The length of drillcollars, L that are required for a particular drilling situation depends on the Weight on Bit, WOB that is required to optimise the rate of penetration of the bit and the bouyant weight per foot, w of the drillcollars to be used, and can be calculated from the following:
L = WOB/w
If the drillpipe is to remain in tension throughout the drilling process, drillcollars will have to be added to the bottom of the drillstring. The bouyant weight of these additional drillcollars must exceed the bouyant force on the drillpipe This will be sufficient to ensure that when the entire weight of the drillcollars is allowed to rest on the bit, then the optimum weight on bit will be applied. The WOB will however vary as the formation below the bit is drilled away, and therefore the length of the drillcollars is generally increased by an additional 15%. Hence the length of drillcollars will be 1.15L.
Institute of Petroleum Engineering, Heriot-Watt University