Little has changed in cement evaluation over the last 50 years, particularly when the method for such evaluation relies on acoustic theory. The interpretation assumes that when a pressure wave emitted by an appropriate acoustic source reaches an interface, normally a steel pipe and the pipe is not cemented (free pipe), the pipe vibrates (rings) and the amplitude of the refracted wave in the casing reflected into the receiver is large. The contrary also holds true, i.e. when the pipe is well cemented, the amplitude of the received signal is small. The consequence of this approach, however, is that when the pipe is free, much of the energy bounces back and hence little energy, if any, is available to diagnose deeper into the wellbore, i.e. increasing radii. Similarly, when the pipe is well cemented, the initial amplitude received is small, and hence much of the energy is "absorbed" by the bond with little being able to return. This half-century-old approach has not yet developed quantitatively beyond the first casing-cement interface leaving other interfaces, including the very important cement-to-formation bond, with very subjective interpretations. It is fair to say that many empirical and qualitative approaches have been used to evaluate the cement in the entire borehole region, but none so far are able to provide quantitative and definite conclusions.
The acoustic corpuscular velocity considers that each molecule in the entire wellbore region is affected by the pressure wave, and hence behaves as a secondary source. As rays are formed in all directions, each ray has a new set of rays once an interface is encountered. The travel times of these rays can be determined using basic principles in wave propagation theory. This approach leads to a series of rays that constantly alter the pattern of the waveform created by the preceding rays. The Quintero Wellbore Index (QWI) uses the timing of these perturbations to create a quantifiable parameter that describes unequivocally the quality of the material present in every radius of the wellbore. The methodology provides for a robust advanced cement evaluation method that is applicable in single, dual, and multiple casing strings.
The paper explains the use of acoustic corpuscular velocity in wellbores, together with a derivation of instantaneous acoustic attributes by means of Fast Fourier Transform and Hilbert Transform. An example in a large diameter well from the Middle East is shown. The redundancy of indicators yields an error in the evaluation of the cement property for a second-string annular space of less than 1.5%, for a QWI of approximately 40% (+/- 1%). These results validate the use of the QWI. The QWI should provide for a much needed new quantitative metric for cement evaluation in wellbores.