A New and Innovative High Frequency Ultrasonic Method for Evaluating Cement Bond and Casing Thickness in Wells with Fiberglass Casing Applied in the French Paris Basin.

Abstract

A new geothermal pilot project has been initiated in the Greater Paris region of France in an effort to promote renewable energy. This work is partially subsidized by the French government and is operated by Semhach Energy. One of the objectives of this effort is to reduce long-term costs for geothermal wells, which in this case incorporate both water injectors and hot water producers with 7-in. fiberglass casing inside an outer 9-5/8-in. steel casing. In order to make this project a success and comply with mining regulations, well integrity must be monitored and maintained immediately after completion and throughout future operations. The use of fiberglass represents an extreme challenge for the evaluation of casing integrity as well as the known associated challenges of cement bond, casing thickness, internal surface condition, and the casing outer diameter.

Many difficulties are involved when evaluating wells with fiberglass casing. Ultrasonic scanning tools typically obtain the cement acoustic impedance in steel casing by inducing a mechanical resonance in the casing structure and then measuring the signal decay. The thickness is determined by applying a Fourier Transform to the frequency of the mechanical resonance. Due to the low sound velocity properties of fiberglass and its resonant frequency, which is below the band limits of typical ultrasonic scanning tools, this method fails. The method of standard cement bond log tools, where the attenuation rate of the first arrival acoustic pulse is measured, also fails in fiberglass casing. This is because the waves traveling through the cement and external steel casing outrun and interfere with the fiberglass arrival pulses, making it nearly impossible to isolate them for use with an attenuation measurement.

To overcome these obstacles, a new and innovative method has been developed and tested for evaluating casing thickness and cement bond quality. The cement acoustic impedance in fiberglass casing is measured using a specially configured high frequency ultrasonic scanner system. The new technique uses the ratio of amplitudes between the echoes reflected from the inner and outer diameters of the casing, thus rendering a quantitative measure of the cement impedance with the same accuracy and resolution as the measurement performed with standard API casings. In addition, the difference in time between the echoes is used to measure the casing thickness. Laboratory test results, along with actual log measurements performed in the French Paris Basin, are provided to confirm the new methodology.