High-Resolution Cementation And Corrosion Imaging By Ultrasound

The UltraSonic Imager (USI) tool is a new wireline tool for cement evaluation and corrosion detection by ultrasonics. A rotating ultrasonic transducer gives full coverage of the casing at high resolution. The principle is similar to existing ultrasonic tools: the transducer emits a short pulse which excites a thickness-mode resonance in the casing. Analysis of the echo gives four measurements: internal radius, rugosity, casing thickness and cement acoustic impedance. The cement impedance indicates cement quality and presence. Good cement has higher impedance than poor cement or drilling fluids. The internal radius and rugosity are derived from the travel time and amplitude of the main echo using a technique that eliminates cycle-skipping problems. The casing thickness and cement impedance are measured by a novel signal-processing algorithm that matches a theoretical model to the measured resonance in the frequency domain, The processing corrects for the effects of casing thickness variations, mud attenuation and transducer variations on the cement impedance. Mud variations are compensated by measuring the mud velocity and impedance downhole. Field results show that channeling, contaminated cement, light cement and gas can be diagnosed and that external hardware such as centralizers can be detected. The corrosion measurements can detect mechanical wear, corrosion and deposits.