The North Sea Oil and Gas industry counts over 7,800 wells drilled. The industry is now entering an era of well abandonment and decommissioning. Current barrier verification for P&A requires appropriate pressure testing and includes surface and downhole monitoring.
Globally, Spectral Noise Logging (SNL) has been utilized in many thousands of cases to detect fluid movement behind completion tubulars and/or across a cement barriers.
In Nov 2017, full-scale verification tests were conducted at the International Research Institute of Stavanger (IRIS). These tests were conducted in a controlled environment to verify current technology thresholds. These showed the technique validated the cement barrier integrity during pressure tests and can diagnose channeling as low as 9 ml/min behind the casing. The threshold matrix for different cement defect versus pressure and flow rates allowed the usage of the technology to support the positive qualification of the barrier elements (
Utilizing a purpose-built test assembly of standard oilfield tubular and cement with fitted end caps, a series of pressure tests operations were conducted to identify the pressure and associate leak rates in conjunction with the SNL. The results clearly demonstrated that the logging tool can provide evidence of barrier verification over a wide range of well applications. Barrier qualification requires that three conditions are met; firstly, cement behind casing is in place and not displaying a micro-annulus or any form of fluid movement behind pipe. Secondly, that a cement plug holds pressure and there is also no fluid leak and finally natural shale barriers are active and create a sufficient barrier. Currently, technology is in its 10th generation, and since the IRIS tests have been used in many wells, covering both onshore and offshore oil and gas wells and wells in highly sensitive environmental areas. On each case the logging operations were used to verify well status before and after the barrier establishment via cement squeeze or section milling and, in several cases, clearly, demonstrate that the barrier status remained ineffective, hidden and further remedial work was required.
This paper discusses the downhole passive noise listening and its spectral analysis technique to prove the effective cement barriers are in place. The concept, methodology and its application which have been successfully tested via yard and field tests are presented in this paper.