The drilling environment encountered in modern wells, especially gas wells, frequently includes higher temperatures, higher pressures and smaller diameters than standard wells. Furthermore, specialized borehole fluids or air may be used to preserve borehole and formation integrity. These conditions frequently limit or even preclude the use of conventional acoustic-logging tools. High rig costs may also result in the elimination of all, or part of the traditional openhole logging program. A new slim, hostile-environment wireline acoustic logging tool has been developed for the cased-hole market The small diameter (1-11/16 in.) allows tool operation in casing or tubing as small as 2-in., and also in standard drillpipe. The tool is rated for operation at temperatures and pressures up to 420℉ and 20,000 psi. A full processor-controlled, digital acquisition system, in combination with a carefully designed arrangement of acoustic transmitters and detectors, provides a simultaneous high-quality cement-bond log, and monopole compressional slowness or radial channel identification. The addition of pulsed-neutron measurements allows for a more complete formation-evaluation analysis. Case histories demonstrate the full range of the tool's measurement capabilities for formation evaluation, seismic correlation, cement evaluation, and stuck-pipe analysis in a variety of borehole environments and in casing sizes ranging from 2-7/8-in. to 9-5/8-in. In one example, twin wells were drilled in an onshore gas field, one using air and the other using conventional borehole fluid. For this type of well, air drilling is a more efficient and lower-cost construction method but at the same time, makes openhole acoustic logging impossible. Logs were run in the fluid-filled open borehole and also in the air-drilled well after setting casing. The cased-hole data proved superior to the openhole data, possibly due to formation alteration by the borehole fluid hi the openhole, and provided more accurate synthetic seismograms and seismic ties. In another example, the bottomhole assembly was stuck in the well and the through-tubing acoustic tool was run through drillpipe to identify the intervals of stuck pipe. Having this information was critical for planning the successful fishing operation because it enabled decisions regarding wash-pipe length, free-point indication and backoff, and allowed estimates of the remaining down time. In situations where openhole logging is either impossible or undesirable, cased-hole acoustic logging can provide high-quality data at a lower cost.
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