Effects of eccentric acoustic source on the amplitude and arriving time of the first arrival in cased boreholes

Abstract

The acoustic fields excited by eccentric acoustic source in cased boreholes bonded by cements with different densities are simulated numerically using 2.5-dimension (2.5D) finite-difference method (FDM). The effects of the source eccentricity on the amplitude and arriving time of the first arrival (FA) in the full waveform are investigated. The numerical results show that the amplitude of the FA will decrease quickly and its arriving time will go ahead as the source eccentricity increases. The change of the arriving time can be estimated approximatively using geometrical acoustics theory. As the eccentric distance of the acoustic source reaches 1/4 of the borehole radius, the amplitude of the FA will reduce to below 20% of the centered case. The quantitative varying trends of the amplitude and arriving time of the FA are the same both in the free pipe and in boned pipe. Therefore, during the cementing evaluation, the tool's eccentricity should be estimated through the arriving time of the first arrival, a...