Unraveling high-pressure gas storage mechanisms in shale nanopores through SANS

As storage rocks rather than source rocks, shale reservoirs can potentially serve as energy storehouses for energy security and sequester CO₂in the long term to mitigate climate change. Despite extensive studies investigating the geochemical and geophysical properties of shale and gas adsorption and transport in the shale matrix, limited studies have been devoted to characterizing nanoscale gas storage mechanisms in shale at elevated high pressure. In this study, contrast-matching small-angle neutron scattering (SANS) has been conducted to quantify the gas storage mechanisms and capacity in three shale samples up to elevated high pressure using deuterated methane. The three-phase Porod invariant method is uniquely used to estimate the average scattering length density (SLD) in open pores over the measured pore range, in which open pores, closed pores, and rock matrix are the three phases. The estimated average SLD in open pores is smaller than the SLD of the bulk phase at the pressure between 10 MPa and the contrast-matched point (∼60-70 MPa, which is sample-dependent), while it is higher than that of the bulk at the pressure below 10 MPa and above the contrast-matched pressure, indicating a variation of average adsorbed phase density over the measured pressure range for the measured shale samples. The average adsorbed phase volume could first increase and then decrease with increasing pressure until the high-pressure region. Three essential factors, including the final injection pressure, total organic carbon (TOC), and accessible porosity, could be used to screen a potential targeted shale reservoir and maximize methane storage and long-term CO₂sequestration.