Written by Domenico Baù, University of Sheffield, who received a funded place to attend the IEAGHG 2nd Modelling and Monitoring Network Workshop (Edinburgh, 5-8 July 2016) as part of the UKCCSRC’s support of this event.
I am a senior lecturer at the University of Sheffield, where I teach modules on hydrogeology and modeling in water engineering for post-graduate students. In a nutshell, I am a computational geoscientist with a particular focus on the development and application of numerical models for subsurface processes such as multiphase flow, solute transport and geomechanical deformation induced by fluid extraction and injection in the subsurface.
I must first thank the UKCCSRC for providing partial funding for my attendance and participation to the 2nd Combined Meeting of the IEAGHG Modelling and Monitoring Network between the 5th and 8th July 2016 at the Edinburgh Centre for Carbon Innovation. This was an enriching professional experience for me.
The meeting kick started on July 5 with a field trip to Siccar Point on the Berwickshire Coast of the North Sea near the town of Dunbar. The site is a clear demonstration of “geology at work” with dipping Devonian sandstone overlying vertically bedded Silurian greywacke with a time gap of around 65 million years. What is in fact sort of a reality check is the degree of heterogeneity exhibited by these geological features, from the scale of centimeters to that of tens of meters.
Worldwide, Carbon Capture and Storage (CCS) is widely acknowledged as a “bridge” technique that − if applied at the industrial scale − has great potential to cut atmospheric CO2 emissions, while more sustainable energy sources than fossil fuels are developed. Estimates from the Department of Energy and Climate Change indicate that deep geological formations underneath the North and East Irish Seas have a CO2 storage capacity of about 70 Gt, which easily exceeds the national targets for reducing CO2 emissions over the next 100 years.
The goal of CCS is to trap CO2 underneath impervious layers (caprock) overlaying the storage site. At the IEAGHG workshop, scientists from Europe, North America, Australia, and Japan presented their experience with several CCS test sites and highlighted once again that CCS is technically feasible, so long as modelling and monitoring efforts are carefully developed and integrated to avoid the unintended environmental side effects associated with CO2 leaking upwards from the storage formations into shallow aquifers or into the atmosphere.
Geological heterogeneities such as those observed at Siccar point give a clear hint that the problem of detecting and preventing CO2 leakage is extremely complex. The variety of conditions found in candidate CCS geological sites, and the uncertainty that exists about these conditions, make the design and management of cost-effective monitoring systems expensive and challenging. This can only be achieved by implementation of cutting-edge site characterization programs and in-situ/ex-situ technologies, together with innovative data assimilation techniques allowing merging observations into simulation/prediction models.
An interesting example of measurements discussed at the meeting is fluid pressure data collected in monitoring ports installed “above-zone”, that is, in brine aquifers overlaying the caprock layer. As the CO2 plume is injected and migrates across the target brine aquifers the pressure pulse propagates much farther, so that if any potential leakage pathways (legacy wells, permeable intrusions, fractures and faults) exist in the caprock, brine first and CO2, possibly much later, will tend to invade above-zone formations, where a change in fluid pressure should be detected well in advance of CO2 leakage. Observations of fluid overpressure can thus be used to infer the presence of weak caprock features through the use of “inverse” multiphase flow reservoir simulators.
An important pro of these data is that they are relatively cheap and, with the adequate instrumentation, can be collected continuously. Potential limitations are associated with low spatial coverage and measurement noise. Still, above-zone pressure data seem to retain a significant value of information and should be possibly included in any CCS monitoring effort. In addition, since strong variations of fluid pressure underground significantly affect stress and strain distributions, spatial coverage can be effectively improved by pairing above-zone pressure data with measurements of ground surface displacement and subsurface deformation, made using InSAR or GPS techniques, tiltmeters, micro-seismic imaging, borehole extensometers, and time-lapse bathymetry.
Given the numerous methods for monitoring subsurface CO2 migration, there is in my view an urgent need to develop methodologies that comprehensively merge these data into modeling results to reduce forecasting uncertainties, support CCS system operations, and ultimately ensure and demonstrate secure geologic storage of CO2.