Behind Pipe Log Evaluation Study: Deepwater Subsea Abandonments in the Gulf of Mexico
Authors: Donovan Glenn , Fadesere Peter , Ware George , Daigle Lou , Suparman F. , Frisch Gary , Fox Phil , Englar Mike , Moake Gordon , Guo Weijun 
Published: September 2015

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 target interval. Gas detection behind pipe in which multiple strings or interior surface and exterior environmental factors hinder ultrasonic detection also makes the method attractive. The techniques described are still evolving as additional wells are logged and interpreted. As historical data are made available from abandonment operations, the modeling will continue to be improved in conjunction with current tool response laboratory characterization.