An experimental study of gas sequestration efficiency using water alternating gas and surfactant alternating gas methods

Capillary trapping plays an important role in the geological storage of Carbon dioxide (CO₂) because of its ability to immobilize a significant fraction of the injected gas. Numerous papers have investigated various factors that affect capillary trapping. Recently, gas-mobility control methods are being investigated through simulation studies, for improving capillary trapping of CO₂ in saline aquifers. The results published so far are inconclusive and sometimes conflicting. In this study, a series of laboratory experiments was conducted to investigate and compare the effects of two mobility control methods namely, water alternating gas (WAG) and surfactant alternating gas (SAG), on gas sequestration efficiency. Efficiency was measured by injectivity and residual trapping capacity. The two methods were tested in both vertical and horizontal flow directions using rock samples of varying lithology and permeability. The different gas injection schemes were compared with the continuous gas injection method. WAG method consistently reduced residual gas saturations in all the experiments while SAG method significantly increased residual gas saturation for both horizontal and vertical flows. Comparison of performance of SAG method in horizontal flow with its performance in vertical flow revealed a significant increase in residual gas saturation when applied in vertical flow direction. However, SAG method resulted in decreasing injectivity as the SAG cycles increase while WAG injectivity remained unaffected throughout its cycles except in tight samples. Furthermore, higher residual gas saturations were observed in high permeability rocks compared to tight rocks when subjected to vertical-SAG sequestration method. The possible reasons for such efficiency in the vertical-SAG method are discussed in detail.