Cement Bond Logging; A Generational Review of Technologies & Log Interpretation

Abstract

AbstractSuccessful placement and curing of cement behind casing is critical for the environment, operational safety, economics and lifespan of any oil and gas well. Cement bond logging technologies have evolved, from basic CBL, to include different measurement physics, sensor technologies and deployment options. Add to that, a better understanding of downhole conditions now enables us to interpret the measurements in a more insightful manner. We will present a modern review of the cement logging technologies including current and best practices in log interpretation.Introduced in the oil and gas industry in the mid-1950’s, Cement Bond Log (CBL) tools have stood the test of time despite the continuous development of increasingly sophisticated technologies. Where used by itself, CBL technology presents several inherent limitations, however, when used together with modern measurements, it frequently provides valuable complementary information. Our methodology consists of using a collection of recent case examples, using multiple technologies and measurement physics, and review both the weaknesses of a standalone CBL and the value it brings when integrated with new methods.Initially designed and tested in laboratory conditions, cement slurries are expected to achieve certain downhole performance with respect to curing time and compressive strength. The casing is also expected as centralized, in good borehole, surrounded by a uniform cement sheath for zonal isolation. A deviation from the ideal conditions, can impact the interpretation of cement logs and lead to erroneous decision making. Low density and or contaminated cements, unexpected downhole pressure and temperature, gasified borehole fluid, enlarged and or rugose open holes, casing eccentricity, multiple concentric casings, fast formations, micro-annulus, etc. are all real-life conditions which influence the condition of cement, as related to both coverage and strength, and challenge the interpretation of the logs. However, a majority of these challenges are effectively overcome, through integration of technologies, as will be demonstrated through presented field examples.Advancement in cement evaluation technologies, over the last 70 years, and our increased understanding of downhole conditions makes the case of an all-encompassing, modern review of cement bond logging. Log responses are easy to understand in extreme conditions of "free" or fully bonded pipe. However, as downhole conditions start to differ from ideal conditions, the log responses can often be misunderstood. This paper proposes to provide a comprehensive and additive information on interpretating the time-tested CBL all the way to modern services in use today. This will lead to unambiguous evaluation of downhole cement status and confident decision making by practicing engineers.