Long Term Integrity of CO₂ Storage – Well Abandonment

Publication Overview

This report focuses on potential hazards to geological storage of CO₂ related to previously abandoned deep oil and gas wells. As many prospective CO₂ storage projects will be situated in mature sedimentary basins, these operations need to accommodate previously drilled and abandoned wells. The current study aims to provide a high order evaluation of abandoned wells and their suitability to CO₂ storage operations. To this purpose an overview of the current state of knowledge on potential degradation mechanisms of typical well materials (i.e. cement and steel) is presented. Several case studies are described, illustrating some typical aspects associated with abandoned wells in the context of geological storage of CO₂. Furthermore, a geographical overview of numerous well abandonment regulations is provided, reflecting significant differences in regulatory demands around the world. Moreover, both abandonment regulations and practices historically gradually developed to the present high standards. As a consequence, especially older wells are considered to be a potential threat to long term storage integrity. Various risk assessment methodologies are described that are tailored to well integrity evaluation for geological CO₂ storage. Finally, an overview of potential corrective measures and monitoring strategies is presented.

Publication Summary

The study references work carried out by Watson & Bachu, 2007, that states that wells can be classified as either existing or future wells. Existing wells are further sub-categorised as abandoned or operational, effectively giving 3 well categories: abandoned, operational and future wells, and this classification is key to understanding and rationalising the risks posed to a field during the initial screening process.

The report demonstrates that there is much experience gained through previous pilot and demonstration operations, and that there is a great deal of knowledge on various abandonment techniques that have been proved suitable for CO₂ storage purposes. Recognition of this knowledge is not always evident, and in communicating with regulators and the general public, this level of understanding and confidence should be expressed. This assessment demonstrates this comprehensive range of techniques implicitly, and demonstrates the accepted limitations where relevant.

Due to this knowledge base, future wells can be designed, drilled and abandoned taking into account the CO₂ storage operation, using state-of-the-art technologies. What remains is for regulatory regimes to stipulate clear guidance on recommended best practices within their spheres of influence and operation, facilitating straight-forward start up and initialisation of projects.

One of the most valuable outcomes of the study is the analysis of the risk management methodologies, and this demonstrates that there are a range of methods available, depending on the aim of the assessment, to perform an in-depth analysis of the risks. Alongside this is the analysis of factors affecting well abandonment and the impact they have on the risk of leakage. This analysis will be very useful in the primary stage of a project, in allowing the site characterisation and screening process to be compared with this matrix to determine an overall risk to long term integrity of CO₂ storage.