A Practical Approach to the Interpretation of Cement Bond Logs

Abstract

Bigelow, E.L., SPE, Gulf Oil E and P Co. Summary The cement bond log has been controversial since its inception. Despite its potential, it is possibly the most maligned logging service available to the industry. Effective zone isolation between permeable intervals in a well requires a cement sheath over an appreciable vertical interval. It is necessary for the annular cement sheath to provide an effective hydraulic seal to withstand subsequent completion and production operations. The oil industry has used wireline well logs to detect the presence or absence of cement behind pipe for more than 20 years. Users have attempted, not always successfully, to evaluate the effectiveness of cement bond to both pipe and formation with cement bond logs. Cement bond logs do not mislead. Poor interpretation habits mislead. Knowledge of the well completion and the inherent physical restraints placed on the log measurements is needed to evaluate the log properly. The purpose here is to dispel some of the myths created by misguided interpretation practices. Examples of cement bond logs that fall into this category will be presented. presented. Introduction The interpretation of cement bond logs is controversial for three primary reasons:dependence on and oversimplified use of the pipe amplitude curve,lack of understanding of the full acoustic waveform, andfailure to compare the physical restraints of the well completion to the log measurements. Most misinterpretations are caused by any or all of these reasons. As with any logging service, the analyst must understand each measurement-not only how it is made, but also its accuracy or inaccuracy under different circumstances. Attempts to quantify cement compressive strength from pipe amplitude measurements are commonly met with pipe amplitude measurements are commonly met with skepticism. No argument will be found here. Borehole conditions must be nearly ideal to measure attenuation rates with enough accuracy to arrive at meaningful calculations of cement compressive strength. Determination of the bond index from attenuation rates is subject to the same rigid judgment of borehole conditions. The user does not need an extensive background in acoustics to acquire a practical understanding of the full acoustic waveform as presented on modern-day logs. Its interpretation is simple, although qualitative. Several service contractors present a single-receiver travel time curve. This measurement may have more potential in the evaluation of cement integrity than the pipe potential in the evaluation of cement integrity than the pipe amplitude curve. Comparison of the full waveform to logs that are sensitive to porosity/lithology can help to evaluate the cement bond log. Caliper information from the open hole is important. Knowledge of the well mechanics and facts about, as well as conditions during, the cement job is extremely important in evaluating the measurements from cement bond logs. The primary purpose for using cement bond logs in the oil industry is to determinethe presence or absence of cement over certain depth intervals, andwhether cement is bonded to the pipe, the formation, or both. A quantitative evaluation should be made whenever conditions make it possible. When conducive conditions are not present, reasonable evaluations of cement integrity can be made using rational, intelligent qualitative methods. The Acoustic Measuring System The transmitter is the heart of the system. It is alternately switched on and off at a fixed rate. When energized by a transducer, which causes a sudden physical change in size, the transmitter vibrates, which in turn produces elastic compressional waves. These vibrational waves are acoustically coupled from the transmitter through a special fluid to a special retaining sleeve, which transfers the energy to the borehole fluid, propagating waves spherically in all directions from the transmitter. Compressional waves are propagated down the sleeve of the instrument, vertically through the borehole fluid, and horizontally across the bottomhole fluid. For cement bond log purposes, the wave front that moves directly to the casing is of primary interest. As it impinges on the casing, some energy is reflected, while the balance is transferred into the steel, the cement sheath, and the formation. See Fig. 1. All materials surrounding the transmitter have been caused to vibrate. These materials allow the passage of sound waves through them in the form of elastic waves. Each type of material exhibits its characteristic effects on the elastic waves. These effects influence wave velocity amplitude, and frequency. The receiver operates essentially as a transmitter in reverse fashion. Passage of waves from the transmitter through mud, casing, cement, formation, and back to the receiver alters the compressional waves. JPT July 1985 p. 1285