A direct measurement of downhole WOB is vital for accurate control of the drilling process during Coiled Tubing Drilling (CTD). Without it, motor stalls will be more frequent and directional control will be more difficult.

In a vertical well with conventional jointed pipe drilling it is a reasonable assumption that the weight applied to the drill bit is directly related to the hook load at surface. This assumption starts to become less valid as soon as the well becomes deviated or horizontal because of the friction between the pipe and the wellbore. But even then, it is possible to compare upward and downward loads to estimate the friction and therefore determine WOB. With Coiled Tubing Drilling (CTD) the situation is complicated further such that the assumption that surface load is directly linked to downhole WOB definitely cannot hold.

With CTD however, you definitely cannot be sure that the load you are reading at surface is the same as the load downhole.

The forces which affect the transfer of weight to the bit are numerous and include:

• Stripper Friction
• Buoyant Coil Weight
• Pipe Friction

With coil, the sliding friction is non-linear because of the CT buckling affect. This in it itself is affected by the actual WOB. If you then add the friction of the stuffing box at surface, which is dependent on the instantaneous pressure applied to it and which in turn will be affected by the variable wellhead pressure.

With CTD the number of forces, their variability, non-linear nature and their interdependency means that it is impossible to determine downhole WOB from a change in surface weight. Bearing in mind that one might experience WOB’s in the order of 2000lbf for effective drilling, having an uncertainty of possibly 5 times that makes WOB determination next to impossible.

The solution to the problem is to directly measure the WOB downhole in the drilling BHA. That way the measurement is taken without the influence of the measurement variables described above.

All downhole WOB sensors are affected by an effect called the ballooning affect which is caused by differential pressure across the tool. The ways to correct for this are a topic for another post.

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Author: Toni Miszewski

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