Surface Casing: When used as surface casing, it isolates shallow formations and freshwater zones, and supports the wellhead.
Intermediate Casing: In deeper wells, it can serve as an intermediate string to isolate problem zones (e.g., high pressure, lost circulation).
Casing Running Procedure
- Preparation:
Casing Inspection: Visual and drift inspections ensure casing joints are within specs.
Dope & Thread Prep: Threads are cleaned, doped, and protected to ensure proper makeup.
Running Tools: Elevator, slips, power tongs, stabbing guides, and fill-up/circulate tools are made ready. - Running:
Joints are picked up and stabbed into the casing already in the hole.
Torque is applied using tongs or an automated tong system to achieve correct makeup.
Centralizers and scratchers may be installed to ensure good cement distribution. - Circulation:
Mud circulation through casing (via fill-up tool or float collar) ensures hole cleaning and cools casing.
Circulating also helps to monitor for lost returns or formation influx. - Float Equipment:
Float collar and shoe at the bottom provide a check-valve to prevent backflow.
Optionally, a guide shoe helps direct the casing through tight spots or ledges. - Running Speed & Drag:
Must be monitored carefully, especially in deviated or long wells to avoid casing sticking.
If problems are expected, reaming or rotating while running may be done.
Challenges & Mitigations
Tight spots / ledges: Use reamers, centralizers, and casing shoes.
Sticking risks: Monitor hookload, apply drag-reducing agents, or rotate casing.
Well control: Keep hydrostatic pressure above formation pressure at all times.
Cementing: Ensure proper displacement rates and volumes for a good cement job.