The impact of heterogeneity on capillary trapping during geological carbon storage

Abstract

Capillary heterogeneity trapping has the potential to significantly improve the security of CO2 storage in underground aquifers. Small-scale heterogeneities could enhance the level of trapping in natural rocks and provide greater storage capacity. Capillary heterogeneities act to trap CO2 at saturations greater than expected from pore-scale residual trapping processes alone as a result of capillary pressure barriers. The aim of this work is to understand the effect of natural rock heterogeneities on capillary trapping over different length scales, during CO2 sequestration. A multiscale approach is applied, combining experimental characterisation and numerical modelling, to create a physics-based representation of capillary heterogeneity trapping. The capillary heterogeneity trapped saturation during CO2 storage post-imbibition is quantified through a 1D analytical model. The analytical model predicts rate-dependency of capillary heterogeneity trapping, validated through experiments. Steady-state core flooding experiments with medical X-ray CT scanning provide a detailed characterisation of continuum multiphase flow properties, including residual trapping characteristics, over cm-scales. These experiments highlight the importance in correctly identifying the mechanism by which saturation is trapped at the core-scale and extracting appropriate trapping relationships when upscaling to the field. This motivated a study which focused on incorporating uncertainty in core and field-scale data into models of the Captain Sandstone, a UK target storage site. To investigate the flow dynamics in heterogeneous sandstone cores, state of the art synchrotron-based X-ray micro-CT experiments at the Australian synchrotron (ANSTO) and European synchrotron (ESRF) have been carried out. These experiments investigated how larger scale capillary heterogeneity trapping processes are impacted by pore-scale filling events, across samples with different heterogeneity types. The resulting saturation distributions demonstrate the impact of cm-scale heterogeneity on pore-scale processes, which in turn influence large scale behaviour. These novel insights develop our understanding of the impact of heterogeneity on fluid migration and capillary trapping from the pore-to-core-to-field scale.