IOGP Well Control Incident Lesson Sharing

21-8 – Subsurface uncertainties, unfamiliar technologies and shallow water flows in a subsea exploration well

We monitor IOGP’s ‘Well control Incident lesson sharing’ which is a regular email that alerts the industry to recent incidents with the hope of sharing knowledge and preventing future well control incidents.

IOGP shared well control lesson 2021-8 ‘Subsurface uncertainties, unfamiliar technologies and shallow water flows in a subsea exploration well’ on 1 October 2021.

During the drilling of tophole sections on a subsea exploration well, the crew experienced a series of water flows in both the initial well and the subsequent re-spud location.

Several lessons were identified relating to subsurface uncertainties, well planning, and the detection of well flow, while operating with a riserless mud recovery system (RMR).

  1. The importance of understanding and planning for subsurface uncertainties in well operations.
  2. Risks associated with the implementation of new technologies in well operations, including the management of risks with crew’s knowledge, skills and ability.

We have created a short explanation about how these lessons relate back to the IWCF Drilling and WIPC syllabi.

What went wrong?

  • A permeable sand formation, capable of flow in the first top-hole section, had not been expected during planning of the well. This same formation with flow occurred again during the re-spud activities. The well plan did not account for this formation and pore pressure regime.
  • The first top-hole section inadvertently drilled into the top, higher pressure, sand formation that was different to the seismic event from the reference well.
  • Pore pressures encountered were 60-100psi higher than predicted.
  • Well flow detection techniques were compromised whilst drilling:
    • The flow check procedure, adapted for use with the mud recovery system during riserless drilling, was not fully documented nor understood by the team. This was subsequently addressed on the re-spud.
    • Well monitoring was complicated when using seawater and sweeps with the mud recovery system in exhaust mode masking well flow events.
    • Traditional flow detection techniques, e.g. bubble monitoring, that have been proven elsewhere in the region were not applicable on this well. Gas bubbles were not seen in association with shallow water flows.

Corrective actions and recommendations

  • Multiple compaction trends and geological scenarios should be considered to support a larger, but still reasonable, pre-drill PPFG uncertainty range.
  • Credible worst-case pore pressures with mitigation action should be included in the well plan.
  • Advances in seismic interpretation tools and technology should be used when determining overburden pressures.
  • Operational safety briefings and training should include awareness that wildcat well tops determined from seismic may not correspond directly to expected lithology and pressure.
  • Well design should be updated to enable sufficient standoff to avoid penetrating the Top sand with the 28” casing setting depth. The seismic event pick for the Top sand formation should also be updated.
  • Develop standard wellbore monitoring procedures when drilling top hole with riserless mud recovery systems and crew resource management training on the following:
    • The lack of bubbling is not an indicator of no-flow
    • Even with good visibility, ROV flow checks need corroboration with mud recovery system pressure data
    • Procedure for flow checking with mud recovery system
    • ECD and sand mining are indicators of flow
    • Operational requirement and procedures to pump out of hole
    • Staged approach to well kill to allow determination of pore pressure
    • The drill bit should be recovered above the wellhead when anticipating extended waiting periods; especially if bad weather is expected (swab risk). The ROV should be deployed to periodically monitor valves.

How does this relate to IWCF syllabi?

It is important to interpret and include subsurface uncertainties into a new well programme. It is also essential to understand the introduction of new technology and equipment in line with existing principles and procedures.

Below we have listed several syllabus items that relate to these topics.

  • Drilling – DR-SF-PNP-01.02.01 /Well Intervention – WI-SF-COM-02.06.01 The need for well control training and assessment

These syllabus items highlight the need to apply critical skills in emergency situations to reduce the severity of well control events.

In the described Lessons Learned, a specific RMR flow check procedure was not fully documented or understood by the team.


    • Drilling – DR-SF-PNP-02.02.01 / Well Intervention – WI-SF-COM-03.02.01Formation pore pressure

The well programme failed to sufficiently include possible geological setting, pre-drill pore pressure and fracture gradient (PPFG) scenarios and document possible mitigations during well planning.

When planning for subsurface uncertainty, the limitations of available seismic data should be clearly understood when creating the well programme. If possible, the programme should verify the subsurface model with the ability to adapt as necessary.


  • Drilling – DR-SF-PNP-04.01.02 / Well Intervention – WI-SF-COM-02.04.01 – The Management of Change (MOC) process

The well programme did not predict a permeable sand formation, capable of flow in the first tophole section. It was also not sufficiently considered when deciding to re-spud the well or when planning the revised casing setting depth.

The management of change (MOC) process was not robust. It did not apply the lessons learned when drilling the first well to the local geological conditions and ensure it was reflected in the revised well programme.


  • Drilling – DR-SF-PNP-06.02.02 – The interpretation of well flow-back (for example: ‘fingerprinting’ and trend analysis).

Drilling with an RMR system is different to standard riserless tophole drilling. The team did not have robust procedures in place for identification of non-gas cut kicks, flow checking with a RMR system or consider the use of an ROV during flow checks.

It is important to have wellbore monitoring procedures specific to RMR operations to differentiate between normal well behaviour and potential kick indicators.

All IOGP Well Control Incident Lesson Sharings

Visit the IOGP website for a full list of all lessons shared to date: