Mitigating capillary barriers in heterogeneous and anisotropic sandstones: Impact of Surfactants on CO2 relative permeability in saline aquifers

Deep saline aquifers are promising candidates for geological CO 2 storage; however, high capillary pressures often restrict the initial CO 2 injectivity, particularly in near-wellbore regions. Although surfactant-induced interfacial tension reduction is known to lower capillary resistance, its direct impact on dynamic multiphase flow across various rock fabrics remains poorly quantified. This study investigated the effects of a nonionic surfactant on CO 2 –brine relative permeability and spatial fluid distribution using a 2D X-ray imaging–integrated core flooding system. To elucidate the influences of rock heterogeneity and flow orientation, primary drainage experiments were conducted under reservoir conditions on three distinct sandstones representing contrasting rock fabrics: homogeneous Berea sandstone, heterogeneous offshore reservoir sandstone (D-1), and vertically cored Otway CRC-2 sandstone. Interfacial property measurements confirmed that the surfactant significantly reduced the capillary factor by lowering the interfacial tension and altering wettability. The dynamic flow results revealed that surfactant pretreatment enhanced CO 2 mobility across all samples. In the heterogeneous d -1 sandstone, the surfactant allowed CO 2 to invade previously bypassed pore volumes, thereby increasing endpoint CO 2 saturation and doubling relative permeability. In the vertically cored CRC-2 sandstone, the surfactant successfully mitigated serial capillary barriers produced by cross-bedding flow. It smoothed the severe localized saturation oscillations observed under baseline and increased endpoint CO 2 relative permeability by approximately 2.5 times. These findings demonstrate that the mechanism of surfactant-assisted injectivity enhancement strongly depends on the heterogeneity and anisotropy of reservoir rock, highlighting the potential of targeted near-wellbore surfactant treatments for optimizing commercial-scale CO 2 storage operations.