Cement Imaging Through Ultrasonics
Authors: Uswak G. , McLafferty N. 
Published: March 2010

A new development in ultrasonics offers improved hydraulic isolation determination through high resolution acoustic impedance imaging providing 360 degree coverage. Additionally, casing thickness and geometry images provide casing inspection data.

The measurement is achieved by using a unique rotating transducer assembly and improved processing methods to analyse digitized waveforms. This data is acquired, processed and colour images presented by powerful mini-computer equipped field units. The tool's rotating transducer acts as both transmiytter and receiver, and permits full casing coverage with fine vertical resolution.

The reflected waveforms are digitized and processed using a physical model to measure wellbore fluid acoustic impedance and acoustic impedance of annulus materials. A vertical resolution of 38 mm and azimuthal resolution of 5 degrees is achieved. Additionally, the model determines casing diameter and thickness.

In several wells, this new technique successfully discriminated cement placement and characteristics with significant improvements over current bond log and pulse-echo technology methods. This new impedance measurement is combined with traditional cement bond log measurements enabling a novel method of predicting hydraulic isolation.

Introduction

The ultrasonic imager (USI) tool is a second generation logging device. providing full azimuthal coverage for both the casing and cement sheath evaluation. It use a single rotating transducer measurement system and a novel full waveform processing technique which employs a three-dimensional physical model.

Cement quality and casing condition are determined using ultra high frequency acoustic pubes. They are presented as high resolution colour images. The acoustic impedance colour image along and around the casing are interpreted in terms of cement distribution and hardness.

The principle of measurement of the USI tool was described by Hayman et al.(1) and can be summarized as shown in Figure I. The transducer fires an ultrasonic burst that travels through the liquid and into the casing wall, resonating the casing in the thickness mode over a small spot. The returning waveform is the summation of the echo waveform from the original burst, and an exponentially decaying waveform from the resonant energy trapped between the inner and outer casing walls. The radius is determined from the measured time for this echo to return. Casing thickness is determined from the value of the fundamental frequency ill the exponentially decaying part or the waveform. Finally, acoustic impedance is determined from the rate at which this fundamentaldecays. Variations in the rate of decay of this waveform represent changes in the acoustic impedance contrast at the casing-amulus inlerface the harder the annular material, the faster the decay.

To determine the radius, thickness and acoustic impedance a three dimensional model solution is filled to this full waveform data. As a departure from the time-domain analyses of earlier generation tools, resonance decay is characterized in the frequency domain on the group delay spectrum of the discreet Fourier transform (Figure 2).

Hayman et al, have described that the advantages of this process, called T3 processing, derive primarily from the aspect that only the fundamental mode of resonance is analysed.