The introduction of new ultrasonic and ratio logging techniques has dramatically transformed cement evaluation. Previous methods [cement bond logs (CBL's)] that used a low-frequency (appx. 20 × 103-cycle/sec [appx. 20-kHz]) acoustic signal and a single-transmitter/dual-receiver combination suffered from inherent measurement problems. To eliminate several of the CBL problems a new dual-transmitter/multireceiver acoustic measurement tool, the ratio bond tool (RBT), has been introduced. The RBT measures attenuation directly through ratiometric techniques, is self-calibrating, and is much less sensitive to tool centralization. For cement evaluation across "fast formation" intervals, a 1-ft [0.33-m] spaced transmitter/receiver signal amplitude is measured.
While techniques such as those implemented with the RBT tool improve on the CBL response, there are still inherent problems that could not be eliminated, such as sensitivity to microannuli and low sensitivity to local channels. An improved tool, the pulse echo tool (PET), uses ultrasonic frequency (appx. 500 X 103 cycles/sec [appx. 500 kHz]) to measure cement/casing bond and casing ID and wall thickness. The use of a double helix array of eight transducers, each independently measuring casing bond, ID, and wall thickness, allows detection of local channels in the cement and a detailed analysis of casing conditions. The radial measurement principle reduces the effect of formation signal and allows the attenuation of the resonant wave even in the presence of a small microannulus.
A "logging well" has been constructed by the U.S. Environmental Protection Agency (EPA) to evaluate downhole tool response to cement channels. This test well was constructed with numerous sizes and weights of casings and has specially constructed flaws on the casing circumference to produce channels of variable lengths and widths. A detailed analysis of the ability of the CBL, RBT, and PET logs to detect these channels is presented.
Cement evaluation logging tools are designed specifically to evaluate casing annular conditions and are constructed to make measurements that can be related to cement quantity, compressive strength, and bonding to pipe and formation, with the determination of cement isolation being the major objective of the log response. For oil and gas production purposes, fluid or gas migration in the annulus is economically undesirable, but in underground injection wells, it is imperative that there is no significant fluid movement into an underground source of drinking water through vertical channels adjacent to the wellbore. EPA underground injection control regulations require all Class II injection wells to demonstrate mechanical integrity before operations begin and at least once every 5 years thereafter.
In July 1981, a research project was founded by the EPA to establish guidelines for mechanical integrity verification. As part of this project, a special logging well was constructed to determine present industry capability to evaluate cement bond between the cement/casing and cement/formation in injection wells. Service companies were invited to run their cement evaluation logging tools and to provide a complete interpretation of the condition of the cement before leaving the job site. The logging tools used in the initial test were of two basic generations: the first was the CBL with a low-frequency acoustic signal and a single-transmitter/dual-receiver combination, and the second was an ultrasonic tool with eight transducers in a helical array. On the basis of the test results, logging procedures for determining mechanical integrity in injection wells were established.
Since the EPA test, new ratiometric logging techniques have been introduced that eliminate many of the inherent CBL problems. Calibrated CBL, RBT, and PET logs have been run on the logging well, and a detailed analysis of the three log responses to channels is presented. The results of this analysis should contribute to a better understanding of the ability of currently available technology to evaluate the cement integrity of the cased wellbore properly.