46231 guidelines_qualification of materials for well suspension and abandonment july 2012

Guidelines on qualification of materials for the suspension and abandonment of wells

Issue 1July 2012

Guidelines on qualification of materials for the suspension and abandonment of wells Issue 1, July 2012 The United Kingdom Offshore Oil and Gas Industry Association Limited (trading as Oil & Gas UK), 2012. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, including electronic, mechanical, photocopying, recording or otherwise, without prior written permission of Oil & Gas UK. Any material within these guidelines that has been reproduced has been done so with the permission of its owners. Contains public sector information licensed under the Open Government Licence v1.0, which can be found at http://www.nationalarchives.gov.uk/information-management/uk-gov-licensing-framework.htm The information contained herein is given for guidance only. These guidelines are not intended to replace professional advice and are not deemed to be exhaustive or prescriptive in nature. Although the authors have used all reasonable endeavours to ensure the accuracy of these guidelines neither Oil & Gas UK nor any of its members assume liability for any use made thereof. In addition, these guidelines have been prepared on the basis of practice within the UKCS and no guarantee is provided that these guidelines will be applicable for other jurisdictions. While the provision of data and information has been greatly appreciated, where reference is made to a particular organisation for the provision of data or information, this does not constitute in any form whatsoever an endorsement or recommendation of that organisation.

ISBN: 1 903 003 85 4 PUBLISHED BY OIL & GAS UK London Office: 6th Floor East, Portland House, Bressenden Place, London, SW1E 5BH Tel: 020 7802 2400 Fax: 020 7802 2401 Aberdeen Office: Exchange 2, 3rd Floor, 62 Market Street, Aberdeen, AB11 5PJ Tel: 01224 577250 Fax: 01224 577251 Email: [email protected] Website: www.oilandgasuk.co.uk


Issue 1, July 2012 1

Contents

Foreword ........................................................................................................................ 4 Glossary ......................................................................................................................... 5 List of abbreviations .................................................................................................... 11 1 Introduction ........................................................................................................ 12

1.1 UK Guidelines for the suspension and abandonment of wells ..................................... 13

2 General considerations for qualification of new technology........................... 15 2.1 Documentation requirements ........................................................................................ 17

3 Functional requirements of permanent barriers .............................................. 18 3.1 Sealing .......................................................................................................................... 18 3.2 Position .......................................................................................................................... 19 3.3 Placeability .................................................................................................................... 19 3.4 Durability ....................................................................................................................... 20 3.5 Removal options and reparability concepts ................................................................ 20

4 Operating conditions .......................................................................................... 21 4.1 Typical conditions for wells in the North Sea ................................................................ 22 4.2 Estimating operational conditions of the permanent barrier ......................................... 23

5 Potential functional failure modes and root causes ........................................ 24 5.1 Shift in barrier position .................................................................................................. 24 5.2 Barrier leakage through the bulk material ..................................................................... 24

5.2.1 Fluid flow through porous media ................................................................................ 25 5.2.2 Diffusive leakage ........................................................................................................ 25

5.3 Leaks around the bulk material ..................................................................................... 26 5.3.1 External / internal stresses ......................................................................................... 28 5.3.2 Shrinkage and expansion ........................................................................................... 28 5.3.3 Chemical degradation ................................................................................................. 29 5.3.4 Creep .......................................................................................................................... 30 5.3.5 Quality of placement ................................................................................................... 30 5.3.6 Thermal expansion differences .................................................................................. 31 5.3.7 Drilling damage ........................................................................................................... 31

6 Material types ...................................................................................................... 32 6.1 Definition of material types ............................................................................................ 32 6.2 Critical material properties related to potential failure modes ....................................... 32

7 Approach to defining acceptance criteria for mass transport properties ...... 34 8 Experimental work plan ..................................................................................... 36

8.1 Literature investigation .................................................................................................. 36 8.2 Permeation testing ........................................................................................................ 36

8.2.1 Permeability ................................................................................................................ 36 8.2.2 Diffusion ...................................................................................................................... 38

8.3 Interaction with fluid ...................................................................................................... 38


2 Issue 1, July 2012

8.3.1 Absorption ................................................................................................................... 38 8.3.2 Leaching, corrosion and chemical resistance ............................................................ 38

8.4 Dimensional stability ..................................................................................................... 39 8.4.1 Expansion / swelling ................................................................................................... 39 8.4.2 Shrinkage .................................................................................................................... 40 8.4.3 Differential thermal expansion .................................................................................... 40 8.4.4 Creep and stress relaxation ........................................................................................ 41

8.5 Mechanical testing ........................................................................................................ 41 8.5.1 Triaxial testing............................................................................................................. 42 8.5.2 Elastic modulus .......................................................................................................... 42 8.5.3 Cohesion ..................................................................................................................... 42 8.5.4 Internal friction angle .................................................................................................. 42 8.5.5 Poissons ratio ............................................................................................................ 43 8.5.6 Hydrostatic compression testing................................................................................. 43 8.5.7 Unconfined compressive strength (UCS) ................................................................... 43 8.5.8 Tensile strength .......................................................................................................... 43 8.5.9 Hardness .................................................................................................................... 43

8.6 Bond strength ................................................................................................................ 44 8.6.1 Shear bond strength ................................................................................................... 44 8.6.2 Tensile bond strength ................................................................................................. 45

8.7 Fatigue life ..................................................................................................................... 45 8.8 Decomposition temperature .......................................................................................... 45 8.9 Density .......................................................................................................................... 45 8.10 Ageing testing ............................................................................................................... 46 8.11 Function test .................................................................................................................. 49 8.12 Field trial ........................................................................................................................ 51

9 Material types specific experimental work plans .......................................... 52 9.1 Type A: Cements, ceramics (setting) ............................................................................ 52 9.2 Type B: Grouts (non-setting) ......................................................................................... 54 9.3 Type C: Thermosetting polymers .................................................................................. 56 9.4 Type D: Thermoplastic polymers .................................................................................. 58 9.5 Type E: Elastomeric polymers ...................................................................................... 60 9.6 Type F: Formation ......................................................................................................... 62 9.7 Type G: Gels ................................................................................................................. 64 9.8 Type H: Glass ............................................................................................................... 66 9.9 Type I: Metals ................................................................................................................ 68

10 References .......................................................................................................... 70 11 Appendix 1: Chemical environment .................................................................. 72

11.1 Seawater and brine composition ................................................................................... 72 11.2 Oil composition .............................................................................................................. 73 11.3 Gas composition ........................................................................................................... 73 11.4 Non-native chemicals .................................................................................................... 74

12 Appendix 2: Radiation ........................................................................................ 75 12.1 NORM activity estimates ............................................................................................... 75



12.2 Lost logging tools .......................................................................................................... 75

13 Appendix 3: Estimating operational pressures ................................................ 76 14 Appendix 4: Calculation of flow rates through a microannulus. ..................... 78 15 Appendix 5: Caprock properties ........................................................................ 79

15.1 Typical North Sea shale properties ............................................................................... 79 15.2 Typical North Sea salt formation properties .................................................................. 79

16 Appendix 6: Flow calculations........................................................................... 80 17 Appendix 7: Diffusion calculations ................................................................... 81 18 Appendix 8: Relevant industry standards ........................................................ 83 19 Appendix 9: Cement material reference data ................................................... 86 20 Appendix 10: Shear bond strength ................................................................... 87 21 Appendix 11: Background to these guidelines ................................................ 88

If you have any feedback on these guidelines, please send your comments (referring to the relevant paragraph number shown in the grey column) to [email protected]. Unfortunately the workgroup may not be able to provide individual responses to feedback submitted. Feedback received will be used to improve the guidelines when they are next revised.



1 Foreword 2 These guidelines have been prepared to provide a reference for well-operators,

manufacturers and regulators on the qualification of materials suitable for the temporary abandonment (suspension) and permanent abandonment of wells in the UK and its continental shelf. The document compiles the current industry expertise to provide guidance on how materials for suspension and well abandonment can be qualified.

3 A separate Oil & Gas UK document entitled Guidelines for the suspension and abandonment of wells [Ref 2] provides minimum criteria to help ensure full and adequate isolation of formation fluids, both within the wellbore and from surface or seabed. This will assist well-operators to comply with the UK Offshore Installations and Wells (Design and Construction, etc) Regulations (SI 1996/913) (hereafter referred to as DCR). Regulations are goal setting in nature and lay down the standard that should be achieved. In essence, DCR requires the well-operators to prevent, on a permanent basis, escape of fluids from the well. Allowance must be made for deterioration of some components of the well over time.

4 Cement is currently accepted as the primary material for permanent barriers in well abandonment, principally on the basis that it is considered to have similar properties to the rock that it is replacing. However, cement has operational limitations, and alternative materials are being proposed and developed by the industry. Although there are potentially many advantages in using such materials, they have currently seen little or no application in well abandonments. One significant reason for this is that well abandonment has an eternal perspective, and uncertainty with regards to long-term integrity of alternative materials acts as a disincentive for their use. The aim of this document is to stimulate consideration of a broader range of materials for abandonment.

5 The deployment of a novel barrier material for suspension or abandonment of wells should follow a sequence of phases from development, qualification, production, storage, transport, installation and verification. Using this sequence, this document covers the qualification steps to ensure that the considered material performs the envisaged function. Product development, production, storage, transport and installation are outside the scope of this document. Also outside the scope of the document is the qualification for health, safety and environmental acceptance.

6 When comparing UK requirements to those of other countries, there are few differences with NORSOK in Norway. Other countries may only refer to cement or indicate that an abandonment material must be equivalent to cement. Some countries may also accept mechanical devices as permanent barriers without further specification.

7 This is the first issue of these guidelines. It is envisaged that updated versions will be released on a three-yearly cycle.



8 Glossary 9 anoxic: A term used to refer to conditions where oxygen is absent.

10 bailer: Container with plugging material that is lowered into a well on wireline to deliver a small amount of plugging material by remotely opening the container at depth. In this context it is more commonly known as a dump bailer that implies the functionality of the equipment.

11 barrier material: Material used in a well to provide a seal as part of a permanent barrier.

12 barrier plug: A volume of barrier material used as either a temporary barrier or permanent barrier.

13 borehole: Hole drilled in the earth crust for extracting fluids from rock formations. Other purposes can include injection, heat exchange or gather data. The borehole can also include the open hole or uncased part of a well. Borehole may refer to the internal diameter of the well and also the formation or rock face that bounds the drilled hole. Also, see well.

14 branch: See sidetrack.

15 bridge plug: Traditionally, a device that can be set in a well to isolate the lower part of the wellbore. Bridge plugs may be classified as either permanent or retrievable and can also be provided as an inflatable device. In the context of well abandonment, a bridge plug can be used as a mechanical device to provide a solid base for setting a permanent barrier such as a cement plug.

16 brine: Water saturated or containing high concentrations of salts, in particular, sodium chloride, potassium chloride and calcium chloride.

17 caprock: The general term for the rocks that trap hydrocarbons in the reservoir. Another term used for these types of rocks is seal.

18 carbon dioxide: A gas which occurs with hydrocarbons in some geographical areas. Deterioration of components in a well can occur from contact with carbonic acid, which is formed when carbon dioxide dissolves in water or there is moisture in the environment. The deterioration is normally referred to as sweet corrosion and typically results in deep pitting and material loss where there is exposure to carbonic acid. There are a number of different quantitative models available to assess the loss of material, however these are partial models using selective parameters.



19 casing or casing string: An assembled length of long hollow cylinders, usually steel in a quenched and tempered condition with threaded connections, configured to suit a specific wellbore. The sections of pipe are connected and lowered into a well, then cemented in place to provide a conduit for fluids to be conducted. Casing is also defined as Oil Country Tubular Good (OCTG) with sizes ranging from 4-1/2 inches to 20 inches in diameter. Casing can be divided into different types which perform different functions. The different types are typically classified as (in order of decreasing diameter); conductor (also known as stove pipe), which is the outermost string of casing through to surface casing; intermediate casing; and production casing. The names assigned to the different types of casing strings also imply the functionality of the particular casing string.

20 casing annulus: The space between the casing and the next larger casing or rock.

21 cement slurry: A suspension of cement and possibly other granular material in water which flows as a liquid.

22 coiled tubing: A long continuous length of pipe wound on a spool. The pipe is straightened prior to lowering it into a wellbore and rewound to coil the pipe back onto the transport and storage spool. Depending on the pipe diameter (typically 1 inch to 4-1/2 inches) and the spool size, coiled tubing can range from 2,000 feet to 15,000 feet or greater length. As a well intervention method, coiled tubing techniques offer several key benefits over alternative well intervention technologies. The ability to work safely under live or pressurised well conditions, with a continuous string, enables fluids to be pumped at any time regardless of the position or direction of travel. This is a significant advantage in many applications. Installing a multicore electric line logging cable within the coiled tubing further enhances the capability of a coiled tubing string and enables relatively complex intervention techniques to be applied safely.

23 completion: A generic term used to describe the assembly of a string of tubing that includes a variety of different subcomponents, which are selected depending on the functional design of a well, that are required to enable the safe and efficient operation of the well. This term can also be used as an all-encompassing term or alternatively as constituent parts such as lower completion, intermediate completion and upper completion.

24 completion fluid: A solids-free liquid that fills the wellbore during the completion phase of the well construction process. The fluid is meant to control a well, should downhole hardware fail, without damaging the reservoir or completion. Completion fluids are typically brines (e.g. solutions of chlorides and bromides), but in theory could be any fluid of proper density and flow characteristics such as seawater or base oil. Also known as kill weight brine as the bottomhole hydrostatic pressure in the well bore (of the completion fluid) is greater than the exposed formation pressure.

25 creep: Permanent deformation resulting from prolonged application of stress below the elastic limit.

26 diffusion: A process where a substance is transported from a region of high concentration to a region of lower concentration.



27 diffusive leakage: The release of fluid above a barrier as the result of diffusion.

28 drill cuttings: Solids removed from a well while drilling.

29 drilling fluid: Fluid that fills the wellbore during the drilling phase of the well construction process. Consists of any of a number of liquid and gaseous fluids and mixtures of fluids and solids (as solid suspensions, mixtures and emulsions of liquids, gases and solids). Also known as "mud which is in general usage, although some individuals prefer to reserve the term "drilling fluid" for a more sophisticated and well-defined "mud". Can also be referred to as drill-in-fluid which is a special fluid designed exclusively for drilling through the reservoir section of a well.

30 drill pipe: A long hollow cylinder, usually steel in a quenched and tempered condition, with threaded connections which are called tool joints. An assembled length of drill pipe which connects the drilling rig surface equipment with a bottom hole drilling assembly and drill bit is collectively known as a drillstring. The drillstring is used to pump drilling fluid to the drill bit for driving a revolving drill bit for drilling wells.

31 ductile: A material characteristic relating to the ability to deform plastically, and thus be drawn out into a wire.

32 dynamic viscosity: A measure of the resistance of a fluid to deformation by either shear or tensile stress. Units = Pa.s.

33 enhanced oil recovery: Techniques employed to increase the amount of oil which can be extracted from a field. This generic term can also be more specifically classified as secondary EOR or tertiary EOR.

34 fluid: Within the context of this document a fluid is a substance in either the liquid or gaseous state.

35 flux: The rate of flow of a fluid through a unit area.

36 formation: Rock present in the crust of the earth.

37 geopolymer: Alkali-activated aluminosilicate cement.

38 hydrogen sulphide: An extremely poisonous gas which is produced during the decomposition of organic matter and occurs with hydrocarbons in some geographical areas. Hydrogen sulphide can also be generated by sulphate-reducing bacteria. Mercaptons can also be a by-product of the interaction of hydrogen sulphide with hydrocarbons. A well that is contaminated with hydrogen sulphide is often referred to as a sour well and should be designed for sour service. Hydrogen sulphide can cause sulphide stress cracking of metals; however this can be addressed with the use of corrosion-resistant alloys. Materials for use in a hydrogen sulphide environment can be controlled by the standard NACE MR0175 / ISO 15156.



39 liner: A casing string that does not extend to the top of the well, but instead is anchored or suspended from inside the bottom of the previous casing string. The device used to anchor or suspend a liner from a casing string is known as a liner hanger. The liner can be fitted with special components so that it can be connected to the top of the well at a later time if required.

40 LSA scale: See NORM.

41 mechanical plug: a device used to produce a seal in a casing through the application of forces by mechanical means.

42 mud: See drilling fluid.

43 naturally occurring radioactive material (NORM): Substances encountered in the environment which contain native, naturally occurring radioactive isotopes. NORM is sometimes encountered as scale formed in parts of a well, where it may have become enriched with radioactive material as a result of the processes it has undergone. NORM is sometimes referred to as Low Specific Activity (LSA) scale.

44 non-native materials: Materials introduced into or around the well as a result of production activities.

45 packer: A device used in well completion to isolate the annulus from the production casing, enabling controlled production, injection or treatment. A packer assembly incorporates a means of securing the packer against the casing or liner, and a means of creating a reliable seal to isolate the annulus, typically by means of an expandable elastomeric element.

46 permanent barrier: A verified barrier that will maintain a permanent seal. A permanent barrier must extend across the full cross section of the well and include all annuli.

47 permanent barrier material: Material used in a well to provide a seal as part of a permanent barrier.

48 permeation: The movement of a substance through a material. Permeation can occur under either a pressure differential or a concentration differential.

49 permeability: a measure of the ability of a porous material to allow transport of fluids.

50 permeable zone: Any zone in the well where there is the possibility of fluid movement on application of a pressure differential.

51 sidetrack: To drill a secondary well away from an original well or a donor well. A sidetracking operation may be done intentionally or may occur accidentally. Intentional sidetracks might be used to bypass an unusable section of the original well or to explore a geological feature nearby. In the bypass case, the secondary well is usually drilled almost parallel to the original well, which may be inaccessible due to problems encountered during the well construction process. It is also possible to have multiple sidetracks in a single well, each of which might be drilled for a different reason. This term can also be used to describe part of a multilateral well where multiple wells or boreholes originate from one donor well or borehole.



52 simulated in situ fluid: A fluid comprising a mixture of water, hydrocarbons and gases in proportions which mimic the composition of a downhole fluid.

53 suspension: Also referred to as temporary abandonment. Action taken prior to leaving the well to ensure adequate isolation of permeable zones, fluids and pressures in any well that will be re-entered or abandoned at a later date.

54 sour well: See hydrogen sulphide.

55 sweet well: A well containing relatively high concentrations of carbon dioxide.

56 temporary barrier: A verified barrier that is designed to maintain a seal over a finite period of time for the purpose of suspension. A temporary barrier is not required to extend across the full section of the well and include all annuli.

57 through-tubing: Activity in which material is placed downhole using tubing or coiled tubing.

58 tubing: See casing.

59 viscous pill: In the context of well abandonment a pill can be used to provide a nominally quasisolid base for setting a permanent barrier such as a cement plug. Pills can take the form of viscous pills, whose viscosity acts to limit mobility, viscous reactive pills whose viscosity derives from a chemical reaction when in contact with cement or other substances. A pill will typically be weighted of suitable density to allow it to locate itself at the correct depth.

60 well: A hole drilled into the Earths surface to extract petroleum oil and fossil natural gas. A well includes the original drilled hole (the wellbore), any sidetrack from it and any hole section. Also see borehole.

61 wellbore: See well.

62 wireline: Deployment method of lowering into and retrieving tools and devices from a well. A generic term used to describe well intervention operations conducted using a single-strand or a multistrand wire or cable in wells. Although applied inconsistently, the term is commonly used in association with electric logging and cables incorporating one or several electrical conductors leading to the terms monoconductor cable or multicore electric line logging cable. Similarly, the term slickline, or piano wire, is commonly used to differentiate operations performed with single-strand wire or braided lines.

63 work string: A generic term used to describe a tubing string used to convey a treatment or for well intervention activities. Both coiled and jointed tubing strings are referred to as work strings.



64 Interpretation

In these guidelines: reference to any legislation or publications includes a reference to that

legislation, publication as may be amended, extended or re-enacted from time to time;

reference to the singular shall include the plural and vice versa; reference to include means including but not limited to; the headings are used for convenience only; and regard has only been made to the jurisdictions within the UK.



65 List of abbreviations 66

API American Petroleum Institute ASTM American Society for Testing and Materials CH4 methane CO2 carbon dioxide CT computer tomography DCR Offshore Installations and Wells (Design and Construction, etc) Regulations 1996 (SI 1996/913) which is commonly known as the Design and Construction Regulations

DSC differential scanning calorimetry DTA differential thermal analysis EOR enhanced oil recovery (see Definitions) H2S hydrogen sulphide ISO International Organization for Standardization ISRM International Society for Rock Mechanics LSA low specific activity MRI magnetic resonance imaging NACE National Association of Corrosion Engineers NMR nuclear magnetic resonance NORM naturally occurring radioactive material ppmv parts per million by volume SEM scanning electron microscope SIF simulated in situ fluid TGA thermogravimetric analysis UCA ultrasonic cement analyser UCS unconfined compressive strength UKOOA United Kingdom Offshore Operators Association now known as Oil & Gas UK



67 1 Introduction 68 Wells that have been used to explore or develop hydrocarbon accumulations

will at some stage require abandonment. This involves placement of barriers in the well to isolate formations from each other and from surface. This is referred to as permanent abandonment if there is no intention to ever re-enter the abandoned part of the wellbore. Where there is an intention to re-enter, it is referred to as temporary abandonment or suspension. Suspension is typically short term (a matter of months). Long-term suspension should be the exception rather than the rule.

69 A simplified schematic of a permanently abandoned well is shown in Figures 1 and 2. It is stressed, however, that wells differ in construction and operation and that the abandonment must reflect this. It should also be emphasised, that the barriers placed during abandonment may not exclusively be cylindrical barriers within the casing for instance, it may be necessary to place barriers in the annulus between the caprock and the casing.

70 Portland cement is currently the most used barrier material in permanent well abandonment. This is because it is considered to have similar properties to the caprock that it is replacing. In existing abandonments, cement is functioning as required in most cases, but there are operational limitations and situations in which cement may not be the most appropriate material. The aim of this document is to stimulate consideration of alternative materials for abandonment.

71 This document covers materials that will cause the sealing action, but not the construction of mechanical devices such as packers or mechanical plugs, nor the placement technique. This document does not cover formation damaging materials. Such materials are currently available, but are not included in existing guidelines. However, they may be included in subsequent versions of this document.

72

Figure 1. Simplified schematic of the abandonment of a well by placement of permanent barriers. See Figure 2 for a close-up of a barrier. In case of suspension there will generally be no casing cut or removed.

Zone A Oil and Gas

Caprockfor zone A

Caprockfor zone B

Water bearing

Casing

Zone B

Cement

Zone A Oil and Gas

Caprockfor zone A

Caprockfor zone B

permanent barrier tozone B

Water bearingZone B

Secondarypermanent barrierto zone A

Primary permanent barrierto zone A

BEFORE AFTER



73

Figure 2. A permanent barrier is an envelope (red dashed line) that encompasses a number of barrier elements (orange boxes), which all need to seal. Cement is assumed. Recommended or best practices (blue boxes) are indicated.

74 1.1 UK Guidelines for the suspension and abandonment of wells

75 In the UK, well design and construction both onshore and offshore is regulated by the Offshore Installations and Wells (Design and Construction, etc) Regulations (SI 1996/913) (hereafter referred to as DCR).

76 Regulation 13 General duty of the DCR states that:

1. The well-operator shall ensure that a well is so designed, modified, commissioned, constructed, equipped, operated, maintained, suspended and abandoned that

a. so far as is reasonably practicable, there can be no unplanned escape of fluids from the well; and

b. risks to the health and safety of persons from it or anything in it, or in strata to which it is connected, are as low as is reasonably practicable.

77 Guidance on DCR is available [Ref 1].

78 Regulation 15 with a view to suspension and abandonment of DCR states that:

The well-operator shall ensure that a well is so designed and constructed that, so far as is reasonably practicable

a. it can be suspended or abandoned in a safe manner; and

b. after its suspensions or abandonment there can be no unplanned escape of fluids from it or from the reservoir to which it led.

Barrier elements

Height of 500 ft MD, containing at least 100 ft MD of good cement.

Plug depth determined by formation (impermeabilityand strength) and primary cementation

Best practices

Pipe stand-off

Good bond, clean surfaces, water wet

Support to prevent cement movement, slumping and gas migration while setting

Sealing abandonment plug

Casings, tubing embedded in cement

Sealing primary cementations

Formation:impermeable andadequate strength to contain future pressures

Tubing sealed with cement, in cement

Restoring the caprockPermanent Abandonment Barrier (red dashed envelope)



79 Through Oil & Gas UK (formerly known as UKOOA), the industry has compiled the Guidelines for the suspension and abandonment of wells [Ref 2] which provide minimum criteria as a means to achieve the specific regulations detailed in DCR.

80 The Guidelines for the suspension and abandonment of wells state that the main characteristics of permanent barrier materials should be as follows: very low permeability - to prevent flow of hydrocarbons or overpressured

fluids through the barrier; long-term integrity - long-lasting isolation characteristics of the material, not

deteriorating over time; non-shrinking - to prevent flow between the barrier-plug / casing annulus; ductile - non-brittle material; to accommodate mechanical loads and

changes in the pressure and temperature regime (conversion of producers to water injectors, steam injection, unconsolidated formations etc);

resistance to downhole fluids and gases (CO2, H2S, hydrocarbons etc); and able to bond to the casing or formation in which it is placed.

81 On the subject of alternative (non-cement) plugging materials, the guidelines add the following:

Cement is currently used in wells as the prime material for abandonment purposes. This does not preclude the use of other materials. Alternative materials should, in principle, conform to the requirements above. The long-term integrity of materials should be documented. Once placed, there should be a means by which the barriers can be verified.

82 The guidelines also state that:

The downhole placement technique of the plugging material is extremely important, especially in cases of through-tubing applications. Allowances will have to be made on the volumes to cater for contamination and shrinkage. A support (such as a bridge plug or a viscous pill) to prevent slumping of the cement slurry, is recommended for all cement plugs.

83 Whilst the Guidelines for the suspension and abandonment of wells have been used as a means of steering the development of the guidance in this document, it should be stressed that, in the case of certain properties, the approach has been taken to require materials displaying appropriate performance in service, rather than specific characteristics.



84 2 General considerations for qualification of new technology

85 Whilst a permanent barrier must be verified in the field, placement of such a barrier must be preceded by qualification of the materials used. This section represents general requirements related to the qualification of new technology, as defined in the Det Norske Veritas document Qualification procedures for new technology (DNV RP-A203, 2001) [Ref 3]. An outline of the process (with some modification) is shown in Figure 3.

86 The first stages of the qualification process involve establishing a plan for qualification. This plan should control the subsequent qualification activities. The qualification should be based on the following philosophy: The qualification process should be based on a systematic approach. Possible failure modes should be identified, and their relevance should be

determined based on their risk, i.e. the combined probability and consequences of a failure mode occurring. Risk in this context is related to the functionality of the new permanent barrier material.

A programme of measurements and tests should be devised an experimental work plan - which is appropriate to evaluate the suitability of the material for its use as a barrier material, giving consideration to the materials in contact with the barrier.

Theoretical analysis and calculations should, when practical, be used as the main tool to record fulfillment of the specifications and margins against failure. Theoretical calculations should be verified by tests, including a function test (see Section 8.11).

The experimental work plan used as the principal means of demonstrating and documenting that manufacture and deployment fulfil the specifications.

87 Furthermore, the following principles should control the qualification process: Specifications and functional requirements should be quantitative. The margins for capacities and margins against failure should be

established based either on recognised methods, standards, or on combinations of all uncertainties used in the data, operation, calculations and tests.

When experience is used as proof of fulfilment of the specifications and reasonable margins, this should be documented.

The limiting material and functional parameters to be used in the analysis should be identified through tests or referenced to recognised literature.



88

Figure 3. Process for the qualification of new technology [Ref 3].

89 Confidentiality of the technology should not limit the information made available for the qualification. These guidelines assume that confidentiality between parties is arranged by contracts to that purpose. The available documentation and insight into the qualification process may follow three alternatives: An open qualification scheme - all information is available. The qualification documentation is made available to a third party

recognised by the involved parties. The original qualification documentation is not accessible. Full function and

endurance tests according to the specifications must document the qualification, in addition to the suppliers statement of qualification. This may imply more extensive testing than the former. These tests must demonstrate acceptable margins for all conditions. It may imply testing of a sufficient number of batches to develop statistically representative data.

Define Qualification Basis

Identification of Failure Modes

Ranking of Failure Modes in Terms of Risk

Data Collection

Selection of Qualification Methods and Planning

Assessment of Probability of Successful Evaluation

Analysis and Testing

Reliability Analysis

Laboratory Testing

Function Test(larger scale)

Documentation

Documentation



90 It is recommended that the qualification method, qualification steps and tests are agreed beforehand with potential users and relevant regulators.

91 2.1 Documentation requirements

92 With reference to DNV RP-A203 [Ref 3], the available documentation related to the qualification process should, when applicable, comply with the following: description and specification; material specifications; quality assurance plan; and detailed drawings of items used in the qualification.

93 The documentation of the qualification should include the following, when applicable:

94 Design criteria:

references with justification to applied standards, rules and regulations; selection of types of tests with justification; selection of test parameters with justification; and expected outcome of each test, with quantified acceptance levels.

95 Documentation of key items in the qualification process:

description and justification of methods (experience, analysis and tests) applied for qualification;

document tests performed, conditions and parameters used, with results, including failures with analysis;

values of key parameters and conditions at the start of a test are to be captured;

resulting margins to failure modes; limit values (maximum or minimum) functions from analyses and tests;

and system reliability.

96 Analysis:

failure mode analysis, including specification of personnel competence; list of all assumptions made in the failure mode assessment; conclusions on operating envelope of the material; statement of conditions or parameters that were not tested or evaluated

in the qualification process; and minimum lifetime estimate describing appropriate extrapolation technique

and assumptions.

97 Manufacturing and installation:

material certificates; manufacturing records; personnel qualification record; installation records; and material samples taken and kept.

98 Revisions:

records of all document revision including content of revision.



99 3 Functional requirements of permanent barriers 100 A permanent barrier has to fulfil a number of functions, which are discussed

below. Functional requirements of temporary barriers do not differ from those of permanent barriers except possibly for a relaxed time scale of required durability.

101 3.1 Sealing

102 The main function of a permanent barrier is to provide a seal against movement of fluids. However, permeation of some kind is possible through most materials. Such fluid movement follows well understood natural processes. Within the context of permanent barrier materials it must, therefore, be recognised that it is inevitable that a fluid within the well will ultimately migrate past a barrier, albeit at a low rate.

103 Thus, appropriate barriers are those through which the rate of permeation is acceptably low. The approach taken in this document is to require that the barrier permits leakage of fluids at the same or a lower rate than the caprock. The permeability of caprock is typically within the range 0.001-1 microdarcy. However, good quality cement (typically with a permeability of 10 microdarcy) is deemed acceptable on the basis of historical industry experience, and on the grounds that barrier length is a controllable parameter (as long as it is opposite the caprock) and the cross sectional area is considerably smaller than that of the caprock. A permanent barrier requires: continuous material, or sequence thereof, with low-permeability; and an appropriate length along the well bore.

104 The rate of permeation in this context is controlled by a number of factors, including: Fluid flow through connected porosity. In this case the driving force for fluid

transport is a pressure differential. Diffusion processes. The driving force for transport is a concentration

difference. Conduits due to defects in the barrier material, such as cracks or channels.

The driving force for fluid transport is a pressure differential. Conduits at interfaces between the barrier material and surrounding

materials, e.g. casing, liner, tubing, rock formations. The driving force for fluid transport is a pressure differential.

105 These factors, plus the length of the barrier, will control the lag time before a specified fluid is released above the barrier. Therefore, the minimum length of barrier will vary depending on the material properties, and the placement technique should be selected based on its capability to place such a volume of material. Fluids will typically include oil, gas, CO2, H2S, water and brine, any of which may contain corrosive components.



106 3.2 Position

107 Once placed, the position of the barrier should not move, either along the well bore or in a lateral direction. This means, for instance, that the barriers should not be pushed upwards by pressure developing below. The barrier material is required to remain attached to interfaces it has been placed against. This is achieved through sealing stresses normal to the casing (Figure 4), friction stress, bonding at the interface, weight and dimensional stability, or a combination of these.

108

Figure 4. Forces acting to maintain the seal and position of a barrier.

109 3.3 Placeability

110 It is a requirement that the permanent barrier material can be placed in a wellbore at depth and is subsequently able to perform its required function [Ref 2].

111 The material should have appropriate properties that allow it to displace the existing fluids and form a continuous sealing medium, even when taking its inevitable contamination into account. The material may be circulated in place to replace the present fluid. This will require a work string, such as drillpipe, tubing or coiled tubing. The work string may be left in place and become part of the barrier. If suitable, a gravity-based placement may be deployed using a bailer device and multiple wireline runs.

112 Where a barrier material undergoes a transformation from a liquid to a solid, this period of transformation must be sufficiently short to prevent escape of fluid and unacceptable disruption of the barrier.

113 Wells may be positioned at an angle to the vertical, and the placement technique employed should take this into account.



114 A means of verifying that the placement of the permanent barrier has been successful is required.

115 For recommendation of placement techniques refer to the Guidelines for the suspension and abandonment of wells [Ref 2].

116 3.4 Durability

117 The barrier material should not degrade such that its sealing capability or position is compromised. For temporary abandonment an anticipated timescale may be specified. Permanent well abandonment has an eternal perspective, meaning that the mindset for design is in terms of geological timescales, which span millions of years. It will not be possible to quantify such a time requirement, and it is clearly not feasible to qualify materials for this timescale.

118 In order to define testing criteria against a quantified service life, a service life of an arbitrary number of a million days (circa 3,000 years) is proposed in the context of this document. This is of the same order of magnitude as requirements for the storage of CO2, which refer to timescales of thousands of years, although it has been suggested that an expectation of more than 30 years will be difficult to prove for most materials [Ref 4].

119 The estimation of long-term performance through ageing testing is discussed in greater detail in Section 8.10.

120 3.5 Removal options and reparability concepts

121 A key objective of permanent well abandonment is that re-entry into the well should be unnecessary. However, in the rare event that a leak through a barrier would develop, there should be a method to remove the barrier(s) in order to remedy the leak. This is in line with the mindset in CO2 storage projects.



122 4 Operating conditions 123 After placement and activation, the permanent barrier material will have to

withstand external loadings, and variations in these loadings, without losing its functionality as part of a permanent barrier. Likely operating conditions are listed below. Qualification will be limited to the stated envelope of each of these parameters.

124 Pressure and variation: Pressure will change during the production life of

a field or due to recharging of a depleted reservoir to original pressure. Pressure in the wellbore may change if suspended particles settle and the fluid changes weight. Wellbores under barriers may eventually become pressurised due to formation and / or wellbore connectivity with deeper strata. enhanced oil recovery (EOR) of an existing field or redevelopment of an abandoned field may require injection of fluids, possibly to pressures exceeding original pressures. Fields may be charged for storage of gas or CO2. During abandonment operations (e.g. pressure / inflow testing, fluid change) rapid decompression may damage certain barrier materials.

125 Temperature and variation: Temperature will change during the

production life of a field, but follows geological patterns when left undisturbed. Redevelopment of an abandoned field may also lead to temperature changes, e.g. during steam injection. Thermal changes may also result from fluid injection, gas storage or CO2 injection in previously abandoned fields.

126 Mechanical stresses and variation: Naturally occurring formation creep,

subsidence or tectonic forces may act on a permanent barrier. Additionally, changes in temperature will cause expansion and contraction. Where materials with very different coefficients of thermal expansion are in contact, particular problems may be encountered in environments where temperatures change, as the differential magnitudes of volume change can create significant stresses. For instance, casings may expand or contract due to temperature and pressure variation, causing mechanical forces to act on the barrier material. Loads on casings may change from tension to compression during abandonment. Rapid short-term stresses may also be created by explosive techniques used in wells to sever or perforate tubular, tagging with the drillpipe for verification and pressure testing.

127 Chemicals: Barrier materials may be exposed to native substances such as

hydrocarbons, CO2, H2S and brine, as well as non-native chemicals deriving from production. Further details of the chemical nature of downhole environments are provided in Appendix 1. Such chemicals may either undergo reactions with the barrier material which lead to deterioration, or leach constituents from the barrier, thus compromising its integrity. Operating temperatures and pressures may influence rates of reaction and leaching.



128 Water: Water may cause swelling of polymers, clays, etc. Osmotic

processes may also cause dimensional instabilities. Degradation of materials through leaching or other dissolution processes may occur. If water contains more than 50,000 mg/L of dissolved solids, it is a brine. Brines can contain a range of types and concentrations of salts, which may contribute towards chemical degradation. Lack of water may also be an issue, since drying of some materials will cause alteration of properties such as shrinkage.

129 Oxygen: Anoxic conditions are a normal state in the well. Low levels of

oxygen may be initially present, but deplete due to chemical processes.

130 Biological: Various microorganisms with the ability to digest barrier

materials or which produce substances capable of attacking them may be present. These include sulphate reducing bacteria, which convert sulphates and sulphites to H2S. Bacteria may be introduced into the well as a result of injection of seawater or drill cuttings, or through circulation of seawater.

131 Radiation: Radiation loading is generally low underground. NORM

deposition in tubulars may be present. Lost neutron logging tools are rarely used, but may occasionally be present. Typical radiation levels from both of these types of source are listed in Appendix 2.

132 Light and UV radiation: There is no visible light or UV radiation down hole.

133 The well abandonment may be only partial the upper part of the hole may be reused for drilling a new branch or sidetrack. The further operation of the field may influence the loading conditions.

134 4.1 Typical conditions for wells in the North Sea

135 There is a wide variation of downhole conditions across fields and wells in the North Sea. The permanent barrier material used should be appropriate for the conditions of a specific well. However, in an attempt to define an envelope for typical conditions, Table 1 is provided. This defines conditions associated with different vertical reservoir depths.

136 Well conditions can also be defined as either sour or sweet, with the fluids in sour wells having high H2S concentrations. Sweet wells are those that do not contain high concentrations of H2S, but can contain a high concentration of CO2.



137 Well type and depth range m (ft)

Typical Conditions

Initial pressure range at reservoir, MPa (psi)

Temperature range at reservoir oC (oF)

Temperature range at surface, oC (oF)

Shallow 1,500 to 3,000 (5,000 to 10,000)

15 to 35 (2,000 to 5,000)

40 to 100 (104 to 212)

0 to 10 (32 to 50)

Medium 3,000 to 4,500 (10,000 to 15,000)

30 to 60 (4,000 to 9,000)

80 to 150 (176 to 302)

0 to 10 (32 to 50)

Deep 4,500 to 6,000 (15,000 to 20,000)

40 to 70 (6,000 to 10,000)

130 to 150 (266 to 302)

0 to 10 (32 to 50)

HPHT 3,000 to 6,000 (10,000 to 20,000)

70 to 105 (10,000 to 15,000)

130 to 200 (266 to 392)

0 to 10 (32 to 50)

Ultra HPHT

3,000 to 6,000 (10,000 to 20,000)

105 to 140 (15,000 to 20,000)

200 to 300 (392 to 572)

0 to 10 (32 to 50)

Table 1. Typical North Sea well conditions.

138 4.2 Estimating operational conditions of the permanent barrier

139 Prior to characterising a material for a given abandonment application, it is necessary to establish the likely operational conditions during the lifecycle of the well and beyond.

140 Wells may be used for future purposes not anticipated prior to abandonment. It is the well-operators responsibility to estimate the likely conditions based on current knowledge of a wells future operation, rather than to attempt to predict unknown future events.

141 The anticipated well status will have to be taken into account and documented (e.g. placement method, deposits, fluids). This will include an evaluation of all the parameters listed previously in this section. Information may be obtained via a number of means including actual field data; generalised field data or calculations. A suggested method for estimating the stresses acting on a barrier under simple pressure conditions is outlined in Appendix 3. The relevant parameters should then be addressed by the experimental work plan.



142 5 Potential functional failure modes and root causes 143 A permanent barrier material may fail in such a way that its functionality is

compromised. A number of failure modes are identified and described in this chapter: shift in barrier position; barrier leakage through the bulk material; and barrier leakage around the bulk material.

144 Placement of a barrier in the field must be accompanied by a documented method for assurance that a barrier performs its intended function. The objective of this verification is to ensure that none of the failure modes are present. Methods for verification will depend on material properties, placement technique used and well configuration. The methods employed should be documented.

145 5.1 Shift in barrier position

146 The position of a permanent barrier in the borehole is important. A permanent barrier should remain at its intended vertical position and retain a seal. Excessive movement of the permanent barrier upwards or downwards may render it ineffective in the long term. Details of requirements for the positioning of barriers are given in the Oil & Gas UK Guidelines for the suspension and abandonment of wells [Ref 2].

147 Regulations in some countries may require that a barrier is placed across perforations. Upwards or downward movement will expose the perforations. Regulations may also require that a barrier is placed across the top of a liner or across a cut casing (casing stump). In these cases, position-holding of the barrier material remains essential.

148 5.2 Barrier leakage through the bulk material

149 The generic behaviour of mass transport through a barrier is shown in Figure 5. This depicts the flux of fluid into, and the resulting outflux from, the barrier. A lag time is common in most situations. Different physical processes may govern behaviour, including absorption, adsorption and displacement of resident fluid.



150

Figure 5. The principle of permeation of a fluid through a material.

151 As previously discussed in Section 3.1, there will be inevitable permeation of fluids. Permeation refers to the transport of a fluid through the barrier material. Permeation requires a physical or chemical driving force, whilst the barrier material will provide resistance to fluid transport. The level of resistance will depend on the materials properties under a given set of operating conditions. Permeation will generally follow two mechanisms fluid flow and diffusion.

152 5.2.1 Fluid flow through porous media

153 Fluid flow will occur in materials which contain porosity. The driving force of fluid flow is a pressure difference between the well interior and the top of the barrier, with a larger differential producing a higher rate of flow. Flow may also be driven by fluid buoyancy effects, where forces arise from fluid below a barrier possessing lower density than that above.

154 The material property governing the rate of flow is permeability, expressed in darcys, which is a function of the volume of porosity, pore size distribution, interconnectivity and the number of fluids and their saturation. Where multiple fluids are present within the pores of a material (gas / liquid, mixtures of immiscible liquids) the permeability of an individual phase is referred to as relative permeability. This will typically be lower than for the situation where the fluid is present in isolation.

155 5.2.2 Diffusive leakage

156 Diffusive leakage is leakage resulting from fluids diffusing through the barrier. The driving force for diffusion is a difference between the concentration of a fluid above and below the barrier, with a higher difference producing a greater rate of diffusion. The material property that governs the rate of diffusion is the diffusion coefficient, expressed in m2s-1 or similar.



157 5.3 Leaks around the bulk material

158 Whilst breakthrough and outflux are inevitable for any barrier, the barrier will have failed in its function if the lag time is shorter and / or the rate of outflux is faster than intended. The main reason for such an occurrence is the formation of a leak.

159 A leak is a breach of integrity which passes completely through the barrier. Such a breach can take the form of a crack or channel development and may be present from the start of placement or develop over a long or short period of time.

160 Failure modes include: debonding (formation of a micro-annulus); dissolution e.g. leaching; and/or cracking.

161 Root causes can include: external/internal stresses exceeding strength limits; shrinkage and expansion; chemical degradation; creep; poor quality of placement; thermal expansion differences; and/or drilling damage.

162 Some modes of barrier failure are shown in Figure 6. The formation of a breach will normally lead to an increase in the flux of fluid passing through the barrier (Figure 7). The various root causes are discussed below.



163

Figure 6. Barrier failure modes. Modified from [Ref 5].

164

Figure 7. Permeation of a fluid material or gas through a barrier where a breach occurs. The time of the breach may vary, e.g. immediately.

Oil & Gas UK WG4b 6

Interface casing/cement(micro-annulus, channel)

Interface casing/cement(micro-annulus)(wax, scale, oil, dirt,etc)

Bulk permeability(connected pores, cracks, channels)

Leak in casing (connection)(corrosion, deformation)

Annulus cement(connected pores, cracks)

Interface rock/cement(micro-annulus, channel)(mudcake, cuttings, oil, etc)

channel

wellcasing

cementfill

formation rock barrier



165 5.3.1 External / internal stresses

166 The external stresses acting on a permanent barrier have been discussed in Section 5. Where the barrier is a solid monolith, and stresses exceed the strength of the barrier or the strength of any bond between the casing and the barrier, failure will occur. Where failure of the material itself occurs, cracks will form in the material. These cracks can exist over a range of dimensions with widths in the millimeter to micrometer scale.

167 Where there is a failure of the casing-barrier bond, debonding will occur, leading to the formation of a micro-annulus between the two surfaces. Cracks or micro-annuli may partially or fully penetrate the length of the barrier, with different implications for the extent to which the rate of permeation is changed.

168 5.3.2 Shrinkage and expansion

169 Shrinkage of barrier materials may occur as drying shrinkage resulting from the evaporation of a liquid constituent of the barrier material, where it is in contact with a gas phase. Alternatively it may occur as the result of processes occurring during the materials ongoing solidification and hardening processes.

170 In the case of cement and similar materials, this process is referred to as autogenous shrinkage in which water in the original slurry is incorporated chemically into solid reaction products, leading to a reduction in volume. In the case of thermoplastics, shrinkage will occur as the material changes from a liquid to a solid, whilst the crosslinking processes occurring during the setting and hardening of resin-based polymer products also lead to shrinkage. With notable exceptions, all metals shrink as they solidify. Temperature reduction may have a similar effect on certain materials.

171 Where shrinkage is restrained (for instance, in a permanent barrier within the casing where the sides of the barrier are bonded to the sides of the casing) stresses will develop in the material. In the case of viscoelastic materials, such as polymers and gels, there is likely to be some relaxation of stress. As stress increases, two possible modes of failure are possible: Where the tensile strength of the bond at the barrier-casing interface is

weaker than the barrier material, de-bonding of the barrier from the casing will occur, leading to the formation and possible growth of a micro-annulus;

Where the opposite is true, cracking of the barrier will occur.

172 Example: in order to determine the relative magnitude of flow though a micro-annulus, the following calculation is provided - if a laminar gas flow obeys Darcys law, then methane flow through an open 25 m microannulus around a plug of 100 ft inside 9-5/8 casing would result in a flow 2 m3 / day with a 1,000 psi pressure differential (all atmospheric conditions assumed). This assumes worst-case conditions in reality cracks will be filled with water, grease, particles and gels. Furthermore, viscosities of gases are higher at downhole pressures. Due to these effects the actual leak rate may be several orders of magnitude lower. Details of this equation are provided in Appendix 4.



173 Expansion of the barrier is also possible, either as a result of hardening processes, chemical reactions with substances below or above the barrier, or as a result of an increase in temperature. Where expansion occurs within the casing, it will have the effect of placing the barrier in compression, and the compressive strength of the barrier material becomes important in such circumstances. Compressive failure of the interface between the barrier and the casing is unlikely. However, as the barrier expands, it is likely that the restraining action of the casing will cause the barrier to undergo strain in a direction parallel to the casing, which will cause the casing-barrier bond to be loaded in shear. The strength of this bond in shear is often relatively weak.

174 5.3.3 Chemical degradation

175 Chemical degradation of a barrier material requires either a reaction between substances within the material itself, or between substances within the material and those entering from outside.

176 Reactions may occur in the gas phase, in solution, at a surface in contact with either a solution or gas phase, or in the solid state.

177 It should be stressed that a chemical reaction need not be detrimental in its effects on the performance of a barrier material, and may in some cases lead to an improvement in properties. However, where a deterioration in properties of the material is observed, it is likely to be the result of either a transformation to a reaction product whose properties (in terms of strength, stiffness, solubility or permeation characteristics) are inferior to the original material, or whose formation leads to the development of stresses capable of causing cracking.

178 Where reactions occur between substances deriving exclusively from the material itself, the rate of reaction may be determined by the nature of the chemical transformation, the rate of diffusion of the substance through the medium in which the reaction is occurring (solution, gas phase or solid), the rate of diffusion of substances through layers of reaction products, or adsorption or desorption of species onto and from surfaces.

179 Where reactions occur between the material and external agents, the rate of reaction is likely to be largely controlled by the rate at which the external species are able to move into the material, either via diffusion or carried by a fluid moving under a pressure gradient. Thus, the requirement for a low permeation rate discussed in Section 3.1 also plays a role in protecting the barrier from failure.

180 Dissolution can occur where barrier material constituents or products deriving from chemical or microbial degradation of the barrier are in contact with a liquid in which they are soluble.

181 Where there is a small and finite volume of liquid, such as in a non-connected pore, the soluble substance will dissolve until the liquid becomes saturated with respect to that substance. Deterioration is only likely to become significant if the liquid in contact with the soluble constituent has a large volume or is connected to a larger volume of itself.



182 In such cases, mass is lost from the material, driven either by diffusion or the transport of liquid under a pressure gradient. The mass loss leads to an increase in porosity and a loss in strength and stiffness. Where diffusion is the driving force, a deteriorated front will move into the barrier at a rate controlled by this process.

183 5.3.4 Creep

184 Materials subjected to a load may undergo both an instantaneous and delayed deformation. This delayed strain is commonly referred to as creep. Some materials may undergo creep over months or years. The mode of barrier failure under creep will either be one of de-bonding of the barrier material from the casing as a result of shear stresses at this interface, or as a result of the formation of micro-cracks in the barrier.

185 5.3.5 Quality of placement

186 The quality of placement is a key functional requirement for two main reasons: Poorly placed barriers may contain channels due to incomplete

displacement. Poor placement technique may result in heavy contamination of the

plugging materials with the displaced fluid, which may significantly affect key material properties.

187 Deposits, including NORM, filter cake, asphaltenes, waxes and hydrates may be present in or around surfaces in the borehole. These should be removed in circumstances where they could compromise the function of the permanent barrier, although many scale types may be difficult to remove.

188 Certain materials exhibit self-healing effects, i.e. a crack or defect may repair itself. Such healing effects normally have limitations in terms of the extent to which the defect can be self-repaired. The properties of the material both before and after the healing reaction will need to be tested and documented.

189 The placement technique of the material is not in the scope of this document. Details of requirements and practices can be found in the Oil & Gas UK Guidelines for the suspension and abandonment of wells [Ref 2].

190 The qualification of plugging material would typically include tests of: Displacement efficiency in wells with relevant wellbore fluids. This may

include segregation (sagging) of the plugging materials components. Sensitivity to contamination with relevant wellbore fluids (due to dilution and

dispersion) that can change key properties of barrier materials.

191 Previous tests of this type have shown that test component size can be critical. For this reason, testing may be required at different scales, e.g. small laboratory test, mid-scale and large-scale testing. Computer simulations may enhance the understanding of the behaviour under varying conditions, provided the output is validated.



192 5.3.6 Thermal expansion differences

193 Where materials with different coefficients of thermal expansion are present, the variation in strain produced by changes in temperature may produce sufficient strain to cause cracking. Such effects may derive from differences in the thermal expansion of the casing and the barrier material, or from the different materials used in composite barriers.

194 The extent to which thermal expansion differences could cause problems is dependent on the magnitude of temperature changes and the rate at which temperature changes occur, with a rapid rate of change being more likely to lead to cracking. Whilst it is unlikely that significant variations in temperature will be encountered after abandonment, the use of barrier materials that evolve heat during hardening or are required to be deployed at an elevated temperature may produce such changes. Additionally, materials used in the annulus may experience temperature cycles of greater magnitude.

195 5.3.7 Drilling damage

196 A further source of leaks can be damage caused to the surrounding formation during the original drilling process. Leakage via cracks in the formation is not directly a barrier material issue, but it may be the case that materials and barrier installation techniques can be chosen such that possible leak paths through damaged formation are sealed by the barrier.



197 6 Material types 198 6.1 Definition of material types

199 The materials potentially suitable as barrier materials are listed in Table 2, which divides materials into material types, based on their chemistry and physical nature.

200 In the case of composite materials, assignment of material type should be for the material acting as the matrix, unless it can be demonstrated through testing that another constituent of the composite defines the behaviour of the material to a greater extent.

201 Barrier configurations employing discrete multiple materials which are interdependent in providing a seal are assigned to multiple types, but require function testing in the envisaged configuration. Multiple barriers comprising different materials should be tested individually.

202 Many materials can be used in a foamed condition. Where it is intended that a barrier material is to be used in such a way, it is essential that testing is carried out on the foamed material.

203 Potential barrier materials that are not assigned to a type are outside the scope of these Guidelines.

204 Type Material Examples

A Cements / ceramics (setting)

Portland API class cement, Pozmix, slag, phosphate cements, hardening ceramics, geopolymers

B Grouts (non-setting) Sand or clay mixtures, bentonite pellets, barite plugs, calcium carbonate and other inert particle mixtures

C Thermosetting polymers and composites

Resins, epoxy, polyester, vinylesters, including fibre reinforcements

D Thermoplastic polymers and composites

Polyethylene, polypropylene, polyamide, PTFE, Peek, PPS, PVDF and polycarbonate, including fibre reinforcements

E Elastomeric polymers and composites

Natural rubber, neoprene, nitrile, EPDM, FKM, FFKM, silicone rubber, polyurethane, PUE and swelling rubbers, including fibre reinforcements

F Formation Claystone, shale, salt.

G Gels polymer gels, polysaccharides, starches, silicate-based gels, clay-based gels, diesel / clay mixtures

H Glass I Metals Steel, other alloys such as bismuth-based materials

Table 2. Material types for permanent barriers

205 6.2 Critical material properties related to potential failure modes

206 The performance requirements of a permanent barrier within in a given set of operating conditions mean that it is necessary to characterize certain properties of a prospective barrier material to ensure that it is appropriate for the application. These properties, along with further definitions, and a discussion of their significance in relation to the potential failure modes, are listed in Table 3.



207 Property Definition Significance

Permeability Measure of the ability of a porous material to transmit fluids under a pressure differential. Units = darcy.

Allows estimation of lag time between placing and break-through and rate of release of fluid below a given length of barrier under a given pressure differential.

Diffusion coefficient

Proportionality constant between the flux of a substance moving as a result of diffusion and the gradient of concentration driving the diffusion process. Units = m2s-1.

Allows estimation of lag time between placing and break-through and rate of release of fluid below a given length of barrier under a given concentration differential.

Absorption Mass of liquid taken up by porosity within a material. Units = % by mass, or % by volume.

Provides an indication of the extent to which the barrier will swell, from which associated stresses can be estimated. Allows calculation of permeability.

Chemical resistance

Indication of whether the material will chemically react with a fluid or fluids. Typically indicated as resistant, limited resistant, not resistant.

Provides an indication of the extent to which the properties of the barrier material will change

Volume change

Change in volume. Units = None (strain) or % by volume.

Allows stresses resulting from expansion or shrinkage to be calculated.

Modulus of elasticity

Ratio of uniaxial stress to uniaxial strain. Units = Pa = Nm-2

Allows calculation of magnitude of deformation of barrier under a given pressure.

Poissons ratio Ratio of lateral to axial strain in a material loaded uniaxially in the axial direction. No units.

Allows calculation of lateral deformation of barrier under a given pressure (in combination with modulus of elasticity)

Cohesion A strength property of granular materials that indicates the cementation strength between the grains under shear stress. Units = Pa = Nm-2.

Allows calculation of shear failure of the plug material. Linked to tensile strength.

Internal friction angle

A strength property that describes a granular materials ability to increase load-bearing capacity (shear stress) with increasing confinement. Units = degrees.

Allows calculation of the increased load-bearing capacity of granular plug material with increase of the confinement. Only strength property after significant plastic deformation resulting in significant reduction in UCS and loss of cohesion.

Hydrostatic yield

Hydrostatic stress (i.e. stress applied uniformly in all directions) at which plastic deformation occurs. Units = Pa = Nm-2.

Indicates the onset of pore collapse in granular materials. This failure mode is plastic and results in irreversible deformation. Beyond this stress level the material will gradually lose cohesion and hence load-bearing capacity.

Tensile strength

Stress at failure under a tensile load. Units = Pa = Nm-2.

Gives maximum tensile stress that can be withstood by barrier.

Unconfined compressive strength

Axial compressive stress at which a material fails. Units = Pa = Nm-2.

Gives maximum compressive stress that can be withstood by barrier.

Hardness Ability of a material to resist penetration of its surface.

Easy quality assurance / quality control test. For certain materials, hardness can be related to yield strength.

Shear bond strength

Stress at which bond between two materials fails under shear loading. Units = Pa = Nm-2.

Allows calculation of pressure differential that can be withstood by the barrier before movement takes place.

Tensile bond strength

Stress at which bond between two materials fails under tensile loading. Units = Pa = Nm-2.

Gives maximum tensile stress that can be withstood at the barrier casing interface.

Creep Non-instantaneous linear deformation under constant load with time. Units = strain rate %/s for a given stress.

Allows prediction of ultimate dimensional change of a barrier under a given pressure differential or other load.

Fatigue life Number of stress cycles of a specified character that an article can undergo prior to failure. Units = Cycles for a given stress range cycle.

Allows projection of longevity of barrier experiencing a specified cyclical stress regime. In the case of materials with an endurance limit (cyclical stress amplitude below which failure through fatigue will not occur), allows assessment of whether there is a risk of fatigue.

Decomposition temperature

Temperature at which barrier material begins to thermally decompose. Units = oC (oF) at given pressure and environment composition

Allows assessment of the upper operating temperature or whether barrier will deteriorate.

Density Mass per unit volume. Units = kg/m3. Easy QA/QC test. Allows assessment of likelihood of barrier moving as a result of imbalance between densities of barrier and well fluids.

Table 3. Properties of barrier materials related to the potential failure modes.



208 7 Approach to defining acceptance criteria for mass transport properties

209 It is necessary to define certain criteria for some of the properties listed in Table 3, such that it can be demonstrated as part of a qualification that a material being considered for use in a permanent barrier is appropriate for the application. In the case of many materials this is based on the probable range of service conditions and the likely characteristics of the different Material Types. However, in the case of mass transport properties (diffusion or flow of fluids through the permanent barrier) the approach has been taken to relate this to the performance of cement. This philosophy is outlined below.

210 Since the permanent barrier is effectively reinstating the caprock, the acceptance criteria are based on the performance of the caprock. Specifically, the length and permeation characteristics (permeability or diffusion properties) of the barrier should be such that the rate of release of fluids in the well should be equal or lower than that of the caprock once breakthrough has occurred.

211 It should be stressed that this is a very conservative requirement, since the cross-sectional area of a barrier will be many orders of magnitude smaller than the area of caprock covering a reservoir. Thus, a moderately higher rate of release per unit area through a barrier relative to the caprock is likely to make only a small difference to the total release rate from a reservoir.

212 North Sea caprock permeability values are discussed in Appendix 5, which are normally better than can be achieved with standard cements the permeability of good cement is typically below 10 microdarcy. However, industry experience has found that 30 m (100 ft) cement barriers perform to a level which has been satisfactory within the historical timeframe of commercial oil and gas production.

213 Example: The Oil & Gas UK Guidelines for the suspension and abandonment of wells [Ref 2] require 30.5 m (100 ft) of good cement. Using a permeability value typical for good cement (20 microdarcy) and a pressure differential of 6.9 MN/m2 (1,000 psi), a release rate of 0.25 m3 of gas per year would be obtained for this length of barrier in a 7 casing, assuming the

46231 guidelines_qualification of materials for well suspension and abandonment july 2012

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