Connection practices vs well inclination (cuttings related stuck pipe risk)
It is common to see companies using the same connection practice throughout the well, which adds significant stuck pipe risk and sometimes leads to major incidents. The way the connection should be performed depends on a number of variables with well inclination, mud gels and torque and drag (TAD) trends being the main ones. Standard practice is to start both rotary and pumps at the same time or rotary first (to break the gels), which is not always the safest approach.
Hole cleaning and cuttings behavior can be categorized into three regimes based on hole inclination – slip, avalanche and stationary. Cuttings behave differently in each regime and understanding this will help us come up with the lowest risk connection strategy.
Figure 1: 3 types of hole cleaning regimes
0° – 30°: A hole cleaning related stuck pipe event is possible after a period of no circulation with poor mud quality. Cuttings can fall on top of the BHA and if an inappropriate circulation start-up procedure is used a pack-off may occur and lead to a stuck pipe scenario.
30° – 65°: Highest sticking risk is on a connection. Connection practices are the key to reducing risk. Rotation could break internal friction between cuttings particles causing an unstable cuttings bed to slide / avalanche on top of the BHA.
65° – 90°: Sticking of a drill string in a static solids bed most often occurs while pulling out of hole after drilling. It can occur whether in open hole or in casing. There is a very low risk over a connection.
One important thing to note – most ERD wells have all 3 regimes to clean! Hence, always be aware of the inclination at the BHA when drilling and tripping.
Ensure you know the mud gels at downhole pressure and temperature. Standard surface tests may not be representative (i.e. Fann 50° vs Fann 70°).
If the BHA is at <65 degrees inclination:
· Extra time should be allowed for cuttings to be circulated sufficiently above the BHA. Either controlled ROP for the last few meters / feet or reaming before the connection should be considered. Torque, drag and hydraulic road-maps will help identify hole cleaning deficiencies and aid in hazard identification.
· Shut down rotation in a controlled manner ahead of the circulation.
· Establish slow circulation first (first 30 seconds or so are critical). Confirm you have established a circulation path i.e. you have returns and SPP / ECD values are aligned with previous readings. Bring pumps to the full rate in stages, confirming adequate SPP response at each interval. Only then kick-in the rotary, starting slowly first (e.g. 30 RPM). Bring it to the full drilling rate after pressure reading stabilizes.
Figure 2: Breaking circulation procedure example
If cuttings are not suspended at high inclinations (>65 degrees) they would just fall a few inches before they are on the low side of the hole, hence the risk of stuck BHA / pack-off is minimal when drilling and / or making connections at such wellbore angles.
Some might say that this approach takes too much time, and we often there is a tendency to cut corners, but we are strong believers that the big prize is avoiding a pack-off / stuck pipe event. Obviously, all of the above are generalized statements and actual local practices could slightly vary between wells.
