Methodology for estimating the prospective CO2 storage resource of residual oil zones at the national and regional scale

Residual oil zones (ROZs) are increasingly gaining interest as potential reservoirs for carbon dioxide (CO₂) storage. Here, we present a national- and regional-scale methodology for estimating prospective CO₂ storage resources in residual oil zones. This methodology uses a volumetric equation that accounts for CO₂ storage as a free phase in pore space and as a dissolved phase in oil and does not assume any oil production associated with CO₂ storage. Reservoir modeling and the CO₂-SCREEN tool are used to demonstrate that CO₂ storage in residual oil zones will predominantly take place in the free phase (approximately 92–97%) with some storage as dissolution in oil (approximately 3–8 %). Based on this preliminary demonstration, the CO₂ storage efficiency for ROZs using this national- and regional-scale method ranges from 0.61 to 7.1 %. This range indicates ROZs have a similar efficiency potential for storing CO₂ as deep saline formations (0.51–5.4 %).