Determination of Formation Properties in Cased Boreholes Using Full Waveform Acoustic Logs

Abstract

Wave propagation in bonded and unbonded cased boreholes is examined through the calculation cif synthetic full waveform acoustic logs. The models consist of a central fluid borehole surrounded by a number of fluid and solid annuli. Waveforms calculated for a variety of formation and cement parameters demonstrate that the first arrivals observed on full waveform acoustic logs in well bonded cased holes are those of the formation and not the casing. Waves refracted along the casing are generally too small to be observed. The presence of the steel and cement can make the determination of formation velocities more difficult than in an open hole. The formation body wave arrivals are decreased substantially if the cement velocities are near or greater than the formation velocities. A fluid layer between the steel and the cement essentially frees the pipe from the cement. The steel arrival then becomes a large, ringing signal which obscures the formation arrival. The presence of this layer is a more important factor than its thickness in causing such behavior. If the fluid layer is between the cement and the formation, the cement can damp out the ringing of the pipe. If a thick cement layer is bonded to the pipe and the fluid layer is thin. the casing arrival is small and the formation arrivals are discernible. A thinner cement layer results in the observation of a body wave that has a velocity that is an average of the steel and cement velocities.