Well Depth Measured with MWD Error Correction and Calculation of Borehole Position Uncertainty

When drilling deep and ultra-deep wells, the high downhole temperature and the large tensile force on the drilling tools in the well result in a large well depth error measured with measurement with drilling (MWD). Therefore, the influence of downhole temperature, thermal expansion coefficient, axial force of drilling tools, and specifications of drilling tools in different well depths was considered, and the drilling tools in the well were segmented at the measurement point. According to the results of the downhole temperature measured with MWD and the force analysis of drilling tools in the well, models of thermal expansion and elastic tension correction for the well depth measured with MWD were established. In addition, the models for the calculation of the error limits of the thermal expansion correction and the elastic tension correction, as well as the calculation method of wellbore position uncertainty after correcting thermal expansion and elastic tension errors of measured well depth with MWD were given. The example calculations demonstrate that during the drilling of ultra-deep wells, the elongation of drilling tools in the well caused by thermal expansion and elastic tension can reach more than 10 m. By correcting the thermal expansion and elastic tension of the measured depth with MWD, the vertical depth errors and the size of the error ellipsoid of the measurement points can be significantly reduced. The results provide a theoretical basis for improving the accuracy of well depth measured with MWD and scientific calculation of wellbore position uncertainty.