Patent ID: 7149146
Filing Date: 2006-12-12
Classification: E21B

Abstract:
1. A method for estimating an impedance of a material behind a casing wall, wherein the casing is disposed in a borehole drilled in a geological formation, and wherein a borehole fluid is filling said casing, the material being disposed in an annulus between said casing and said geological formation, said method using a logging tool positionable inside the casing and said method comprising: (i) exciting a first acoustic wave in said casing by insonifying said casing with a first pulse, the first acoustic wave having a first mode that may be one of flexural mode or extensional mode; (ii) receiving one or more echoes from said first acoustic wave, and producing a first signal; (iii) extracting from said first signal a first equation with two unknowns, where first unknown is an acoustic property of said material and second unknown is an acoustic property of said borehole fluid; (iv) exciting a second acoustic wave in said casing by insonifying said casing with a second pulse, the second acoustic wave having a thickness mode; (v) receiving one or more echoes from said second acoustic wave, and producing a second signal; (vi) extracting from said second signal a second equation with said two unknowns; (vii) extracting said acoustic property of said material from said first and said second equations.