Feasibility and prospects of symbiotic storage of CO2 and H2 in shale reservoirs

Storing CO2 and H2 in underground reservoirs represents an effective approach to sequester increasing amounts of captured CO2 for carbon neutrality and to store H2 to promote clean energy revolutions. However, commercial/pilot-scale CO2/H2 storage sites are mainly restricted to conventional oil reservoirs or salt caverns – both capacity and geographic-location limited. This paper presents a systematic review of the feasibility and prospects of CO2 and H2 storage in fractured shale reservoirs as secure repositories. Both field pilot and laboratory studies of CO2 injection in shales are cross-analyzed across various spatial and time scales, to provide a reliable and substantial basis to support new findings. The presence of suitable injectivity and adequate sealing capacity in shale are demonstrated. The fracture networks are shown to provide major storage space in shales, contrasting with the pore system in conventional reservoirs. This difference in storage mechanisms results in an overestimation in the storage capacity of shales when applying porous-medium-based methods. An underestimation in the mass of injection, however, is apparent from a single well for the reported cases due to unknown characteristics of underground fracture networks. The symbiotic storage of CO2 and H2 in shale is discussed in its feasibility and ability to improve both CO2 storage but principally H2 recovery – due to the presence of a gas cushion. A new equivalent-fracturing method is proposed as a supplement to recalibrate the over- and under-estimated prospects of CO2/H2 storage in shales – a necessary component in reducing carbon emissions and accelerating the energy transition.