MANUAL (SAMPLE)
CASING DESIGN GUIDE
PTS 40.018
DECEMBER 1992
PREFACE
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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
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 drilling a) Internal pressure profile b) External pressure profile
c) Special cases
Air, foam or aerated drilling Salt loading
Formation compaction Blowout
6.2.2.2 Evacuation during production a) Internal pressure profile b) External pressure profile
c) Special cases Artificial-lift wells Salt loading Formation compaction Blowout 6.2.3 Burst loads
6.2.3.1 Burst during drilling
a) Internal pressure profile b) External pressure profile
c) Special cases
Over-pressured aquifer in borehole below casing Salt loading
6.2.3.2 Burst during production
a) Internal pressure profile b) External pressure profile
c) Special cases
Gas-lift wells Salt loading
Gas-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
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 fluid
density
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 surface b) Casing hangers at seabed
7.5.4 Thermal growth of wellhead
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
9.2.4 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
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
12.5.1 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 Lamé 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)
14.7 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
A.0 Overview
A.1 Foreword
Casing design is an integral part of the effort required to design, build and operate Quality Wells, contributing monetary value over their entire life cycle, without compromising safety 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 bearing capacity of the casing strings, require particular attention.
Extremely important is also that good documented information on the casing design intent is known at the wellsite, in order to ensure that the operating envelope remains at all times within the design criteria.
• Optimisation of the commercial aspects, i.e. ensuring fit for purpose, cost effective designs and standardisation. In 1991, some $350 million was spent on casing/tubing (±16% of the Group's drilling expenditure), hence a determined effort will lead to considerable savings. Early involvement of the Operations disciplines in greenfield exploration ventures and field development plans is regarded as the prime vehicle for the preliminary casing scheme optimisation. Computing tools now available will speed up the subsequent detailed design calculations, allowing the casing designer to concentrate on high value input and alternative design 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 casing design equivalent to that provided in Round II. It is recommended that a Casing Design focal point be established in each Opco to collate relevant local expertise, develop it further where required, and address more complex issues.
This Guide interfaces with other SIPM supported documents, to which reference is made where appropriate. Due attention has been paid to relevant international standards. Local Opco staff may depart from the advice given in this Guide, provided the proper control procedures are followed and documented.
This Casing Design Guide is one of the functional documents issued by and with the authority of EPO/51, the Head of Drilling Engineering. Any comments or observations for subsequent revisions 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 has become obsolete.
A.2 Acknowledgements
The author wishes to thank all staff in SIPM, KSEPL and Opcos, who have contributed to the compilation of this Guide.
He would like to extent this especially to KSEPL staff, who have contributed to the writing of the relevant 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 the following 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
1.0 Introduction 1.1 Introduction
Field experience and the results of research carried out both within and outside the Group indicate that casing costs for both exploration and development wells can be cut without compromising safety, and without adverse effects on the environment over the entire life cycle of the well, if an approach to "fit-for-purpose" casing design embodying the following features is adopted:
- 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 the well [2];
- accurate definition of the various load cases to which each casing string is likely to be subjected [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 and external pressure loads, and to the axial loads encountered during installation of the casing, and
• triaxial design methods involving detailed calculation of the radial, tangential and axial stresses on each volume element of the casing to determine the resistance to the actual service 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 above characteristics, together with all the background information required for their effective deployment.
The present chapter discusses the various functions which casing has to perform, defines the different types of casing used in a well, and describes the casing design process with its different elements - which will be dealt with in full in subsequent chapters.
International standards relevant to casing design are currently in a process of evolution. The position of the various standardisation bodies involved is explained in Appendix 1. Departure from these external standards is acceptable provided this is properly documented and discussed.
1.2 Purpose of casing
For drilling and completing a well it is usually necessary to line the walls of the hole with steel pipe called casing. This casing, together with the cement which holds it in place and seals the annulus [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;
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 is called 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 of casing 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 from erosion by the drilling fluid and, in the event of an offshore application, reduce the wave and current loads imposed on the inner strings. Where the formation is sufficiently stable, this string may be used to install a full mud circulation system. It also serves to guide the drillstring and subsequent casing into the hole. The name given to this string is primarily related to the type of drilling 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-drilled hole. The stove pipe often carries the subsequent conductor casing, but once the latter string is cemented the stove pipe is released from this axial load. Therefore, subsequent casing strings will be hung off the conductor casing string.
Marine conductors may form a part of the piling system for a wellhead jacket or piled platform and are therefore often designed by the structural engineers. They provide centralisation for the inner casing strings against column buckling, but do not carry direct axial loads except during initial installation of the conductor string. The marine conductors serve to reduce the wave and current loads imposed on the inner casing strings and provide sacrificial protection against oxygen corrosion in the splash zone.
On gravity structures, they are also required to minimise the transfer to the inner casings of stresses 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 no Temporary Guide Base is used, they support the Main Guide Base which carries and aligns all future wellhead components, BOPs, Xmas tree and casing/tubing strings for both the drilling and production 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 by the environment via marine riser and BOP.
1.3.2 Conductor string
The conductor string is used to prevent poorly consolidated formations from sloughing into the hole, to provide a full mud-circulation system, to protect fresh water sands from contamination by the drilling mud and to provide protection against shallow hydrocarbons. This string is usually cemented to surface or seabed and is always the first casing on which one or more BOPs are mounted.
For onshore wells the conductor string usually supports the wellhead, the BOP, the Xmas tree and 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 often the most critical design parameters for this casing. Above the top cement, the conductor must be centralised to prevent column buckling. The annulus between the marine conductor and conductor string is usually left uncemented above the mudline, in order to minimise load transfer from 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, and stays in tension.
1.3.3 Surface string
The next string is the surface string which provides blowout protection during deeper drilling. Its setting depth is often chosen so that it also isolates troublesome formations, loss zones, shallow hydrocarbons, 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 isolate formations or deeper hole profile changes that can cause drilling problems. It is recommended to set an intermediate casing string whenever there is a chance of encountering an influx that could cause breakdown at the previous casing shoe, and/or severe losses in the open hole section. A string is therefore nearly always set in the transition zone above or below significant overpressures, and in any potential cap rock below a severe loss zone. Similarly, it is good practice when appraising untested, deeper horizons, to case off the known hydrocarbon intervals as a contingency against the possibility of encountering a loss circulation zone. Obviously this latter advice applies primarily to massive reservoir sections rather than sand-shale sequences with numerous small reservoirs and sub-reservoirs.
An intermediate string may also be set to shut off a swelling shale, a brittle caving shale, a creeping 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 designer should design the well to combine many of these objectives in a single casing point. A liner may be used instead of a full intermediate casing, and difficult wells may contain several intermediate casing strings and/or liners.
1.3.5 Production string
This is the string through which the well will be completed, produced and controlled throughout its service life. While on some exploration wells this will amount to only a short testing period, on most development wells it will span a significant number of years during which many recompletions may be performed. In most cases, the production casing will serve to isolate the productive intervals, to facilitate proper reservoir control and to prevent the influx of undesired fluids. 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 well deliverability, 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 production casing and impose additional thermal stresses. The loads to which a production casing is subjected 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 production string. Special consideration is required where drilling takes place below the production casing since it may suffer some damage, e.g. in barefoot completions, open-hole gravel packs, liner completions and deep- zone appraisal. In a liner completion both the liner and casing form the production 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 the production casing should be rotated and/or reciprocated during cementing. This imposes additional 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, but is suspended a short distance above the previous casing shoe. It is usually cemented over its entire length to ensure a seal with the previous casing string. It is indeed important to ensure that the 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 of slimhole 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 additional distinction is usually made between drilling liners and production liners, which are defined as follows.
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 into the 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 a variety of external and internal pressures, thermal loads and loads related to the self-weight of the casing. These casing strings are subjected to time-dependent corrosion, wear and possibly fatigue, which downrate their resistance to these loads during their service life. The interaction between the casing strings - which may lead e.g. to annular pressure build-up or wellhead movement [9,10] also merits attention.
This section briefly surveys the structure of the process used to arrive at a technically and economically sound casing design.
Casing design as described in this Guide is divided into two main phases, preliminary design and detailed design, with the former further subdivided into data collection and casing-scheme selection. As illustrated in Flowchart A-1, it will generally be necessary to repeat these phases in an 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 tools required to obtain the necessary information.
To be effective, data collection must be carried out at an early stage in the design process, by means of a multidisciplinary team including local Opcostaff from the Petroleum Engineering and Operations departments in addition to the casing designer. Recent developments in downhole technology will lead to the introduction of novel ideas resulting in reduced well costs. The casing designer should be familiar with these developments and evaluate their merits for application in his 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 development plan can play a major role in cutting overall well costs, and guaranteeing formation integrity during drilling under all realistic loading conditions [2]. The structure of the selection procedure is illustrated inFlowchart 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. Continual vigilance is required to avoid overdesign and other forms of unnecessary expenditure. It would go beyond the scope of the present Guide to deal fully with all the considerations leading to proper choice of the casing scheme; however, the main lines of this topic are dealt with in
The outcome of the casing-scheme selection is a specification of the minimum external diameter and minimum casing-shoe depth for each casing section in the proposed well. The casing diameter is mainly determined by the availability of downhole drilling equipment and logging tools, and by production requirements determining the sizing of the production or evaluation conduit. The casing-shoe setting depth is usually a function of the strength of the formation to be drilled through and the loads on the wellbore during drilling or lithological/geological related considerations. 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 the casing diameters from the inner strings towards the outer strings and by evaluating the casing setting depths from the total depth upwards to surface.
The preliminary casing scheme may have to be modified on the basis of the results of later stages of the design process.
1.4.2 Detailed design
In the detailed design phase, the casing designer determines the material grade and casing wall thickness for each section of the casing scheme selected, which will allow it to withstand all realistically expected loads throughout the life of the well. The structure of this phase is illustrated in Flowchart A-3. In general, it will be most effective to design the individual casing sections in the 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 designer must decide which load cases can realistically be expected to occur. This topic is dealt with in Chapter 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 API
Bull. 5C2 [11] on the basis of the formulae published in API Bull. 5C3 [12] (see Chapter H), after these values have been corrected to take the influence of corrosion, wear and fatigue (see
Chapter I) into account and divided by the relevant design factor (see Chapter K). The casing design obtained in this way is then checked to see whether the casing selected can withstand the loads occurring during installation (in particular the axial forces due to the total weight of the casing string down to the depth considered, and the shock and torsional loads experienced during setting the casing).
The principle of this uniaxial design Process is illustrated in Figure A-3, and the design calculations involved are dealt with in sections 2 and 3 of Chapter G, and sections 1.1 to 1.3 of Chapter H. In general, uniaxial design often leads to a conservative choice of tubular grade and wall 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 becoming increasingly common. The interrelationship of design loads can now be analysed using a combination of Hooke's law, the Lame equations and the Von Mises yield criterion. These computer analyses relieve the designer of a lot of repetitious calculations and allow him to concentrate on more accurate estimation of the service-life load conditions - a task for which computer software has also been developed [3,4]. As with the uniaxial approach, the influence of corrosion, wear and fatigue should be taken into account before the triaxial design factor is applied. This extension of the design process, made possible by the advent of desktop computing power, should lead to an optimised casing design fine-tuning the simplified conventional approach [4]. Designs that previously did not meet the uniaxial design rules may know be acceptable following a detailed triaxial stress analysis.
The principle of triaxial design is illustrated in Figure A-4. The design calculations involved are dealt 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 also withstand the loads to which they are subjected. Recent developments have led to a wide diversity of connection types and sealing compounds for use with casing connections. The salient aspects of connection design are highlighted in Chapter L, with ample references to the relevant 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, special design 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 design considerations applicable to such cases are discussed briefly, with ample references to the literature, in Chapter N.
Operational aspects
Practical details which need to be taken into account in the design of any casing string are discussed in Chapter O. The casing designer should be familiar with the relevant purchasing specifications [16,17] and should be aware of the procedures and tools available to help the operator responsible for installing and maintaining the casing strings. Early incorporation of these aspects 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 casing design in the future [18,19]. Such methods, permitting quantification of the risk of failure associated 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/51
Making the most of well planning
EP 92-2500 [3] SIPM, EPO/51
OSCP User Guide - version 2.3
EP 91-2156 [4] Pittman, W.
Commercial casing design software - detailed evaluation EP 92-0473
[5] SIPM, EPO/5
Management, Technology and Human Resources, Programme 1991-1993
EP 91-3000 [6] SIPM, EPD
Technology development programme 1992-1994
EP 92-0350 [7] SIPM, EPO/51
Drilling Spearhead Documentation, Volume 1, 2 and 3
EP 89-0115 [8] SIPM, EPO/51
Cementing Manual, Volume I - Primary Cementing of Casing
EP-50500
[9] MacEachran, A. and Adams, A.J.
Impact on casing design of thermal expansion of fluids in confined annuli
SPE/IADC 21911 [10] Adams, A.J.
How to design for annulus fluid heat-up
SPE 22871
[11] American Petroleum Institute
Bulletin on performance properties of casing and tubing
Bull. 5C2, Twentieth edition, 31 May 1987 [12] American Petroleum Institute
Bulletin on formulas and calculations for casing, tubing, drillpipe and line pipe properties
Bull. 5C3, Fifth edition, July 1989 [13] Klementich, E.F. and Jellison, M.J.
A service-life model for casing strings
SPE 12361
[14] Klementich, E.F., Jellison, M.J. and Johnson, R.
Triaxial load capacity diagrams provide a new approach to casing and tubing analysis
[15] Bax, D.J.M. (SIPM) and Bontenbal, P.J. (KSEPL)
Casing connections
Contribution to the upgrade of the SIPM Casing Design Manual
EP 92-1563
[16] American Petroleum Institute
Specification for casing and tubing
Spec. 5CT, Third edition, 1 December 1990 [17] SIPM, EPO/512
Technical suggestions for ordering non-API tubulars
DEN 17/92
[18] Payne, M.L. and Swanson, J.D.
Application of probabilistic reliability methods to tubular design
SPE 19556
[19] Reeves, T.B., Parfitt, S.H.L. and Adams, A.J.
Casing system risk analysis using structural reliability
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 well documented that less conservative casing designs still meet stringent demands on safety and environmental friendliness, and comply with local legal requirements.
SIPM is working with industry partners (and competitors) to make these international standards reflect 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 developed by a range of organisations and national, regional and international standardisation bodies. A standard is a document providing rules, guidelines or characteristics for activities or their results, aimed at the achievement of the optimum degree of order in a given context [1]. The industry 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 for the broadest possible application. Worldwide use of the standards is seen as being for the mutual benefit of users and manufacturers [3,8]. The E&P industry has in many areas adopted local, 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 have proven so useful to the E&P industry that they are extensively used and hence basically adopted by 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 Protection Agency) Standards provide the upstream industry with standards that support activities worldwide. ANSI (American National Standards Institute) is the recognised standardising body for the USA [2].
However, new developments are underway, as explained hereafter. The E&P industry is adapting to the changing political and economic climate. Until recently the API was the leading oil industry organisation. With the upcoming European Market the situation is changing. The Committee 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 has been involved with the international use of its standards. The manufacturing and use of tubulars has recently expanded to all corners of the world. Committee 5 has extended its membership to qualified users and manufacturers regardless of their location. As a result, the tubular- goods standards have developed the necessary provisions needed in any international standard. During the recent years Committee 5 has worked with CEN representatives from the European Community (EC) to prepare for EC 1992. Some progress has been made [5]. Topics that have been discussed include Oil-Country Tubular Goods (OCTG) and line-pipe items, and the Committee 5's goal is to review all the topics and to handle the higher priority items that might help 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 API standards. 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 Group Standards. Group Standards are generated with the specific aim of providing a means to enhance 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 feedback loops between users and Custodians of Standards;
• to positively influence External Standards bodies, thereby increasing the number and improving the quality of External Standards applicable to Group use.
In the next paragraphs the organisations that have developed and are maintaining E&P standards 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 designs were developed, based on many years of experience, to cater for the specific needs of this industry. The API in particular has played a significant role in developing standards for those areas which are unique to this specialised industry. API has served the E&P industry since 1923 and developed standards initially for domestic U.S. use, and later for broad international use as the industry spread around the world [8]. API is involved in International Standards through its activities 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 Goods which publishes, and continually updates, a series of Specifications, Standards, Bulletins and Recommended Practices covering the manufacture, performance and handling of tubular goods. They also license manufacturers to use the API Monogram on material that meets their published specifications, so that field personnel can identify materials that comply with the standards. Their pronouncements are almost universally accepted as the basis for discussions on the properties of tubulars. However, this does not mean that everyone accepts the published performance data as the best theoretical representation of the parameters. The forum consists both of users and manufacturers.
1.6.2.2 API Committee 5 documents
The documents published by Committee 5 of particular relevance to casing design and specification are described below.
1. API SPEC 5CT, "Specification for casing and tubing". Covers seamless and welded casing and tubing, couplings, pup joints and connectors in all grades. Processes of manufacture, chemical and mechanical property requirements, methods of test and dimensions are included.
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 as instruments 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 plain end 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 the correct 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 tubing connections". Describes tests to be performed to determine the galling tendency, sealing performance and structural integrity of tubular connections.
7. API BULL. 5A2, "Bulletin on thread compounds". Provides material requirements and performance 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". Covers
collapsing pressures, internal yield pressures, and joint strengths of casing and tubing and minimum yield load for drill pipe.
9. API BULL. 5C3, "Bulletin on formulas and calculations for casing, tubing, drillpipe and line pipe properties". Provides formulas used in the calculations of various pipe properties, also background information regarding their development and use.
10. API BULL. 5C4, "Bulletin on round thread casing joint strength with combined internal pressure and bending". Provides joint strength of round thread casing when subject to combined 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 difficulties with the application of specifications in the use and the ordering of products. Enquiries under consideration 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 and the 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 the participation of manufacturers and users. However, manufacturers generally oppose any additional specification restrictions. Oil Country Tubular Goods (OCTG) manufacturer attendance significantly exceeds user attendance at committee meetings. This continues to lead to products that are several years behind those currently being purchased by knowledgeable operators using their upgraded specifications.
1.6.3 International Standardisation Organisation (ISO)
ISO describes itself as "the specialised international agency for standardisation". Its members are the national standards organisations of 91 countries. ISO publishes International Standards emanating 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 the International 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 was increasingly recognising the need for good international standards that could be accepted and applied worldwide.
As part of the reactivation, the scope of ISO/TC 67 was extended to the standardisation of the materials, equipment and offshore structures used in drilling, production, refining and the transport by pipelines of petroleum and natural gas. The work programme developed was primarily in the fields of drilling and production but also includes machinery and equipment used in 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 standards organisations of the EC countries. It aims to achieve the goal of the EC, i.e. to improve the international 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 European Norms (ENs);
- introducing directives (which will become law at national level, referring to relevant ENs); - harmonising certification.
1.6.5 Cooperation between ISO, CEN and API
As all CEN-members are also ISO members, a close cooperation exists. The cooperation between ISO and CEN has been formulated as follows:
"It is declared policy of the community that whenever possible CEN/CENELEC shall implement international standards in a uniform way but where international standards have not yet been developed or where existing standards need to be adapted to European situations, CEN and CENELEC will develop ENs in anticipation of international ones."
As part of the Harmonisation Legislation for Europe 1992 the EEC commission requested the CEN to introduce ENs. As the upstream oil and gas industry is dominated by API standards, the CEN requested the ISO to investigate the feasibility of converting API standards into ISO standards 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 international
standards.
In 1988 API offered more than 70 of its Standards to ISO, to be the basis of International Standards. In 1989 an ISO Advisory Group classed several of these as suitable for adoption without technical modification and ISO/CS agreed to "fast-track" these to become International Standards. "Fast-Track" means that the API document is given an ISO Number, front cover and foreword but is otherwise presented as is. So far API Bull. 5C3, API RP5Cl and API Std 5B have been "fast-tracked".
The ISO foreword addresses issues such as equivalent references to American national references, certification and the API Monogram.
The industry is now three years into the process of "transferring" API Standards. It is no longer seen as appropriate that all the API Standards offered should become ISO Standards. Some may be better left with API because the helpful and discursive style of many (RPs and Bulletins in particular) is lost when re-formatted to comply with ISO Directives.
1.7 References
[1] SIPM, MFSO/3
Procedural Specification - DEP Publications
DEP 00.00.00.30-Gen. [2] Wilson, D.E.
Internationalisation of oil industry standards
OTC 6921 [3] Arney, C.E.
Toward one set of international standards for the petroleum industry worldwide
OTC 6922
[4] Thomas, G.A.W., Throp, G. and Jenham, J.B.
The upstream oil and gas industry's initiative for international standards
OTC 6920
[5] Bartlett, L.E., Kohut, G.B, Dabkowsky, D.S. and McGill, R.
Activities of the API Committee on Standardisation of Tubular Goods
SPE Drilling Engineering, September 1991, 215-218 [6] SIPM, EPO/7
Standardisation Spearhead-Standardisation Pointers
EP 90-3300 [7] SIPM, EPD/15
Standardisation Spearhead - A Progress Report
EP 90-3301 [8] E & P Forum
Position Paper, Development and Use of Standards
March 1992 [9] Gundlach, H.C.W.
Testing and certification in Europe
OTC 6924 [10] E& P Forum
Report 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 precede the detailed casing design calculations: data collection (Chapter C) and preliminary selection of the casing scheme for the planned well, specifying the minimum casing diameter and minimum casing shoe setting depth for all strings (Chapter D).
Fit for purpose design is impossible without early collection off all the relevant data. This should be done by a multidisciplinary team. Chapter C discusses the types of data required for casing scheme selection and the subsequent detailed design calculations, and indicates briefly how these data should be processed to produce suitable input for the design process. Appendix 2 shows, 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 the formation evaluation requirements and drilling and production equipment sizes. Recent developments in drilling, evaluation and completion techniques have increased the application of slimhole drilling and monobore completions to allow for slimmer casing-scheme selection.
Casing setting depths are determined by comparing formation strength with the loads to which the formation may be subjected. The primary method of estimating formation strength is still the use of leak-off and limit tests, though other methods are available and under development. The main means of determining wellbore pressure loads during drilling, mud circulation and tripping is physical modelling. SIPM recommends use of the HYDRAUL and SWABSURGE computer models, available via OSCP, for this purpose. For the modelling of wellbore pressure loading during 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 are discussed in Chapter C, and recommended procedures for carrying them out are given in Appendix 4. Appendix 5 gives an example of the calculation of for Nation strength from the results of leak-off tests.
3.0 Design parameters 3.1 Introduction
Considerable effort is required from the Petroleum Engineering and Operations departments when planning, designing and drilling/completing a well. In view of the high costs of these operations and the severe penalties attached to failure, the data set used for casing design must be as complete as possible right from the start. Some of these data are laid down in the development 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 be presented. The topic of data collection is not covered exhaustively in this chapter; it is the task of the 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 address the basic geological and reservoir related design parameters that the casing designer requires from various departments prior to the design of a well. These are: the lithological column, the formation-strength, pore-pressure and temperature profiles, the hydrocarbon composition and the H2S/CO2 concentrations.
Derived design parameters such as required mudweight or gaslift pressure will not be discussed here.
It will be clear that the design parameters can be obtained either from actual measurements or from (computer-based) modelling tools. Reworking and translating these data into a usable format will obviously assist all parties involved. Several Opcos are now streamlining their data flow [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 discussed in this chapter, in the design equations presented in Chapters G, H and J, for the relevant load cases discussed in Chapter F.
3.2 Lithological column
The lithological column is the description of the sequence of formations that are prognosed to be present in the well to be drilled.
Every formation has its characteristic properties with regard to formation strength, drilling problems , reservoir potentials, etc. Advanced knowledge of these properties will be time and cost 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 for
the various casing shoes, as the type of formation and its depth will give a good indication of formation strength. More details are to be found in the formation breakdown profile paragraph.
- The presence (if an aerobic environments can be an indicative for H2S which may be formed by 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 zones may be predicted. More details on this topic are to be found in the pore pressure profile paragraph.
In case of an exploration well the casing designer may not have much information available. Well planning and design will be based entirely on information from seismic and regional geological information. However, with the progress of time and the increase of the available data the geological 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 related activities. This local information could be further summarised to reflect a base case lithological column. Figure C-1 gives an example.
Note that the predicted depths always have an error margin. The reason is that the prognosed depth 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. Rock failure 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 hydrocarbon accumulations and mechanisms of overpressures.
• Drilling Operations, e.g. casing setting depth, maximum safe drilling depth and losses caused by circulating pressures, surge pressures, and cementing operations,
For a complete theory of Rock Mechanics we refer to a suitable textbook or manual [271. A good summary 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 ability of the borehole wall to contain wellbore fluids, both for an intact and a fractured borehole. The following paragraphs will address the relevant definitions, followed by the evaluation and description of the different formation strength test methods. The preferred test method will be discussed 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 typical formation breakdown test, (see Figure C-3). In a plot of the downhole pressure exerted on the formation of a closed-in well versus time (or volume pumped), several characteristic points can be identified.
Figure C-3 Relation between leak-off, formation-breakdown, propagation, fracture-closure and instantaneous shut-in pressures.
FIGURE C-3 : RELATION BETWEEN LEAK-OFF ,FORMATION-BREAKDOWN,FRACTURE PROGRATION, FRACTURE-CLOSURE AND INSTANTANEOUS SHUT-IN PRESSURE
Initially, the pressure - time relationship is linear. The Leak-Off Pressure (LOP) is the pressure at which the curve deviates from the initial linear build-up.This deviation is associated with a noticeable intake of fluid into the formation either by filtration through the mudcake or by the formation of small cracks in the borehole wall.
At the Formation Breakdown Pressure (FBP) the borehole wall fails and a major fracture is initiated. Powered by the decompression of the wellbore fluid, this fracture grows almost instantaneously and the wellbore pressure reduces sharply. The stress concentrations around an intact 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 the minimum in situ stress of the formation. This explains the reduction in strength of a fractured borehole.
If pumping is continued, the fracture propagates into the formation in a controlled manner, and the pressure stabilises at the Fracture Propagation Pressure (FPP). Due to the frictional pressure losses 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 Instantaneous Shut -In Pressure (ISIP). At this stage, the fracture is open, but does not propagate any more.
