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"Weight on bit" is a quantitative term used to express the amount of weight or force placed on the drill bit at the bottom of an oil well by the lowering of the drillstring and collars onto the bit. To cut rock, the drill bits used in these operations are forced against the bottom of the wellbore by the weight of the entire drillstring and, in particular, specially-designed, heavy tube sections known as drill collars. When the entire assembly is still suspended from the derrick, it is weighed and the weight noted. The string is then carefully lowered until the bit is on the bottom of the wellbore. From that point onward, the suspended drillstring weight is carefully monitored as it is progressively lowered, the decrease in the initial reading as the drill bit takes the weight of the collars and string being designated as the "weight on bit."
To understand the concept of weight on bit, the difference in applied force between conventional drilling and oil well drilling has to be understood. For example, to drill a hole through a piece of timber with a normal household electric drill, the handyman exerts direct pressure on the drill, and consequently on the tip of the drill bit, by pushing against it. In the case of deep drilling of wells and boreholes, this type of action is not practical. In these instances, the drill bit is lowered to the bottom of the wellbore where it is forced against the bottom face of the hole by steadily lowering weights onto it as it rotates. The amount of weight added to the drill bit section of the drillstring is known as "weight on bit."
To achieve this rather complex operation, the entire drillstring, consisting of the drill bit section, drill pipe, and drill collars, is suspended with the bit off of the bottom of the wellbore. The exact weight of the combined assembly is carefully noted and the string lowered until the bit is just on the bottom of the hole. Obviously, as the string is lowered, some more the drill bit starts to take up a portion of the drillstring weight, with a corresponding drop in the suspended weight. If, for instance, the initial suspended weight was 200,000 lbs (90,718 kg) and the reading once some of the weight is on the bit is 150,000 lbs (68,038 kg), then the weight-on-bit value is 50,000 lbs (22,679 kg).
These readings are typically taken from a drillstring weight indicator located on the drill platform. Some indicators are simple single-scale instruments that require the operator to continually calculate the weight on bit values as in the example in the previous paragraph. Others are a little more functional and feature dual scales — a primary scale that indicates suspended weight and a secondary scale for bit weight. The secondary scale is zeroed once the bit bottoms out and basically reads backward as the suspended weight decreases, with operators then simply reading the weight-on-bit values directly from it. Alternately, measurement while drilling (MWD) sensors located downhole just above the bit section can also be used to send more accurate weight-on-bit values to a surface readout interface.
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