Stage Collar is a tool in the casing that allows cement to be pumped in stages, ensuring precise placement and protecting sensitive formations.
Reasons for Multistage Cementing
-Reduce the hydrostatic column and control ECD in weak zones or zones with low fracture gradient.
-Use multiple cement slurries in different stages.
-Avoid long cementing periods.
-Keep some zones without cement when necessary to reduce costs or protect very weak formations.
Where to Place Stage Collar
1- Problem Formation salt & gas zones
200 ft below for salt zone
500 ft below for gas zone
To reduce slurry volume.
2- problem Formation Weak & Lost Circulation Zones:
Above the LC zones
to reduce hydrostatic pressure on the formation
Stage Collar Doesn’t Open Troubleshooting
1-No indication of increased pressure:
Opening bomb may not have arrived.
Monitor the volume counter and recalculate pumping volume.
If pumping WBM and bubbles are present, measure once the mud weight by mud balance with pressurized and once without pressurized then calculate over displacement:
Over displacement ={ (mud weight pressurized / mud weight unpressurized ) -1 } x theoretical volume
2-Pumping over displacement without opening stage collar:
Increase pump pressure (max < Burst Pressure of casing).
3-Run drill pipes in the hole and try to open mechanically.
4-If all else fails, perform a Top Cement Job through the annulus.
Key Points About Stage Collar :-
-A stage collar is considered a weak point in the casing. When dropping the first wiper plug, required displacement volume must be calculated accurately, with continuous monitoring of the volume counter during pumping Before the volume counter reaches the calculated displacement volume
the pumping rate should be reduced to 2–3 bpm to avoid accidental opening.
Volume displacement= capacity casing x ( height from surface to S.C)
-At the end of the first stage, open the stage collar immediately and circulate the excess cement.
-The 1st stage lead slurry should have enough thickening time to avoid any problems when circulating the excess cement.
-There is no wiper plug between drilling fluid (mud) and slurry, So use good spacer and washer to avoid contamination and achieve a wet shoe.