Acoustic Logging Techniques for Guiding Casing Recovery Operations in Deepwater Gulf of Mexico

Abstract

While ultrasonic logging devices have been used in recent years to determine the likelihood that casing strings are unbonded and thus free to be removed in a casing recovery, this application is still under development. The uniqueness of this application lies in the fact that the bonding forces present on the outside of the casing are caused by drilling fluids in varying stages of decomposition as opposed to cement. The fact that the exact composition of these altered fluids is unknown makes it difficult to quantify their compressive strength, leaving no clear method for anticipating an acoustic response. Virtually all of the acoustic devices in use today, when used for any application other than borehole imaging, were developed to quantify the ability of cement in the casing-formation annulus to provide a pressure barrier between zones connected to one another by the wellbore. In every such case, the cement used to provide the isolation is of a known chemical composition and extensive laboratory experiments are conducted to precisely measure its compressive strength and other properties. Since there exists no such opportunity to measure the properties of the material in an uncemented annular space, theoretical solutions have been developed which estimate the extent of adhesion these materials cause. This paper discusses the applicability of such theoretical solutions and substantiates its conclusions with case studies. Introduction In order to sidetrack out of a previously completed well, it is generally necessary to remove one or more of the existing casing strings to gain access to a larger casing interval from which to kick the new well off. This allows for the use of a larger casing in the new well than could be placed were the internal casings not removed. In this case, the internal casing strings, while well cemented in their deeper intervals as required by sound well completion practices, are generally not cemented in their upper sections. The removal of the internal casings can be accomplished more efficiently if the precise point at which they are no longer held in place by adhering forces of the material surrounding them is known. While it is known that the adhering forces are caused by barite settling out of the drilling mud placed in the casing-to-casing annuli, acoustic information has been collected for homogenous mud, fluid, and cement samples only. (Table 1) Additionally, the information gathered is limited to acoustic impedance, which is not easily directly related to material strength, as can be seen by the tabulated values which show that impedance is more closely related to cement slurry density that it is to compressive strength.

Behind Pipe Log Evaluation Study: Deepwater Subsea Abandonments in the Gulf of Mexico

“…• Log indicates solids in annulus. Misinterpretations of ultrasonic logs give rise to the assumption solids are present in the annulus when actually the shallow reading nature of the measurement is merely identifying a thin layer of material present on the casing's exterior (Donovan, 2009). In this case, an unnecessarily conservative approach is taken, extending the abandonment as smaller sections of casing are retrieved from the well.…”