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type     June, 2005

Vol 2 Chapter 32: CO2 Storarge in Coalbeds: Risk Assessment of CO2 and Methane Leakage

Shaochang Wo, Jenn-Tai Liang and Larry R. Myer

Abstract: The practice of testing seal integrity is not routinely employed in coalbed methane projects. With injection of CO2, changes in stress caused by potential high injection pressure and rate may open previously closed fractures and faults, thus generating new leakage pathways. The research presented in this chapter focuses on assessing potential leakage pathways and developing a probabilistic risk assessment methodology. A study was performed to evaluate geomechanical factors that need to be taken into account in assessing the risk of CO2 leakage in CO2 storage in coalbeds. The study revealed that geomechanical processes lead to risks of developing leakage paths for CO2 at each step in the process of CO2 storage in coalbeds. Risk of leakage is higher for old wells that are converted to injectors. Risks of leakage are much higher for open cavity completions than for cased well completions. The processes of depressurization during dewatering and methane production, followed by repressurization during CO2 injection, lead to risks of leakage path formation by failure of the coal and slip on discontinuities in the coal and overburden. The most likely mechanism for leakage path formation is slip on pre-existing discontinuities that cut across the coal seam. A mathematical model for probabilistic risk assessment was developed. The model consists of six functional constituents:

  1. initiators,
  2. processes,
  3. failure modes,
  4. consequences (effects),
  5. indicators,
  6. and inference queries.

Potential leakage pathways are usually coupled with identified failure modes. In assessing the risk of CO2 storage in geological formations, inference rules can generally be categorized into seven different types. The inference logic of this model is based on set theory, which is superior to the traditional decision-tree based inference logic in terms of flexibility, generality, capability in dealing with uncertainties and handling large, complex problems, such as cascading phenomena. The model was designed to be implemented on a relational database.

Carbon Dioxide Capture for Storage in Deep Geologic Formations – Results from the CO2 Capture Project Capture and Separation of Carbon Dioxide from Combustion Sources - Volume 1
Edited by:
David C. Thomas, Senior Technical Advisor, Advanced Resources International Inc, USA

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