Modeling Thermal-Hydrological-Mechanical (THM) Processes for CO₂ Sequestration in Depleted Gas Reservoirs

Over the past two decades, researchers have devoted enormous efforts into developing numerical models for simulating CO₂ geo-sequestrations. Due to the large temporal and spatial scales associated with such processes, numerical modeling has proven to be the most 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 and thermal 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 storing CO₂ in depleted petroleum reservoirs can serve as an interim solution for storage until large-scale storage in saline aquifers reaches maturity. 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 thermalhydrological-mechanical modeling. Our research could fill the gap and provide a reference for future studies in this regard.