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I94 ACOUSTIC K'ELL LOGGING <br />A CBT (Cement Bond Tool) has been developed specifically to evaluate cemented <br />casines so as to eliminate the deficiencies of the conventional CBL tool, i.e. sensitivity to <br />centerine, fluid attenuation, receiver sensitivity, temperature drifts and calibration. This <br />?'/," sonic tool provides a BHC ratio attenuation measurement with two receivers <br />symmetrically located between an upper and lover transmitter. The measurement is self <br />calibrating and not affected by mud attenuation. <br />The CBL is used to: <br />(a) Locate [he top of the cement. <br />(b) Determine the qualiry o(the casine-cement bond. <br />(c) Check the effectiveness of cement injections. <br />Id) Check damaee caused by very hieh pressure inside the casine. <br />(e) Evaluate different cementing techniques. <br />5.3.].2. Variable densih• recordinc <br />The recordine of the complete +vace train, as cxn be seen in Fie. 5.7, is of ereat interest in <br />cases that are difficult to interpret because the information is much richer. It is usually <br />done +vi[h a receiver located 5' from the transmitter. The complete or upper half of the <br />sienal may be recorded every foot or nvo feet. A Variable Density Log i1'DL), <br />Srh/umher;er trademark. is also used in +vhich the positive part of the sienal prin[s a <br />rectilinear plot which is all the blacker as the sienal is stronger. In principle therefore, it is <br />possible to make a clearcut distinction ben+een arches corresponding to casine arrivals <br />and those corresponding to formation or drilling-mud arrivals. <br />Figures ~.8a and b show interpreted VDL logs. <br />Figure S.Sa is a VDL in an uncentented or free casine. Strong signals can be seen due to <br />the casine. and +veak sienals due to the formation. The casing joints stand out clearly. <br />Fieure 5.8b represents a VDL run in a cemented casing. The sienals from the casine are <br />+veak but the sienals from the formation are strong, indicating good casing-cement and <br />cement-formation bonds. <br />Recordings of complete +vave trains of the VDL t}pe are thus a precious auxiliary for <br />CBL in doubtful cases. <br />5.3.2. High-frequency sonic beam <br />5.3.2.1. Cement Evaluation Tool (CET) <br />A tool built by Sclilunlberger uses a radial ultrasonic beam. This tool has eight <br />ultrasonic transducers housed in a c}'lindrical body in a spiral pattern at ~SS° intervals. The <br />casing is separated from ttte transducer faces by 2 to 5 cm (I to ?"). The wave thus passes <br />back and forth through the Iluid contained in the borehole. The pulse emitted corresponds <br />to a frequency band o(300 to 600 kHz. Ttte pulse reflected by thecasing is tlteconvolution <br />of the emitted pulse and the pulse response of the casing. In practice, a measurement <br />window is selected to obtain maximum contrast between the free and properly cemented <br />casine. A t}'pical diagram produced is slto++'n in Fig. i.9. <br />ie <br />I <br />r <br />strong <br />Cos'~nq -~ <br />s~gnol <br />Csq. collar { <br />Weak - <br />Formalion -= <br />Siq nal - <br />Csq. Collor { <br />