Flue gas injection into depleted tight hydrocarbon reservoirs in the context of global warming mitigation: Computed evolution of some reservoir parameters

CO2 storage in depleted tight hydrocarbon reservoir seems to be a promising solution to mitigate greenhouse gas emissions. However, flue gas contains not only CO2 but also minor gaseous impurities due to CO2 production or capture processes. In the case of oxy-combustion, the main impurity can be O2 in concentration up to 7%. O2 injection into the reservoir can lead to the oxidation of the residual hydrocarbons and therefore, it is necessary to assess the thermal consequences on the reservoir. COMSOL Multiphysics (R) has been used to model an axisymmetric fractured porous reservoir with its cap and base rocks and containing n-octane as a model compound of residual oil. A global kinetic model for n-octane oxidation has been derived from a previous detailed free-radical model, and implemented into the reservoir model. Simulated injections of N2/O2 mixture (representing a simplified flue gas) have been performed to compute temperature, pressure and n-octane concentration profiles. The results show that the oxidation exothermicity may have a strong influence on temperature profile, especially when the heat capacity of the rock is rather low. In these conditions, the remaining hydrocarbons may be consumed in some months. However, the influence of the thermal conductivity seems negligible. Therefore, it appears safer to select reservoirs whose rocks compositions, in particular the cap and base rocks, have a high heat capacity to promote heat dissipation. It is a criterion to consider when selecting a storage site containing residual hydrocarbons, especially for CO2 captured after oxy-combustion.

Automated summary

This study investigates the feasibility of storing CO2 in depleted tight hydrocarbon reservoirs. The injection of oxygen into the reservoir causes oxidation reactions of residual hydrocarbons, which has significant impact on the temperature profile. It is found that rocks with high heat capacity are more effective in dissipating heat and mitigating the influence of temperature rise on residual hydrocarbons.