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

Abstract

• Injecting flue gas captured after oxy-combustion in depleted tight hydrocarbon reservoir can lead to the oxidation of the residual hydrocarbons. • The temperature increase due to the oxidation exothermicity depends strongly on the heat capacity of the rocks. • A high heat capacity makes the temperature increase negligible. • Thermal conductivity and its potential anisotropy have almost no impact on temperature profile. CO 2 storage in depleted tight hydrocarbon reservoir seems to be a promising solution to mitigate greenhouse gas emissions. However, flue gas contains not only CO 2 but also minor gaseous impurities due to CO 2 production or capture processes. In the case of oxy-combustion, the main impurity can be O 2 in concentration up to 7%. O 2 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® 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 N 2 /O 2 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 CO 2 captured after oxy-combustion.