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.

New techniques for well abandonment log evaluations have been under study since 2012 in the Gulf of Mexico (GOM). Legacy practices typically used acoustic methods consisting of cement bond log 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. Behind pipe evaluation techniques (BPET) is the overall solutions package described within the paper. When properly modeled and analyzed, this data has the potential of significantly reducing the cost of removing casing strings during plug and abandon activities.A new development in CBL refracted waveform processing provides relative amplitude mapping within four concentric cylindrical volumes between the CBL transmitters and receivers. These regions extend from the first casing and its annular region and outwards within the wellbore. The density and neutron nuclear sensors allow grouping of detector count rate ratios in data clusters which have been interpreted to be responding to annulus materials spanning from cement, heavier liquids, settled mud solids, lighter liquids, and gas. Neutron responses are useful in trending the relative hydrogen index of the annular contents. Distribution imaging of settled solids from ultrasonic measurements have been helpful in supplementing the interpretation of nuclear and refracted waveform indications.More than 27 log runs were conducted with the applied abandonment evaluation methods in the deepwater sector of the GOM. One of the benefits derived from conducting the evaluations in abandonment operations is the ability to validate interpreted log predictions with additional surveys after the cut and extraction of primary casing strings. An interesting example from time-lapse surveys is included. Detection of hydrocarbon gas before release of the casing hanger and during circulation operations can reduce risks from a health, safety, and environment (HSE) perspective. Frequently, during the abandonment phase, casings can have accumulated mud solids on the exterior surface. This phenomenon can mask the detection of gas from shallow-reading ultrasonic measurements. The density and neutron nuclear sensors allow slightly deeper detection of annular material responses. Examples where gas was detected through casing and associated post-cut results are shared. Cut-and-pull rig tension prediction has been another deliverable developed from the described methods. Results from this technique are displayed along with available actual applied rig tensions during casing extractions. The paper describes current downhole logging tool configurations allowing variable depth of measurement and shares the interpretation methods practiced through well case history study examples.Although primary applications of the method were developed and applied for well abandonment purposes, other uses of the new technique could include sidetrack window positioning in wells without previous cement evaluation logs available over the targe...