The Drilling Engineering Research Group at the University of Calgary has been seeking better ways of improving drilling operations and decreasing costs by the use of advanced real-time modeling and simulation technologies. It is well known that the performance of a drill bit directly affects the overall drilling performance. The bit performance is often evaluated by the rate of penetration (ROP) which is dependent on the weight on bit (WOB). Therefore, obtaining actual downhole weight on bit (DWOB) is crucial in achieving good performance of a drill bit.

This paper defines the procedures or steps to measure DWOB and an analytical model for calculating DWOB using typical surface collected drilling data. Field test data is used to initially calibrate the analytical model. The calibrated analytical model is next used in a forward calculation to predict DWOB on the same well. DWOB is also predicted on a second well using the same drilling rig.

The results from the calibrated model are compared to the DWOB collected by the CoPilot, a Baker Inteq downhole measurement tool. The comparison shows that the DWOBs from the model match those from the CoPilot well.

The model can be integrated in a new directional Autodriller system, which can in real time set the DWOB from surface measurements. The directional Autodriller can automatically conduct real-time analysis and calculations of DWOB as well as maintaining a precise DWOB for the drill bit. This will improve drilling efficiency and reduce cost.


A drilling mechanics sub (DMS) was developed to measure DWOB, torque on bit, bending moments, shear force, accelerations and differential pressure [1]. Measurement-while-drilling (MWD) tools have increased drilling efficiency in many directional wells. With information provided by downhole weight on bit and torque sensors, more timely and accurate decisions have been made, resulting in lower costs per foot. Specific applications of this technology include bit optimization, directional feedback, and drillstring-friction analysis [2]. The theoretical calculation of DWOB in this paper was done using an analytical torque and drag model [3]. The calculation of the friction coefficient and DWOB was also attempted using finite element analysis FEA [4, 5]. The calculations and analysis in these works have laid a solid foundation for a new type of Autodriller [6]. The future Autodriller will be installed at surface with an operating panel and connected to a powerful computer with high speed and a big memory, which can gather large amounts of real-time drilling data and other necessary information. This paper states the procedures or steps of measuring DWOB. An analytical model for calculating DWOB is introduced herein and some of the field test data is used to initially calibrate the analytical model. Finally, the calibrated model DWOB is compared to the collected downhole DWOB for two wells.

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