Ah.. the Air Weight or Buoyant weight discussion again!
The correct answer is ...a bit of both!
I have uploaded a couple of slides from my Stuck Pipe course to illustrate the following.
OK ...some basics...
- String weights calculated using buoyancy factors only give the right buoyed tension value at surface...no-where else in the string.
- Buoyancy is caused by force imbalances from hydraulic pressure acting on flat surfaces of the pipe. They start on the bottom of the string and then increase or decrease as you go up the string. What you see at surface is the cumulative value of all of them.
Remember its quite easy in field units.. pressure (psi) x area (square inches ) = Pounds of more or less tension!
- Before the pipe got stuck it was all subject to buoyancy.. that didn't disappear when the pipe got stuck
This scenario is illustrated in the slides.
The most critical number you need is the free weights up down and rotating just before the pipe gets stuck. (The Driller DOES record that every stand doesn't she/he?!)
If you have moved the actual hookload away from that value then you will have changed the tension in the pipe above the stuck point.
At some point of the back off operation you will want to reduce the ACTUAL tension at the back off point to around zero. So, first you need to know what it was prior to being stuck. If there are no pipe size or weight changes between the stuck point and surface you can see from the slides that the free tension at the stuck point would have been the free string tension at surface (free hook weight tension less weight of the blocks) less the weight in air of the string above the stuck point. (NB: use TVDs for string lengths!) If there is a change in pipe OD or ID then you also need to allow for the buoyancy acting at the point that either the ID and/or the OD changes. It usually means the stuck point tension will be GREATER than you expected by the buoyancy force acting at the crossover.
Say this tension is 50,000lbs. This means you will need to set the hookload to the free hookload value less 50,000lbs to get the tension at the stuck point to be around zero. Now, in a deviated hole you need to consider drag:
- if you have to pick up to achieve the required hookload then use the free weight up value
- If you have to slack off the string then use the free weight down value
I hope this makes sense.