In many drilling operations, the drill string is stuck while pulling out after first having experienced tight hole, swabbing and overpulls.
This indicates the following shortcomings:
- Pulling out has continued for too long without remedial action.
- Drilling personnel have been allowed to struggle on their own without supervision or advice.
- Deterioration of the hole or of the mud properties has not been detected or recognised in time prior to pulling out.
- Incorrect practices, of which supervisory staff are not aware, have been used.
Usually, the causes or contributing factors can afterwards be recognised as follows:
- Drilling through long sensitive shale sections.
- Mud properties not up to standard, i.e. shale inhibition, waterloss, density, lubricity, etc.
- Insufficient annular velocity and insufficient circulating times to effect optimum hole cleaning and/or optimum mud conditioning and treatment.
- Insufficient solids removal in the mechanical mud treating equipment, i.e.: shakers, desanders, desilters, clayjector, etc. Also, not utilising the effective lifting capabilities of viscous pills.
In addition during drilling a hydrostatic over pressure due to the column of mud exists over the formation pressure. This results in some risk of differential sticking, therefore the following precautions should be taken:
1. Use spiral drill collars (keep the number run to a minimum) and use HWDP to apply more WOB if necessary.
2. Use stabilisers at 30 or 40 m.
3. Maintain a low waterloss and minimum solids content.
4. Use a friction reducer additive in the drilling mud if needed, in consultation with Base.
5. Avoid leaving the pipe stationary in open hole for any longer than is absolutely necessary.
6. When running multi or single shot surveys drop the barrel, take reading then pull back into shoe to fish the instrument.
7. When adding a single:
a) Circulate at half pump rate with bit on bottom for approximately two minutes.
b) Pick up kelly plus one single.
c) Lower pipe down into the slips.
d) Shut off pumps.
e) Add single.
f) Start the pumps at half rate.
g) Pick up the string.
h) Lower bit slowly to bottom.
i) Increase pump to full rate and resume drilling.