Assessment of the Conditions of Abandoned Wells in Potential CO2 Storage Reservoirs

Abstract

This project is part of the larger REX-CO2 project, which assesses the re-usability of abandoned wells in reservoirs targeted for CCS operations. It comes as a necessity to also assess the abandoned wells that will not be re-purposed, by assessing the risk they pose in terms of allowing the stored CO2 to resurface across them, which is the focus of this work.

In order to assess these wells, a decision making framework was developed that will consider the main mechanisms that can lead to leakage across an abandoned wellbore. The objective is that this framework could be used by an operator of a prospective reservoir for a CCS project, prior to the start of the operations, in order to understand the risk associated with the abandoned wells present, and take decisions for remediation if necessary and possible. This framework considers three main aspects relevant for the formation of leakage paths. These are the effectiveness of the abandonment process itself, the chemical processes that can lead to the degradation of the isolation elements, and the mechanical processes that can lead to loss of integrity.

The framework consists of two main parts. The first is a qualitative analysis, based on a thorough literature review, assessment of experts and testing with case studies. This qualitative assessment consist of decision trees, formed by a series of questions which answers will dictate the final outcome that will reflect the state of the abandonment. The second part is a quantitative risk analysis. For this purpose "Bayesian Belief Networks" are used which is a probabilistic tool used for calculating the relative probability of a combination of factors. The BBNs are constructed and populated based on a thorough literature review including data on experimental results. In addition, geomechanical simulations were carried to populate these models. All this data was processed and converted into normal random distribution functions, which were used to infer the probabilities that would be included in the BBNs. The final outcome of the BBNs reflect the probability for leakage to occur.

Both the qualitative and quantitative parts of the framework are integrated together in order to obtain a complete analysis. The complete framework was tested with case studies based on real wells for which the abandonment states are known and reported by the operator, to observe how the outcome of the framework analysis matched the outcomes of the analysis performed by the operator.

With this study, a complete and systematic framework was developed that can be used to aid decision making for prospective CCS projects in the future. The framework is constructed such that it has the capacity to be improved and expanded in the future, to increase its consistency and broaden its applications. A series of potential improvements are also explained at the end of this report. Furthermore, the framework can be used to challenge the current abandonment standards and assess what will have to be changed to adapt the current abandonment techniques so that they include CCS projects.