Abstract
A plug and abandon (P&A) operation aims to isolate the subsurface from the surface and success is traditionally gauged by fulfilling arbitrary criteria such as a physical length of an annular barrier. Multiple examples exist where a perfectly legal barrier has leaked. This paper intends to demonstrate that a sufficiently sensitive noise tool can provide the closest measure possible to verification of "no leakage".
The principle that fluid moving in a turbulent fashion will emit acoustic energy is very well established and is used in many successful noise logging tools. When the fluid movement is very small, the level of noise is correspondingly small, and conventional tools may miss them. A tool optimised for low energy sensitivity was evaluated in a series of text fixtures and cemented casing sections recovered from real wells to quantify the lower levels of noise detectability. Following this, logs were recorded in previously abandoned wells, detecting minute leakage despite full compliance with guidelines.
An industry leading acoustic tool was redesigned from the sensor through to the electronics to enhance its detection range to as low as possible. The sensor to electronics design was optimised to lift the low energy response by a factor of 10, while the intrinsic baseline noise was lowered to significantly increase the dynamic range available. A field test confirmed the low energy response to be 25 decibel (dB) better than pre-modification. Controlled testing under a range of conditions for both liquid and gas leakage resulted in the detection threshold being delineated for a variety of conditions. In addition to this the examination of statistical sampling parameter revealed meaningful acoustic signatures associated with varying flow regimes at very low rates. Field validation of the tool in a previously abandoned well confirmed the low threshold of the optimised tool and an intermittent leak of extremely low rate successfully mapped.
This paper demonstrates through controlled testing and field deployment that compliance with P&A barrier condition regulations absolutely does not guarantee zero leakage, but that an optimised acoustic tool is able to detect even very tiny leaks. The judicious use of such technology during the abandonment procedure to verify a barrier is actually sealing will prevent costly returns to remediate faulty procedures.
