing has been cemented in place.

New design tools [3,4] and technology spearheads [5,6,7] support this approach.

This Guide gives full details of SIPM-approved casing design methods having the abovecharacteristics, together with all the background information required for their effectivedeployment.

The present chapter discusses the various functions which casing has to perform, defines thedifferent types of casing used in a well, and describes the casing design process with itsdifferent elements - which will be dealt with in full in subsequent chapters.

International standards relevant to casing design are currently in a process of evolution. Theposition of the various standardisation bodies involved is explained in Appendix 1. Departurefrom these external standards is acceptable provided this is properly documented anddiscussed.

1.2 Purpose of casing

For drilling and completing a well it is usually necessary to line the walls of the hole with steelpipe called casing. This casing, together with the cement which holds it in place and seals theannulus [8], performs one or more of the following important functions (see Figure A-1):

- to keep the hole open from sloughing and swelling shales;

- to keep the hole open from moving salt formations;

- to prevent contamination of fresh-water horizons;

- to provide a means of controlling fluid influxes;

- to provide a container for drilling and completion fluids;

- to confine produced fluid to the wellbore;

- to provide a smooth conduit for drilling, logging and completion tools;

- to provide a smooth conduit for future casing and tubing strings;

- to support wellhead equipment and subsequent casing strings;

- to provide a means of anchoring the blowout preventers and Xmas tree.

FIGURE A-1 : PURPOSE OF CASING

1.3 Casing Types And Functions.

The total length of casing run in the well and hung off at the wellhead during a single operation iscalled a casing string. A liner is a string of casing which does not extend all the way to surface,but is suspended a short distance above the previous shoe. There are five principal types ofcasing string:

1. Stove Pipe, Marine Conductor or Foundation Pile;

2. Conductor String;

3. Surface String;

4. Intermediate String(s);

5. Production String.

The function of these strings is described below and summarised in Panel A-1. See also Figure A-2.

1.3.1 Stove pipe, marine conductor or foundation pile

The purpose of this first string of pipe is primarily to protect the incompetent surface soils fromerosion by the drilling fluid and, in the event of an offshore application, reduce the wave andcurrent loads imposed on the inner strings. Where the formation is sufficiently stable, this stringmay be used to install a full mud circulation system. It also serves to guide the drillstring andsubsequent casing into the hole. The name given to this string is primarily related to the type ofdrilling operation:

Stove Pipe : Onshore drilling.

Marine Conductor : Offshore drilling with surface BOPs.

Foundation Pile : Offshore drilling with subsea BOPs.

Stove pipes and marine conductors are either driven, drilled/driven or cemented in a pre-drilledhole. The stove pipe often carries the subsequent conductor casing, but once the latter string iscemented the stove pipe is released from this axial load. Therefore, subsequent casing stringswill be hung off the conductor casing string.

Marine conductors may form a part of the piling system for a wellhead jacket or piled platformand are therefore often designed by the structural engineers. They provide centralisation for theinner casing strings against column buckling, but do not carry direct axial loads except duringinitial installation of the conductor string. The marine conductors serve to reduce the wave andcurrent loads imposed on the inner casing strings and provide sacrificial protection againstoxygen corrosion in the splash zone.

On gravity structures, they are also required to minimise the transfer to the inner casings ofstresses resulting from platform settlement and rotational movement of the platform.

Foundation piles are usually either jetted into place or cemented in a pre-drilled hole. If noTemporary Guide Base is used, they support the Main Guide Base which carries and aligns allfuture wellhead components, BOPs, Xmas tree and casing/tubing strings for both the drilling andproduction phases. If a Temporary Base Guide is used, the foundation pile is landed in tension.The foundation pile directly carries both the axial and bending loads imposed on the wellhead bythe environment via marine riser and BOP.

PANEL A-1 : CASING TYPES AND FUNCTION

FIGURE A-2 : TYPES OF CASING STRING AND LINERS

1.3.2 Conductor string

The conductor string is used to prevent poorly consolidated formations from sloughing into thehole, to provide a full mud-circulation system, to protect fresh water sands from contamination bythe drilling mud and to provide protection against shallow hydrocarbons. This string is usuallycemented to surface or seabed and is always the first casing on which one or more BOPs aremounted.

For onshore wells the conductor string usually supports the wellhead, the BOP, the Xmas treeand subsequent casing strings.

For offshore wells with a surface BOP, the conductor string also usually supports the wellhead,the BOP, the Xmas tree and subsequent casing strings. Compressional loads are therefore oftenthe most critical design parameters for this casing. Above the top cement, the conductor must becentralised to prevent column buckling. The annulus between the marine conductor andconductor string is usually left uncemented above the mudline, in order to minimise load transferfrom the environment and hence bending stresses in the conductor string.

For offshore wells with a subsea BOP, the conductor string is landed on the foundation pile, andstays in tension.

1.3.3 Surface string

The next string is the surface string which provides blowout protection during deeper drilling. Itssetting depth is often chosen so that it also isolates troublesome formations, loss zones, shallowhydrocarbons, water sands, or protects the build-up section of deviated wells.

1.3.4 Intermediate string

This string is used to ensure adequate blowout protection for even deeper drilling, and to isolateformations or deeper hole profile changes that can cause drilling problems. It is recommended toset an intermediate casing string whenever there is a chance of encountering an influx that couldcause breakdown at the previous casing shoe, and/or severe losses in the open hole section. Astring is therefore nearly always set in the transition zone above or below significantoverpressures, and in any potential cap rock below a severe loss zone. Similarly, it is goodpractice when appraising untested, deeper horizons, to case off the known hydrocarbon intervalsas a contingency against the possibility of encountering a loss circulation zone. Obviously thislatter advice applies primarily to massive reservoir sections rather than sand-shale sequenceswith numerous small reservoirs and sub-reservoirs.

An intermediate string may also be set to shut off a swelling shale, a brittle caving shale, acreeping salt, an over-pressured permeable stringer, a build- up or drop-off section, a high-permeability sand or partly depleted reservoir that causes differential sticking. The designershould design the well to combine many of these objectives in a single casing point. A liner maybe used instead of a full intermediate casing, and difficult wells may contain several intermediatecasing strings and/or liners.

1.3.5 Production string

This is the string through which the well will be completed, produced and controlled throughoutits service life. While on some exploration wells this will amount to only a short testing period, onmost development wells it will span a significant number of years during which manyrecompletions may be performed. In most cases, the production casing will serve to isolate theproductive intervals, to facilitate proper reservoir control and to prevent the influx of undesiredfluids. In other cases, accumulation conditions are such that the well can be left with an open-hole completion below the production string.

It is also possible that the casing itself could be used as a conduit for maximising welldeliverability, for minimising pressure losses during a frac job, for injecting inhibitor or for lift gas.This may require Annular Safety Valves, which impose severe loads on the uncemented casing.It should be remembered that production operations will affect the temperature of the productioncasing and impose additional thermal stresses. The loads to which a production casing issubjected are therefore quite different from those imposed during drilling.

Care has to be taken in the selection of the steel type and the connections for a productionstring. Special consideration is required where drilling takes place below the production casingsince it may suffer some damage, e.g. in barefoot completions, open-hole gravel packs, linercompletions and deep- zone appraisal. In a liner completion both the liner and casing form theproduction string and must be designed accordingly.

The quality of the primary cement job is of paramount importance for the production casing,especially where zonal isolation is critical. It is therefore strongly recommended that theproduction casing should be rotated and/or reciprocated during cementing. This imposesadditional design requirements.

1.3.6 Liner

As discussed before, a liner is a string of casing which does not extend all the way to surface, butis suspended a short distance above the previous casing shoe. It is usually cemented over itsentire length to ensure a seal with the previous casing string. It is indeed important to ensure thatthe liner overlap has a good seal. In cases of suspected cement seal quality a mechanical seal,in the form of a liner packer, should be installed.

Drilling from a production liner is becoming a common practice. This is an important feature ofslimhole and monobore designs, where multiple liners may be used [2].

Although in principle the same types exist as discussed for the casing string above, an additionaldistinction is usually made between drilling liners and production liners, which are defined asfollows.

Drilling liners are set:

- to provide a deeper and hence a stronger shoe;- to keep the hole open from unstable formations;

- to achieve a drilling casing at low cost;- because of rig limitations on tensional loads;

- to minimise the effect of a reduced internal diameter on drilling hydraulics.Production liners are set:- to achieve a production casing at low cost;

- because of rig limitations on tensional loads;- to allow the installation of a larger flow conduit.

Either type of liner may subsequently be tied back to surface with a string of casing stabbed intothe top of the liner.

1.4 The design process

The objective of casing design is to design a set of casing strings, capable of withstanding avariety of external and internal pressures, thermal loads and loads related to the self-weight ofthe casing. These casing strings are subjected to time-dependent corrosion, wear and possiblyfatigue, which downrate their resistance to these loads during their service life. The interactionbetween the casing strings - which may lead e.g. to annular pressure build-up or wellheadmovement [9,10] also merits attention.

This section briefly surveys the structure of the process used to arrive at a technically andeconomically sound casing design.

Casing design as described in this Guide is divided into two main phases, preliminary design anddetailed design, with the former further subdivided into data collection and casing-schemeselection. As illustrated in Flowchart A-1, it will generally be necessary to repeat these phases inan iterative process to obtain an optimum design [2].

1.4.1 Preliminary design

1.4.1.1 Data collection

The outcome of the casing design process is strongly influenced by the quality of the initial data-collection exercise. Chapter C (Design parameters) addresses this issue and reviews the toolsrequired to obtain the necessary information.

To be effective, data collection must be carried out at an early stage in the design process, bymeans of a multidisciplinary team including local Opco staff from the Petroleum Engineering andOperations departments in addition to the casing designer. Recent developments in downholetechnology will lead to the introduction of novel ideas resulting in reduced well costs. The casingdesigner should be familiar with these developments and evaluate their merits for application inhis specific case [5,6].

FLOWCHART A-1 : OVERALL STRUCTURE OF THE CASING PROCESS

1.4.1.2 Casing-scheme selection

Selection of the optimum (most cost-effective) casing scheme for the anticipated developmentplan can play a major role in cutting overall well costs, and guaranteeing formation integrityduring drilling under all realistic loading conditions [2]. The structure of the selection procedure isillustrated in Flowchart A-2.

Casing-scheme selection is a complex matter involving the global issues of well configuration,which are mainly driven by the well objectives and field- development economics. Continualvigilance is required to avoid overdesign and other forms of unnecessary expenditure. It wouldgo beyond the scope of the present Guide to deal fully with all the considerations leading toproper choice of the casing scheme; however, the main lines of this topic are dealt with inChapter D.

FLOWCHART A-2 : GENERAL PROCEDURE FOR CASING-SCHEME SELECTION

The outcome of the casing-scheme selection is a specification of the minimum external diameterand minimum casing-shoe depth for each casing section in the proposed well. The casingdiameter is mainly determined by the availability of downhole drilling equipment and loggingtools, and by production requirements determining the sizing of the production or evaluationconduit. The casing-shoe setting depth is usually a function of the strength of the formation to bedrilled through and the loads on the wellbore during drilling or lithological/geological relatedconsiderations. The total depth of the well will be mainly determined by the well objectives.

In general, the preliminary casing scheme selection should be addressed by considering thecasing diameters from the inner strings towards the outer strings and by evaluating the casingsetting depths from the total depth upwards to surface.

The preliminary casing scheme may have to be modified on the basis of the results of laterstages of the design process.

1.4.2 Detailed design

In the detailed design phase, the casing designer determines the material grade and casing wallthickness for each section of the casing scheme selected, which will allow it to withstand allrealistically expected loads throughout the life of the well. The structure of this phase is illustratedin Flowchart A-3. In general, it will be most effective to design the individual casing sections inthe order specified in Flowchart A-4.

1.4.2.1 Selection of relevant load

Before design calculations can be performed for a given casing section, the casing designermust decide which load cases can realistically be expected to occur. This topic is dealt with inChapter F.

1.4.2.2 Uniaxial design

The design loads for the load cases selected are determined with the aid of the relevant data(see Chapter G). They are compared with the resistances to burst or collapse tabulated in APIBull. 5C2 [11] on the basis of the formulae published in API Bull. 5C3 [12] (see Chapter H), afterthese values have been corrected to take the influence of corrosion, wear and fatigue (seeChapter I) into account and divided by the relevant design factor (see Chapter K). The casingdesign obtained in this way is then checked to see whether the casing selected can withstand theloads occurring during installation (in particular the axial forces due to the total weight of thecasing string down to the depth considered, and the shock and torsional loads experiencedduring setting the casing).

FLOWCHART A-3 : STRUCTURE OF DETAILED DESIGN PHASE

FLOWCHART A-4: DESIGN SEQUENCE FOR CASING STRINGS OR LINERS

FIGURE A-3 : PRINCIPLE OF UNIAXIAL CASING DESIGN

The principle of this uniaxial design Process is illustrated in Figure A-3, and the designcalculations involved are dealt with in sections 2 and 3 of Chapter G, and sections 1.1 to 1.3 ofChapter H. In general, uniaxial design often leads to a conservative choice of tubular grade andwall thickness.

1.4.2.3 Triaxial design

With increasing acceptance of triaxial stress analysis and the appearance of commercial casing-stress-analysis software [4,13,14], use of triaxial analysis to optimise casing design is becomingincreasingly common. The interrelationship of design loads can now be analysed using acombination of Hooke's law, the Lame equations and the Von Mises yield criterion. Thesecomputer analyses relieve the designer of a lot of repetitious calculations and allow him toconcentrate on more accurate estimation of the service-life load conditions - a task for whichcomputer software has also been developed [3,4]. As with the uniaxial approach, the influence ofcorrosion, wear and fatigue should be taken into account before the triaxial design factor isapplied. This extension of the design process, made possible by the advent of desktopcomputing power, should lead to an optimised casing design fine-tuning the simplifiedconventional approach [4]. Designs that previously did not meet the uniaxial design rules mayknow be acceptable following a detailed triaxial stress analysis.

The principle of triaxial design is illustrated in Figure A-4. The design calculations involved aredealt with in Appendix 6.

1.4.2.4 Further design considerations

Connections

It is important to ensure that the connections between successive lengths of casing should alsowithstand the loads to which they are subjected. Recent developments have led to a widediversity of connection types and sealing compounds for use with casing connections. Thesalient aspects of connection design are highlighted in Chapter L, with ample references to therelevant literature, as a basis for technically justified selection of the right connection types [15].

Design for special cases

The design steps outlined above are applicable to any casing string or liner. However, specialdesign measures are needed to ensure adequate design in special cases such as high-temperature/high-pressure wells, squeezing salt wells and horizontal wells. The special designconsiderations applicable to such cases are discussed briefly, with ample references to theliterature, in Chapter N.

FIGURE A-4 : PRINCIPLE OF TRIAXIAL CASING ANALYSIS AND DESIGN

Operational aspects

Practical details which need to be taken into account in the design of any casing string arediscussed in Chapter O. The casing designer should be familiar with the relevant purchasingspecifications [16,17] and should be aware of the procedures and tools available to help theoperator responsible for installing and maintaining the casing strings. Early incorporation of theseaspects into the design process will yield an optimum design.

Probabilistic approach to casing design

Probabilistic methods of risk analysis now under development may become applicable to casingdesign in the future [18,19]. Such methods, permitting quantification of the risk of failureassociated with a given casing design, might allow further rationalisation to be brought about.

1.5. References

[1] SIPM, EPO/51 Proceedings of the PW82 Well Design Workshop - 1-5 October 1990 EP 90-3460

[2] SIPM, EPO/51Making the most of well planningEP 92-2500

[3] SIPM, EPO/51OSCP User Guide - version 2.3EP 91-2156

[4] Pittman, W. Commercial casing design software - detailed evaluation EP 92-0473

[5] SIPM, EPO/5Management, Technology and Human Resources, Programme 1991-1993EP 91-3000

[6] SIPM, EPDTechnology development programme 1992-1994EP 92-0350

[7] SIPM, EPO/51Drilling Spearhead Documentation, Volume 1, 2 and 3EP 89-0115

[8] SIPM, EPO/51Cementing Manual, Volume I - Primary Cementing of CasingEP-50500

[9] MacEachran, A. and Adams, A.J.Impact on casing design of thermal expansion of fluids in confined annuliSPE/IADC 21911

[10] Adams, A.J.How to design for annulus fluid heat-upSPE 22871

[11] American Petroleum InstituteBulletin on performance properties of casing and tubingBull. 5C2, Twentieth edition, 31 May 1987

[12] American Petroleum InstituteBulletin on formulas and calculations for casing, tubing, drillpipe and line pipe propertiesBull. 5C3, Fifth edition, July 1989

[13] Klementich, E.F. and Jellison, M.J.A service-life model for casing stringsSPE 12361

[14] Klementich, E.F., Jellison, M.J. and Johnson, R.Triaxial load capacity diagrams provide a new approach to casing and tubinganalysisSPE/IADC 13434

[15] Bax, D.J.M. (SIPM) and Bontenbal, P.J. (KSEPL)Casing connectionsContribution to the upgrade of the SIPM Casing Design ManualEP 92-1563

[16] American Petroleum InstituteSpecification for casing and tubingSpec. 5CT, Third edition, 1 December 1990

[17] SIPM, EPO/512Technical suggestions for ordering non-API tubularsDEN 17/92

[18] Payne, M.L. and Swanson, J.D.Application of probabilistic reliability methods to tubular designSPE 19556

[19] Reeves, T.B., Parfitt, S.H.L. and Adams, A.J.Casing system risk analysis using structural reliabilitySPE/IADC 25693

1.6 Appendix 1: International standards for tubular goods

1.6.1 Introduction

Opcos may depart from international standards relevant to casing design when it can be welldocumented that less conservative casing designs still meet stringent demands on safety andenvironmental friendliness, and comply with local legal requirements.

SIPM is working with industry partners (and competitors) to make these international standardsreflect this new vision. However, the process of change is justifiably a slow one.

This Appendix describes the framework within which these changes will have to be made.

The oil and gas exploration and production industry uses a great number of standards developedby a range of organisations and national, regional and international standardisation bodies.

A standard is a document providing rules, guidelines or characteristics for activities or theirresults, aimed at the achievement of the optimum degree of order in a given context [1]. Theindustry uses standards with the specific aim of providing a means to enhance technical integrity,transfer knowledge and carry out business efficiently.

It is the E&P industry's goal to foster the development of standards on an international level forthe broadest possible application. Worldwide use of the standards is seen as being for themutual benefit of users and manufacturers [3,8]. The E&P industry has in many areas adoptedlocal, national or regional standards for non E&P-specific equipment such as pressure vessels,lifting equipment, materials, electrical gear, etc. Certain regional or national standards haveproven so useful to the E&P industry that they are extensively used and hence basically adoptedby this industry.

In many areas, American standards and in particular API (American Petroleum Institute), ASME(American Society for Mechanical Engineers), ASTM (American Society for Testing Materials),NACE (National Association of Corrosion Engineers) and NFPA (National Fire ProtectionAgency) Standards provide the upstream industry with standards that support activitiesworldwide. ANSI (American National Standards Institute) is the recognised standardising bodyfor the USA [2].

However, new developments are underway, as explained hereafter. The E&P industry isadapting to the changing political and economic climate. Until recently the API was the leading oilindustry organisation. With the upcoming European Market the situation is changing. TheCommittee for European Normalisation (CEN) and the International Standards Organisation(ISO) seem to be setting the pace [3,4].

For many years, API's Committee 5 the Committee on standardisation of Tubular Goods hasbeen involved with the international use of its standards. The manufacturing and use of tubularshas recently expanded to all corners of the world. Committee 5 has extended its membership toqualified users and manufacturers regardless of their location. As a result, the tubular- goodsstandards have developed the necessary provisions needed in any international standard. Duringthe recent years Committee 5 has worked with CEN representatives from the EuropeanCommunity (EC) to prepare for EC 1992. Some progress has been made [5]. Topics that havebeen discussed include Oil-Country Tubular Goods (OCTG) and line-pipe items, and theCommittee 5's goal is to review all the topics and to handle the higher priority items that mighthelp the EC transition before the opening of the European Common Market in 1992 [5].Committee 5's latest effort is to gain ISO approval and acceptance of many of the existing APIstandards. Several Committee 5 documents are now under review [5].

SIPM has a clear view on standardisation, as defined in [1,6,7]. SIPM uses so called GroupStandards. Group Standards are generated with the specific aim of providing a means toenhance technical integrity, transfer knowledge and carry out business efficiently.

It is Group policy:

to rely, to the maximum possible extent, on External Standards; to aim for minimum additional requirements in Group, Opco and Project Standards; to actively, pursue the proper technical/commercial authorisation processes whereby

variations are justified, both for technical and business reasons;

to consistently improve the quality of Group Standards by creating/ maintaining feedbackloops between users and Custodians of Standards;

to positively influence External Standards bodies, thereby increasing the number andimproving the quality of External Standards applicable to Group use.

In the next paragraphs the organisations that have developed and are maintaining E&Pstandards are reviewed in more detail.

1.6.2 American Petroleum Institute (API)

Some of the equipment used for exploration and production is highly specialised and designswere developed, based on many years of experience, to cater for the specific needs of thisindustry. The API in particular has played a significant role in developing standards for thoseareas which are unique to this specialised industry. API has served the E&P industry since 1923and developed standards initially for domestic U.S. use, and later for broad international use asthe industry spread around the world [8]. API is involved in International Standards through itsactivities as Technical Advisory Group Administrator to ANSI [2].

1.6.2.1 API Committee 5 - tubular goods specifications and publications

The API has appointed a Committee, named Committee 5, on Standardisation of Tubular Goodswhich publishes, and continually updates, a series of Specifications, Standards, Bulletins andRecommended Practices covering the manufacture, performance and handling of tubular goods.They also license manufacturers to use the API Monogram on material that meets their publishedspecifications, so that field personnel can identify materials that comply with the standards. Theirpronouncements are almost universally accepted as the basis for discussions on the propertiesof tubulars. However, this does not mean that everyone accepts the published performance dataas the best theoretical representation of the parameters. The forum consists both of users andmanufacturers.

1.6.2.2 API Committee 5 documents

The documents published by Committee 5 of particular relevance to casing design andspecification are described below.

1. API SPEC 5CT, "Specification for casing and tubing". Covers seamless and welded casingand tubing, couplings, pup joints and connectors in all grades. Processes of manufacture,chemical and mechanical property requirements, methods of test and dimensions areincluded.

2. API STD 5B, "Specification for threading, gauging, and thread inspection for casing, tubing,and line pipe threads". Covers dimensional requirements on threads and thread gauges,stipulations on gauging practice, gauge specifications and certifications, as well asinstruments and methods for the inspection of threads of round thread casing and tubing,buttress thread casing, and extreme line casing, and drill pipe.

3. API RP 5A5, "Recommended practice for field inspection of new casing, tubing, and plainend drill pipe". Provides a uniform method of inspecting tubular goods.

4. API RP 5Bl, "Recommended practice for thread inspection on casing, tubing and line pipe".The purpose of this recommended practice is to provide guidance and instructions on thecorrect use of thread inspection techniques and equipment.

5. API RP 5Cl, "Recommended practice for care and use of casing and tubing". Covers use,transportation, storage, handling, and reconditioning of casing and tubing.

6. API RP 5C5, "Recommended practice for evaluation procedures for casing and tubingconnections". Describes tests to be performed to determine the galling tendency, sealingperformance and structural integrity of tubular connections.

7. API BULL. 5A2, "Bulletin on thread compounds". Provides material requirements andperformance tests for two grades of thread compound for use on oil field tubular goods.

8. API BULL. 5C2, "Bulletin on performance properties of casing and tubing". Coverscollapsing pressures, internal yield pressures, and joint strengths of casing and tubing andminimum yield load for drill pipe.

9. API BULL. 5C3, "Bulletin on formulas and calculations for casing, tubing, drillpipe and linepipe properties". Provides formulas used in the calculations of various pipe properties, alsobackground information regarding their development and use.

10. API BULL. 5C4, "Bulletin on round thread casing joint strength with combined internalpressure and bending". Provides joint strength of round thread casing when subject tocombined bending and internal pressure.

1.6.2.3 Items under review

In 1987 Committee 5 formed an ad hoc group to develop a list of topics that caused difficultieswith the application of specifications in the use and the ordering of products. Enquiries underconsideration that will have substantial impact on the specifications are listed below [5].

1. The adoption of a super-K grade in the minimum strength level of 65,000 to 70,000 psi andthe elimination of grade K-55.

2. Combination of Grades P105 and P110 into a single grade with modified strength levels.

3. Evaluation of test frequency on tubular goods and couplings.

4. The effect of full-body normalising on corrosion.

5. Better methods of evaluating electric resistance weld tubular seams.

6. Inspection methods to include transverse and ID inspections.

7. Premium connections for casing and tubing.

8. Toughness requirements for casing, tubing, drillpipe, and couplings.

9. Suitability of NACE testing of C-90 and T-95 thin-wall tubulars.

10. Quality limits.

11. A complete revision of STD 5B.

1.6.2.4 Shortcomings of API standards

The API emphasises "voluntary, consensus standards." The consensus results from theparticipation of manufacturers and users. However, manufacturers generally oppose anyadditional specification restrictions. Oil Country Tubular Goods (OCTG) manufacturer attendancesignificantly exceeds user attendance at committee meetings. This continues to lead to productsthat are several years behind those currently being purchased by knowledgeable operators usingtheir upgraded specifications.

1.6.3 International Standardisation Organisation (ISO)

ISO describes itself as "the specialised international agency for standardisation". Its membersare the national standards organisations of 91 countries. ISO publishes International Standardsemanating from several Technical Committees and sub-committees.

ISO is governed by a Technical Board comprising one representative from each national body.The Central Secretariat coordinates ISO operations, administers voting and approval procedures,maintains and interprets the Directives that set out the procedures and rules, and publishes theInternational Standards.

ISO is responsible for all fields of international standardisation except electrical and electronic.

1.6.3.1 ISO Technical Committee 67 (ISO/TC 67) oil industry matters

ISO/TC 67 was reactivated in 1988, because the international upstream industry wasincreasingly recognising the need for good international standards that could be accepted andapplied worldwide.

As part of the reactivation, the scope of ISO/TC 67 was extended to the standardisation of thematerials, equipment and offshore structures used in drilling, production, refining and thetransport by pipelines of petroleum and natural gas. The work programme developed wasprimarily in the fields of drilling and production but also includes machinery and equipment usedin refining and petrochemicals.

1.6.4 Committee for European Normalisation (CEN)

CEN is the European counterpart of ISO. It consists of the members of the national standardsorganisations of the EC countries. It aims to achieve the goal of the EC, i.e. to improve theinternational competitive position of European industry.

One of the methods to achieve this is the removal of technical trade barriers by:

- harmonising standards (with emphasis on health, safety and environment) into EuropeanNorms (ENs);

- introducing directives (which will become law at national level, referring to relevant ENs);- harmonising certification.

Testing and certification in Europe are reviewed in [9].

1.6.5 Cooperation between ISO, CEN and API

As all CEN-members are also ISO members, a close cooperation exists. The cooperationbetween ISO and CEN has been formulated as follows:

"It is declared policy of the community that whenever possible CEN/CENELEC shall implementinternational standards in a uniform way but where international standards have not yet beendeveloped or where existing standards need to be adapted to European situations, CEN andCENELEC will develop ENs in anticipation of international ones."

As part of the Harmonisation Legislation for Europe 1992 the EEC commission requested theCEN to introduce ENs. As the upstream oil and gas industry is dominated by API standards, theCEN requested the ISO to investigate the feasibility of converting API standards into ISOstandards and subsequently into ENs.

It was decided to divide the API standards into three classes:

- Class 1: API standards to be circulated by the ISO central secretariat under the "fast-track"procedure, meaning 1-2 years [10].

- Class 2: API standards to be further discussed to modify them prior to submittal to the ISO.

- Class 3: API standards requiring significant study prior to moving forward as internationalstandards.

In 1988 API offered more than 70 of its Standards to ISO, to be the basis of InternationalStandards. In 1989 an ISO Advisory Group classed several of these as suitable for adoptionwithout technical modification and ISO/CS agreed to "fast-track" these to become InternationalStandards. "Fast-Track" means that the API document is given an ISO Number, front cover andforeword but is otherwise presented as is. So far API Bull. 5C3, API RP5Cl and API Std 5Bhave been "fast-tracked".

The ISO foreword addresses issues such as equivalent references to American nationalreferences, certification and the API Monogram.

The industry is now three years into the process of "transferring" API Standards. It is no longerseen as appropriate that all the API Standards offered should become ISO Standards. Somemay be better left with API because the helpful and discursive style of many (RPs and Bulletinsin particular) is lost when re-formatted to comply with ISO Directives.

1.7 References

[1] SIPM, MFSO/3Procedural Specification - DEP PublicationsDEP 00.00.00.30-Gen.

[2] Wilson, D.E.Internationalisation of oil industry standardsOTC 6921

[3] Arney, C.E.Toward one set of international standards for the petroleum industry worldwideOTC 6922

[4] Thomas, G.A.W., Throp, G. and Jenham, J.B.The upstream oil and gas industry's initiative for international standardsOTC 6920

[5] Bartlett, L.E., Kohut, G.B, Dabkowsky, D.S. and McGill, R.Activities of the API Committee on Standardisation of Tubular GoodsSPE Drilling Engineering, September 1991, 215-218

[6] SIPM, EPO/7Standardisation Spearhead-Standardisation PointersEP 90-3300

[7] SIPM, EPD/15Standardisation Spearhead - A Progress ReportEP 90-3301

[8] E & P ForumPosition Paper, Development and Use of StandardsMarch 1992

[9] Gundlach, H.C.W.Testing and certification in EuropeOTC 6924

[10] E& P ForumReport of Status of "Fast - Track" API Standards in ISO (monthly report)January 1992

2.0 Introduction

This part of the Casing Design Guide deals with two important operations which must precedethe detailed casing design calculations: data collection (Chapter C) and preliminary selection ofthe casing scheme for the planned well, specifying the minimum casing diameter and minimumcasing shoe setting depth for all strings (Chapter D).

Fit for purpose design is impossible without early collection off all the relevant data. This shouldbe done by a multidisciplinary team. Chapter C discusses the types of data required for casingscheme selection and the subsequent detailed design calculations, and indicates briefly howthese data should be processed to produce suitable input for the design process. Appendix 2shows, by way of example, a number of data-collection sheets for single-string ventures.

As discussed in Chapter D, casing diameters should be the minimum feasible given theformation evaluation requirements and drilling and production equipment sizes. Recentdevelopments in drilling, evaluation and completion techniques have increased the application ofslimhole drilling and monobore completions to allow for slimmer casing-scheme selection.

Casing setting depths are determined by comparing formation strength with the loads to whichthe formation may be subjected. The primary method of estimating formation strength is still theuse of leak-off and limit tests, though other methods are available and under development. Themain means of determining wellbore pressure loads during drilling, mud circulation and tripping isphysical modelling. SIPM recommends use of the HYDRAUL and SWABSURGE computermodels, available via OSCP, for this purpose. For the modelling of wellbore pressure loadingduring well control Shell Research, Rijswijk, has developed the relevant software, WELLPLAN/WINDOWS, which will be available by mid-1993.

The basic elements of rock mechanics are reviewed in Appendix 3. Leak-off and limit tests arediscussed in Chapter C, and recommended procedures for carrying them out are given inAppendix 4. Appendix 5 gives an example of the calculation of for Nation strength from theresults of leak-off tests.

3.0 Design parameters

3.1 Introduction

Considerable effort is required from the Petroleum Engineering and Operations departmentswhen planning, designing and drilling/completing a well. In view of the high costs of theseoperations and the severe penalties attached to failure, the data set used for casing design mustbe as complete as possible right from the start. Some of these data are laid down in thedevelopment plan, well proposal or well objectives. However, in depth and "fit-for-purpose"casing design demands more detailed information on all strata to be penetrated.

This chapter is specifically aimed at stressing the importance of a good, complete data set,collected by a multidisciplinary team, prior to the design of a well.

The relevance of the data set will be addressed and examples will illustrate how the data can bepresented. The topic of data collection is not covered exhaustively in this chapter; it is the task ofthe Opco to establish a structured data-collection organisation including at departments involved,and to arrange for internal audits to highlight shortcomings in the data flow [1].

The parameters involved will be called the design parameters. This chapter will mainly addressthe basic geological and reservoir related design parameters that the casing designer requiresfrom various departments prior to the design of a well. These are: the lithological column, theformation-strength, pore-pressure and temperature profiles, the hydrocarbon composition andthe H2S/CO2 concentrations.

Derived design parameters such as required mudweight or gaslift pressure will not be discussedhere.

It will be clear that the design parameters can be obtained either from actual measurements orfrom (computer-based) modelling tools. Reworking and translating these data into a usableformat will obviously assist all parties involved. Several Opcos are now streamlining their dataflow [2], others have developed special data-collection sheets (see Appendix 2).

In simple terms, casing design then involves use of the relevant design parameters, as discussedin this chapter, in the design equations presented in Chapters G, H and J, for the relevant loadcases discussed in Chapter F.

3.2 Lithological column

The lithological column is the description of the sequence of formations that are prognosed to bepresent in the well to be drilled.

Every formation has its characteristic properties with regard to formation strength, drillingproblems , reservoir potentials, etc. Advanced knowledge of these properties will be time andcost saving.

Lithological information is important in casing design for the following reasons:

- The column may provide a warning for potential drilling and casing hazards (see Figure C-1).

- The parameter will assist the Drilling Engineer in making a tentative design of the depth forthe various casing shoes, as the type of formation and its depth will give a good indication offormation strength. More details are to be found in the formation breakdown profileparagraph.

- The presence (if an aerobic environments can be an indicative for H2S which may be formedby bacterial action. More details to be found in the H2S/CO2 profile paragraph.

- In combination with offset well pore pressure profiles potential over/under pressure zonesmay be predicted. More details on this topic are to be found in the pore pressure profileparagraph.

In case of an exploration well the casing designer may not have much information available. Wellplanning and design will be based entirely on information from seismic and regional geologicalinformation. However, with the progress of time and the increase of the available data thegeological prognosis can be compared to the actual lithological column as shown in Figure C-2.The geological summary sheets reflect in a concise way all the relevant mud logging data.Common data bases, like EPIDORIS, will supply more detailed information on the drilling relatedactivities. This local information could be further summarised to reflect a base case lithologicalcolumn. Figure C-1 gives an example.

Note that the predicted depths always have an error margin. The reason is that the prognoseddepth is derived from a two way travel time of a soundwave through the various formations.Seismologists and geophysicists quantify these margins leading to a shallow or deep estimation.Narrowing this margin down will lead to a more "fit-for-purpose" well design.

3.3 Formation-strength profile

3.3.1 Introduction

Formation strength refers to the ability of rock to withstand a certain load without failure. Rockfailure and the opposite, formation integrity are important phenomena in Petroleum Engineering.Different measures of formation strength are used in the different disciplines in the industry:

Geology, e.g. modelling of geological structures, trapping mechanisms of hydrocarbonaccumulations and mechanisms of overpressures.

Drilling Operations, e.g. casing setting depth, maximum safe drilling depth and lossescaused by circulating pressures, surge pressures, and cementing operations,

Production Operations, e.g. well killing, sand control operations and well stimulation.

FIGURE C-1 : LITHOLOGICAL COLUMN

FIGURE C-2 GEOLOGY SUMMARY SHEET OF WELL 14/25a-3

For a complete theory of Rock Mechanics we refer to a suitable textbook or manual [271. A goodsummary of the aspects relevant to the Drilling Engineer is presented in Appendix 3 [3].

The main importance for casing design is the relation between wellbore pressures and the abilityof the borehole wall to contain wellbore fluids, both for an intact and a fractured borehole. Thefollowing paragraphs will address the relevant definitions, followed by the evaluation anddescription of the different formation strength test methods. The preferred test method will bediscussed to offset the accuracy of the results against the risk to reduce the formation strength.

3.3.2 Borehole failure

The mechanism of borehole failure can be shown and discussed with the results of a typicalformation breakdown test, (see Figure C-3). In a plot of the downhole pressure exerted on theformation of a closed-in well versus time (or volume pumped), several characteristic points canbe identified.

Figure C-3 Relation between leak-off, formation-breakdown, fracture-propagation, fracture-closure and instantaneous shut-in pressures.

FIGURE C-3 : RELATION BETWEEN LEAK-OFF ,FORMATION-BREAKDOWN,FRACTUREPROGRATION, FRACTURE-CLOSURE AND INSTANTANEOUS SHUT-IN PRESSURE

Initially, the pressure - time relationship is linear. The Leak-Off Pressure (LOP) is the pressure atwhich the curve deviates from the initial linear build-up.This deviation is associated with anoticeable intake of fluid into the formation either by filtration through the mudcake or by theformation of small cracks in the borehole wall.

At the Formation Breakdown Pressure (FBP) the borehole wall fails and a major fracture isinitiated. Powered by the decompression of the wellbore fluid, this fracture grows almostinstantaneously and the wellbore pressure reduces sharply. The stress concentrations around anintact borehole provide the strength of a borehole. Once formation breakdown has occurred,these stress concentrations disappear, and the strength of the borehole is reduced to theminimum in situ stress of the formation. This explains the reduction in strength of a fracturedborehole.

If pumping is continued, the fracture propagates into the formation in a controlled manner, andthe pressure stabilises at the Fracture Propagation Pressure (FPP). Due to the frictional pressurelosses in the fracture, the FPP will increase if the flowrate increases.

When pumping is stopped, flow into the well and into the fracture stops almost immediately;frictional pressure losses disappear, and the pressure drops to a value called the InstantaneousShut -In Pressure (ISIP). At this stage, the fracture is open, but does not propagate any more.

The fluid in the fracture then leaks away, through the faces of the fracture into the formation, andthe pressure decreases. The pressure at which the fracture closes is the Fracture ClosurePressure (FCP). It can be shown that this pressure is equal to the minimum in-situ stress.

After the fracture has closed the fluid leaks away very slowly, through the mud cake into theformation. The pressure will, given enough time, reduce to the hydrostatic pressure of the mudcolumn. There is no clear transition between these last two situations on the pressure decaycurve. Techniques have been developed to derive the FCP, by determining the intersectionbetween the two "trend" lines in the pressure time plot.

If the fractured borehole is pressured up again, the fracture opens up at the Fracture ReopeningPressure (FRP), which in most cases is equal to the FCP. The fracture continues to propagate atthe FPP. The original FBP will not be reached anymore; the strength of the borehole is reducedcompared to the original unfractured situation. In some situations the strength of the boreholemay be restored. This process is called "clay-healing". However, the mechanism is notunderstood, and should not be relied upon.

3.3.3 Formation-strength gradient and equivalent mud weight

Formation strength is often expressed as a gradient by dividing the pressure by the true verticaldepth relative to a reference level. This way formation strength measurements at different depthscan be compared and formation strength can be related to mud weight. The different conventionsare given below.

In geophysics and rack mechanics, the Formation Breakdown Gradient (FBG) is calculated bydividing the FBP by the true vertical thickness of the overburden. This way formation strengthand overburden gradient can be compared. The conversion is different for land and offshorewells, (see Figure C-4):

Land wells : FBG = dfed

FBP

form (relative to surface) (C-1)

Offshore : FBG = )surfacetorelative(dd

)dd(xFBP

seabedform

FWLseabedsw

(C-2)

(Similar expressions can be given for LOG, FPG and FCG.)

However, for Drilling Engineering in casing design and for well control, the FormationBreakdown Pressure is expressed as an equivalent mud gradient. This is the mudgradient of a mud that will give a hydrostatic pressure equal to the FBP at the formationof interest. This can be calculated as follows:

FB form = formd

FBP (relative to derrick floor) (C-3)

(Similar expressions can be given for LO, FP and FC , for LOP, FPP and FCP resp.)

where:

dform = true vertical depth of the formation below derrick floor

dfe = drillfloor elevation above reference level (usually ground surface)dFWL = true vertical depth of Free Water Level, below derrick floor

dseabed = depth of seabed below derrick floor

sw = seawater density (equivalent mud gradient)

FB,form. = equivalent mud gradient of the FBP.

3.3.4 Measuring the formation strength

3.3.4.1 Introduction

Formation strength measurements are performed to determine the strength of the wellbore. Thisinformation is used for planning of subsequent downhole operations and provides a database forfuture well designs in the area.

Different methods exist for determining the strength of a formation:

Limit Test or Leak-Off Test - During these tests the well is slowly pressured up, taking carenot to break down the formation. As soon as a predetermined pressure is reached or whenleak-off is observed, the test is stopped. These tests confirm that the wellbore can withstandthe test pressure without breakdown of the formation.

FIGURE C-4a : DEFINITION OF FORMATION-STRENGTH AND PORE-PRESSURE GRADIENTS(LAND LOCATION )

FIGURE C-4b : DEFINITION OF FORMATION-STRENGTH AND PORE-PRESSURE GRADIENTS(OFFSHORE LOCATION)

Formation Breakdown Test - The well is pressured up until formation breakdown isobserved. The test is sometimes continued with a series of fracture opening and closingcycles (microfrac, minifrac test). The results of these tests will give information on the state ofstress of the formation, (e.g. minimum and intermediate in-situ stress, see Appendix 3).

Measurements on core material - These measurements will give additional information onrock properties and the orientation of the in-situ stress.

Wireline testing - Several new wireline tools for improved rock property evaluation orformation strength measurement are under development and will become available in thenear future.

Analysis of loss events - The strength of the open hole can be inferred from a carefulanalysis of any operational event that causes losses. If losses occur, they should be treatedas an opportunity to derive valuable information.

The different tests are carried out with different objectives. In this paragraph, the methods andobjectives of each type of test will be addressed, and the advantages and disadvantages of eachtype will be discussed.

3.3.4.2 Available measurement methods

Limit tests and leak-off tests

Leak-off and Limit tests are carried out during the actual drilling of the well. The BOP is closedaround the drillpipe, and the well is slowly pressured up, using mud. Usually the pressure ismeasured and recorded at surface, but for high mud weights the application of downhole gaugeswith a surface read out should be considered. At the first indication of fluid leak-off into theformation the pumping is stopped. Limit tests are carried out until a pre- determined test pressureis reached. The test pressure is usually determined by the minimum formation strength requiredto drill the next open hole section. Leak-off tests are carried out until leak-off is observed. InAppendix 4 a detailed procedure is given for the preparation, execution and reporting of Limit andLeak-off tests. An example of a typical trend curve for a leak-off test is shown in Figure C-5.

The formation should not be fractured during a Limit or Leak-off test. If the pressure suddenlydrops during the test, this may be an indication of formation breakdown. This should be treatedas an opportunity to determine FBP and FCP (see formation breakdown test on next page).

The objective of a Limit or Leak-off test is to:

investigate the capability of the formation in open hole below the casing shoe to withstandpressure;

confirm the quality of the cement bond around the casing shoe.

FIGURE C-5 : TYPICAL TRENDS FROM LEAK-OFF TEST

The results of the tests will be used to plan the operations in the next hole section and designsubsequent wells in the same area by:

- calculating the Maximum Allowable Annular Surface Pressure (MAASP) for well controlpurposes;

- confirming the soundness of the well design by checking the maximum safe drilling depth,against the planned depth;

- preventing losses to the formation during subsequent drilling operations (during circulating,tripping, running casing and cementing);

- storing and analysing the information in a database, for future well design optimisation,contingency planning and regional geological studies. This will result in graphs like FiguresC-6, C-7 and C-8.

Formation breakdown test

During a formation breakdown test, the well is closed in at the BOP or wellhead, and is slowlypressured up until formation breakdown is observed. Then pumping is stopped, and the pressuredecay curve is recorded until the pressure stabilises. The test may be continued with anotherpumping period, to determine when the fracture re-opens.

This test will determine some real strength properties of the borehole and the formation (FBP,FCP and FRP ). If the pore pressure and overburden stress are known, the minimum andintermediate in situ stress can be estimated, using Eq. App. 3-6 to Eq. App. 3-9.

FIGURE C-6 : MINIMUM HORIZONTAL STRESS AS A FUNCTION OF DEPTH FOR US GulfCoast, North Sea and onshore Netherlands

FIGURE C-7 : LEAK-OFF PRESSURE FROM NORTH SEA CENTRAL GRABEN AS A FUNCTION OFDEPTH

These data can be used to:- plan the operations in the next openhole section;

- design subsequent wells in the same area;- construct a regional in-situ stress model and provide a good formation strength prediction.

A simple example calculation is provided in Appendix 5. 'This shows how the FBP for a futurewell can be predicted based on a given dataset. Note the dependency of the FBP on the holeangle.

It should be noted that after a formation breakdown test the maximum pressure to be applied tothe formation in the FCP.

Microfrac test, minifrac test

During a microfrac or minifrac test a fracture will be created and a series of fracture opening,fracture propagation and fracture closing cycles are carried out. The pressure at which thefracture opens and closes will be measured over a number of cycles. For a minifrac test, theinjection rate during each of the pumping cycles is higher , and the injection volume larger thanfor a microfrac test.

Methods and procedures for carrying out these tests are described in [4].

A micro-frac test requires little more effort than a formation breakdown test, but provides moreaccurate data on in-situ stress and additional data on fracture propagation. It is thereforepreferred to carry out micro frac tests. Mini-frac tests also provide accurate data, but will only becarried out for well stimulation planning. Results are used to:

- construct a regional in-situ stress model and provide a good formation strength prediction;- design well stimulation treatments by the determination of the fluid loss coefficient.

Measurements on cores

More information on the ratio of horizontal in-situ stress and the properties of rock can beobtained with rock mechanical measurements on core samples. Examples of suchmeasurements are:

- thick walled cylinder tests;- unconfined compressive strength tests;- triaxial strength test;

- differential strain analysis;- inelastic strain recovery tests;- compaction experiments.

Wireline testing

Some wireline evaluation tools may offer additional information related to in-situ stress andformation strength:

- With Borehole Geometry Surveys inferences can be made about the orientation of the in-situstress.

- Sonic Evaluation - Measurement and processing of shear wave and compressional wavevelocities may give some elastic rock properties (Poisson's ratio and Young's modulus).Under some specific assumptions, the ratio between the effective vertical stress and theminimum horizontal effective stress can be derived from these measurements. This allowsthe minimum in-situ stress to be calculated.

New concepts and tools for improved formation strength testing are under development:

- Several service contractors are developing tools to perform a small formation breakdown testdownhole between inflatable packers. Typical results are formation breakdown, fracturepropagation and fracture closure pressures.

- Downhole acoustic emission measurement techniques may offer a better way to determineimpending formation breakdown during a formation strength test. This method has so faronly been proven under laboratory conditions, and is not yet commercially available.

Analysis of "loss events"

Formation strength may also be inferred from a careful analysis of some "drilling events". Thesecould be situations where losses are observed during drilling, tripping, cementing or a wellcontrol situation. These events should be treated as an opportunity to derive some real formationstrength data [26].

The mechanism that induced the losses should be identified, and the pressures that occurred atthe moment the losses started will be an indication of the FBP. The sequence of events afterformation breakdown, may give some information about the minimum in-situ stress.

There is no uniform method to perform the analysis. It requires common sense, and acommitment to improve the regional strength model. A system should be in place to gather andprocess formation strength data acquired this way.

3.3.4.3 Choosing the right method

The process of deciding which formation strength testing method to choose is characterised bytwo, usually conflicting considerations:

the required accuracy and significance of the results;

the requirement to avoid the risk of reduced formation strength, caused by formationbreakdown.

In the design stage a trade-off has to be made between the risk of formation strength reductionand the need for realistic formation strength data. These two aspects are discussed below.

Accuracy of formation strength testing method

The different methods for formation strength testing are summarised in the table below. They aregiven in order of increasing accuracy and significance of the results. Laboratory tests andwireline testing methods are not mentioned here, because they only offer additional informationor are not yet operationally available.

A successful Limit test only confirms that the Formation Breakdown Pressure of the formationbelow the casing shoe is higher than the limit pressure (LP). If the required capacity is confirmed,drilling can continue. However, little is actually learned about the formation itself, and therefore,this information cannot be transferred to other wells.

The occurrence of Leak-off is the first indication related to the strength of the borehole and themudcake. It is traditionally used as an indication of impending formation breakdown, and the LOPhas been often used as an estimate of FBP or minimum in-situ stress. However, the use of theLeak-off test as a method to characterise formation strength has the following drawbacks:

- Sometimes breakdown occurs without indications of Leak-off.

- Leak-off is dependent on parameters that are not related to formation strength (e.g. mudtype, length (if the open hole section, whether any natural or drilling induced fractures areexposed, borehole condition).

- Leak-off testing relies on subjective interpretation of the engineer. Interpretation is difficult,especially in unconsolidated formations.

- Leak-off tests do not repeat well. Leak-off pressures tend to increase with time, especially insandstone.

- Leak-off pressures have been shown [5] not to correlate closely with other, more significantformation strength parameters (for example FBP and FCP or minimum in-situ stress).

During a Formation Breakdown test, the FBP is determined, and the FCP can be estimated.The FCP is equal to the minimum in-situ stress, which is to be preferred as a measure of formation strength. Its value is not dependent on the mud or the borehole orientation orgeometry, and can be correlated regionally from well to well. Knowledge of the minimum in-situstress also offers the possibility to predict FBP for nearby wells at different deviations (see Eq.App. 3-6 to Eq. App. 3-9).

A micro-frac or mini-frac test allows the minimum in-situ stress to be derived with a higherdegree of accuracy. Data from these tests can be used to derive regional models of in-situ stressand formation strength. Additional information about fracture propagation is obtained from thesetests. This can be used to design well stimulation treatments.

TABLE OF TEST TYPES AND USES

Operational considerations

The main consideration that co