ABSTRACT

The cement-bond log (CBL) is a conventional and widely used cement quality evaluation technology for vertical wells. With the increase in horizontal wells around the world, the existing cement evaluation technologies are not appropriate. We have explored the possibilities of utilizing CBL in horizontal wells through investigating the effects of a noncentralized tool on CBL measurements. The parallel finite-difference numerical simulation method and experiments in calibration wells were adopted in the study. The numerical and experimental results matched very well, and indicated that the CBL amplitude decreases linearly with increasing tool eccentricity in a well with free pipe (i.e., a cased but uncemented well). For a standard pipe with a diameter of 5.5 in (139.7 mm) and a thickness of 7.72 mm, an eccentricity of 1/4in (17% of the maximum eccentricity) could cause the CBL amplitude to be reduced by about 20%. The numerical simulations of CBL in wells with fluid channels in the cement showed that tool eccentralization could either increase or reduce the CBL amplitude relative to a centered tool, depending on the channel azimuth relative to eccentered direction. To explain this phenomenon, we investigated numerically the polarizations of casing waves in a well with free pipe and in a well with a fluid channel, and casing waves at higher frequencies in a well with free pipe. The relationship between the CBL amplitude and the percentage of cemented area for a borehole-centered tool was also studied. Our results provided some insights into understanding CBL measurements in horizontal wells.

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