Non-uniform temperature distribution's impact on downhole weight on bit (DWOB) measurement and the novel compensatory method.

Non-uniform temperature distribution of DWOB measuring devices has a great effect on the accuracies of weight on bit (WOB) readings. To study the influence, two types of non-uniform temperature distributions of the WOB measuring device were created. The relationship between the two temperature distributions was studied with the further transform of heat conduction formulas. Then the strain of the measuring device was acquired under the two temperature distributions. And the relations of the strain in the two conditions was obtained with the further transform of compatibility equations and constitutive equations. To quantify the influence of non-uniform temperature distribution, a contrast simulation without non-uniform temperature distribution was carried out. The simulated results showed that the WOB errors were 9.33 kN and 46.25 kN. This paper proposed a novel compensating method based on the deduced relationships above, with which the WOB error was eliminated to 0.4 kN. To further validate the new method, the laboratory heating and cooling experiments were carried out. The experiment showed that the biggest WOB error was eliminated to 1.7 kN while the original error was up to 60 kN. Then a field experiment was conducted. Eight connection processes are chosen to further elaborate the temperature difference's impact and the related compensatory method. The economic loss caused by the WOB measuring inaccuracy was calculated, which was up to 1.17% of the drilling cost per meter. The work presented herein can serve as guidance for the DWOB measurement. • Temperature difference inside the WOB measuring tool results in great inaccuracies. • Temperature difference varies with the formation and drilling working conditions. • The WOB measuring error was up to 60 kN in heating and cooling experiments. • The WOB measuring error was eliminated to 1.7 kN using the proposed method. [ABSTRACT FROM AUTHOR]