The internal and external casing condition is an important part of well integrity evaluation. The public is concerned with environmental issues at sea and on land. In areas where shale exploitation is in full swing, the protection of the underground aquifers is of concern. Tools with increased resolution in corrosion and thickness evaluation are being upgraded to meet these challenges.
Ultrasonic scanning tools measure casing thickness by inducing a resonance in the casing in the thickness-mode. At the same time, they measure the cement acoustic impedance at higher radial and vertical resolution than standard CBL and sector tools. Traditionally, the thickness measurement is delivered after computing the resonant frequency of the pipe from the raw waveforms telemetered to the surface via the communication system. This method is fine except there is a time delay between the acquisition and the final delivered thickness log and map. Another disadvantage of this method is that in order to cover the pipe circumference for thickness, the logging speed needs to be reduced during acquisition.
A modified Digital Fourier Transform algorithm implemented in the ultrasonic radial scanner tool improves the circumferential and vertical coverage over previous methods while extending the thickness measurement range, resolution and accuracy. External corrosion is evaluated by ultrasonically measuring the pipe ID and adding it to the thickness. The algorithm is performed by the down-hole tool at high speed and resolution, covering the pipe in both circumferential and vertical dimensions, in real time and at normal logging speeds, thus saving operation and rig time.
Log examples from test wells constructed to simulate internal and external corrosion, as well as results from field testing corroborate the methodology and the algorithm. The ultrasonic radial scanner with real time thickness (RTT) capability gives a higher resolution thickness measurement and an improved internal and external corrosion evaluation.