Differences in Logging While Drilling Versus Wireline Sonic Data, Their Origins, and Implications on Geomechanical Model Results

ABSTRACT: Traditionally, logging while drilling (LWD) sonic data are compared visually against equivalent wireline sonic data based on depth track representations. In doing so, LWD and wireline sonic data may ‘overlay’ and thus can be said to be similar or even equivalent. However, depth tracks may be two to three centimeters wide and represent a 200 (for compressional slowness) up to 500 (for shear slowness) microsecond per foot slowness range. Thus, if a sonic slowness curve is represented by a one-millimeter-thick trace on such a depth track, the line thickness itself already represents 10 us/ft up to 25 us/ft formation slowness uncertainty. This paper proposes an objective comparison method for LWD and wireline sonic data, focusing on processed data from logging service companies. After depth matching the LWD and wireline logs, the data is compared using slowness differences, histograms, or cross-plots. This analysis reveals slowness shifts of up to 30 microseconds per foot between the two data types. Differences arise from tool design, data acquisition modes (drilling vs. wireline), and tool centralization effectiveness, all impacting data quality and processing. The paper also examines how these discrepancies affect geomechanical properties such as formation elastic moduli, with an example from a well where both LWD and wireline data were collected. 1. INTRODUCTION Traditionally, logging while drilling (LWD) sonic data is visually compared to equivalent wireline (WL) sonic data using depth track representations (Varsamis et al. 2000, Market and Canady 2006, etc). This approach often suggests that LWD and wireline sonic data are similar or even equivalent when they appear to ‘overlay’ each other. However, depth tracks can vary in width from two to three centimeters and may represent slowness ranges from 200 microseconds per foot (for compressional slowness) to 500 microseconds per foot (for shear slowness). Consequently, a sonic slowness curve depicted with a one-millimeter-thick trace on such a depth track can correspond to a slowness range of 10 to 25 microseconds per foot. This inherent thickness in the visual representation makes this comparison method subjective.