What is weight on bit (WOB)?
Weight on bit is the downward force applied to the drill bit during drilling, usually measured in thousands of pounds (klbs) or kilonewtons (kN). It is one of the two parameters a driller controls directly, alongside rotary speed. Too little weight and the bit scrapes rather than cuts, producing slow penetration. Too much and the bit wears prematurely, or the string enters a dysfunction such as stick-slip.
How is weight on bit measured?
There are two measurements.
Surface WOB is calculated from hookload. The driller notes the hookload with the string hanging free and off bottom, then the hookload while drilling. The difference is the weight transferred off the hook and onto the bit.
Downhole WOB is measured directly by sensors in the bottom hole assembly, close to the bit. It reflects the force the bit actually receives.
How do you calculate weight on bit?
Surface WOB is the free-rotating hookload minus the hookload while drilling, expressed in klbs:
WOB = hookload off bottom − hookload on bottom
The calculation assumes all weight removed from the hook reaches the bit. That assumption holds reasonably well in a vertical well and does not hold in a deviated well.
What is the optimal weight on bit?
There is no single figure. Optimal WOB depends on bit type, formation strength, rotary speed, and hole cleaning, and it changes as the bit wears and the formation changes.
The shape of the relationship is consistent. Dupriest described the founder point, the level beyond which adding weight stops producing proportional gains and begins to work against you (SPE 92194, 2005). MSE analysis gives a way to find it: optimal WOB is where mechanical specific energy stays close to the rock's confined compressive strength (SPE 24584, Pessier and Fear, 1992).
How does weight on bit affect rate of penetration?
Not linearly. Up to a point, more weight means faster penetration. Past the founder point, the returns flatten and then reverse, because the additional energy goes into vibration and bit wear rather than into cutting rock.
What happens if weight on bit is too high?
The bit wears faster and can be damaged. Excess weight also drives drilling dysfunctions: stick-slip, where the bit alternately stops and releases, and in slender strings, buckling. A damaged bit usually also costs a trip to replace it.
What happens if weight on bit is too low?
Penetration slows, and the bit scrapes rather than cuts, wearing the cutting structure without making hole.
Why doesn't surface weight reach the bit?
Because the drill string is long, flexible, and in contact with the wellbore wall. Weight applied at surface travels through thousands of feet of pipe, and friction against the wall absorbs part of it on the way down.
In directional wells, friction can consume 50 to 70% of the weight applied at surface (SPE 173024). The proportion changes with hole angle, hole cleaning, and formation, so surface readings do not reliably indicate what the bit is experiencing at a given moment.
How is weight on bit controlled in horizontal and extended reach wells?
The conventional method is to add drill collars and let gravity load the bit. In a horizontal section the string lies on the low side of the hole rather than hanging above the bit, so gravity contributes nothing and friction governs what reaches the bit.
In extended reach wells this reaches a limit. Beyond a certain measured depth, adding weight at surface produces no additional weight at the bit, because the string loads up against the wellbore wall instead.
How is weight on bit controlled downhole?
By generating the force at the bottom hole assembly rather than transmitting it from surface. A downhole system grips against the wellbore and pushes the bit forward using its own mechanism, so the weight does not depend on what survives the journey down the string.
NexTitan works this way. It grips the wellbore and applies controlled thrust directly at the bit, holding the force steady as the formation changes. Because the force is generated and measured in the same place, the control loop closes at the bit instead of at surface.
What is the difference between surface WOB and downhole WOB?
Surface WOB is what the driller applies. Downhole WOB is what the bit receives. In a vertical well the two are close. In a deviated well they diverge by an amount that is not constant and cannot be read from surface.
Can a downhole tool apply weight on bit directly?
Yes. A downhole drive system grips the wellbore and applies force at the bit itself, which makes weight on bit independent of drill string friction and hole angle. NexTitan does this, and because it also measures the force it is applying, it can hold a setpoint rather than estimate one.
The drilling cost calculator models what steadier weight at the bit is worth on a specific section: enter your section length, spread rate, and bit cost.
If you want to know whether this applies to a particular well or field, our team can walk through the geometry and the numbers.
