Nigeria 1993

Problem: Severe corrosion created a leak at an unknown depth in the 3 1/2" tubing string of this 3507m TVD gas well located in Nigeria. A leaking wellhead and the inability of the casing annulus to hold the flowing pressure complicated a shut-in and workover operation. This well was the source for a gas lift operation in a prolific oil field. A possible catastrophic failure was to be avoided for economic and environmental considerations.
Remedial Strategy: Risk analysis results showed that surface and subsurface intervention phases would be instrumental to the success. A relief well would be positioned within a 1.5m radius target 1m above the producing formation at 3417m TVD. The relief well would kill the blowout and ultimately replace the gas producer. Once the relief well was in place, a surface operation would replace faulty wellhead valves. If a casing failure and a consequent surface broach occurred, a subsurface kill would commence. The relief well surface location was positioned 957m away, requiring an "S" shaped wellpath to locate the target well and then to establish a non-collision trajectory. Avoidance of a direct intersection with the target well was mandatory to insure successful well replacement .
Special Services: John Wright Company (JWCO) was contracted to supervise the special services required for this project. This would include pre-planning and on-site supervision of directional drilling, surveying, and casing detection. Vector Magnetics, responsible for electromagnetic ranging, was coordinated through JWCO on this project and worked together as one team. Also, Well Flow Dynamics simulated various kill scenarios for the two proposed kill operations..
Challenges: The project was complicated by the following:
  • The project was complicated by the following:
  • Environmental concerns required a 957m relief well surface location separation.
  • An "S" shaped well design was necessary to locate the target well.
  • The 1.5m radius target at 3417m TVD also required a non-collision trajectory.
  • Only low accuracy TOTCO and Dipmeter surveys existed for the target well.
  • Difficult low inclination directional control and surveying would be mandatory.
  • Electromagnetic ranging would be critical to achieve well placement accuracy.
  • Kill operation was influenced by poor pressure integrity of wellhead equipment.
  • The 'team concept' for special services was successfully and efficiently utilized.
  • A large target well positional uncertainty complicated the relief well design.
  • A target well cross-by without a plugback was highly successful and effective.
  • Directional drilling and surveying techniques proved successful with proper QC.
  • Electromagnetic ranging techniques were instrumental to meet target objectives.
  • The target was intersected with a diverging trajectory. See Figure 2.
  • An uneventful surface operation prevented the use of the relief well kill option.