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.