Applications for a Full Wave Sonic LWD Tool in the Middle-East

Abstract In recent years MWD/LWD formation evaluation applications have evolved to apoint where many applications usually reserved for wireline logging can now beattained from LWD data. The latest area is the applications attained from LWDsonic measurements. In a drilling environment a sonic tool has applicationsthat extend beyond standard porosity-type evaluations. MWD/LWD sonic logs canalso provide real-time seismic tie-in as well as assist in drillingoptimization. Conventional perceptions are that there are two key aspects thatcould minimize or preclude the use of sonic technology in an MWD/LWDenvironment. First, drilling noise is assumed to exist within the frequencyrange of the measurements, and minimization of it was considered impractical.Second, the interference of the tool-body mode (acoustic waves coupled to thereceivers as a result of tool body waves) is thought to be dominant, to theextent where significant manual post-processing is required to generatereasonable logs. This paper presents some fundamental design choices and case histories fromthe Middle-East where a new Full-Wave Dual-Mode MWD/LWD sonic tool has recentlybeen run. The tool is constructed such that it operates in two modes (operatorselectable and available as either/or or both): (i) classical compressionalwave sonic tool (the equivalent to the traditional monopole source mode); (ii)enhanced shear-wave detection mode (dipole-type source). The tool has multipletransmitters and multiple receivers (arranged in a combination of arrays) thatallow operation in both modes. The tool utilizes a unique set of isolationsections that minimize the effects of drilling noise and tool-body mode. Thedesign of these isolation sections was based on extensive lab testing withscale models, as well as, the use of sophisticated 3-D analytical models. Thesestudies led to a comprehensive understanding of the dynamics of the tool bodymode and drilling noise and the best ways to reduce them. Implementation of thefinal design resulted in a negligible tool mode and nearly complete isolationfrom drillstring noise. The success of this design has been verified indrilling environments throughout the world where sonic LWD logs have beensuccessfully obtained in hole sizes from 30" down to 5 7/8". Case histories will be presented from Abu Dhabi where the sonic LWD tool wasrecently run. The tools response in the varied formations of the Arabian Gulfand its application as a porosity device will be examined. Further applicationssuch as fracture detection will also be discussed.