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type     June, 2005

Vol 2 Chapter 8: Reactive Transport Modelling of Cap-rock Integrity During Natural and Engineered CO2 storage

James W. Johnson, John J. Nitao and Joseph P. Morris

Abstract: Long-term cap rock integrity represents the single most important constraint on the long-term isolation performance of natural and engineered CO2 storage sites. CO2 influx that forms natural accumulations and CO2 injection for EOR/storage or saline-aquifer disposal both lead to geochemical alteration and geomechanical deformation of the cap rock, enhancing or degrading its seal integrity depending on the relative effectiveness of these interdependent processes. Using our reactive transport simulator (NUFT), supporting geochemical databases and software (GEMBOCHS, SUPCRT92), and distinct-element geomechanical model (LDEC), we have shown that influx-triggered mineral dissolution/precipitation reactions within typical shale cap rocks continuously reduce microfracture apertures, while pressure and effective-stress evolution first rapidly increase then slowly constrict them. For a given shale composition, the extent of geochemical integrity enhancement in the cap rock is nearly independent of key reservoir properties (permeability and lateral continuity) that distinguish EOR/sequestration and saline formation settings and of CO2 influx parameters (rate, focality, and duration) that distinguish engineered disposal sites and natural accumulations, because these characteristics and parameter have negligible (indirect) impact on mineral dissolution/precipitation rates. In contrast, the extent of geomechanical integrity degradation is highly dependent on these reservoir properties and influx parameters because they effectively dictate magnitude of the pressure perturbation. Specifically, initial geomechanical degradation has been shown inversely proportional to reservoir permeability and lateral continuity and proportional to influx rate. Hence, while the extent of geochemical alteration is nearly independent of filling mode, that of geomechanical deformation is significantly more pronounced during engineered storage. This suggests that the currently secure cap rock of a given natural CO2 accumulation may be incapable of providing an effective seal in the context of an engineered injection, a potential discrepancy that limits the extent to which natural CO2 reservoirs and engineered storage sites can be considered analogous. In addition, the pressure increase associated with CO2 accumulation in any compartmentalized system invariably results in net geomechanical aperture widening of cap-rock microfractures. This suggests that ultimate restoration of pre-influx hydrodynamic seal integrity—in both EOR/storage and natural accumulation settings—hinges on ultimate geochemical counterbalancing of this geomechanical effect. To explore this hypothesis, we have introduced a new conceptual framework that depicts such counterbalancing as a function of effective diffusion distance and reaction progress. This framework reveals that ultimate counterbalancing of geochemical and geomechanical effects is feasible, which suggests that shale cap rocks may in fact evolve into effective seals in both natural and engineered storage sites.

Carbon Dioxide Capture for Storage in Deep Geologic Formations – Results from the CO2 Capture Project Geologic Storage of Carbon Dioxide with Monitoring and Verification - Volume 2
Edited by:
Sally M. Benson, Lawrence Berkeley Laboratory, Berkeley, CA, USA

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