Schlumberger

Technical Paper: New-Generation Ultrasonic Measurements for Quantitative Cement Evaluation in Heavy Muds and Thick-Wall Casings

Society: SPE
Paper Number: 181450
Presentation Date: 2016
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Abstract

In this paper, we describe major improvements to ultrasonic logging techniques used for cement evaluation. Specifically, we explain engineering solutions developed to tackle known limitations such as logging in heavy (ultrasonically attenuative) muds, and very large and very thick pipes.

On the hardware side, new heavy mud/thick casing transducers are presented together with dedicated large-diameter logging heads for both pulse-echo and pitch-catch (flexural attenuation) techniques. On the processing side, we present a novel pulse-echo inversion scheme, based on real time inversion using accurate 3D models for metrology accuracy and a new scheme that does not require a prior knowledge of mud properties (specifically its acoustic impedance, Zmud). Through this, Zmud becomes an output parameter of this novel processing and we present field examples of such logs where we now image features such as mud deposition and segregation in deviated pipes. This new processing enables easier and more accurate interpretation of the cement sheath together with critical information on the logging fluid. The validity of the measurement across the operating range is demonstrated in this paper through results from both high precision laboratory measurements and several logs acquired during the last few years' field test campaign. Together with the fundamental sensor and processing improvements, we present engineering changes to the logging platform, enabling up to three times faster logging and accurate well site logs delivered right after rig down. Finally, we demonstrate that the combination of the pulse-echo and pitch-catch techniques, with its high azimuthal resolution, can probe the annulus up to the cement/formation or cement/second casing interface, making it a powerful technique for cement evaluation.

Previous limitations (such as mud weight, casing size and thickness) have been overcome and this has been fully demonstrated in both the laboratory and the field. Thus ultrasonic cement evaluation techniques now enable accurate and critical decisions to be made–from well construction to plugging and abandonment–in all conditions, casing sizes, and mud and cement types.

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