Advanced Ultrasonic Scanning Tool and Evaluation Methods Improve and Standardize Casing Inspection

Abstract

Abstract Evaluating the inner and outer condition of casing has been problematic. Typical casing inspection logging tools such as calipers, phase-shift tools, and flux-leakage devices often have been unable to function mechanically in larger size casing. Additionally, small pipe defects can go unnoticed due to poor azimuthal resolution of the measurements or to the averaging techniques, in both casing and risers, using previous casing inspection methods. The high horizontal (radial) and vertical sampling rates of ultrasonic devices can provide precise casing inspection for most casing sizes. Use of a navigation package permits data to be corrected for orientation and tool rotation. Data such as casing inner diameter and radius, casing ovality, and 40 to 200 caliper traces may be recorded in real time in conventional or imaging log formats, permit monitoring over time. This allows easier monitoring of the deterioration in specific regions of the casing. With the use of risers for sub-sea trees and large casing in the deep offshore arena, the problems of pipe inspection are escalated. Riser monitoring represents one of the unique and economically important applications of this tool. Previous attempts at using conventionally configured ultrasonic scanning tools to evaluate large casing have been unsuccessful because the significant distance between the tool transducer and the casing wall causes the reflected signals to be highly attenuated and virtually unusable. Therefore, a new large-radius adjustable scanning head was developed for use with the ultrasonic scanning tool. Its inclusion allows the positioning of an ultrasonic transducer at an optimum distance from an inspection target, such as the wall of a casing or a marine riser, to provide consistent and accurate internal measurements in large-diameter casing. This tool and head combination has successfully monitored risers with an internal diameter larger than 19 1/2 inches. New real-time and post processing analytical procedures correct for tool position and are required to accurately detect and measure casing wear. These processes will provide a "quality grade" for each joint of downhole casing. Improved processing software provides two- and three-dimensional images, a tabular listing of evaluation results, automatic counting of joints, and segmented data plots. Several field logs illustrate a range of wellbore conditions and casing damage. The ultrasonic scanning tool provides both quantitative and qualitative evaluation and diagnosis of casing problems. Ultrasonic Tool Theory Downhole ultrasonic scanners have been used primarily in openhole imaging applications. These ultrasonic tools acoustically scan the circumference of the wellbore or casing to furnish improved images. Improved tool design, telemetry, and waveform analysis provide remarkable borehole and casing inspection and cement evaluation capabilities. The new scanning tool described in this paper utilizes extremely high vertical and horizontal sampling rates that are necessary to detect small casing deformities. The ultra-sonic imaging tool uses two ultrasonic transducers. The primary transducer is mounted in a rotating scanner head that transmits ultrasonic signals and receives reflections from the casing or formation. The secondary transducer is secured in a fixed position with the signals reflecting from a flat target plate and provides data concerning the wellbore fluid travel time1. The two-way travel time of this signal, which is measured throughout logging, is used to compute the acoustic slowness of the borehole fluid and is required to convert time to distance. This fluid travel time is required for the internal diameter and impedance calculations and is used in quality control determinations.