Acoustic Field In A Cased Well With A Sectorial Crossing Channel

The crossing channel in a cased well is a column filled with fluid in the cement layer between the steel pipe and formation. It is formed during well cementing sometimes and is very harmful to the well. People have been trying to detect crossing channels by various methods. Some new types of tools were developed as well.In order to study the possibility of detecting and sizing the crossing channel by using a traditional logging tool, the acoustic field in a cased well with a crossing channel of various degree is studied for the first time in this paper. To authors' knowledge only situations of well bonded and 360° crossing channels were considered, much different from the practical situations.2.5-D finite difference method developed by Leslie and Randall are used which transform all Finite Difference(FD) equations into axial wave-number domain and, after the FD iteration in this domain, the result is transformed back to the time-spatial domain by Fourier transform. The most advantage of this 2.5-D method lies in that it solves some kinds of 3-dimensional acoustic field problems with small computer memory.The results of the 2.5-D finite difference method for the cases of 0° and 360° are compared and consistent with that of the well known real axis integration method. Snapshots of normal stress and synthetic time-domain waveforms are displayed. The two-dimensional spectrum in wave-number and frequency domains is also calculated, where the influence of the channel is clearer than that in the waveforms. Numerical study demonstrated that a crossing channel can be detected and sized if its angle is larger than 30 degree, and larger offset and lower frequency source are favorable to detect and size the crossing channel.The acoustic field in an asymmetric borehole was also investigated experimentally and compared with the result of numeric simulation with 2.5-D finite difference method, and a consistent result was obtained. This work provides an accuracy check of our investigation of asymmetric borehole with 2.5-D finite difference method.