Introduction

Weight on Bit (WOB) is defined as the downward force applied to the drill bit. This force is applied by using weight of drill collars. To control the Weight on Bit, the drillstring is lowered by the driller and effective weight of the drillstring hanging at the travelling block decreases due to compression. This reduction in the weight of drillstring is translated to the Weight on Bit.

Weight on Bit is essential to drill a hole. It directly affects the rate of penetration (ROP). When the weight on bit and rotary speed are increased, ROP should increase proportionately.

Proper control of WOB is necessary to ensure proper drilling. If WOB is not controlled:

• Compression of the drillstring can cause bucking of drillstring.
• Increase in WOB can also affect vibration susceptibility of the drilling system.
• If the WOB is not controlled during drilling, increased compression can damage the drill bit.

Calculation Approaches

The calculation approach used to calculate WOB is as follows:

• First, the algorithm checks if rig state at first data points is ‘drilling’.
  • In this case, Weight on Bit cannot be calculated due to lack of reference points
• If there are non drilling state data points for before drilling state,
  • Current ‘rigstate’ is checked,
  • If current rig state is ‘drilling’, reference hookload is selected based on hookload at data point just before the first drilling state.
  • Current Weight on Bit = Reference Hook Load – Current hook load
  • If current rig state is not ‘drilling’, Weight on Bit is 0.

Metadata Required

Only metadata required to calculate Weight on Bit is Rigstate.

References