December 15, 2014 | Mark Paradies

Monday Accident & Lessons Learned: OPG Safety Alert 262 – Shallow Gas Leads to Well Control Incident


  • The well is located in a well-known, shallow gas prone area.
  • Deep gas wells with high pressurized layers.
  • Crowded platforms with wells anti-collision complex management.
  • SIMOPS including construction and well intervention
  • After each incident, procedures for shallow section drilling were enhanced.

The sequence of events were:

  • 0:00 – Skid rig on well. Batch drilled 12 ¼’’ hole section + 9 5/8’’ intermediate casing
  • 08:30 – Cleaned out CP 24’’ with 17 ½’’ BHA to 131m
  • 16:30 – Drilled 12 ¼’’ hole to 286m with 1.15+ SG mud. Heavy losses (67 m3/h)
  • 20:20 – Homogenize mud to 1.12 SG
  • 20:33 – Resume drilling to 296m. Heavy losses (70 m3/h)21:10 – Spot 10m3 LCM pill. POOH wet.
  • 22:20 – Well swabbing and started to flow. Closed diverter. Started pumping 1.12 SG mud at high flow rate.
  • 23:04 – Pumped kill mud 1.50 SG, followed by sea water at high rate.
  • 00:30 – Flow outside CP. Well out of control. Full rig evacuation.

What Went Wrong?

The cause of the incident could be listed as follows:

1. Supervision on rig

  • POOH wet (no pump out)
  • Continue with pulling operations, despite swabbing, until well kicked in. Shallow gas procedure not followed

2. Mud weight

  • Inconsistency in MW control and reporting
  • Pack off at 291m interpreted as a (new) loss zone

3. Documentation

  • No comprehensive instructions concerning total loss situation

Corrective Actions and Recommendations

  • Maintain a continuous awareness on shallow gas hazard, even when the shallow gas section has already been penetrated in other wells. This aims at avoiding routine approach hence complacency.
  • The standard drilling Instructions should be enriched and reinforced with lessons learnt e.g. Management of Change, the required concentration of KCl for the top hole section, the threshold of heavy losses, hole cleaning procedure for the top hole, responsibility assignment for key personnel, ‘Ready to drill’ checklist.


Whilst every effort has been made to ensure the accuracy of the information contained in this publication, neither the OGP nor any of its members past present or future warrants its accuracy or will, regardless of its or their negligence, assume liability for any foreseeable or unforeseeable use made thereof, which liability is hereby excluded. Consequently, such use is at the recipient’s own risk on the basis that any use by the recipient constitutes agreement to the terms of this disclaimer. The recipient is obliged to inform any subsequent recipient of such terms.

This document may provide guidance supplemental to the requirements of local legislation. Nothing herein, however, is intended to replace, amend, supersede or otherwise depart from such requirements. In the event of any conflict or contradiction between the provisions of this document and local legislation, applicable laws shall prevail.

Safety Alert Number: 262
OGP Safety Alerts

Root Cause Analysis
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