Logging While Drilling Versus Wireline Sonic Data: An E&P Operator’s Perspective

Traditionally, logging while drilling (LWD) sonic data are visually compared to equivalent wireline sonic data using depth track representations. 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. This paper introduces an objective approach for comparing LWD sonic data with wireline sonic data, utilizing the final processed and validated data provided by logging service companies. The first step in this proposed method is to depth-match the LWD sonic data to the wireline sonic data. It is well- established that LWD and wireline depths often differ and must be aligned to a common depth reference. This alignment ensures that geological features recorded by both LWD and wireline sonic data correspond to the same reference depth, thereby eliminating depth discrepancies as a potential source of variation in sonic slowness measurements. The second step involves generating slowness difference logs for both compressional and shear sonic data by calculating the simple difference between the wireline and LWD sonic measurements. The final step is to analyse these difference logs using histograms and cross-plots with other logs, providing a comprehensive assessment of the discrepancies. Using this approach, we have observed significant differences in slowness measurements, with discrepancies reaching up to 20 microseconds per foot or more, particularly for shear slowness. The factors contributing to these differences between LWD and wireline sonic data are multifaceted: 1. Data Acquisition Environment: Wireline logging is performed in an acoustically quiet environment, whereas LWD sonic data is acquired under more challenging and noisier conditions. Factors such as bottom hole assembly rotation, shocks, vibrations, and the effects of mud gas can significantly impact the quality of LWD sonic data. 2. Tool Centralization: The effectiveness of LWD sonic tool centralization while drilling affects recorded data, leading to poor or ineffective data processing, and thus misleading results. 3. Frequency Range and Modeling: Variations in shear sonic data arise from differences in frequency ranges analysed and the models used to derive formation shear slowness from LWD data. 4. Collar Size: The size of the LWD sonic tool collar can influence the distribution of differences, impacting the accuracy and consistency of LWD sonic measurements. These factors collectively contribute to the observed discrepancies between LWD and wireline sonic data. The impact of these factors on data quality is evident from the receiver array plots. Wireline sonic receiver array plots typically exhibit strong visual coherency across all receivers, while the coherency in LWD sonic receiver array plots is noticeably reduced, highlighting