Resolving nanoscale pore attributes in geological reservoirs for CO₂ storage assessment

Understanding gas storage potential in porous unconventional reservoirs, like shale, involves resolving complex pore morphology, pore accessibility as well the inherent heterogeneity and anisotropy of the reservoir. Quantifying the nanopore attributes in shale requires a combination of special techniques, such as low-pressure gas adsorption (LPGA), small-angle neutron/X-ray scattering (SANS/SAXS) and high-resolution microscopy. In this work, we discuss the influence of shale composition, thermal maturity of organic matter and geological setting on the total pore volume in shales. Pore characteristics determined, through gas adsorption and small-angle scattering techniques, provide new insights towards the pore accessibility in shales showing that total pore volume can be 66%-100% higher than accessible pore volume. Although pore volume increases, pore accessibility decreases with decreasing pore size, and the effect is more pronounced in deeper shales due to higher overburden pressure. Thermal treatment of the shales under anoxic and oxic environment shows a starkly opposite behaviour. Changes in pore morphology and increase in pore volume is more prominent in oxic-heated shales, which is further catalysed due to presence of siderite. A comparative gas-adsorption and SAS studies confirmed pore closure by bitumen in shales subjected to anoxic heating till 300 °C. A scanning SAXS approach was adopted to quantify the inherent bedding-parallel pore heterogeneity and pore localization in shales. The study shows that the degree of heterogeneity increases with maturity of shales, and pores are preferentially abundant in organic matter as compared to the mineral matrix. (author)