Approach Enables Vertical Well Re-Entry for Safe and Efficient Legacy-Well Reabandonment

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 225865, “Vertical Well Re-Entry Solutions for Safe and Efficient Reabandonment of Legacy Wells,” by Benjamin Cannell, SPE, Aquaterra Energy. The paper has not been peer-reviewed. _ Cost-effective re-abandonment of legacy wells is essential to the success of offshore carbon capture and storage (CCS) projects, but re-entry poses a significant challenge, particularly when wells have been severed below the mudline. The complete paper discusses the challenges of accurately locating, and then successfully re-entering, these wells. The author writes that a key part of addressing these challenges is the innovative use of the recoverable abandonment frame (RAF). Finding the Well: Locating Subsurface Legacy Infrastructure Successfully locating a legacy well often is a complex and uncertain task, particularly for wells that were fully severed below the mudline with imprecise historical positioning data. Early technologies had significant accuracy limitations. The advent of satellite-navigation technology improved accuracy to 5 m but was primarily used as a secondary positional check until becoming routine by 2000. To locate legacy wells, a multitiered geophysical approach is employed. Initially, magnetometers and gradiometers detect magnetic signatures of steel casing buried beneath the seabed. These tools are towed behind a vessel and can cover large areas, narrowing down the search to approximately 5 m of accuracy. Autonomous vehicles are increasingly used in this phase to further reduce costs. Once a potential location is identified, more-precise instruments are deployed to confirm the well’s position to within a few cm. Tools such as seabed-fixed sonar devices, acoustic corers, and sub-bottom profilers are used to obtain high-resolution subsurface images, often providing 3D imaging of the well structure. Wide-Area Detection Techniques The following methods are used to scan large sections of the seabed to detect features or anomalies indicative of legacy-well infrastructure. - Side-Scan Sonar. This method produces acoustic images of the seabed surface. This technique is effective at flagging areas of interest for further inspection. - Sub-Bottom Imager. An acoustic picture of subsurface layers beneath the seabed is provided with resolutions ranging from 10 to 30 cm, depending on frequency. This method helps detect buried casing or cement. - Magnetometer and Gradiometer. These tools detect the strength and gradient of magnetic fields, allowing detection of ferrous components even when buried beneath sediment. This is particularly valuable for severed wells where no hardware remains above the mudline. - High-Resolution Pinpointing. Once a likely location has been identified, high-resolution sensing methods are used to validate the well’s position and condition. Seabed static pinpoint sensors can generate 3D images of the seabed and subsurface to depths of 40 m with a resolution of approximately 10 cm. Remotely operated vehicles may be deployed to confirm visual characteristics of the wellhead or surrounding features. While not fixed to the seabed, when combined with positioned seabed monuments or markers, this method can measure the relative position of the well center to the marker or monument, allowing for an extremely accurate well-center position (Fig. 1).