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
- An "S" shaped well design was necessary to locate
the target well.
- The 1.5m radius target at 3417m TVD also required a
- Only low accuracy TOTCO and Dipmeter surveys existed for the
- Difficult low inclination directional control and surveying
would be mandatory.
- Electromagnetic ranging would be critical to achieve well
- Kill operation was influenced by poor pressure integrity of
- The 'team concept' for special services was successfully and
- 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
- The target was intersected with a diverging trajectory. See
- An uneventful surface operation prevented the use of the
relief well kill option.