SIST EN 15112:2006
(Main)External cathodic protection of well casings
External cathodic protection of well casings
This European Standard specifies methods used to evaluate the external corrosion hazards of well casings, as well as cathodic protection means and devices to be implemented in order to prevent corrosion of the external part of these wells in contact with the soil.
This European Standard applies to any gas, oil or water well with metallic casing, whether cemented or not.
However, in special conditions (shallow casing: e.g. 50 m, and homogeneous soil), EN 12954 can be used to achieve the cathodic protection and assess its efficiency.
This European Standard also describes techniques allowing determination of the current required for protection and ensuring correct operation of the cathodic protection devices installed.
Äußerer kathodischer Korrosionsschutz von Bohrlochverrohrungen
Diese Norm präsentiert Methoden, die benutzt werden, um äußere Korrosionsgefahren sowohl von
Bohrlochverrohrungen als auch Mittel und Geräte, die eingesetzt werden, um Korrosion der mit dem Boden
kontaktierten äußeren Teile der Verrohrungen zu verhindern, zu bewerten.
Diese Norm gilt für jedes Gas-, Öl- oder Wasserbohrloch mit metallischer Verrohrung, egal ob einzementiert
oder nicht.
Unter besonderen Bedingungen jedoch (flache Verrohrung: z. B. 50 m und homogener Boden) kann die Norm
EN 12954 genutzt werden, um kathodischen Korrosionsschutz zu erreichen und dessen Wirksamkeit
einzuschätzen.
Diese Norm beschreibt auch die Techniken, die die Bestimmung des für den Schutz erforderlichen Stromes
erlauben und die korrekte Betriebsweise der eingebauten Geräte für den kathodischen Korrosionsschutz
sicherstellen.
Protection cathodique externe des cuvelages de puits
La présente norme européenne a pour objet de présenter les méthodes permettant d’évaluer les risques de
corrosion externe des cuvelages de puits, ainsi que les moyens et dispositifs de protection cathodique a
mettre en oeuvre pour prévenir la corrosion externe de la partie de ces puits en contact avec le sol.
Cette norme s’applique a tous les puits de gaz, de pétrole ou d’eau a cuvelage métallique, cimenté ou non.
Néanmoins dans certaines conditions particulieres (cuvelages de faible profondeur : par exemple 50 m, et
sols homogenes) la norme EN 12954 peut etre utilisée pour réaliser la protection cathodique et évaluer son
efficacité.
La présente norme européenne décrit également les techniques qui permettent de déterminer le courant
nécessaire a la protection de l’ouvrage et de s’assurer du bon fonctionnement des dispositifs de protection
cathodiques mis en place.
Zunanja katodna zaščita globinskih zaščitnih cevi
General Information
Relations
Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.External cathodic protection of well casingsProtection cathodique externe des cuvelages de puitsÄußerer kathodischer Korrosionsschutz von BohrlochverrohrungenTa slovenski standard je istoveten z:EN 15112:2006SIST EN 15112:2006en25.220.40Kovinske prevlekeMetallic coatingsICS:SLOVENSKI
STANDARDSIST EN 15112:200601-oktober-2006
EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN 15112July 2006ICS 23.040.99; 77.060 English VersionExternal cathodic protection of well casingsProtection cathodique externe des cuvelages de puitsÄußerer kathodischer Korrosionsschutz vonBohrlochverrohrungenThis European Standard was approved by CEN on 19 June 2006.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the Central Secretariat or to any CEN member.This European Standard exists in three official versions (English, French, German). A version in any other language made by translationunder the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the officialversions.CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania,Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre: rue de Stassart, 36
B-1050 Brussels© 2006 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN 15112:2006: E
EN 15112:2006 (E) 2 Contents Page Foreword.3 Introduction.4 1 Scope.5 2 Normative references.5 3 Terms and definitions.5 4 Description and assessment of corrosion risks.9 4.1 General.9 4.2 Description of corrosion risks.9 4.3 Corrosion risk assessment.9 5 Prerequisites for application of cathodic protection.10 5.1 General.10 5.2 Electrical continuity.10 5.3 Electrical isolation.10 5.4 Cathodic protection equipment.11 5.5 Groundbed.11 5.6 Safety requirements.11 6 Design of the cathodic protection.12 6.1 General.12 6.2 Voltage drop profile method.12 6.3 Polarisation curve method.13 6.4 Mathematical approach based on a field test.13 6.5 Simulation of the cathodic protection for a well.13 7 Measurement of the well-casing-to-soil potential at the wellhead.14 7.1 General.14 7.2 Measuring points.14 7.3 Method used for potential measurement - Interpretation.15 8 Additional cathodic protection equipment.15 Annex A (normative)
Voltage drop profile.16 Annex B (informative)
Polarisation curve method applied to a well.23 Annex C (informative)
Determination by calculation of the potential shift at the bottom of the well and the well to soil resistance.26 Bibliography.36
EN 15112:2006 (E) 3 Foreword This document (EN 15112:2006) has been prepared by Technical Committee CEN/TC 219 “Cathodic protection”, the secretariat of which is held by BSI. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by January 2007, and conflicting national standards shall be withdrawn at the latest by January 2007. According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EN 15112:2006 (E) 4 Introduction Gas, oil and water well casings are usually cemented for the proposes of anchoring the pipes in the borehole and isolating the various geological layers from each other. This is necessary to avoid liquid exchanges between these. Steels in contact with the cement are in a passivation status and, thus, protected from any kind of external corrosion, except if the cement contains chloride ions. However, it is not always possible to obtain a continuous cementation on all the external steel surfaces. These bare residual surfaces may be in contact with more or less aggressive layers. Furthermore, these surfaces may constitute electrochemical cells with the cemented metallic parts. The anodic areas, which are the poor cemented parts, correspond to corrosion areas. In general, external corrosion effects are rare, particularly on recent wells, since most of them are well cemented. However, borehole cementation programmes sometimes result in cementation failures, and studies have shown that, corrosion phenomena being progressive, the mean time for the appearance of leaks is dependent on different factors such as geological formation, thickness of the layers and of the steel casing. Experience has also shown that the situation may be significantly improved by applying external cathodic protection to wells. Environmental aspects with regard to gas, oil or water wells should be considered when deciding on whether or not to apply cathodic protection.
EN 15112:2006 (E) 5
1 Scope This European Standard specifies methods used to evaluate the external corrosion hazards of well casings, as well as cathodic protection means and devices to be implemented in order to prevent corrosion of the external part of these wells in contact with the soil. This European Standard applies to any gas, oil or water well with metallic casing, whether cemented or not. However, in special conditions (shallow casing: e.g. 50 m, and homogeneous soil), EN 12954 can be used to achieve the cathodic protection and assess its efficiency. This European Standard also describes techniques allowing determination of the current required for protection and ensuring correct operation of the cathodic protection devices installed. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 12954:2001, Cathodic protection of buried or immersed metallic structures — General principles and application for pipelines EN 60079-10, Electrical apparatus for explosive gas atmospheres — Part 10: Classification of hazardous areas (IEC 60079-10:2002) 3 Terms and definitions For the purposes of this document, the terms and definitions given in EN 12954 and the following apply (see also Figure 1). 3.1 casing (or well casing) heavy steel pipe string used to line a borehole from the ground surface, and secured in the formations generally by cementing NOTE Casing is generally externally cemented over its total depth or over a length sufficient to obtain anchoring and stability between the production or storage zone and the ground surface or other intermediate layers. This pipe string allows: - to prevent the ingress of fluid from upper strata; - to keep the hole from collapsing due to the pressure of the geological layers crossed; - to isolate the inside part of the well from the surrounding soil; - to continue drilling to the production or storage zone; - to drive down the tubing string from the surface to the production or storage zone. There may be two or more strings of casing, one inside the other, in a single well:
EN 15112:2006 (E) 6 -
surface casing: casing that extends from the surface to a depth sufficient to avoid any entering of surface waters or earth into the well; - intermediate casing: casing set from the ground surface down to an intermediate depth. This intermediate depth is situated between the surface casing shoe and the production or storage zone; - production casing: casing that extends through the surface casing and intermediate casing to the production or storage zone. The extremity of the production casing can be at the top or bottom of this zone. 3.2 cellar excavation at ground surface, intended for housing the wellhead and safety shut-off devices. EXAMPLE safety valves 3.3 cementation process, and its result, which ensures the anchoring of well casing in the borehole and the tightness between different geological levels.
NOTE In the same time, this cementation can mitigate corrosion 3.4 centralizer device constituted by a set of metallic blades which are fitted around the pipes of a string to keep them centred, either in the open hole (hole drilled in the ground), or inside pipes of larger diameter in which the considered string is installed. This device can also be used to ensure electrical continuity between the two concentric pipe strings 3.5 completion process, and its result, which consists of fitting a well with the tubing to allow well operation in accordance with the applicable codes of practice and safety rules 3.6 flow-line pipe connecting a well to a station 3.7 liner (bottom hole) pipe having the same function as the casing but hung inside a casing (or another liner) and not at the wellhead like a conventional casing 3.8 packer (production) device ensuring tightness of a pipe annulus. The production packer seals the annulus between the tubing and the production casing or liner 3.9 shoe cylindrical element attached to the lower part of the casing, and allowing to place the casing in the borehole (guide shoe). If equipped with a valve, it makes easier the borehole cementation (cementing shoe) 3.10 tubing (production tubing) pipe string, with its additional equipment, inside the production casing to allow the flow of oil, gas or water between the production or storage zone and the ground surface
EN 15112:2006 (E) 7 3.11 wellhead device installed at the top of the well, designed to hang the different pipe strings and to ensure tightness between the various annular spaces. The wellhead is fitted with valves to allow access (pressure monitoring, sampling) to the different annuli. Such fitted wellhead allows well operation and the intervention on the different components of the well. This device allows a good electrical continuity between all the pipe strings
EN 15112:2006 (E) 8
12453 1210635789
Key 1
ground surface 2 surface casing 3
cementation 4
production casing 5
shoe 6
production annulus 7
tubing 8
liner (bottomhole) 9
packer (production) 10 intermediate casing Figure 1 — Typical well completion equipment
EN 15112:2006 (E) 9 4 Description and assessment of corrosion risks 4.1 General Corrosion may occur on the external surface of well casings. This corrosion, if not controlled, may lead to harmful damage such as losses of products, water, gas or oil, damage to the well and its completion (internal equipment), damage to the environment, for instance in allowing exchange between different geological formations. There is also the possibility of harm for people living near such installations. The risks of corrosion should be considered in order to decide if cathodic protection shall be applied to the structure. 4.2 Description of corrosion risks In general, for technical reasons, well casings should be covered by cement. In such conditions steel is passive, its potential is uniform under the cement and the corrosion hazards are reduced. In this case, cathodic protection should not be necessary. In fact, due to the heterogeneity of the soils which are crossed during drilling and specifically due to the heterogeneity of the mechanical properties of these soils, it is not always possible to guarantee that a continuous cement layer covers the whole steel surface. Because of this non-homogeneous cement layer, some parts of the casing surface are in contact with the external medium. Macro-electrochemical cells (steel/cement and steel/medium) are then established and this results in a corrosion of the anodic parts of the cells (steel in the medium). If there is no isolating joint between the well and surface piping, such detrimental macro-cells may also appear between the casing and the bare or poorly coated parts of the buried structure surface which become the anodic parts of the macro-cell. Corrosion caused by the currents generated by macro-cells is more severe where soil layers with low resistivity are crossed. Risks of corrosion damage shall be considered particularly where:
- the designed service life is long (depending on location, operational conditions);
- the procedure and execution of the cementation results in areas not or incorrectly cemented;
- there are stray current sources;
- the geological layers crossed are of a different nature. 4.3 Corrosion risk assessment The previous information is only intended to provide a general idea on the corrosion risks involved. Usually, a corrosion risk is assessed by measuring the structure-to-electrolyte potential. However, these potential measurements require installation of a reference electrode in the electrolyte in the immediate vicinity of the metal. For a well casing, access is limited to the upper part of the well and it is thus impossible to perform any measurement on the deep borehole. During drilling, samples of drill cuttings should be checked and recorded at regular depths, particularly if their make up changes, to assess corrosivity and composition if the strata changes.
EN 15112:2006 (E) 10 As an alternative to the above method, another way could be to carry out an accurate analysis of the electric log surveys which have been recorded in the open borehole. Another approach consists in establishing whether current coming from the outside environment (ground) enters in or, conversely, exits from the casing, by using the method known as voltage drop profile (Annex A), which allows this determination by following the direction and intensity of currents circulating in the casing along the well. This method allows localization of all areas where there is corrosion. Furthermore, according to the voltage drop observed, it is possible to assess the importance of the current intensity exiting from the casing, which determines the rate of corrosion. Nevertheless, this method is difficult to implement. If available, the usual logs performed after borehole cementation can be usefully analysed to ascertain quality and homogeneity of the borehole cementation, especially in the areas with low electrical resistivity. 5 Prerequisites for application of cathodic protection 5.1 General The requirements defined in EN 12954 shall be met. However, it should be taken into account that the well casing is bare and in contact with the soil in the borehole through the cement. 5.2 Electrical continuity If a well is to be cathodically protected, a number of precautions shall be taken during completion. In addition to the external parts in contact with the borehole cementation or the soil, for which protection is required, the well generally includes other parts which are not in contact with the surrounding soil. The latter comprise the production string and all or part of intermediate and production casings depending on the type of completion, operation mode, the depth and the diameter. It is necessary to avoid current flow through an electrolyte located in the annular space, since it could cause corrosion. Annular spaces which are not cemented are generally filled with a liquid which may be brine, mud water and so on. Under such conditions, current flow through the electrolyte shall be prevented by the use of bonds between each string. Therefore it is necessary: - to establish metallic bonds to ensure perfect electrical continuity of each casing part, at upper (wellhead) and lower (shoe) levels, and - to install metallic centralisers where geological layers may promote a flow of current into the casing. 5.3 Electrical isolation 5.3.1 General In principle, there should be no electrical continuity between the well to be protected and the foreign structures and particularly the flow-line. For this purpose, an isolating joint is installed between the well and its flow-line. In this case, a special attention shall be given to avoid undesirable electrical shunts which may be caused by metallic bonds due to the small diameter pipes which are used for well control and safety devices. Another problem may appear when the fluid or a part of the fluid conveyed in the flow-line is a low-resistivity electrolyte. An internal corrosion risk may exist due to a possible voltage drop between both sides of the isolating joint. In such a case, the isolating joint shall be internally coated with a suitable and electrically isolating material. Moreover, the internal coating shall be:
EN 15112:2006 (E) 11 - applied over a suitable length to reduce the corrosion rate to an acceptable level; - only applied on the side with the most negative potential. 5.3.2 Particular situations It may be sometimes necessary to ensure electrical continuity (direct or resistive) between the well to be protected and neighbouring structures, typically the flow-lines. - Such may be the case for some wells remote from any electrical current source. A method to allow the well cathodic protection consists in the use of the flow-line as a return conductor by short circuiting the isolating joint. Then, the protection is carried out on both structures. However, as both structures have very different coating resistances, it can be necessary to fit the bond of the isolating joint with a resistor. - Such may be also the case if the well is subject to the influence of stray currents. The bonding of the isolating joint (with or without resistor) can sometimes allow the installation of a drainage on the flow line to mitigate the influence of the stray currents. - Wells have very low resistance against earth. A device to protect the isolating joint against over voltages should be installed if such a risk exists. In some cases, it may be impossible to insulate the well from foreign structures. Such is the case for offshore platforms where wells are always connected to the main structure. 5.4 Cathodic protection equipment Considering the low resistance to earth of the casing and its length, it is generally only possible to obtain protection down to the well shoe by use of impressed current even when borehole cementing is of good quality. Offshore, where it is difficult to obtain potentials more negative than - 1,00 V measured with an Ag / AgCl / sea water reference electrode at the wellhead, whatever the platform protection method, the borehole cementing needs to be of very good quality to protect the entire casing. For installations affected by stray currents, suitable equipment (e.g. resistive drainage bonds) shall be considered. Whatever the method selected, the equipment shall be chosen and provided in accordance with EN 12954. 5.5 Groundbed To allow the protective current to reach the lower extremity of the well, the groundbeds should be at a sufficient distance from the casing in order to obtain a good current distribution. The distance depends on: - soil resistivity along the well casing; - amount of protective current (dimensions of the well casing and cementing quality); - depth of the well casing. 5.6 Safety requirements The national and local safety rules and procedures concerning gas, crude oil and water drilling installations, shall be complied with. These rules and procedures concern surface installations, wells and equipment related to these structures. They may cover the following:
EN 15112:2006 (E) 12
- electrical insulation;
- permanent or temporary earthing;
- perfect electrical continuity throughout the installations to avoid any sparking risk, even during maintenance and workovers;
- materials and equipment;
- classified hazardous areas, according to EN 60079-10, where it is possible to install cathodic protection equipment both with regard to access to the wellhead and any explosion risk. A close co-operation between specialists on safety and cathodic protection shall be established to comply with the safety rules as well as cathodic protection requirements (assurance of its correct installation and operation, as well as absence of influence risks for neighbouring buried metallic parts which are not protected by means of cathodic protection). 6 Design of the cathodic protection 6.1 General In general, design of the cathodic protection of a structure includes as a first step the definition of the minimum initial of protective current demand required to meet the basic criterion for cathodic protection E ≤ Ep’ as defined in the European Standardization (see EN 12954). However, as mentioned above (Clause 4) it is impossible to verify that the basic criterion for cathodic protection of well casings is correctly fulfilled along the entire structure to be protected. Consequently, to begin the study of cathodic protection of a well, it is necessary to use methods and measurement procedures specific to this type of structure. The methods described hereafter allow the determination of the currents required for cathodic protection. Other methods, based on specialist experience, may be used, if they are documented and can lead to a comparable result. 6.2 Voltage drop profile method This method, as mentioned in 4.2 for corrosion risk assessment and described in Annex A, may be used to determine the protective current to ensure effective cathodic protection. The aim of this method is to make sure that all segments of the voltage drop profile have a positive slope which means that the entire structure no longer has anodic areas. For this purpose a temporary cathodic protection station has to be installed. The temporary groundbed should be far away from the well to allow a good repartition of the current. Groundbed selection shall take account of safety, particularly electrical hazards, for the personnel in charge of tests and also for the structure under test. For a chosen protective current, the voltage drop is recorded along the entire well in accordance with the procedure described in Annex A. If the protective current used during this test is not sufficient (Annex A, Figure A.2, case B), the voltage drop, recorded at each measuring point with the measuring tool shows negative slope segments. These represent those areas which remain anodic. This is always the case if the voltage drop shows negative values. If the protective current used during the test is sufficient (Annex A, Figure A.2, case C), all segments of the curve of potentials have a positive slope. The entire well structure is cathodic. In some cases, the test may be performed again with a lower current to determine the minimum protection current which makes the entire structure cathodic.
EN 15112:2006 (E) 13 6.3 Polarisation curve method The principle and performance of this method are described in Annex B. From a test cathodic protection installation, a protection current is injected. For this purpose a temporary cathodic protection station can be used (see 6.2). At the end of a defined time period, when the structure to electrolyte potential becomes stable, the structure-to-electrolyte potential value of the casing Eoff at the ground surface is recorded (see 7.1). After this first measurement, the current is increased by a pre-defined increment and maintained at the new value over the same time period. This procedure is repeated until a sufficient number of adjustments have been made to the applied current to enable line II of the curve to be drawn in Figure B.1. Once the curve is plotted, the tangents to the two linear parts I and II (see Figure B.1) are used to determine the minimum current which ensures that the well casing is considered to be cathodically protected. 6.4 Mathematical approach based on a field test
Even the mathematical calculation in Annex C is presented for two concentric pipes (see C.5); it may be applied using computer means where casings consist of several concentric pipes. It allows expressing the casing potential shift at ground surface as well as protection current, according to the:
- well features (length, section, thickness, steel resistivity);
- contact resistance with environmental medium (structure-to-soil resistance);
- minimal potential shift expected at the lower extremity (shoe). Well protection is considered to be achieved over its whole length if the potential shift, related to the free corrosion potential EN, calculated at the lower extremity of the well is at least equal to 300 mV. A current injection test is performed; the current injected and the potential at the top of the well are recorded. When transferred in the equation system of Annex C, these two values allow the determination of the protection current to be adopted for the well. This method is very easy to implement with a maximum of two concentric casings. It leads to determination of excess values of the required current protection. Indeed, it does not take into consideration the electrochemical polarisation phenomena which progressively appear on the well metal surface. It may be used for the whole well life to establish, whenever required, the optimum protection current adjustment. 6.5 Simulation of the cathodic protection for a well Cathodic protection for a well can be simulated by numeric methods in order to determine how protection current is distributed on the external surface along the casing, depending on the characteristics of the geological layers, on the electrochemical interface between the steel and the cementation (polarisation curves of steel in cementation) and on ohmic characteristics of the columns. These numeric methods also allow the potential values all along the casing to be predicted. NOTE The simulation can be associated with field measurements in order to verify its agreement and to adjust the required current.
EN 15112:2006 (E) 14 7 Measurement of the well-casing-to-soil potential at the wellhead 7.1 General A general indication of protection level is given by measuring the well-casing-to-soil potential at the wellhead in compliance with requirements of EN 12954. However, as mentioned in Cla
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