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`g's~~0 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY ,,..,_ ~ REGION 8 , ~~~/`J ~ 99S118TM STREET -SUITE 300 GENVER, CO 80202.2466 http://www.epa.gov/regi on OB January 17, 2001 CEMENT EVALUATION NOTES Compiled for the MIT Workgroup by Jerry T. Thornhill USEPA, Robert S. Kerr Research Lab. Edited by Paul S. Osborne USEPA, Region VIII Background-Acoustic Cement Bond Logging The Reasons for cementing wells aze: 1) to support the casing; and 2) to isolate zones (hydraulic seal), such as producing horizons, injection reservoirs, and underground sources of drinking water (USDW). When a well is completed, a cementing record will be submitted as part of the well completion record. This information will riot address the question regarding the adequacy of the cement to isolate the various zones. One of the methods utilized to assess the adequacy of the cementing of a well to isolate the various zones is by using an acoustic cement bond log (CBL). Although an acoustic cement bond logs does not directly measure hydraulic seal, the measured bonding qualities do provide inferences of sealing adequacy (zone isolation). The bonding of cement to the casing can be: measured quantitatively using a CBL. The bonding of cement to the formation, however cannot be measured quantitatively using a CBL, but it does provide a qualitative estimate of the bonding to the formation. Determination of cement integrity is accomplished by an analysis of the full acoustic waveform, the amplitude or attenuation rates of the casing arrivals, and a single receiver travel-time measurement. The Acoustic CBL tool used to mab:e the cement bond log puts energy into the well and measures the energy returned. The operating frequency for all conventional instruments is 20 kHz. The time it takes for energy to return and the amplitude of the returned energy are detemuned by the cement bonding. Elastic compressional waves are propagated down the sleeve of the instrument, vertically through the borehole fluid, and horizontally across the borehole fluid. Of primary interest to the CBL log is the wavefront moving directly toward the casing. As the wave front impinges upon the casing, some energy is reflected, while the balance is transferred into the steel, the cement sheath and the formation. Acoustic energy propagates through fluid at about 180-220 microseconds per foot, and about 57 microseconds per foot through steel. At each of these interfaces, some energy will be reflected, and some will be transferred into the adjoining medium. The reflected waves coming back from the various Printed on Recycled Paper