Despite its potential, the cement bond log is probably one of the most abused, misused, and misunderstood logs used in the oil field today. Miscalibration, inadequate information, and a severe lack of standardization are enough to push petroleum engineers into a morass of bewilderment.
Well cementing technology in both relatively straight and high-angle directional holes has advanced dramatically since the first casing was cemented in 1903. Besides the everyday cementing needs in "problem-free" boreholes, recent engineered improvements successfully deal with cementing of arctic wells, ultradeep and hot holes, water-sensitive formations, and proper placement opposite incompetent, fractured, or proper placement opposite incompetent, fractured, or highly permeable formations. The basic requirements for obtaining a successful primary cement job have been known for years. Good primary cement job have been known for years. Good design characteristics are based on a knowledge of formation, cement, and pipe properties, and controlled placement techniques that consider fracture gradients. Also important is an understanding of
minimum practical mud density and viscosity,
turbulent flow conditions,
the optimum size of preflushes,
centralizing of casing, the use of scratchers, and the handling of pipe, and
the proper choice of casing.
For more than a decade now, the oil industry has used wireline well logging, such as cement bond logging, to detect the presence of cement behind pipe and to evaluate the bond of the cement to both the casing and the formation. The validity of Cement Bond Log (CBL) interpretation has been a subject of controversy since its introduction; and the CBL, despite its great potential, is probably one of the most abused, misused, and misunderstood logs run in the oil field today. To make matters worse, tools run by service companies use various gating systems, spacings, frequencies, etc. This lack of standardization, in addition to poor sonde centering, miscalibration of tools, and inadequate information on log headings, has more than once confused unsuspecting petroleum engineers. In the present discussion, CBL's are reviewed as to the information obtainable and as to how they are interpreted. Comparative field tests and specific observations illustrate some of the pitfalls and possible misinterpretations if logging operations are not designed properly or run correctly. The CBL, if properly run and interpreted, is an efficient aid in estimating cement bond quality. Usually the log consists of an amplitude curve measuring a specific part of the acoustic signal; and since interpretation of the amplitude curve alone may be inconclusive and misleading, supplemental data are normally included. The latter may be one or more of the following:
transit time to the first event of the acoustic signal reaching a minimum or predetermined amplitude,
amplitude of the formation predetermined amplitude,
amplitude of the formation signal,
variable intensity, and
oscilloscope pictures. Additional measurements, although not pictures.
Additional measurements, although not directly related to cement bonding, can also be included on the CBL. These usually include the gamma ray curve and casing collar log.