Impact of fault rock properties on CO2 storage in sandstone reservoirs

The aim of the IMPACT project was to improve risk assessment when planning and developing potential reservoirs for CO2 sequestration. This was achieved by increasing our understanding of the processes and products of faulting in porous sandstone in order to forecast the distribution and impact of faults and deformation bands on reservoir performance and seal properties. The IMPACT Project lasted for 4 fruitful years (2011-2014). The status of the project is very satisfying with regard to fulfillment of the project milestones. Totally, six Master students have been graduated as part of the IMPACT Project. Both PhD students, Reza Alikarami and Elin Skurtveit have successfully defended their theses last year. Our research results have been published in high ranked journals and have been disseminated through presentations in relevant international conferences, where we emphasized the application of our research for CO2 storage underground. Some of our results are summarized below: -In-situ measurements of stiffness and permeability of deformed/ faulted sandstone can be used to predict the behaviour of these rocks in response to stress change and/or fluid flow. -Intense grain crushing in immature and poorly consolidated sandstone could occur at shallow burial depth during deformation independent of mineralogy of the rock, which will locally reduce the permeability of these rocks up to two orders of magnitude. -The degree of consolidation of sandstone at the time of faulting affects the sealing property of sandstone, resulting in variable CO2 column heights in fault related rocks. -Slip surfaces in poorly consolidated sandstone could form conduits to fluid, which might cause a risk of CO2 leakage in these kinds of reservoirs. -Faults can change from a baffle to a conduit to fluid flow during different deformation episodes. -Triaxial experiments on poorly consolidated sandstone and sand are used to understand the mechanism of deformation at different stress levels and predict the final deformation products and their effects on reservoir quality. -Structural imaging using seismic attribute analysis and analogue fault models help understanding the kinematic and possible reactivation scenarios of faults in Snøhvit Field (Barent Sea). - Our numerical modelling reveals that loading rates and pore pressure increase affect the localization and configuration of the observed shear bands in a deformed sandstone reservoir.

An optimal reservoir for CO2 storage should ideally have high porosity and permeability and exhibit predictable communicational properties. Within sandstone reservoirs, faults and deformation bands may act as barriers and baffles, thus introducing compart mentalization and areas of flow retardation, deviation or acceleration and hence make sub-surface fluid flow less predictable. This will in turn affect the injection rate and the total capacity of the reservoir or compartments. CO2 injection in aquifers creates a fluid pressure increase, which causes changes in the stress state of the aquifer and the reservoir seals. This might affect and reactivate faults, fractures or other deformation structures both within and around the reservoir. (Li et al., 2007). The proposed project aims to increase our understanding of the processes and products of faulting in porous sandstone in order to forecast the distribution and impact of faults and deformation bands on reservoir/aquifer performance and seal properties. Th is will contribute to improved risk assessment when planning and developing potential reservoirs for CO2 sequestration. To achieve these aims, a comprehensive integrated and cross-disciplinary study, which combines analysis of empirical outcrop and subsu rface data, experiments using physical analogues, micro-structural analysis, is called for. The experiments will utilize the existing CIPR database and the state-of-the-art geo-mechanical testing equipments at the Norwegian Geotechnical Institute, support ed by numerical back calculations of the experiments. The final results will be also utilized to verify and, where possible, improve the theories for development of faults and deformation bands.