WHAT CAN WE LEARN FROM NATURAL ANALOGUES?

Abstract

The timescales needed for the geological storage of carbon dioxide (CO₂) are potentially thousands of years. Therefore, before large-scale underground CO₂ storage can take place, it will be necessary to demonstrate that the processes are well understood, risks to the environment and human populations are low, and environmental disturbances can be minimised. One way of demonstrating that CO₂ can remain trapped underground for geologically significant times is to provide evidence from existing naturally occurring accumulations. These accumulations occur in a variety of geological environments and many can be demonstrated to have retained CO₂ for periods longer than those being considered for CO₂ storage. This fact will build confidence in the concept with non-specialist policy-makers, environmental NGOs and the public. Studies of natural analogues can be used to further validate predictive geochemical and geomechanical models, increasing confidence in these models to predict how CO₂ will behave during and after storage, helping to determine how much of the CO₂ will be permanently trapped through mineral reactions. The results have identified that kinetic reaction data need to be improved. It is unlikely that in reservoirs similar to those investigated here, significant mineral trapping can be expected, except over long geological timescales. Natural accumulations can be used to test methodologies for monitoring CO₂ leakage that may be appropriate for use above repositories, both onshore and offshore, to establish baseline conditions and to monitor sites at the surface during and after storage. Soil gas surveys and analyses of gas leakage rates can define how CO₂ migrates through the near surface environment. Techniques for determining the sealing capacity of caprocks have been tested on natural seals known to retain CO₂ and caprocks from future potential storage sites can be compared with these datasets.