Improving Gas Sequestration by Surfactant-Alternating-Gas Injection: A Comparative Evaluation of the Surfactant Type and Concentration

Gas injection into porous subsurface geological formations for geological storage is currently considered the most preferable and practicable means of reducing greenhouse gases due to the huge capacity of deep saline aquifers. Residual trapping of gas plays an important role in immobilization of injected gas into an aquifer. Surfactant-alternating-gas (SAG) injection can be used as a mobility control method in gas sequestration and several simulation studies explained different aspects of residual-gas trapping. However, research in this field is inconclusive and needs more attention to develop a better understanding. In this work, we used 3 different surfactants from different classes at various concentrations to assess gas-sequestration efficiency using SAG methods in carbonate and sandstone rocks. This work is the first of its kind on the comparative performance of different surfactants for gas sequestration in different rocks. The surfactants were hydrocarbon zwitterionic, fluorinated zwitterionic, and nonionic. It was found that residual-gas saturation increases by increasing the surfactant concentration. The increase in residual trapping with surfactant concentration also depends on the type of surfactant and rock. In both rock types considered, the best performance was achieved using fluorinated zwitterionic surfactants followed by hydrocarbon zwitterionic surfactants. In addition, a synergetic effect between hydrocarbon and fluorinated surfactants further improves the residual-gas saturation. The residual-gas trapping was higher in the tight sample compared to highly permeable samples. This will help in developing an understanding of surfactant optimization and selection for gas sequestration using the SAG method.