Abstract: Based on analysis of field logs, fluctuation and amplitude increase in sonic log caused by fractures in carbonate rocks are investigated. It is found that the attenuation of sonic waves by fractures leads to severe amplitude decrease at receivers far from the source. As a result, the travel time has an increase of 3～4 μs/ft. This attenuation mechanism successfully interprets the fluctuation and amplitude increase in sonic logs. Combined with electric and acoustical imaging logs, the conditions for the amplitude increase are investigated. The results show that fractures with lower angle and larger open-width lead to more significant increase in sonic logs.

Key words: fluctuation in sonic logs, increase in sonic logs, attenuation of P-wave, fractured reservoir