Cement-Bond-Log Interpretation Reliability

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 101420, "Reliability of Cement-Bond-Log Interpretations Compared to Physical Communication Tests Between Formations," by Douglas Boyd, SPE, Salah Al-Kubti, SPE, Osama Hamdy Khedr, SPE, Naeem Khan, and Kholoud Al-Nayadi, SPE, Zadco; Didier Degouy, Adma-Opco; and Antoine Elkadi and Zaid Al Kindi, SPE, Schlumberger, prepared for the 2006 SPE Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, UAE, 5-8 November. Two kinds of cement-bond-log (CBL) tools are run as part of a standard cement-evaluation program. The effectiveness of these tools and their evaluations often are challenged, and they are not regarded as a replacement for reservoir interzonal-communication tests performed between producing reservoirs. The value of continuing to run these tools was questioned by management. In response, the reliability of these tools and their ability to determine the existence of poor behind-casing cement quality were examined. Introduction Historically, three methods are used to check for isolation between reservoir units. Pressure tests are restricted to localized areas of the casing such as the casing/liner shoe and squeeze perforations. Communication tests, regarded as the most definitive method of testing behind-casing isolation, jeopardize casing integrity and are costly. Cement-evaluation logs are time-efficient, cover the majority of the casing, and are inexpensive compared to communication tests. Interpretations of these logs sometimes do not predict behind-casing communication, creating a perception of unreliability. Determining the confidence that can be placed in cement-log interpretations is an objective of this task force. Behind-casing communication of hydrocarbons via the cemented annular space requires expensive remedial cement squeezes to cure the problem. Because of the importance and criticality for reservoir management of the zonal isolation between oil-bearing formations, Zadco's policy is to confirm isolation by physical communication tests regardless of the quality of cement interpreted from CBLs. Only cement barriers between formations where production is planned are tested. Intervals between reservoirs not planned for production are not tested. Because the majority of cement-squeeze decisions are based on the results of physical-communication tests, the value of continuing to run CBLs was questioned. A task force comprising representatives from drilling, well integrity, petroleum engineering, petrophysics, and the primary service company was formed to review Zadco's communication-test policy and determine the reliability of CBLs to determine behind-casing cement quality and from this infer the presence or lack of isolation between formations.