Plug and Abandonment: A Barrier Design Philosophy and Integrity Assurance 

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Table of Contents 

  1. Introduction – The Basis of Barrier Philosophy 

  2. Regulatory Framework and Global Practices 

  3. The Well Integrity Lifecycle 

  4. Well Barrier Elements and Their Functions 

  5. Barrier Design, Placement, and Acceptance Criteria 

  6. Verification, Testing, and Documentation 

  7. Case Study – Delta-12 Offshore Well (Training Example) 

  8. Common Barrier Failures and Mitigation Measures 

  9. References 

1. Introduction – The Basis of Barrier Philosophy 

Barrier design and integrity assurance form the cornerstone of Plug and Abandonment (P&A) operations. The ultimate objective is to achieve permanent isolation of all hydrocarbon-bearing and overpressured formations, preventing any fluid movement or cross-flow that could pose safety, environmental, or regulatory risks. 

Modern P&A practices are built around the principle of multiple, independent, and verified barriers. This approach ensures that if one barrier fails, the second can still contain formation fluids. 

Industry guidance such as API RP 65-3 (Wellbore Plugging and Abandonment), NORSOK D-010 (Well Integrity in Drilling and Well Operations), and regional regulations (e.g., BSEE/30 CFR in the U.S. and NSTA in the U.K.) all require operators to design and verify redundant barriers before well abandonment. Each jurisdiction requires the submission of an abandonment program, a barrier schematic, and a verification plan before approval. 

2. Regulatory Framework and Global Practices 

The regulatory foundation of P&A operations is based on the same universal principle: each potential flow path must be isolated by two independent barriers that can individually contain the formation pressure. 

Each barrier envelope consists of one or more well barrier elements (WBEs) such as cement plugs, mechanical plugs, casing shoes, or packers. These elements, when combined, provide full mechanical and hydraulic isolation. 

Commonly referenced standards and guidance include: 

  • API RP 65-3 – Wellbore Plugging and Abandonment (2021). 

  • NORSOK D-010 – Well Integrity in Drilling and Well Operations (latest revision). 

  • OEUK/OGUK Well Decommissioning Guidelines – for UK North Sea operations. 

  • ISO 16530-1 & 2 – Well Integrity for the Life Cycle of the Well. 

While specific technical requirements (e.g., plug length, test pressure) vary by jurisdiction, the underlying philosophy remains consistent: permanent containment, redundancy, and verification. 

3. The Well Integrity Lifecycle 

Well integrity is not limited to the abandonment phase. It spans the entire life of the well, from design and drilling to production and final decommissioning. 

During the P&A phase, the objective is to restore the subsurface to a state equivalent to natural containment, meaning no pathways exist for hydrocarbons or pressured fluids to reach the surface or migrate between formations. 

Key steps in lifecycle integrity management during abandonment include: 

  • Reviewing well construction and cementing history. 

  • Identifying potential leak paths, such as annuli, perforations, or micro-annuli. 

  • Designing and verifying barriers to ensure redundancy and reliability. 

  • Selecting appropriate materials (cement systems, mechanical plugs, sealants). 

  • Documenting verification results to demonstrate regulatory compliance. 

4. Well Barrier Elements and Their Functions 

Each barrier element contributes to the overall containment system. Typical Well Barrier Elements (WBEs) used in both land and offshore wells include: 

  • Cement Plug 
    A primary sealing element that provides hydraulic isolation of permeable zones. The barrier function, formation characteristics, and regulatory requirements must justify plug length and placement. There is no universal fixed length; however, most guidance requires the plug to be long enough to ensure a competent seal, with placement verified through tagging or logging. 

  • Mechanical Plug 
    Bridge plugs or cement retainers serve as mechanical foundations or secondary seals. They are often used to support cement plugs or provide temporary isolation during multi-stage abandonment. 

  • Tubing Hanger and Packer 
    These components provide internal isolation when production tubing remains in the well. They can serve as verified mechanical barriers if tested and documented. 

  • Formation Integrity (Caprock) 
    A natural caprock or low-permeability formation may be credited as a barrier if pressure data, well history, or geomechanical evidence confirms its sealing capacity. Verification must be formally documented. 

5. Barrier Design, Placement, and Acceptance Criteria 

Effective barrier design ensures isolation of all potential inflow zones — including producing formations, overpressured intervals, and any fluid migration paths. The goal is to achieve permanent containment and eliminate the risk of future leakage. 

Key Design Considerations 

  • Formation Pressure and Strength: Barriers must withstand formation pressure without allowing fluid movement or failure. 

  • Casing Condition and Cement Bonding: Evaluate casing integrity and cement bond quality to identify and repair any potential leak paths. 

  • Barrier Overlap: Primary and secondary barriers must overlap to prevent unsealed intervals. 

  • Verification Access: Design should allow for tagging or logging to confirm plug placement and integrity. 

Cement plug placement must ensure proper overlap with competent formation and adequate bonding to casing or open hole.  

Engineering Justification 

Each barrier’s location, length, and type must be supported by sound engineering reasoning, which includes: 

  • Plug Coverage: The plug must extend fully across the flow zone and at least some distance (typically 100 ft or more) into a stable formation below. 

  • Barrier Overlap: Adjacent barriers should overlap to eliminate any unsealed intervals. 

  • Annular Remediation: Any detected annular flow or communication must be repaired before final abandonment, often through squeeze cementing or similar methods. 

Each plug’s design and location must be justified in the P&A program and supported by calculations, test results, and logs as required by the relevant regulatory authority. 

6. Verification, Testing, and Documentation 

Barrier verification confirms that the installed barriers perform their intended sealing function. The verification program is defined before abandonment and approved by the regulator or the operator’s responsible engineer. 

Common verification methods include: 

  • Pressure Testing 
    Test pressures and durations vary by region and operator. Typically, a pressure test is conducted on the installed barrier or annulus to demonstrate mechanical integrity. Acceptance criteria — including maximum allowable pressure drop and duration are defined in the verification plan based on casing, formation, and safety margins. 

  • Tagging 
    Tagging confirms the top of the cement or the mechanical plug. A tolerance of ±10 ft from planned depth is commonly used but must be verified according to company or regulatory criteria. 

  • Cement Bond Log (CBL) 
    Evaluates the cement bonding between the casing and the formation across the sealing interval. It is one of the most widely used verification tools. 

  • Ultrasonic Imaging and Temperature Logs 
    Provide advanced diagnostic information in high-pressure, deepwater, or complex wells. They help assess cement integrity and detect any internal flow paths. 

All test and log results must be recorded and archived in the Final Well Abandonment Report, which serves as the permanent record of compliance and verification in accordance with NORSOK D-010 and corresponding national regulations. 

7. Case Study – Offshore Well 

A well, located in a mature offshore field, was selected for permanent abandonment after reservoir depletion. 

The abandonment program adhered to the dual-barrier philosophy outlined in NORSOK D-010 and the OGUK Well Decommissioning Guidelines. The main barrier configuration included: 

  • Primary barrier: Bridge plug set at 4,700 ft MD, capped with a 500 ft cement plug (4,200–4,700 ft). 

  • Secondary barrier: Cement plug placed across the 13-3/8" casing shoe from 2,000–2,500 ft MD. 

  • Surface casing: Cement verified to surface from original drilling records and confirmed by CBL. 

Verification results: 

  • CBL confirmed a continuous bond across the plug interval. 

  • A pressure test at 1,000 psi was held for 30 minutes with no pressure loss. 

  • Tag depth confirmed within ±8 ft of planned top. 

This design met the required engineering and regulatory standards, demonstrating the value of a structured barrier philosophy and systematic verification. 

8. Common Barrier Failures and Mitigation Measures 

Understanding how well barriers can fail is critical for maintaining long-term integrity after abandonment. Each mechanism has distinct causes and preventive measures that should be addressed during design, placement, and verification. 

  • Microannulus 

    A microannulus is a tiny gap that develops between the casing and the cement sheath. It usually occurs due to poor bonding, casing movement, or repeated pressure and temperature cycles that cause expansion and contraction. These gaps can create leak paths for fluid or gas migration. 
    Prevention: 

    • Ensure proper casing centralization to maintain uniform cement coverage. 

    • Optimize mud removal with effective spacers and flow rates. 

    • Control cement displacement to ensure full contact between the cement and the casing/formation. 

  • Cement Retrogression 

    At high temperatures, standard cement can lose its strength over time through a chemical process known as retrogression, where the cement minerals change their structure and weaken. This is a concern in deep or geothermal wells. 
    Prevention: 

    • Use high-temperature cements with silica or other additives designed to resist retrogression. 

    • Allow sufficient curing time before pressure testing or verification. 

  • Gas Channeling 

    Gas channeling occurs when formation gas migrates through unset or porous cement before the slurry fully hardens, leading to poor zonal isolation. 
    Prevention: 

    • Use gas-tight or low-permeability cement slurries. 

    • Pump preflush spacers to clean the wellbore and improve bonding. 

    • Apply controlled pressure during and after placement (pressure-managed cementing) to prevent gas entry. 

  • Casing Corrosion 

    Over time, casing in abandoned wells can corrode, especially in the presence of water, CO₂, or other corrosive fluids. Corrosion can compromise the long-term integrity of barriers. 
    Prevention: 

    • Ensure a strong and continuous cement sheath to isolate the casing from corrosive environments.

    • Repair or seal annular communication through squeeze cementing before final abandonment.

    • Use corrosion-resistant materials or inhibitors where applicable.

9. References 

  • API RP 65-3 (2021) – Wellbore Plugging and Abandonment. 

  • NORSOK D-010 (2021) – Well Integrity in Drilling and Well Operations. 

  • OEUK/OGUK Well Decommissioning Guidelines (latest issue). 

  • ISO 16530-1 & 2 – Well Integrity for the Life Cycle of the Well. 

  • Schlumberger Cementing Handbook (2022 Edition). 

  • DNV-RP-E103 – Risk-Based Abandonment and Environmental Assessment.