Effects of fluvial sedimentary heterogeneity on CO2 geological storage: Integrating storage capacity, injectivity, distribution and CO2 phases

Abstract

Fluvial system deposits often form suitable reservoirs for CO2 geological storage (CGS). These potential storage sites usually present heterogeneous fluvial architectures at multiple scales. This heterogeneity can exert varied effects on different aspects of CGS, resulting in significant storage efficiency variability. Here, we investigate the effects of variable fluvial lithofacies associations on CO2 storage, using the Puig-reig anticline in the SE Pyrenees (Spain) as a reservoir analog. To test this, we employ a multidisciplinary approach that integrates field sedimentology, reservoir modeling, and numerical simulation of CO2 injection to produce models akin to different fluvial lithofacies associations. The storage volume and injectivity of CO2 are found to decrease in reservoirs with decreasing fractions and sizes of high-permeable facies from the proximal to the medial-distal lithofacies associations. The flow barriers created by low-permeable facies can hinder the vertical migration of the CO2 plume and prevent it from reaching the reservoir top, hence reducing the direct contact between the CO2 plume and the overlying caprock. Furthermore, an optimal amount of low-permeable layers (around 30% in this study) can increase the swept area of CO2 and reduce the proportions of free CO2 phase. These aspects can collectively increase the amount of permanently trapped CO2 and reduce the leakage risks of the injected CO2. Based on the characteristics of the resulting models (i.e., storage volume, injectivity, distribution and phases of CO2), a multi-criteria decision-making method has been used to quantitatively rank the different lithofacies associations according to their suitability for CO2 storage. In this analysis, the proximal-medial fluvial lithofacies associations are assessed to be the most suitable ones because they feature low proportions of the injected CO2 reaching the reservoir top and in free phase while maintaining the high storage volume and injectivity. This study reveals that heterogeneous reservoir architectures have mixed effects on CO2 storage, and that reservoirs featuring moderately heterogeneous architectures (i.e., fractions of low-permeable facies ranging from 30% to 40%) are beneficial to keeping the balance among different aspects of CO2 storage. This provides new insights for the screening and selection of potential geological sites for CO2 storage.