Abstract
Demonstrating conformity between observed and simulated plume behaviour is one of the main high-level requirements, which have to be fulfilled by an operator of a CO2 storage site in order to assure safe storage operations and to be able to transfer liability to the public after site closure. The observed plume behaviour is derived from geophysical and/or geochemical monitoring. Repeated 3D seismic observations have proven to provide the most comprehensive image of a CO2 plume in various projects such as Sleipner, Weyburn, or Ketzin. The simulated plume behaviour is derived from reservoir simulation using a model calibrated with monitoring results. Plume observations using any monitoring method are always affected by limited resolution and detection ability, and reservoir simulations will only be able to provide an approximated representation of the occurring reservoir processes. Therefore, full conformity between observed and simulated plume behaviour is difficult to achieve, if it is at all. It is therefore of crucial importance for each storage site to understand to what degree conformity can be achieved under realistic conditions, comprising noise affected monitoring data and reservoir models based on geological uncertainties. We applied performance criteria (plume footprint area, lateral migration distance, plume volume, and similarity index) for a comparison between monitoring results (4D seismic measurements) and reservoir simulations, considering a range of seismic amplitude values as noise threshold and a range of minimum thickness of the simulated CO2 plume. Relating the performance criteria to the noise and thickness threshold values allows assessing the quality of conformance between simulated and observed behaviour of a CO2 plume. The Ketzin site is provided with a comprehensive monitoring data set and a history-matched reservoir model. Considering the relatively high noise level, which is inherent for land geophysical monitoring data, a reasonable conformance between the observed and simulated plume behaviour is demonstrated.
Highlights
On the European and international level, there is a variety of regulations defining the legal framework for CO2 storage in the six lifecycle phases of a storage site: (1) assessment, (2) characterization, (3) development, (4) operation, (5) post closure, and (6) post transfer (Kühn et al, 2013)
In this study we present an approach we applied on the monitoring and simulation results of the Ketzin pilot site, taking into account uncertainties related to the identification of the CO2 distribution from the monitoring data and related to the detection limits, affecting the ability to image the full extent of the CO2 plume
It is assumed that the simulated plume footprint area and the simulated plume volume predicted by reservoir simulations on the basis of history-matched models are in conformance with the “real” CO2 distribution, even if the exact shape of the CO2 plume cannot be reproduced by the simulation
Summary
On the European and international level, there is a variety of regulations defining the legal framework for CO2 storage in the six lifecycle phases of a storage site: (1) assessment, (2) characterization, (3) development, (4) operation, (5) post closure, and (6) post transfer (Kühn et al, 2013). According to the EU CCS Directive, Article 18, the site operator has to demonstrate that three high-level requirements are fulfilled in order to assure the long-term safety of the storage site and to be able to transfer the liability to the public. Comparing the performance measures of the geophysical signature, assuming a range of noise thresholds, with the performance measures of the simulated CO2 plume will enable to determine the site-specific achievable conformance between simulated and observed plumes and the detection limits of the geophysical measurements considered in this context
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