Dynamic Depth Correction To Reduce Depth Uncertainty and Improve MWD/LWD Log Quality
Authors: Dashevskiy D. , Dahl T. , Brooks A. G. , Zurcher D. , Lofts J. C. , Dankers S. 
Published: March 2008

Accuracy of measurement while drilling/logging while drilling (MWD/LWD) depth measurements can be improved by considering the dynamic variation in drillstring length caused by pipe loading under changing drilling conditions. This paper details a new method that uses surface torque, hookload, and temperature measurements to determine force distribution in a drillstring and to compute apparent drillstring length. When available, torque, weight on bit (WOB), and temperature measured downhole are used to increase accuracy and robustness of the method.

Although logging depth is referred to as a measurement, in reality only the drilling block position is measured. Depth is inferred from it using drillstring length. In recent publications, physical phenomena affecting this were analyzed and quantified. Elastic pipe stretch and thermal expansion were found to be most significant. Techniques to compensate for these effects on the basis of empirical formulae have been proposed (Brooks et al. 2005), but they provide an averaged correction that has insufficient accuracy for many drilling and formation evaluation applications.

This paper presents experiments covering various wellbore profiles, temperature profiles, and drilling modes, which show that the depth fluctuation may be as much as 2.7 m with a 7,000–m long drillstring even when only the current rig operation mode changes. Among other factors considered in the paper, apparent depth fluctuation is the most significant contributor to commonly observed MWD/LWD log discrepancies when bed boundaries or other features are not logged at the same depth with each sensor. These errors lead to inaccurate petrophysical calculations, distortion of borehole images, and lost time caused by depth matching. Case studies illustrate the positive effect of dynamic depth correction on formation evaluation log quality.

The accuracy of a depth measurement is normally estimated in terms of its bias and uncertainty. A significant portion of the depth bias is caused by elastic stretch and thermal expension (for example in a 7,000-m long vertical drillstring they can be 9 and 6 m, respectively). The proposed method removes this bias and allows improved depth uncertainty.

Measured depth uncertainty (1cr) in the Industry Steering Committee on Wellbore Surveying Accuracy (ISCWSA) MWD model caused by drill string stretch is 2.2×10−7 m−1, multiplied by measured depth (MD) and by true vertical depth (TVD). Uncertainty in measured depth cannot be completely eliminated even by applying corresponding corrections because of the modeling and input data inaccuracies. Nevertherless, it is estimated that the proposed method significantly reduces this uncertainty (e.g., 50% and more, depending on the wellbore, available data, etc.).

Improved depth accuracy, in turn, reduces the uncertainty in computation of reservoir characterization parameters, such as net-to-gross and structural dip, especially when data from multiple wells are evaluated together.