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For purposes of cement evaluation, the CAST tool evaluates the type of material in the <br />annulus adjacent to the outside of the casing. This is accomplished by evaluating the <br />acoustic impedance as measured by the decay of an ultrasonic frequency signal imposed <br />upon the casing. The rotating head provides a high resolution measurement around the <br />azimuth of the wellbore. CAST interpretation focuses on the variability of the measured <br />acoustic impedance. Liquids behind the casing will have flat consistent readings with <br />low variability on the acoustic impedance whereas cement will have higher variability. <br />Schlumberger IBC (Image Behind Casing) Log <br />Much like the Halliburton CAST log, this log utilizes an ultra -sonic technology that <br />allows determination of the acoustic impedance of the material behind the casing string. <br />Based on a second set of transducers that impart an angled signal to the wellbore, the <br />flexural attenuation is also measured. Cross plotting the acoustic impedance with the <br />flexural attenuation and comparing the data set against a known model allows one to <br />distinguish gas from liquid and both of these from solid. The log also presents a <br />hydraulic communication map based upon the identified material behind casing. <br />CBL/VDL (Cement Bond Log, Variable Density Log) <br />CBL/VDL measures the quality of the cement sheath adjacent to the casing by measuring <br />the amplitude of the first arrival of a 3 ft spaced transmitter- receiver 20 khz sonic signal. <br />The transit time of the first arrival is used for log quality control. When casing is well <br />cemented, this signal is attenuated and thus reduced in amplitude in comparison to a <br />signal in un- cemented free pipe. By knowing the attenuation in free pipe and in 100% <br />bonded pipe for a given cement type, a bond index that represents the percent of bonded <br />pipe around the circumference of the wellbore can be calculated. Since light cements and <br />foamed cements behind casing attenuate the signal much less than regular cements, <br />calibration of the amplitude readings and thus the bond index becomes extremely critical. <br />The VDL displays the 5 ft transmitter- receiver wave form with the darker images <br />indicating higher amplitudes and lighter showing lower amplitudes. The appearance of <br />formation arrivals as strong variable readings in the middle of the display indicates that <br />the cement to formation bond is strong and therefore acoustically coupled. Formation <br />arrivals can be recognized in contrast to the straight "ringing" casing arrivals near the left <br />hand side of the VDL display. The section below outlines my determination of bond <br />index for the TM -2, TM -3, TM -5 and TM -6 wells. <br />CALIBRATION OF BOND INDEX WITH FOAMED CEMENT <br />All of the Cement Bond Logs discussed below were run on wells with 4 l inch casing. <br />In order to convert the amplitude readings to a more definitive assessment of bond <br />quality, I have converted the amplitude (mV) responses to attenuation (dB /ft) and <br />calculated the equivalent bond index for various amplitudes. For the 0% bonding (free <br />pipe) I used an amplitude reading of 80 mV. For the 100% bonded pipe reading, I <br />converted cement compressive strength to attenuation based on the data shown in SPE <br />