Numerical simulation of CO₂-SO₂ co-sequestration in shale gas reservoirs coupled with enhanced gas recovery at reservoir scale

Due to the favorable affinity of SO₂ adsorption on shale over CO₂ and CH₄, CO₂-SO₂ co-sequestration in shale gas reservoirs coupled with enhanced natural gas recovery has been proposed recently. To evaluate the feasibility of injection impure CO₂ containing SO₂ for shale gas recovery and CO₂ storage at reservoir scale, we established a field-scale shale gas production model which incorporates multiple fluid flowing mechanisms including the slip flow, viscous flow, Knudsen diffusion and also gas adsorption/desorption, based on accurate CO₂-SO₂-CH₄ mixtures properties prediction. Results show that the presence of 3 mol% SO₂ in the CO₂ stream can increase CH₄ production by 9.55 % via increasing the pressure differential of the production well and promoting the migration of CO₂ with displacement and replacement and it has negligible impact on CO₂ sequestration. The CH₄ production capacity increases with the SO₂ content in the CO₂ stream. However, excessive adsorption of SO₂ over CO₂ on shale is not advantageous for shale gas recovery because high adsorption of SO₂ in the CO₂ stream can alleviate the pressure build-up induced by fluid injection and hinder SO₂ migration. In addition, larger reservoir pressure and temperature, artificial fracture half-length, fracture permeability and lower reservoir permeability can exaggerate the positive impact of SO₂ on CH₄ production. The increase of the aforementioned factors plays negative role in CO₂ storage security with the exception of temperature.