This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 103094, "Dynamic Depth Correction To Reduce Depth Uncertainty and Improve MWD/LWD Log Quality," by D. Dashevskiy, SPE, T. Dahl, A.G. Brooks, SPE, D. Zurcher, J.C. Lofts, SPE, and S. Dankers, SPE, Inteq, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September. Accuracy of measurement-while-drilling (MWD) and logging-while-drilling (LWD) depth measurements can be improved by considering the dynamic variation in drillstring length caused by pipe loading under changing drilling conditions. A new method that uses surface torque, hookload, and temperature measurements was developed to determine force distribution in a drillstring and to compute apparent drillstring length. When available, downhole measurements of torque, weight on bit (WOB), and temperature are used to increase accuracy and robustness of the method. Introduction With longer and deeper wells in deeper provinces around the world and the use of drillpipe conveyance of MWD/LWD data, accurate depth measurements are critical. Depth accuracy is vital for calculating structural dip from borehole images, picking perforation points, and correlating geological units. Measured depth (MD) is used directly in the calculation of true vertical depth (TVD) (the primary depth used for reservoir delineation). Errors in depth are difficult to detect with a single scalar measurement; however, comparing several logging curves with similar character (Fig. 1) can reveal these artifacts. Cross correlation and a depth "rubber-banding" technique are used to bring MWD/LWD measurements from different sources (even within the same bottomhole assembly) together, using measurements from the sensor nearest the bit as a reference. This method addresses the symptom, not the cause. Misalignment of curves serves as a good indicator of the formation-evaluation (FE) -measurements depth-placement uncertainty and should be used instead for validation of methods used to convert MWD/LWD data to depth logs. Background All data from MWD/LWD tools are referenced to time. The moment in time when every measurement value was taken is known. However, it is necessary to know where the measurement was taken. To determine where measurements are taken, the following are needed.The wellbore profile—wellbore location in space.The time/depth profile—the location (with respect to the wellbore) of the bit at each moment in time. This information can be used to place MWD/LWD measurements along the wellbore and, hence, in space. The assumption that the time/depth profile represents the TVD of the bit (and hence logging-sensor depths) at any moment in time often is incorrect. Bit depth is seldom equal to the summation of lengths of all steel run in the hole and the hook-height displacement, in part because of forces acting on the string that cause pipe to stretch, compress, or buckle. All errors in measuring time and depth will affect the quality of MWD/LWD logs.