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RECOMMENDED PRACTICE

DNVGL-RP-E103 Edition April 2016

Risk-based abandonment of offshore wells

DNV GL AS

The electronic pdf version of this document found through http://www.dnvgl.com is the officially binding version. The documents are available free of charge in PDF format.

FOREWORD
DNV GL recommended practices contain sound engineering practice and guidance.
© DNV GL AS April 2016

Any comments may be sent by e-mail to [email protected]

This service document has been prepared based on available knowledge, technology and/or information at the time of issuance of this document. The use of thisdocument by others than DNV GL is at the user's sole risk. DNV GL does not accept any liability or responsibility for loss or damages resulting from any use ofthis document.

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GeneralThis is a new document.

Recommended practice, DNVGL-RP-E103 – Edition April 2016

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CONTENTS
CHANGES – CURRENT .................................................................................................. 3

Sec.1 Introduction.................................................................................................. 51.1 Objective................................................................................................51.2 Scope .....................................................................................................6

1.2.1 General ........................................................................................61.2.2 Exclusions.....................................................................................61.2.3 Application....................................................................................6

1.3 Users......................................................................................................61.4 References to standards.........................................................................71.5 Definitions..............................................................................................7

1.5.1 Definitions ....................................................................................71.5.2 Abbreviations ................................................................................91.5.3 Verbal forms .................................................................................9

1.6 System description.................................................................................91.6.1 Marine environment .......................................................................91.6.2 Geology......................................................................................111.6.3 Wellbore .....................................................................................11

Sec.2 Risk assessment framework for well abandonment design ......................... 122.1 Establishing the risk context ................................................................12

2.1.1 Well abandonment design .............................................................132.1.2 Flow potential sources ..................................................................142.1.3 Permanent well barrier principles ...................................................142.1.4 Number of well barriers ................................................................15

2.2 Permanent well barrier failure modes ..................................................182.3 Risk analysis ........................................................................................19

2.3.1 Flow potential..............................................................................192.3.2 Valued ecosystem components ......................................................192.3.3 Dispersion modelling ....................................................................202.3.4 Impact analysis ...........................................................................20

2.4 Risk evaluation.....................................................................................202.4.1 Risk acceptance criteria ................................................................202.4.2 Environmental and safety risk evaluations ......................................21

2.5 Treatment of uncertainties...................................................................21App. A Input for risk-based abandonment of offshore wells................................... 22App. B Well barrier failure modes for P&A wells .................................................... 23


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SECTION 1 INTRODUCTION
All offshore hydrocarbon wells that are drilled will eventually require permanent abandonment in order to control subsurface pressures and prevent the free flow of pore fluids to the seafloor. The ultimate objective of plug and abandonment [P&A] of wells is to protect the environment, while maintaining safety standards.

Offshore, there is a large variety of well types to be plugged and abandoned. Certain wells require less complex P&A operations and can be plugged either by existing platform drilling equipment, or by a simple rig-less P&A solution. Other more complex wells will need a rig that can handle more challenging and heavy P&A operations including the retrieval of tubing and casing, milling and cement repairs. Wells drilled decades ago often lack essential documentation needed for P&A, especially if the ownership of the well has changed. Missing or incomplete documentation means that larger uncertainties will present themselves in the well abandonment design.

The plugging and abandonment of offshore wells represents a significant cost and liability to operating companies and national authorities, while at the same time being governed by prescriptive downhole requirements.

Current requirements are prescriptive as to the number and size of permanent well barriers required and the requirements are the same for all types of wells. Alternatively, risk-based methods can be applied to well abandonment design and acceptance criteria. By introducing a risk-based approach, tailor-made design solutions can be devised, which better suit the different wells and allow cost-saving benefits to be gained from the least critical wells.

There is an ongoing paradigm shift towards differentiating P&A requirements on a well-by-well basis instead of having prescriptive requirements, which will not only provide the appropriate focus for complex wells, facilitate development of new technology, but will also potentially reduce P&A expenditures.

This recommended practice [RP] is intended to provide an alternative approach, based on functional requirements and risk acceptance criteria to assess abandonment designs. This is consistent with offshore engineering practice and is intended to facilitate cost efficient solutions including the development of new technology. By calculating the risk levels for the proposed solutions and cross-checking them with the risk acceptance criteria, more cost-effective solutions can be identified and implemented.

Advantages to this approach are that it has:

— explicit criteria for environmental protection— P&A spending focussed on higher-risk wells— the ability to optimise well abandonment design— flexibility to make use of new plugging technology in the future— site specific considerations.

This RP can be used in the design and planning of abandonment activities for offshore wells. This RP recommends, based on DNV GL’s opinion, an industry practice for that purpose in a risk-based manner.

This RP is applicable worldwide with adaptation to local industry and regulatory requirements.

1.1 ObjectiveThe objective of this RP is to provide a risk-based framework for well abandonment design and permanent offshore well abandonment.


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1.2 Scope
1.2.1 GeneralThis RP presents principles and practices for:

— establishing site-specific environmental risk acceptance criteria— confirming compliance with safety criteria for the field/installation— determining the functional requirements for permanent well barrier materials— differentiating the environmental risk exposure relative to hydrocarbon composition.

1.2.2 ExclusionsThis RP is not applicable for:

— onshore wells— suspended wells— temporarily abandoned wells.

1.2.3 ApplicationThis RP may be applied as a basis for risk-based decision making. It can be applied for:

— evaluation of well abandonment designs — well abandonment design optimisation in relation to cost and material— evaluation of environmental performance for P&A wells— independent assessment and verification— guidance and quality assurance of P&A planning— stakeholder communication.

The requirements in this RP are intended to be subordinate to local regulations.

1.3 UsersUsers of this RP may be:

— well operators— regulators— suppliers— independent verifiers or examination bodies— investors, co-ventures, partners or other financial stakeholders.


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1.4 References to standards
1.5 Definitions

1.5.1 Definitions

Table 1-1 References

Document code TitleAPI RP 65 Cementing Shallow Water Flow Zones in Deep Water WellsAPI RP 65-2 Isolating Potential Flow Zones During Well ConstructionAPI Bulletin E3 Well Abandonment and Inactive Well Practices for U.S. Exploration and Production Operations,

Environmental Guidance DocumentCBD Convention on Biological DiversityISO 16530-1 Well integrity Part 1: Life cycle governanceISO TS 16530-2 Well integrity Part 2: Well integrity for the operational phaseISO 17776 Petroleum and natural gas industries – Offshore production installations – Guidelines on tools

and techniques for hazard identification and risk assessmentISO 31000 Risk management – Principles and guidelinesNORSOK D-010 Well integrity in drilling and well operationsNORSOK Z-013 Risk and emergency preparedness analysisOil & Gas UK Guidelines for the suspension and abandonment of wellsOil & Gas UK Guidelines on qualification of materials for the suspension and abandonment of wellsOil & Gas UK Well integrity guidelinesOSPAR Convention for the Protection of the Marine Environment of the North-East Atlantic UNEP/CBD Marine and Coastal biodiversity: Decision Adopted By the Conference of the Parties To the

Convention on Biological Diversity At Its Ninth MeetingUnited Nations United Nations Convention on the Law of the Sea

Table 1-2 Definitions

Term Definitionbiogenic gas naturally occurring methane generated by biological activity at shallow depths below the

seafloorconsequence outcome of an event affecting objectivescontainment prevention of flow at rates or in total mass sufficient to cause adverse impactcross-flow hydrocarbon flow from one formation to another oneecological receptor organisms living in the offshore marine environment or the habitat which supports such

organismsfailure mode potential or observed manner of failure on a specified level of a well barrier or well barrier

elementflow potential [source] a formation which contains moveable fluids in the form of hydrocarbons or abnormally

pressured watergeological fault a displacement of rocks along a shear surface

The surface along which displacement occurs is called the fault plane (often a curved surface).hazard potential source of harmlevel of risk magnitude of a risk or combination of risks, expressed in terms of the combination of

consequences and their likelihoodlikelihood chance of something happening expressed either qualitatively or quantitatively and described

using general terms or mathematically, such as a probability or an expected frequency over a given time period

overburden sedimentary succession (stratigraphic column) overlying a reference underground formation, normally a petroleum reservoir

permanent [well] barrier

combination of one or several well barrier elements (WBE’s) that contain fluids within a well to seal a source of inflow

permeability measure of the ability of a soil or rock to transmit fluids


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plug and abandonment

action taken to ensure permanent isolation of fluids and pressures from exposed permeable zones along well trajectory by installation of well barriers

porosity ratio of the volume of void pore space in the rock relative to the bulk volume of the rockregulator relevant national, state or provincial authority and/or international regulatory bodyreservoir a subsurface body of rock having sufficient porosity and permeability to store and transmit

fluidsrisk effect of uncertainty on objectives, where risk may be expressed in terms of a combination of

a likelihood of occurrence of an event and the associated severity of potential consequences that may arise as a result of the event, where this definition is the equivalent to the definition of risk in ISO 31000

risk acceptance criteria

terms of reference against which the significance of a risk is evaluated, where this definition is the equivalent to the definition of risk criteria in ISO 31000

risk analysis process to comprehend the nature of risk and determine the level of risk

risk assessment overall process of risk analysis and risk evaluationrisk evaluation process of comparing the results of risk analysis with the evaluation risk criteria to determine

whether the risk and/or its magnitude are/is acceptable or tolerablesafety aspects related to humans’ life and health.

This term may be used in conjunction with safety risk or safety hazards.seepage the slow escape of a liquid or gas through porous material or small holesstakeholder individual, group of individuals, or organisation whose interests are substantially affected by

the project, which may include employees, shareholders, community residents, suppliers, customers, non-governmental organisations, governments, regulators, labour unions, and other individuals or groups

suspended well suspension well status, where the well operation is suspended without removing the well control equipment

temporary abandonment

the well status, where the well is abandoned and/or the well control equipment is removed, with the intention that the operation will be resumed within a specified time frame

thermogenic hydrocarbons

naturally occurring gas, condensate or oil generated by the thermal alteration of kerogens and bitumens at depth

well abandonment design

a design for the execution of the plug and abandonment for the well

well barrier element a physical element which by itself does not prevent flow but in combination with other WBE’s forms a well barrier

well integrity the ability of a well to perform its required function effectively and efficiently while preventing uncontrolled release of formation fluids along the wellbore throughout the life of the well

well operator natural or legal, private or public person, business organisation(s) or government entity who operates and controls the hydrocarbon reservoir operation or to whom decisive power over the hydrocarbon reservoir operation has been delegated according to regulations

well release uncontrolled release of hydrocarbon fluids from a wellwellbore the physical hole that makes up the well

Table 1-2 Definitions (Continued)

Term Definition


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1.5.2 Abbreviations
1.5.3 Verbal forms

1.6 System descriptionThe systems referred to in this RP include offshore wells, their surrounding geology and overlying marine environment. Figure 1-1 illustrates the system.

1.6.1 Marine environmentThe marine environment around an abandoned well includes the seafloor, the water column and the sea surface. Together these form habitats for living organisms that may be mobile or fixed, such as benthic invertebrates, fish and plankton. Water depth, ocean currents, sea water properties (e.g. salinity, temperature) and the type of sediments on the seafloor vary from place to place, but one common factor is that the surface casing of abandoned wellbores is cut off below the seafloor (mud line).

All wells interact with the marine environment to some degree and abandoned wells are no exception. Soft sediments below the seabed contain organic debris and generate biogenic methane that is buoyant and seeps into the marine environment as a source of nutrients. Wellbores form preferential pathways for gas migration and natural seepage thus has a tendency to aggregate around wells. In the case that soft sediments have collapsed into the hole left by surface casing removal, natural seepage may be dispersed by buoyant percolation through the infilled sediment.

The same processes apply to hydrocarbons that seep from thermogenic sources deeper in the well. These may be pure methane or contain heavier hydrocarbon fractions. Thermogenic and biogenic methane are identified by their isotopic signatures, but are indistinguishable to biodegradation. Heavier hydrocarbon fractions are less soluble in seawater than methane and take longer to react and be broken down by living organisms. Hydrocarbons may accumulate on the seafloor or form buoyant droplets that detach from the seafloor and rise within the water column before continuing to dissolve and be broken down. These droplets have the potential to reach the sea surface, or heavier fractions may re-settle on the seafloor after lighter fractions have been removed.

Whereas methane gas is a natural and important component of marine ecology, no environmental standards exist for setting acceptance criteria for upper limits of methane in seawater. This is not the case for heavier fractions of hydrocarbons that are measured in terms of the total hydrocarbon [THC] content of seawater or sediment.

Table 1-3 Abbreviations

Term DefinitionALARP as low as reasonably practicableEBSA ecologically or biologically significant marine areasP&A plug and abandonmentPSA Petroleum Safety Authority NorwayRP recommended practiceTHC total hydrocarbonsTVD true vertical depth of the wellVEC valued ecological componentsWBE well barrier element

Table 1-4 Definitions of verbal forms

Term Definitionshall indicates a mandatory requirement to be followed for fulfilment or compliance with this

Recommended Practiceshould verbal form used to indicate that among several possibilities one is recommended as

particularly suitable, without mentioning or excluding others, or that a certain course of action is preferred but not necessarily required

may verbal form used to indicate a course of action permissible within the limits of the document


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Figure 1-1 System diagram illustrating the main components of P&A wells

Wellbore

Fish and larvae

Coral and other benthic fauna

Geological formationsoverburden & reservoir

Not to scale

Seafloor sediment

Surface barrier

Secondary barrier

Primary barrier


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1.6.2 Geology
The geological sequence above an oil or gas reservoir is referred to as overburden and may also contain lesser quantities of hydrocarbons in certain formations. Reservoirs themselves may consist of a single homogenous formation or a complex sequence of more or less permeable lithologies within the reservoir interval.

The mechanism of hydrocarbon generation changes with the depth and temperature, from biogenic processes close to the surface to thermogenic processes at greater depth. Thermogenically derived hydrocarbons that seep up to shallow depths may in turn be altered by biogenic processes that mask their thermogenic origin.

All pore fluids, including hydrocarbons, are mobile within the geological sequence at a range of timescales and fluid movement is controlled by sealing formations that have a low permeability relative to other formations, such as shales. Seals control the rate at which formation fluids are squeezed out as formations get buried to greater depth, where buoyant hydrocarbons accumulate and are preserved before the hydrocarbons continue their buoyant migration to the surface. Seals may be compromised by natural processes such as faulting or overpressure, or by man-made drilling activity.

Wells are exposed to a variety of geological processes during their operational lifetime and may be viewed as a passive component within an active geological system once they are abandoned.

1.6.3 WellboreThe wellbore is the conduit for production from, or injection to a reservoir. A well that accesses the reservoir penetrates the overburden and forms a potential passageway for pore fluids to migrate to the seafloor.


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SECTION 2 RISK ASSESSMENT FRAMEWORK FOR WELL
ABANDONMENT DESIGN

International standards advocate for a risk-based approach to well integrity management, and suggest that risk-based approaches are used to assess wells relative to their potential loss of containment. This RP applies this approach to the permanent abandonment of offshore wells by including threats to long-term well integrity.

The objective of performing risk-based abandonment assessments is to systematically assess the well abandonment design to defined acceptance criteria in order to safeguard the environment and maintain safety standards.

2.1 Establishing the risk contextWell abandonment risk assessment establishes, analyses and evaluates the risks. The risk assessment uses a systematic approach for identifying the main contributors to the risk profile. The result of the risk analysis provides valuable knowledge whether the proposed well abandonment design is suitable for use and whether it is necessary to implement measures to reduce the risk. The risk assessment should include environmental and safety risks.

The well abandonment risk assessment may be qualitative or quantitative depending on the context. The focus of this RP is on a quantitative approach.

ISO 31000 describes the risk management processes and emphasizes the importance of establishing the context prior to starting or executing any of the elements included in the risk assessment process, and updating the context throughout the process. Before assessing well abandonment design(s), an evaluation of the flow potential of the producing reservoir(s) and evaluations of the in-situ formation where flow potential, permanent well barrier solutions and potential cross-flow between formations should be performed.

The main elements in the well abandonment risk assessment are illustrated in Figure 2-1. The risk assessment process illustrated in Figure 2-1 is described in detail in the subsequent sub-sections. The four main categories of inputs for the risk assessment are:

— well specific data (well design, well history and current status)— geology data (reservoir and overburden condition)— environmental data (environmental resource overview) — metocean data (ocean current including salinity and temperature profiles).

Sample input data is provided in App.A.


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Figure 2-1 Elements in well abandonment risk assessment

During the risk assessment process, documentation should be provided in a transparent, traceable and consistent manner as to how each of the activities has been performed.

Quality assurance of the analysis assumptions, inputs and results should be done as part of the risk assessment.

2.1.1 Well abandonment designThe main objective with the well abandonment design should be to prevent environmental harm until the original geological barriers are re-established while maintaining safety standards.

The well abandonment design(s) should be assessed as comprehensively as reasonably practicable, as outlined in this section. The design should be based on the established context through the investigation of


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hydrocarbon-bearing formations, where the maximum anticipated flow potential should form the basis for
the well abandonment design.

For abandonment of multiple wells, it should be recognised that each well is unique and the well abandonment design should be considered on an individual basis.

2.1.2 Flow potential sourcesAn overview of the formations should be compiled, where hydrocarbon-bearing formations with flow potential are identified and examined. The overview should illustrate the potential flow from each formation including descriptions of their magnitude.

A flow potential, in this context, is defined as a hydrocarbon-bearing formation containing moveable hydrocarbons large enough to have a potential environmental or safety impact. The flow potential for hydrocarbon-bearing formations should be categorised according to Table 2-1. Categorisation of the flow potential should be performed for a distribution of the anticipated flow potential for the identified hydrocarbon-bearing formations, including potential re-charging, re-development for hydrocarbon extraction, or use for other projects such as geothermal projects, storage of energy or CO2.

For hydrocarbon-bearing formations penetrated by the well with moderate or significant flow potential, potential consequences to the environment should be mitigated with permanent well barriers in accordance with ALARP principles, with reference to the UK Oil & Gas Guidelines for the suspension and abandonment of wells.

2.1.2.1 Cross-flowCross-flow between hydrocarbon-bearing formations should be prevented. Multiple hydrocarbon-bearing formations may be treated as one formation, if the cross-flow does not impact the environmental acceptance criteria.

Note:This may be done for multiple hydrocarbon-bearing formations located within the same pressure regime.

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2.1.3 Permanent well barrier principlesThe permanent well barrier design should be fit-for-purpose and take into account the effects of any reasonably foreseeable chemical and geological process. Its function should be to control hydrocarbon-bearing formation(s) with moderate or significant flow potential. The duration of the requirement for the permanent well barrier should be site specific and should be dependent on the well barrier’s design functionality.

The permanent well barriers should be installed at a depth where the formation is strong enough to contain the hydrocarbon-bearing formations.

A permanent well barrier may consist of any material or combination of well barrier elements [WBE] as long as it provides the following functionalities:

— withstand the maximum anticipated combined loads to which it can be subjected— function as intended in the environments (pressures, temperature, fluids, mechanical stresses) that can

be encountered— prevent unacceptable hydrocarbon flow to the external environment.

Table 2-1 Categorisation of flow potential for hydrocarbon-bearing formations

Categories of flow potential DefinitionNo or limited flow potential Hydrocarbon-bearing formations where moveable hydrocarbons present or in the

future cannot under any circumstances have an environmental or safety impactModerate flow potential Hydrocarbon-bearing formations where moveable hydrocarbons present or in the

future may have an environmental impact, but no safety impactSignificant flow potential Hydrocarbon-bearing formations where moveable hydrocarbons present or in the

future may have both an environmental and safety impact


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The formation surrounding the well should be an element of the permanent well barrier. The permanent
well barrier should be positioned adjacent to an impermeable formation and at a depth where formation integrity is higher than the potential pressure below.

2.1.4 Number of well barriersFor hydrocarbon-bearing formations with moderate or significant flow potential, the number of independent barriers to be included in the well abandonment design should be evaluated by risk analysis. Having multiple independent permanent well barriers can increase the level of reliability.

Note 1:The primary and secondary well barrier may be combined into a single well barrier, provided it is as effective and reliable as having both a primary and secondary well barrier. The effectiveness and reliability of such solutions should be evaluated quantitatively.

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In addition to having primary and secondary barrier(s) for sealing the hydrocarbon-bearing formations, the well should have a surface barrier to isolate flow paths in the wellbore.

Note 2:There is no depth requirement with respect to formation integrity for the surface barrier.

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Figure 2-2 provides an example of a well abandonment design with one hydrocarbon-bearing formation with limited flow potential including a sample well barrier schematic.

Figure 2-3 provides an example of a well abandonment design with two hydrocarbon-bearing formations with moderate flow potential.

Figure 2-4 provides an example of well abandonment design with one hydrocarbon-bearing formation with limited flow potential in the overburden and one hydrocarbon-bearing formation with moderate flow potential.


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Figure 2-2 Example of permanent abandonment for one hydrocarbon-bearing formation with limited flow potential


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Figure 2-3 Example of permanent abandonment for two hydrocarbon-bearing formations with moderate flow potential in overburden


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Figure 2-4 Example of permanent abandonment for hydrocarbon-bearing formation with moderate flow potential and with limited flow potential in the overburden

2.2 Permanent well barrier failure modesPermanent well barrier failure mode identification is performed for each well abandonment design. The process should ensure consideration of all relevant failure modes, and document identified threats, events and consequences in a transparent, traceable and consistent manner.


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The failure mode identification process includes:
— identification of failure and degradation mechanisms and categorisation of threats according to established consequence categories

— identification of additional threats related to unique aspects of the well abandonment design, for example:

— unique features of the subsurface under consideration— technical or organisational aspects that are outside the well operator’s experience.— well completion design and integrity.

— identification of interdependencies between different failure modes , including the potential for cascading

— identification of effects that may increase likelihood of occurrence or severity of consequences.

Additionally, upside potential for the well abandonment design, such as cost saving potentials should be evaluated in the failure mode identification.

App.B lists potential failure modes for such analysis.

2.3 Risk analysisRisk analysis aims to enhance the understanding of risks, including the nature of the risk itself, the likelihood of occurrence and the severity of potential consequences.

For P&A wells, risk analyses should be performed for the well barrier failure modes identified in the previous step. The steps from Figure 2-1 provide a structured approach to the risk analysis. The risk analysis should include safety and environmental risks.

Recognised sources of data should be used when possible. Data should be adapted to the objectives of the analysis. When limited data sets are available or where the data sets are uncertain, assumptions may be applied. In that case, conservative values should be applied.

2.3.1 Flow potentialFlow potential analysis should be performed for all hydrocarbon bearing formations penetrated by the well. Flow potential analysis should be performed to determine the maximum flow potential and hydrocarbon content and composition in hydrocarbon bearing formations penetrated by the well. The objective of the flow potential analysis is to assess the magnitude of the consequence of hydrocarbon flow. The assessment should be performed using the maximum anticipated flow potential from the identified hydrocarbon-bearing formation(s).

As part of the flow potential analysis, the likelihood of well seepage should be evaluated with the objective to characterise the identified well barrier failure mode(s), in terms of the likelihood for them to occur. The degree of detail to be achieved in the likelihood analysis may depend on the type of analysis performed. A quantitative analysis may require a high degree of detail, while fewer details are required for a qualitative analysis.

For further guidance on qualitative vs. quantitative risk analyses, see ISO 31000.

2.3.2 Valued ecosystem componentsThis step aims to establish a site specific background map of the Valued Ecosystem Components (VEC’s) around a given well. The mapping should provide a list and geographical distribution of valued biological resources and habitats and a categorisation of their values, which should be used in the risk evaluation.

Site specific environmental data should be assessed for valued ecosystem components and habitats by using criteria in the Ecologically or Biologically Significant Marine Areas [EBSA] approach.*) For areas where limited knowledge of the environmental resources is available or there is large uncertainty in the criticality of the environmental resources, environmental mapping should be performed.*) UNEP/CBD, 2008. IX/20 - Marine and Coastal biodiversity

This activity will provide a list and map of valued ecosystem components and a categorization of their values. The results are used in the risk analysis.


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2.3.3 Dispersion modelling
Three-dimensional dispersion modelling should be used to calculate the transport of the identified hydrocarbon flow potentials. The model should calculate and record the distribution (as mass and concentrations) of hydrocarbons on the water surface, in the water column, and in the sediments. To provide insight into how potential seepages will behave under a wide range of ocean conditions, a probabilistic approach should be applied.

The results from the dispersion modelling are used in the risk analysis.

2.3.4 Impact analysisThe flow potential analysis results combined with the dispersion modelling results make up the consequence portion of the risk picture. The likelihood portion of the risk picture is produced in the analysis of the flow potential.

The environmental risk picture builds on assessing the potential for environmental impact from the degree of overlap between defined VEC(s), which should be identified from resource mapping, and hydrocarbon concentrations, as predicted with the dispersion model, see Figure 2-1.

The safety risk picture should be a compilation of the likelihood and consequence of safety risk for the well abandonment design.

For each specific well abandonment design, the risk result is the combined output from the consequence analysis (flow potential, mapping and valuing of VEC(s) and marine dispersion) and the likelihood analysis. The overall risk picture provides all of the associated well abandonment design risk results for the environmental risk and safety risk.

2.4 Risk evaluationThe purpose of risk evaluation is to use the outcome of risk analysis to assist in decision making and comparison of the well abandonment design relative to risk acceptance criteria.

Should the outcome of the risk evaluation determine that the risk level of the well abandonment design be not acceptable it is necessary to take measures in order to reduce the risk. Re-analysis and revision of the well abandonment design may be necessary.

2.4.1 Risk acceptance criteriaWhen evaluating the level of risk for the permanent abandonment of offshore wells, risk acceptance criteria should be determined for the environment and safety.

2.4.1.1 Environmental risk acceptance criteriaThe environmental risk acceptance criteria should be based on hydrocarbon exposure of the identified VEC(s). This should be done by overlaying the modelled footprint of hydrocarbons with the identified VEC(s). Environmental risk acceptance criteria for different compartments (sea surface, water column, sediments) should be based on the following:

— proportion of identified VEC(s) exposed to a defined threshold value for hydrocarbons— the probability that the proportion of VEC’s is exposed to a concentration above the defined threshold

value.

If background levels of THC in seawater are available, these should be included when setting the environmental risk acceptance criteria.

2.4.1.2 Safety risk acceptance criteriaWells for permanent abandonment should be categorised based on their potential for adverse safety consequences.

Hydrocarbon bearing formations with flow potential should be controlled by well barriers during P&A operations. Following the finalisation of the P&A operations, permanent well barrier(s) should control the hydrocarbon bearing formations.

Following the completion of a P&A operation of subsea wells, the safety risk may not be relevant.

For platform wells, the established risk acceptance criteria for the platform should be applied.


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Note:
See for example NORSOK Z-013, Risk and emergency preparedness analysis.

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2.4.2 Environmental and safety risk evaluations Environmental and safety risk evaluations should be performed in order to compare the result obtained from the risk analysis relative to risk acceptance criteria.

Various well abandonment designs may be quantitatively compared to the risk acceptance criteria and compared relative to each other.

For each well abandonment design, the risk evaluation compares the risk level relative to risk acceptance criteria. The associated risk level can be considered acceptable or not according to the applicable risk criteria. In order to improve the associated risk level from a well abandonment design, changes can be performed at the well abandonment design definition stage. The modified well abandonment design should be re-evaluated through the risk assessment process in order to quantify the impacts of the changes on the associated risk level.

Risk evaluations results may be used as support for decision making in cost benefit analysis.

2.5 Treatment of uncertaintiesCare should be exercised to ensure that the results of the risk assessment exhibit reasonable accuracy. If significant uncertainty related to risk magnitude exists, relative to the risk acceptance criteria, then the degree of uncertainty should be modelled through sensitivity studies or scenario analyses and be used to provide reasonable uncertainty bands. Critical parameters such as pressure, hydrocarbon volume and temperature may be studied in sensitivity studies, depending on their influence.

With regard to quantification of uncertainty, a probability distribution may be used as an interpretation of a set of weighted probabilities. The method for establishing the probability distribution depends on the available knowledge (including available data, expert opinions and analyst judgements), and if the knowledge changes over time.

The quality and quantity of the available data for P&A planning should be studied for each potential well. Critical pieces of information that may reduce the uncertainty for a given well abandonment design should also be identified and recorded.


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APPENDIX A INPUT FOR RISK-BASED ABANDONMENT OF
OFFSHORE WELLS

Table A-1 Tabular listing of generic input needed for risk-based P&A

Detail Notes

Gen

eral

Well details number, field and location of wellstype of well (production/injection)future usage plans for the well

Field architecture subsea or platform, high level descriptionWater depth water depth

Res

ervo

ir &

ove

rbur

den

Number of flow potential overburden formations

any formation which contains moveable fluids in the form of hydrocarbons or abnormally pressured water.

Hydrocarbon-bearing formation 1 name & geological formationtrue vertical depth [TVD] range (top & bottom)contents of formation, including composition of hydrocarbons and volume capacityoriginal, current and future pressures

Additional hydrocarbon-bearing formations

name & geological Formation, TVD range (top & bottom), contents formation, including hydrocarbon composition volume original, current and future pressures, cross-flow potential

Subsurface factors hydrogen sulphide [H2S], carbon dioxide [CO2], geological faults, pore- and fracture gradients

Geological barrier formations formations that are or can be qualified as barrier

Wel

lbor

e

Well history summary well barrier diagram & schematic annuli fluids and annuli operating limitsprimary well barrier status including status of tubing/casing/linersecondary well barrier status including status of casing/cement including cement qualityprevious abandonment activities, including sidetrackswellbore stability diagrams, temperature plots, mud logs, pressure tests, openhole logschallenges during well construction – caving, losses, washouts, cementing problems, borehole instability issues/geological challenges known well integrity issues – leaks, degraded components pressure containment issues.

Current and previous well operational status

well status details including the well’s operational mode and whether the well has additional equipment, for example, gas-lift.

Well flow assurance history wax, sand, hydrate & scale issues

Site

spe

cific

Metocean data ocean current including salinity and temperature profilesEnvironmental resource overview uniqueness , rarity or importance of environmental resources

special importance for life-history stages of species Site specific safety general or site-specific safety requirements


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APPENDIX B WELL BARRIER FAILURE MODES FOR P&A WELLS
Table B-1 Generic well barrier failure modes for P&A wells

Potential failure mode Potential cause mechanism Risk management strategy

Mai

nbor

e

Insufficient barrier length in mainbore

— low top of barrier— barrier slippage— density miscalculation

include functional barrier length assessments into quantitative models

Barrier function degraded in mainbore

— incorrect barrier density— operational issues— permeable barrier— high barrier shrinkage leads to increased

porosity and stresses that may cause a micro-annulus to form

perform sensitivity studies as to the flow potential through and around these barriers

Cas

ing

Corrosion of casing — well fluids exposure or long term exposure perform sensitivity studies as to the flow potential through and around these barriersInclude formation aspects and time perspectives

Yielding of casing due to pressure in well

— well loading over time including geological forces

— formation loads

Ann

ulus

Insufficient barrier length in annulus

— slippage due to inadequate density or losses— not able to perform squeeze job

include functional barrier length assessments into quantitative models with sensitivity studies

Degradation of annulus barrier

— channelling/lack of bonding— CO2 corrosion— H2S corrosion— magnesium chloride degradation— thermal cracking and/or de-bonding (micro-

annulus) due to Joule-Thomson effect during injection into, e.g., depleted gas reservoir

— pre-existing channels — pre-existing micro-annulus

perform sensitivity studies as to the flow potential through and around these barriers

Contamination of annulus barrier

— poor mud and filter cake removal leaves a route for hydrocarbons to flow up the annulus

— high barrier shrinkage leads to increased porosity and stresses that may cause a micro-annulus to form

Form

atio

n

Overpressure of formation

— build-up of pressure over time— injection nearby

evaluate the formation characteristics, the need for cross-flow prevention and natural leakage/seepage.Fluid exposure — degradation effects over time

Geological barrier formations

— potential to use formations as an additional well barrier, if possible

identify if compacting formations or aquifers can be used as permanent barriers.


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