A Review of Natural CO₂ Occurrences and Releases and their Relevance to CO₂ Storage

Publication Overview

The aim of this study was to evaluate natural occurrences of CO₂ leakage and compare and contrast them with engineered storage of CO₂ in geological formations. The objective of the study was to provide a reference manual for IEA Greenhouse Gas R&D Programme members and others interested in the subject, to provide a factual and balanced review of natural CO₂ releases and their relevance to geological CO₂ storage.

Publication Summary

The main conclusions that can be drawn are that the most notable CO₂ emissions that are commonly referred to like Lake Nyos and the Dieng release all arise in volcanically active regions of the world. These volcanically active regions are not being actively considered for CO₂ storage in geological formations. Rather it is the volcanically inactive sedimentary basins around the world that contain extensive hydrocarbon accumulations and some natural CO₂ accumulations that are being actively considered for geological storage.

The major natural CO₂ emissions that have occurred and lead to significant loss of life, like Lake Nyos and the Dieng volcano can be regarded as representing fairly unique geological situations. These are either tropical craters lakes that do not overturn and are filling with CO₂ due to volcanic activity, or are the result of shallow CO₂ accumulations again resulting volcanic activity that are released as a precursor to a volcanic eruption as fractures in the strata open up prior to an explosion occurring. Such geological situations have nothing in common with the stable sedimentary formations, where it is proposed to store CO₂.

It must be noted that natural CO₂ emissions do occur in sedimentary basins. There are numerous natural CO₂ fields within sedimentary basins around the world. Some of these fields have held CO₂ for millions of years without evidence of leakage. Others, however, do leak along faults and as a result of wells drilled within the fault zones. Leakage generally manifests itself as carbonated springs or dry seeps (called moffettes) and can result in localized ecosystem damage. In general terms the environmental impact resulting of these leaks are significantly smaller than those occurring in volcanic regions.

The study of natural CO₂ releases and accumulations has provided reference information that can be drawn upon in the selection of sites for geological CO₂ storage. Sites need to be in regions that are volcanically inactive, they require a thick overlying succession of impermeable rocks to prevent upward migration and they must be sited away from areas that are faulted. Wells provide a potential transmissive pathway for leakage from storage reservoirs. The frequency and abandonment status of all wells must be a critical analysis that is undertaken when selecting a potential CO₂ storage reservoir. Old wells may need to re-cemented to maintain their integrity and inadvertent drilling into these storage sites need to be controlled in the future to prevent accidental release of the stored CO₂.