Cement Evaluation Tool: A New Approach to Cement Evaluation (1982)

Summary A new generation of cement and casing evaluation tools is being introduced with construction permitting detailed examination of cement behind casing. Classical cement bond logging systems measure amplitude or attenuation of plan waves propagating axially along the casing. The design described in this paper exploits the principle of casing thickness resonance to overcome previous tool limitations. Applications and results from field tests illustrate the advantages of this approach. Cement bond logging achieves its greatest utility when it provides the production engineer with precise indications of cement strength and distribution around the casing. Zone isolation is of critical importance in production. Previous logging systems have yielded measures of cement bond that were circumferential averages of cement quality. These were difficult to interpret. Additionally, they were sensitive to the degree of shear coupling between pipe, cement, and formation and thus were affected by microannulus. The cement evaluation tool (CET) described here overcomes these difficulties. It provides a measurement of cement presence and strength, which is largely insensitive to microannulus. Its log output is interpreted easily. Tool design allows examination of the casing circumferentially at each depth. Impedance behind casing is measured. Laboratory calibration measurements allow this to be presented in terms of cement compressive strength. Cement channels are distinguished easily, and a zone isolation indicator can be presented. Additionally, casing internal diameter and distortion are displayed. European and North American field tests have been completed, and performance for a variety of well conditions is discussed. The ability of the tool to identify channels is confirmed. Sequential runs with and without excess pressure demonstrate immunity to microannulus in cases where CBL is affected but where microannulus is small enough to prohibit hydraulic communication. Geometrical measurements have been good indicators of casing deformation and have identified casing corrosion and wear. Introduction Conventional cement bond logs (CBL's) are run in cased hole using a sonic logging tool in a single receiver mode (or a simplified sonde specially designed for CBL). The transmitter and receiver are more or less omni-directional, with an operating frequency around 20 kHz. The most rapid wave path from transmitter to receiver is a plate-type mode in the casing with a compressional wave in the mud path from transmitter to casing and casing to receiver. At this frequency, the casing is very thin with respect to the wavelength, and the velocity is close to the compressional velocity in steel. Attenuation of this wave depends mainly on loss of energy to the materials on either side of the casing (mud, cement) and since the particle movement at the surfaces of the casing is predominantly parallel to the casing axis, more loss occurs through shear coupling to a solid medium (cement) than to a liquid (mud). Thus, the amplitude of the first arrival at the receiver is indicative of the presence of cement around the casing: low amplitude indicates cement, and high amplitude indicates free pipe. There are several drawbacks to this system.The omnidirectional characteristics imply good centralization to ensure simultaneous first arrivals from all azimuths.The omnidirectional characteristics make it difficult to distinguish high-strength cement with a channel (hydraulic communication) from an even distribution of low-strength cement (no communication) because the amplitude may be the same in both cases.The necessity for good shear coupling to the cement implies practically free pipe amplitude in the case of a microannulus.In hard formations where the compressional velocity is higher than the plate mode velocity in steel, the first arrival is no longer the casing wave, and the calibration is no longer valid. JPT P. 1835^