REVEALING HIDDEN INFORMATION; HIGH RESOLUTION LOGGING-WHILE-DRILLING SLOWNESS MEASUREMENTS AND IMAGING USING ADVANCED DUAL ULTRASONIC TECHNOLOGY

摘要

A new logging while drilling (LWD) acoustic tool hasbeen developed with novel ultrasonic pitch-catch andpulse-echo technologies. The tool enables both highresolutionslowness and reflectivity images, whichcannot be addressed with conventional acoustic logging.Measuring formation elastic-wave properties incomplex, finely layered, formations is routinelyattempted with sonic tools that measure slowness over areceiver array with a length of 2 ft or more dependingupon the tool design. These apertures lead to processingresults with similar vertical resolutions, obscuring thetrue slowness of any layering occurring at a finer scale.If any of these layers present significantly differentelastic-wave properties than the surrounding rock, thenthey can play a major role in both wellbore stability andhydraulic fracturing but can be absent fromgeomechanical models built on routine sonicmeasurements.Conventional sonic tools operate from approximately0.1 kHz to 20 kHz and can deliver slowness informationwith approximately 1 ft or more depth of investigation.This is sufficient to investigate the far field slownessvalues but makes it very challenging to evaluate the nearwellboreregion where tectonic stress redistributioncauses pronounced azimuthal slowness variation. Thisstress-induced slowness variation is important because itis also a key driver of wellbore geomechanics.Moreover, in the presence of highly laminatedformations there can be a significant azimuthal variationof slowness due to layering that is often beyond theresolution of conventional sonic tools due to theiroperating frequency. Finally, in horizontal wells,multiple layer slownesses are being measuredsimultaneously because of the depth of investigation ofconventional sonic tools. This can cause significantinterpretational challenges.To address these challenges, an entirely new designapproach was needed. The novel pitch-catch technologyoperates over a wide frequency range centered at250 kHz and contains an array of receivers having a 2 in.Receiver aperture. The use of dual ultrasonic technologyallows the measurement of high-resolution slowness dataazimuthally as well as reflectivity and caliper images.The new LWD tool was run in both vertical andhorizontal wells and directly compared with bothwireline sonic and imaging tools. The inch-scaleslownesses obtained show characteristic features thatclearly correlate to the formation lithology and structureindicated by the images. These features are completelyabsent from the conventional sonic data due to itscomparatively lower vertical resolution. Slownessimages from the tool reflect the formation elastic-waveproperties at fine scale and show dips and lithologicalvariations that are complimentary to the data from thepulse-echo images. The physics of the measurement arediscussed along with its ability to measure near wellboreslowness, elastic-wave properties and stress variations.Additionally, the effect of the stress-induced nearwellborefeatures seen in the slowness images and thepulse-echo images is discussed with the wireline dipoleshear anisotropy processing.