Modeling CO₂ sequestration in depleted gas reservoirs considering the mutual solubilities in CO₂-hydrocarbon gas-brine systems

Abstract

Over the past two decades, researchers have devoted enormous efforts into developing numerical models for CO2 geo-sequestrations. Due to the large temporal and spatial scales associated with such processes, numerical modeling has proven to be the most effective and cost-effective to evaluate them. presently, the majority of the existing models were developed for CO2 sequestration in saline aquifers. In contrast, the number of models for depleted gas reservoirs is relatively small, and only a fraction of them have the capability to consider geomechanical coupling or complicated natural gas compositions. CO2 sequestration in depleted gas reservoirs is considered a promising technique for combating global warming. The use of depleted gas reservoirs has several advantages over saline aquifers, such as the availability of existing facilities and the proven capacity and stability of gas storage. It is believed that CO2 storage in depleted petroleum reservoirs can provide intermediate-scale storage until large-scale storage in saline aquifers becomes mature. Here we propose an Equation of State (EOS) module for CO2 sequestration in depleted gas reservoirs or CO2-enhanced gas recovery. It takes into consideration the mutual solubilities in the CO2-hydrocarbon gas-brine system based on the equality of the chemical potentials of the aqueous and non-aqueous phases. In such a way, it can properly address CO2 dissolution into brine when hydrocarbon gas is present. Hydrocarbon gas includes methane, ethane, and propane. It employs well-recognized approaches to calculate the thermodynamic properties of the gaseous and aqueous phases. This EOS module has been applied to our in-house simulator, TOUGH2-CSM, to enable thermo-hydrological-mechanical modeling. Our research could fill the gap and provide a reference for future studies in this regard.