The efficiency of a drilling operation is to a great extent governed by how well one is able to optimize the rate of penetration (ROP) throughout each stage of the operation. ROP optimization normally involves balancing drilling speed on the one hand with acceptable wear to the drill bit on the other. The bit lifetime is largely determined by the mechanical conditions at the bit-formation rock interface and the weight on bit (WOB) and torque on bit (TOB) provide important information related to the working condition of the bit. The accuracy of WOB and TOB measurements can thus become a determining factor for the overall drilling efficiency.
Due to the low bandwidth of downhole mud pulse-based telemetry systems, the WOB and the TOB are generally derived from surface measurements, i.e. from the hook load and the top-drive torque. Field experience indicates that a WOB derived from surface measurements can be of limited accuracy, such as when surface measurements suggest a negative WOB even though the ROP is positive, or when high sampling rate and high precision downhole measurements confirm a large discrepancy between the memory recorded downhole data and the estimated values based on the measurements made at the level of the hoisting equipment and the top-drive.
The reason for these inconsistencies is simply that there are numerous physical processes taking place between the bit and the surface measurements that are normally not accounted for when WOB and TOB are estimated. This paper reviews and analyses the sources of these deviations and models the physical processes in order to quantify the precision for which the WOB and the TOB can be ascertained using solely surface measurements. Methods are also proposed that compensate for certain side effects by utilizing real-time torque and drag and hydraulic calculations.