Summary

Simulations of cement bond logging (CBL) have shown that wellbore fluid effects can be segregated from sonic-signal response to changing cement strengths. Traditionally, the effects have been considered negligible and the CBL's have been interpreted as if water were in the wellbore. However, large variations in CBL's have become apparent with the increasing number of logs run in completion fluids, such as CaCl2, ZnBr2, and CaBr2, To study wellbore fluid effects, physical and numerical models were developed that simulated the wellbore geometry. Measurements were conducted in 5-, 7-, and 9 5/8-in. casings for a range of wellbore fluid types and for both densities and viscosities. Parallel numerical modeling used similar parameters. Results show that bond-log amplitudes varied dramatically with the wellbore fluid acoustic impedance--i.e., there was a 70% increase in signal amplitudes for 11.5 lbm/gal (1370-kg/m3) CaCl2 over the signal amplitude in water. This led to the development of a fluid-compensated bond log that corrects the amplitude for acoustic impedance of various wellbore fluids, thereby making the measurements more directly related to the cement quality.

Introduction

Traditionally, CBL interpretation is based on the acoustic signal contrast of free pipe (Fig. 1) and 100% cemented casing in the wellbore (Fig. 2) for calibrated tools, under standard calibration conditions of temperature, pressure, and water in the borehole. An experimental and numerical wave-mechanics study of CBL's indicated that the recorded bond-log amplitude is affected by wellbore conditions. These effects must be identified and compensated for to obtain an amplitude signal that truly represents the cement quality. Acoustic-wave generation, propagation, and reception depend on the surrounding environment. The oilfield environment presents challenging conditions for cement bond logging because of the occasional absence of free pipe (e.g., in the case of a liner) and the custom-tailored pipe (e.g., in the case of a liner) and the custom-tailored cement slurries with variable compressive strengths. The variety of wellbore-fluid types, densities, hydrostatic pressures, and temperatures further complicates the pressures, and temperatures further complicates the measurement. These conditions are manifested throughout the measurement process-ranging from the transmitter's output pulse to the actual wave propagation in the wellbore and casing, and finally to the reception mode. The following is a summary of the primary effects. 1. The transmitter's output power depends on the acoustic impedance (fluid density and acoustic velocity) of the surrounding medium (wellbore fluid). 2. Attenuation and transmission of sound waves in the wellbore depend on the viscous damping and scattering by the wellbore fluid. 3. Acoustic wave transmission and reflection at the casing interface depend on the acoustic-impedance difference between the wellbore fluids and the casing. 4. Attenuation of the casing vibrational mode will depend not only on the cement but also to a small degree on the wellbore fluid properties. 5. The receiver's characteristics are influenced in the same manner as those of the transmitter. 6. All the above conditions vary with depth, pressure, and temperature. As can be seen, the true cement quality and the degree of isolation can be masked by any one of the variations. The misinterpretation of the bond logs that results could impair judgment regarding hydraulic isolation and thereby lead to high squeeze costs and isolation failures. Having recognized these various environmental effects associated with CBL'S, the task is to develop a complete interpretation package that will identify and compensate for most variations. The package will have to relate all variables through a set of software transformations at the wellsite and normalize them to the current interpretation charts. Experimental and numerical models were developed to address these problems. Most of the work has been completed, and the rest of the study will be finished this year. A preview of the work completed so far is demonstrated in Figs. 3 and 4, which show variations of wellbore-fluid effects on the amplitude signal. The temperature and pressure relations for both types of transducers currently used pressure relations for both types of transducers currently used are shown in Figs. 5 and 6. These effects are very large by any standards. They must be recognized and compensated for so that the log response will accurately represent cement quality and the degree of isolation.

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