There are some misconceptions about storage site monitoring. It’s commonly believed that because there are a lot of potential monitoring tools out there, they will all have a part to play as CCS is rolled out. The abundant use of monitoring tools in pilot-scale projects has also perhaps left the impression that costs of monitoring tools are low, so they are barely a cost consideration. This is not necessarily the case.
In fact, future industrial-scale storage monitoring programmes are likely to be fairly simple with a limited portfolio of tools deployed. Industries are likely to look for the cost-optimal mix of techniques that meets site-specific requirements imposed by regulators.
To decide which techniques are to be used, we need to know what monitoring has to show. The EU Directive on the geological storage of carbon dioxide leaves us with four main requirements: we have to demonstrate that the site is working as expected, that we understand current processes and can make reliable predictions of future ones, that there is no evidence of leakage, and that operations will be safe to people and the environment.
Most of this concerns the deep underground, so deep-focussed monitoring systems will be the principal means of ensuring that the legal requirements are met. Here, the optimising starts. At industrial-scale storage sites, existing wells are typically several kilometres apart with wellbores difficult or costly to access. New surveillance wells are very expensive. Therefore, deep-focussed monitoring is likely to comprise a judicious combination of down-hole and surface technologies.
In practice, two key deep-focussed monitoring techniques seem to stand out: down-hole pressure and temperature measurement, and time-lapse seismic imaging. These tools allow for the calibration and verification of predictive simulations of CO2 plume migration and also of reservoir pressure changes. As such, this combination can provide detailed information on processes in the storage reservoir and also give early warning of unexpected CO2 migration: either out of the storage reservoir or, more seriously, out of the Storage Complex. The Sleipner, In Salah, Snøhvit and Weyburn projects all demonstrate in different ways how important these two tools are for monitoring storage performance. An example of a seismic image representing the Sleipner CO2 plume is shown in the accompanying figure.
3D seismic image of the CO2 plume (bright reflections under the reservoir topseal) at Sleipner in 2006
While deep-focussed techniques can satisfy the first two EU Directive criteria for monitoring, shallow-focussed monitoring makes essential contributions to addressing the latter two. A robust pre-injection baseline against which all future measurements can be compared is a priority for all monitoring. For shallow-focussed monitoring in particular, however, the baseline is crucial for capturing the full range of natural variation. This can be significant, and future exceptions might be interpreted as leakage if the baseline is not measured over a sufficiently long period of time. Shallow monitoring can also play a role in public engagement, as it provides an accessible way of giving credibility to statements that things are going according to plan.
If things do go wrong, monitoring takes a central role and additional tools could be necessary. Monitoring might have to detect and measure emissions over large areas but with a high accuracy. This is technically challenging, with a range of different issues onshore and offshore.
The CO2ReMoVe project has investigated a much wider range of monitoring techniques than a CO2 storage project, operating normally, is likely to ever use. Still, this effort is far from a waste of money. Although not all techniques will be used widely at the industrial scale, the value of pilot-scale monitoring lies in two aspects: the testing, to a high level of detail, of the reliability of our predictive models, and the availability, if something goes wrong, of specific monitoring techniques that have been tested in real time. Testing and validating a variety of alternative tools in early CO2 storage projects will allow for effective cost optimisation when CCS is rolled out on the Gigatonne scale.
Andy Chadwick (British Geological Survey)
Andy has been involved with CO2 storage since 1998 participating in many European CO2 storage projects and a number of UK government and industrially-funded ones. Current research directions include quantitative analysis of time-lapse seismic data to characterise CO2 plumes, and history-matched flow modelling to understand CO2 migration in reservoirs. He has advised a number of national and international regulatory bodies and is particularly interested in developing pragmatic integrated monitoring systems and strategies for industrial-scale storage sites.
