A new acoustic tool has been developed to measure formation acoustic properties through casing. This measurement is important for oil and gas production in mature fields, and for wells that are cased without logging due to borehole stability issues. Conventional acoustic logging through casing in poorly bonded boreholes has been a difficult task due to the presence of overwhelming casing waves that mask the formation acoustic signal. To overcome this difficulty, we developed an acoustic tool using dual-source transmitters and the processing technique for the data acquired by the tool. This paper elaborates the operating principle of the new dual-source technology and demonstrates its application to casedhole acoustic logging. By using the dual-source design, the overwhelming casing waves from the poorly bonded casing are largely suppressed. On the basis of the casing-wave suppression and the condition that the formation is acoustically slower than casing, the formation acoustic-wave amplitude is significantly enhanced in the dual-source data-acquisition process. Subsequent processing of the data reliably obtains the acoustic velocity of the formation. The new tool has been tested in many cased wells with proven performance for various cement-bond conditions. The success of this technology makes casedhole acoustic logging an effective operation that can be routinely used to obtain reliable formation information through casing for slow to moderately fast formations.

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