Tracer-Based Monitoring of P&A in Offshore Fields
Authors: Silva Mário , Brandvoll Øyvind , Hubred Per-Arne , Viig Sissel Opsahl 
Published: June 2023

Thousands of offshore wells are planned to be permanently plugged and abandoned worldwide up to 2050. Fractures are known to occur and develop in the materials used as seals/plugs, creating channels that compromise P&A integrity. This risk is increased by the foreseeable use of oil and gas assets in CCS and hydrogen storage projects. Existing monitoring methods may fail to provide early warning of issues with plug integrity. There is presently no regulation in place on Norwegian Continental Shelf about long-term monitoring of permanently plugged and abandoned wells, however, this situation is likely to change in the future. Thus, cheap, and effective methods to monitor the integrity of long-term P&A of offshore wells are necessary.

The existing technology for P&A is sufficiently mature to dramatically reduce the risk of a "catastrophic" failure in its deployment. Thus, if the integrity of permanent barriers is compromised, it is likely that this will happen through the establishment of micro-fractures where very small flowrates of reservoir fluids can escape or diffuse to the marine environment. If such early signs are detected, then a more thorough diagnose and mitigation campaign can be deployed in a timely manner. Tracer technology can provide this type of early warning. By including tracers in the P&A projects, either deploying a slug just before the installation of barriers, or embedded in the barrier material matrix, these substances will be detected in the waters around the plugged well head, if channels allow for the circulation of fluids between the bottom and top of the wells. Most (if not all) existing tracers are detectable in the ng/L (ppt) level, thus providing a warning system even if just diffusion occurs. The tracers qualified for use in oil & gas reservoirs are not exposed to the presence of materials used in plugs/barriers in P&A. Many of these materials (e.g., cement-based) exhibit surfaces with relatively high energies and activities. Thus, it is critical to evaluate tracer response for this type of new application.

In the present document, we present an initial study to evaluate long-term stability and dynamic flooding properties of existing water-based tracers in the presence of Portland cement and CO2 originated ions. The stability experiments were performed at two different temperatures. The dynamic flooding properties were evaluated relatively to tritiated water in a reference barrier cell constructed with known defects. The cell used (referred to in the present document as plug) consisted of a well section with cemented casing (Portland), constructed on purpose to allow for fluid circulation in a "microannulus". Results show that further evaluation and development of the concept of using tracers as an early warning system for permanent P&A is necessary. However, of the 8 tested tracers, 4 exhibit an ideal or close to ideal flooding behavior in the microannulus system that open the doors to such an application. Results also suggest that tracer tests using conventional inter-well water tracers can be devised to assess the quality of both borehole and casing cementing in conventional well-completion operations.