Acoustic isolator for downhole applications

Abstract

A plurality of heavy mass irregularities attached to an inner wall of the drill collar attenuate waves traveling through the collar. The plurality of heavy mass irregularities are spaced and sized for the maximum attenuation of acoustic pulses in a predetermined frequency range. The mass irregularities may be rings secured to the inner surface of the collar by neck pieces, extending outwardly from the outer circumference of the ring. The mass irregularities may be made of steel or tungsten and are between six and ten in number. The spacing of the irregularities may lie between twelve and fourteen centimeters. A center pipe may be included to isolate the irregularities from the fluid flow associated with the drilling operation. The pipe may be of a soft material such as rubber to reduce transfer of acoustic noise along the drill string. The irregularities may be in an oil based fluid with the pipe fitting closely in the center of the rings. In another embodiment of the invention, each of the mass irregularities is attached to the drill collar over substantially the entire length of the mass irregularity, enabling the attenuation of high frequencies. In yet another embodiment of the invention, the attenuator comprises a substantially cylindrical body with a plurality of recesses on the inside and/or outside of the cylindrica body, with the length of the recesses selected to provide attenuation within a specified band. In another embodiment of the invention, the attenuator comprises a plurality of sections each having an inner diameter and an outer diameter, each section acting like a waveguide with an associated passband and reject-band.