A Study in Ultra-Thin Cement Sheath Evaluation

Abstract

Reliable evaluation of the cement-bonding and identification of isolation zones of a cased-hole well are challenging problems. The CBL tool can provide a cement map for identifying cement top and cement channels of a well. Ultrasonic tools have been developed and deployed to conduct pitch-catch and/or pulse-echo measurements for cement evaluation at high spatial resolutions. However, there are limitations in tools based on pitch-catch or pulse-echo measurements, particularly for ultra-thin cement sheath. This paper presents a systematically study of effects on responses of sonic and ultrasonic cement evaluation tools caused by cement sheath thickness and propose a workflow for the reliable evaluation of ultra-thin cement sheath. To quantify limitations of these types of cement evaluation tools, a systematically study has been conducted to evaluate effects caused by casing annulus spacing on tool responses. The test fixtures with varied annulus spacings were designed, constructed, and cemented. Multiple tests were designed and conducted to verify the annulus spacing effects on amplitudes of arrival wave peaks, the inverted acoustic impedance of casing annulus, attenuation rates of waveforms and quantify the annulus spacing size limits that cause the failures of tools. The tests results confirmed the attenuation rates of shear horizontal wave measurements will not be influenced by casing annulus if it is greater than 0.4 inch. The cement sheath must be greater than 0.75 inch if a CBL tool is utilized. For the first time, different types of cement evaluation tools are evaluated on the same test fixtures. This experimental study provides a guideline for selecting tools to conduct cement evaluation.