Real-Time LWD Sonic Semblance Image and the Step Change in LWD Sonic Applications—A Case Study from Latin America (Mexico)

Abstract

Abstract The need for reliable logging-while-drilling (LWD) sonic data in real-time has been of great interest to many operators since inception. However, providing such information reliably requires many challenging tasks. Unlike wireline tools, where waveforms are sent and processed on the surface in real-time, the waveforms of an LWD sonic tool cannot be sent in real-time due to bandwidth limitations. This requires the data to be processed downhole and only the processed data to be sent in real-time. Traditionally, windows were used to define the range of the real-time compressional velocity. However, a major drawback to this approach is the requirement of knowing the compressional arrival range beforehand and the possibility of receiving a different arrival. For example, fast shear could be confused with slow compressional data. Sending a complete semblance image in real-time is the best way of capturing all arrivals. However, data must be inelegantly compressed to maintain the arrival times yet reduce the bandwidth to a minimum. A new sonic LWD tool has been developed to provide high-quality real-time sonic semblance images, creating opportunities for obtaining this information while drilling. An offshore case study comparing the real-time sonic images to the recorded memory data in terms of resolution and data quality will be investigated. The paper will discuss the uses and applications of the sonic semblance image and its real-time value. This proven data acquisition process is crucial in allowing the end user to access all the formation evaluation data, thus improving real-time decision making. Technological advances in LWD services have had a direct impact on the ability to evaluate a reservoir and provide production planning on a foot-by-foot basis. The amount of downhole data processing and data compression required plays a critical role in delivering real-time sonic data while drilling. High-resolution, real-time semblance images can be obtained in real-time, and are expanding the uses and applications over traditional delta-t data acquisition. Operating companies have formed an outdated opinion that real-time LWD sonic data is unreliable and requires post-processing. With newer technology, high quality semblance images can be provided in real-time and, as a result, there is a shift in the way operating companies use the data for analysis and planning. The offshore environment has always presented a difficult challenge for acoustic tools in general; however, with the scientific advances in LWD tools, continuous compressional and shear arrival from the real-time semblance images can prove useful. This case study will also provide basic background information on the lithology in the region, highlighting why sonic data in particular was required in real-time.