High-Frequency, Azimuthal, Sonic LWD Tool Helps to Improve Compressional Velocity Processing in Formations with Harsh Logging Conditions: Case Study

Abstract The oil and gas industry drills wells in harsher conditions year after year. Besides drilling challenges, these harsh conditions make it difficult to obtain accurate and consistent petrophysical measurements, particularly sonic and density data, because of high temperature, high pressure, and high mud weight (MW) in highly deviated wells. Such conditions, in particular the high MW, attenuate sonic waves significantly, affecting the detection of sonic arrivals as well as ultrasonic caliper measurements. In addition, extremely high MW affects density data and invalidates photoelectric effect (Pe) measurements because of the high barite content. The Pe is typically used as an input to volumetric lithology interpretations in a four-mineral model. When the Pe is unreliable, sonic velocities can be applied to the interpretation workflow instead, if they are accurate. This paper discusses engineering solutions for software and hardware challenges and operational and technical aspects of acquiring the sonic log in combination with conventional triple-combo data. As a result of the introduction of a high-frequency sonic logging-while-drilling (LWD) tool and advanced processing techniques, signal coherence tripled from an initial 0.3 (30%) to almost 0.9 (90%), significantly improving the quality of the data. Additionally, ultrasonic caliper coverage quadrupled from 25 to 100% of the logging interval, compared to previous runs in offset wells. Furthermore, acoustic velocities play an important role as an input to geomechanical models, which study the stress regime and allow for estimation of rock moduli. Thus, an azimuthal acoustic tool measuring velocities 360° around the borehole provides information on formation stress anisotropy. Estimating formation rock moduli requires accurate density and acoustic properties provided by high- frequency sonic measurements. For wells with inclination greater than 60°, it becomes difficult to run wireline logging. Therefore, LWD quad combo with a high-frequency, azimuthal acoustic tool is a viable solution for this environment. Additionally, it is advantageous that formation-evaluation measurements can be acquired while in drilling mode, saving rig time and reducing well-construction costs.