Cement-Bond Evaluation: A Step Change in Capabilities

Abstract

A new, small-diameter ultrasonic scanning logging tool has been field tested and commercially deployed for simultaneous cement-sheath and casing evaluation. The tool is designed for use in 9-5/8-in„ heavy-weight casing down to 4-1/2-in. casing. Enhancements in downhole electronics, computing capabilities, and telemetry transmission allow the new circumferential acoustic scanning tool to operate on 7/32-in., 5/16-in., and 9/32-in. mono-conductor e-line cables, which allow for the data acquisition to be performed off a small foot print cased-hole unit with a smaller, lighter tool at the same logging speed as previous generation tools run off a seven-conductor e-line cable on an open-hole unit. The enhanced portability and evaluation capability in casings as small as 4-1/2 in. make the system ideally suited for land operations. Analysis of the cement data is accomplished with the same statistical variance process that has been widely used over the years to analyze data provided by all ultrasonic logging tools available in the industry, resulting in a robust and easy-to-interpret computed product in traditional, light-weight, and foam cement. The cement-evaluation process uses the measured acoustic impedance of the material in the casing-formation annulus. The casing's internal diameter is obtained from the twoway travel time and the measured borehole-fluid travel time. The casing-wall thickness is determined from the frequency content of the reflected waveform data. Both internal diameter and wall thickness are used to determine internal and external casing wear. Legacy ultrasonic scanning tools could not provide quality data in small-diameter casing because of tool size and tool-fishing constraints and near-field transducer ring-down effects. A new, interactive job-planner program has been developed to optimize data acquisition for casing size, weight, and borehole fluids. The planner determines the number of shots per scan and maximum logging speed required for the tool to provide 100% circumferential coverage without sector gaps or interpolated data. Examples comparing the multi-conductor circumferential acoustic scanning tool and the mono-conductor circumferential acoustic scanning tool in conventional and nonconventional cement slurries are presented.