Wells are complex structures of cement and steel that stretch kilometers into the ground. They can never be removed, but need to be permanently plugged when they have performed their duty as exploration wells, production wells, injection wells or monitoring wells. The plugging procedure involves filling discrete sections of the well with an appropriate plugging material, typically cement. On the Norwegian Continental Shelf (NCS), 2-3 cement plugs of 50-100 m lengths are typically used to permanently seal wells. It is important to avoid leakage through these plugs, as wells provide a passage from the reservoir all the way up to the surface. Avoiding leakage is especially important for wells drilled into CO2 storage reservoirs, as the pressure here will be higher than in depleted oil and gas reservoirs. CO2 is also a buoyant and reactive fluid, meaning that better plug sealing is required for CO2 wells compared to oil and gas wells.
The present project has aimed at understanding in more detail how we can safely and efficiently plug and abandon CO2 wells to ensure long-term geological CO2 storage. Several important topics have been studied, namely: (i) material choices for well plugging, (ii) optimal plug placement and hardening, and (iii) plug degradation in relevant CO2 well conditions. The studies have been experimental and numerical, and have resulted in research-based advice to industry on how to secure long-term integrity in CO2 wells.
Three material types have been investigated in the project, namely ordinary Portland G cement, cement with silica additions and a type of thermosetting polymer material. Through experimental and numerical work we have determined how the rheological properties of well barrier materials should be to ensure that they can be placed in the well without the creation of channels/pockets that can cause leakage. This work has been thoroughly described in a textbook written in this project, which has received good international evaluations. We have also focused on how to measure and improve cement bonding to steel and rock, and we have proposed that applying an electric voltage on the casing can be a simple way of achieving a good bond. With the use of synchrotron radiation, we have made the world's first in-situ study of how defects in cement close up under CO2 exposure. This happens because CaCO3 precipitates in voids and closed regions. This study is the first to be able to quantify how fast this process occurs, and under which conditions it takes place. These results have just been published in the journal Environmental Science and Technology. One disturbing finding that has resulted from the project work is that cement with silica additions (which is used in approximately half of the wells on the Norwegian Continental Shelf), has a low resistance towards CO2. It is therefore important to dedicate future research to understanding how to cost-efficiently remediate existing wells that have not been constructed for CO2 service.
Leakage rates and plug lengths are thoroughly discussed in the deliverables of the project, and it is underlined that to be able to recommend plug lengths it is first necessary to agree on what an "acceptable leakage rate" is. It is also necessary to obtain a detailed description of well defects to be able to calculate leakage in a realistic way. This requires better logging and defect detection possibilities than what we have today.
This project will study the solidification process of plugs set for permanent abandonment of CO2 wells. Focus will be on cement (with relevant additives) and emerging plugging material types. High resolution X-ray tomography, using a for-purpose built X-ray transparent pressure cell, will be the primary analysis technique. This will enable studies of solidification fronts, phase transformations and emerging heterogeneities in the materials. A special focus will be given to the latter, as voids and fractures in/along a permanent plug can compromise its long-term sealing ability towards CO2 leakage. Complementing materials characterization studies and numerical modelling will therefore be used to improve our understanding of the physics of leak path development, and its consequences in terms of estimated leakage rates. The project results will be used to rank the performance of candidate plugging materials for CO2 purposes, to advise safe plug lengths for each material type, and to suggest structural/chemical changes of the materials to improve their long-term sealing ability in CO2 wells.