To complement the now traditional Cement Bond Log (CBL/VDL) a device called CET* (Cement Evaluation Tool) was commercially introduced in 1982. It features an array of eight ultrasonic transducers allowing a radial inspection of the casing and its annulus. Its response can be interpreted in terms of cementation quality. In particular, distinction between acceptable cement, unset slurry or liquid, or gaseous cement and free gas can be made. Analysis of the radial and vertical provides the basis for the identification of possible causes of poor cementation. With these indications primary cementing procedures can be controlled and improved. Also, procedures can be controlled and improved. Also, remedial cementing methods can be more rigorously adapted to individual situations.
The causes of poor cementation can be classification in two broad categories:
Poor mechanical systems when pumping the Poor mechanical systems when pumping the slurry. Namely, poor pipe centralization, washed out hole, incorrect pre-flush excessive local pressures. The characteristic consequences are incomplete mud removal or loss slurry into the formation.
Degradation of cement quality during curing. Until recently little attention has been given to the pressure phenomenon during curing. It is now acknowledged pressure phenomenon during curing. It is now acknowledged that the differential pressure between cement pore pressure and formation pressure is the cause of many cement pressure and formation pressure is the cause of many cement failures. Proper chemicals to counter these effects are becoming available. The characteristic consequences of an uncontrolled pressure history during curing as a cement quality degradation through both water loss and formation fluid intake.
With the advent of sophisticated and expensive primary cementing operations came the need for techniques to validate procedures and chemistries in situ.
Laboratory experiments have revealed a linear relationship between the logarithm of E1 the amplitude of the first arch of the CBL wavetrain and the percentage of pipe circumference not cemented, all other percentage of pipe circumference not cemented, all other conditions remaining constant. Also, when pipe is fully covered by at least 3/4 inch of cement, it has been shown that thee is a relationship between cement compressive strength and E1. Based on these data, the nomogram of figure 1 derives E1 the percentage of pipe circumference cemented also referred to as "Bond Index" in previous publications. The determination of E1 in free previous publications. The determination of E1 in free pipe and bonded pipe is valid for a well calibrated tool in pipe and bonded pipe is valid for a well calibrated tool in water. Recent investigations have shown that in other fluids, the raw E1 requires the correction reproduced on figure 1.
The CET wavetrains, similar to the one illustrated on figure 2, are firs normalized. The parameters useful for cement evaluation that are extracted for each transducer i (i=1, to 8) are:
W2Ni-The energy integrated and normalize din the time window W2.
W3Ni-The energy integrated and normalized in the time window W3.
Accurate internal pipe dimensions are also measured and are used to estimate the pipe wall thickness assuming a constant outside diameter. A built-in-pendulum references the radial position of each transducer to the high side of the pipe in deviated wells.