To maintain high rate of penetration (ROP) and to realize automatic drilling, it is necessary to monitor and control the weight on bit (WOB). Generally, the WOB is measured at the wellhead, and it is called surface WOB (SWOB). During directional drilling, there is big difference between the SWOB and downhole WOB (DWOB). In this paper, an analytical model for torque and drag of the drillstring is developed to calculate the DWOB. When building the model, the well profiles are separated to two categories, including straight section and curved section. Moreover, for the curved section, the azimuth and inclination of a well are considered while modeling, adequately. Based on a set of field tests, the friction factors and DWOB are calculated. The results indicate that there is a good match in value and trend between the data calculated and the one measured. In practice, the model can be used to predict and adjust the DWOB in autodriller system for real-time drilling monitoring. In addition, the model can also be applied to estimate downhole conditions and diagnose drilling status.

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