Recent changes in guidelines for isolation of potentialflow zones during well construction present newchallenges for cement bond logging (CBL) tools. Onechallenge is cement evaluation logging in large thickwalledcasing, i.e., casing with an outside diameter(OD) of 14 in or larger and a wall thickness of 0.7 in orthicker. To address the task, a pad-based segmentedCBL tool can be configured to provide the cementevaluation in these conditions, providing a uniqueability to produce a 60-degree attenuation map in casingas large as 24 in OD. To manage the large casingrequirements, a detailed study of the thick casingresponse is performed for the cemented and free pipe.The study includes numerical modeling as well asempirical data analysis, and results in the compilationof a comprehensive tool response chart for thedemanding thick-walled casing environment. The datain the chart is further verified using the tool loggingresponse in real case studies.Larger, thicker casing comprises a reduced cementresponse, e. g., the difference between the free-pipe andcemented pipe, putting greater requirements on allcement evaluation systems. Extant CBL and pulse-echodevices rely on the internal casing fluid to enabletransfer to and reception of acoustic energy from thecasing or “mud path”. The energy flow in the presenceof large, thick-walled casing can be problematicbecause of the geometric considerations such ascentralization and the mud attenuation effects from thelonger travel path in larger casing sizes. A padmountedtransducer system on articulating arms placesthe transducers into direct contact with the casing,eliminating the “mud path” and mud attenuation effects.The pad approach and compensated attenuationmeasurement adopted in the pad-based systemaccomplishes the following:1 Attenuation effects of the mud path arevirtually eliminated, preserving theavailable signal energy for measuring theeffects on the signal by annular cement.2 Dispersion effects on the acoustic signalsare minimized because the pads are indirect contact with the casing.3 Compensated measurement enablesremoval of individual transducersensitivity and need for calibration; itcancels measurement discrepanciesbecause of changes in transducer response(both transmitters and receivers) withtemperature and pressure.The paper outlines the results of the numerical andempirical CBL study, demonstrates the loggingresponses and discusses the CBL challenges in largecasing.
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