New methods of eliminating downhole WOB measurement error owing to temperature variation and well pressure differential

Abstract The downhole weight on bit (WOB) measurement sensor can produce significant inaccuracies in its readings owing to the effects of temperature variation and well pressure differential. To improve the accuracy of downhole WOB measurement, the impacts of temperature variation and pressure differential on WOB measurement were studied via theoretical derivations, laboratory experiments, and field experiments. Based on the constitutive equations of thermoelastic materials and the strain theory of strain gauge, the equation of error induced by temperature variation was deduced. Through laboratory experiments, the conventional temperature compensatory method, and the new compensatory temperature method proposed herein were compared. The experimental results show that the proposed new temperature compensatory method performed better in eliminating the effects of temperature variation. The shortcoming of the conventional WOB correction method for eliminating the pressure differential impact is that it does not consider the pressure drop in the drill motor. A new WOB correction method is proposed based on the momentum equation of flowing fluid and floating body theory. The field experimental results indicate that the measurement error of the conventional correction method could reach up to 80 kN. The work presented herein can serve as guidance for the downhole WOB measurement.