Study on the downhole measurement method of weight on bit with a near-bit measurement tool

Abstract

The downhole weight on bit (DWOB) measurement method using a near-bit measurement tool can directly measure the force between the well bottom and drilling bit, which is crucial for safe and efficient drilling. However, its accuracy is highly influenced by the downhole drilling fluid pressure and temperature difference between the inner and outer walls. Previous studies have proposed compensatory methods. However, these methods are either extremely complex or not sufficiently accurate. In this study, we proposed a DWOB measurement method based on the force-strain principle. We theoretically and quantitatively studied the impact of various factors, including fluid pressure and temperature difference, on strain measurements. The results showed that the strain output is linear with respect to axial force, fluid pressure, and temperature difference. Using this linear relationship, we determined the downhole axial force. Subsequently, we combined the linear relationship with that between the DWOB and obtained axial force and derived a final DWOB measurement equation that incorporated the nozzle jet force, flow friction force, and lift force. Finally, we quantified the three forces using drilling-fluid mechanics and applied the proposed method to both a finite-element simulation and a real-field drilling experiment. The calculated DWOB measurement error in the simulation and field experiment was less than ±1000 N and ±5000 N, respectively. In short, the proposed method may be used to achieve improved WOB measurements and can significantly improve the bit life and increase the rate of penetration (ROP).