Impacts of fluvial reservoir heterogeneity on connectivity: Implications in estimating geological storage capacity for CO2
Résumé
Our awareness of global warming and increasing greenhouse gas emissions emphasises the need to develop counteractive technologies. One promising tool in this respect would appear to be the geological storage of CO2, but there still remain uncertainties regarding the geological complexity of the subsurface. As in the oil and gas context, determining the scale of the heterogeneities that impact the reservoir storage capacity and fluid flow efficiency is crucial. In this paper, we propose to study the impact of two fluvial heterogeneity scales: (1) the architectural scale (megascopic scale) which consists in the connectivity between the main channel belts and (2) the scale of the channel belt (macroscopic scale) which considers the internal sedimentary fill, mainly composed of silty-sandy abandoned channel. To assert geological consistency, this study relies on a conceptual geological model discussed here and based on fieldwork in Saudi Arabia. This model incorporates sedimentary bodies ranging from fluvial braided to high- and low-wandering meandering type. Following the established concept, the largest reservoir bodies are found at the base and top of the system. A workflow is then proposed to statistically analyse the impact of the two considered heterogeneity scales onto CO2 storage characteristics. A code has been implemented to stochastically generate two series of 3D numerical models that account for the conceptual geological model. Each pair of models from the two series share the same architectural structure and only differ in their internal channel body infill. These two series of homologous 3D models support a statistical analysis of the impact of the heterogeneity representation on reservoir capacity. For a 25 km by 25 km by 60 m thick 3D model with a net to gross of 15%, the results show that estimated storage capacities can reach 5.7 Mt and 7.5 Mt respectively for the uppermost and the basal bodies. The presence of oxbow lakes, however, can lead to an estimated loss of capacity of about 11% (∼0.9 Mt) in the basal volumes and 20% (1.3 Mt) in the uppermost volumes.