Behind-Pipe-Log-Evaluation Study: Deepwater Subsea Abandonments
Authors: Wilson Adam 
Published: December 2016

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 174777, “Behind-Pipe-Log-Evaluation Study: Deepwater Subsea Abandonments in the Gulf of Mexico,” by Glenn Donovan, SPE, Peter Fadesere, SPE, George Ware, and Lou Daigle, SPE, Shell Offshore, and Suparman, SPE, Gary Frisch, SPE, Phil Fox, Mike Englar, Gordon Moake, SPE, and Weijun Guo, SPE, Halliburton, prepared for the 2015 SPE Annual Technical Conference and Exhibition, Houston, 28-30 September. The paper has not been peer reviewed.
Techniques for well-abandonment log evaluations have been studied in the Gulf of Mexico since 2012. Legacy practices typically used acoustic methods consisting of cement-bond-log (CBL) and ultrasonic-scanner devices. The new methods described in this paper consist of adding nuclear sensors to supplement the acoustic measurements and introduce novel processing methods. The overall solutions package consists of behind-pipe- evaluation techniques (BPETs). When properly modeled and analyzed, these data have the potential to reduce the cost of removing casing strings significantly.
Introduction
Much of the nonproductive time associated with well abandonments is attributed to difficulty circulating the stagnant material left behind in the casing-to-casing annuli during well construction. Many improvements have been made in the design of these fluids; however, drilling muds using barite as the weighting agent still suffer phase degradation over time, leaving a column of material with a density that increases with depth, except when solids bridging occurs. In extreme cases, the precipitated barite adheres to the outside of the casing wall, impeding the removal of the severed casing section.
When traditional sonic and ultrasonic logs are run over concentric casing strings, the results can be misleading. Sonic logs such as the CBL do not yield quantitative results in concentric strings of casing because of the inability to predict the influence of the outer casing on the log response. Misleading data and experiences in well abandonments have left the industry actively seeking alternatives to the conventional method of identifying the material in the annulus, to increase the efficacy of the abandonment process.
Abandonment Log-Evaluation Methodologies
Historically, it has been common practice to use ultrasonic imaging technologies in combination with traditional sonic-waveform data to characterize the annular space of wells that are candidates for casing-retrieval operations. However, this approach is inadequate for evaluating a phenomenon that is often observed in older wells that contain concentric casing strings that were not cemented to the top of the string. In those cases, the original drilling fluid between the casing strings experiences degradation. As the drilling fluid degrades, solids in the fluid begin to settle out and migrate toward the bottom of the well or toward areas of high deviation or dogleg severity. The settling of these solids is commonly referred to as barite sag, mud sag, or barite settling.