CO₂ and N2 injection to enhance shale gas recovery

We investigate the potential benefits of injecting the mixtures of CO₂ and N2 into shale reservoirs to both optimize recovery of natural gas and to sequester CO₂. In this work, we develop dual-porosity, dual-permeability finite element models to simulate multicomponent gas flow in porous media coupled with shale deformation and sorption behavior. We use this model to explore the injection of pure CO₂, pure N2, and mixtures of CO₂ and N2 in different ratios to enhance shale gas recovery (ESGR). This behavior necessarily includes the evolution sorption-induced strain by competitive adsorption and its influence on permeability of matrix and fractures to ultimately define cumulative production history of CH4. The results show that injecting pure CO₂ can increase shale gas recovery by ∼20%. Injection of N2 works as an ESGR agent and can increase shale gas recovery by ∼80%. Injecting a mixture of N2 and CO₂ of different gas ratios results in recovery ranging between ∼20% and ∼80% (pure component end-member cases). Moreover, injecting N2 in higher proportions relative to CO₂ is more beneficial since N2 shows higher recovery in ESGR. Therefore, increasing injection pressure is not the only means to achieve higher production, instead, increasing N2-CO₂ gas ratio can achieve the same goal. The observations also show that although CO₂ is not sensitive to pressure in performing ESGR, it is sensitive to pressure when accounting for sequestered CO₂. Furthermore, the sequestration of CO₂ by injecting CO₂-N2 mixtures does not simply increase as CO₂-N2 gas ratio increases. Since higher CO₂-N2 ratio results in a decrease in shale gas recovery, which leads to more CH4 left in the reservoir to compete with CO₂ for sorption sites, this also results in reduced CO₂ sequestration.