Challenges Around Incorporating Regional Aquifers in CO2 Storage Simulation Models

Abstract Historical literature on Carbon Dioxide (CO2) storage has often focused on the modelling of storage sites, while the effect of regional aquifers tends to be mentioned in passing, usually in the form of closed versus open systems. Recognising the outsized importance of regional aquifers to storable quantities, this paper focuses on the characterisation and modelling of regional aquifers in reservoir simulation. We also share key observations and lessons from an actual Asia Pacific project that we have worked on, which involved creating a notional development plan for the injection of CO2 into a saline aquifer connected to an existing oilfield. The project used reservoir simulation extensively, and demonstrated the large impact of regional aquifer size on storable quantities. Through the work, we found that existing data collected during hydrocarbon extraction was effective in reducing uncertainty around regional aquifer characterisation, e.g. pressure and production history helped to characterise aquifer size and responsiveness, while the understanding of gross depositional environments helped us to infer how a storage site may be connected to the regional aquifer. When performing the reservoir simulation study, we found the incorporation of the regional aquifer using a numerical aquifer approach to be flexible to implement; however, this flexibility introduced additional degrees of freedom associated with setting up the numerical aquifer. In this paper, we share the various numerical aquifer configurations we used to obtain a match against known production history; these configurations varied in cell count, dimensions and properties. In addition, we also show that connecting the same numerical aquifer to disparate parts of the model resulted in a worm holing effect whereby pressure changes at one boundary was transferred to another boundary via the numerical aquifer; this effect can be avoided by creating a separate numerical aquifer for each boundary.