Characterization of an Angstrom-Scale Pore Structure in Organic-Rich Shales by Using Low-Pressure CO2 Adsorption and Multifractal Theory and Its Role in CH4/CO2 Gas Storage

Knowledge of CH4 and CO2 storage in pore systems of organic-rich shale can provide valuable perspectives on gas-bearing properties and CO2 sequestration in shale reservoirs. To finely characterize the angstrom-scale pores and investigate their role in CH4/CO2 storage behaviors, this study examines 14 Lower Cambrian Niutitang shale samples by using an array of experiments, including total organic carbon (TOC) content tests, optical observations, section analysis, X-ray diffraction analysis, field-emission scanning electron microscopy (FE-SEM), low-pressure CO2 adsorption (LPGA-CO2), and isothermal adsorption experiments. FE-SEM images reveal the presence of three main types of pores in the studied samples: intraparticle organic pores, dissolved intrapores within quartz particles, and intercrystal pores of clay minerals. Furthermore, the pore size distribution (PSD) curves from LPGA-CO2 possess two prominent peaks, and the pore structure parameters show significant linear covariations with the TOC, clay, quartz, and dolomite contents. The pore structure information exhibits multifractal behavior, and the Q-type cluster analysis on generalized fractal dimension spectrum parameters reveals two distinct types of samples. Type I samples have a stronger degree of PSD heterogeneity, whereas type II samples have better pore connectivity. By virtue of the spherical pore and conceptual pore-filling models, we demonstrate that CO2 has a higher storage volume in the angstrom scale than CH4, whereas CH4 has a higher ratio of the filling volume to the maximum adsorbed capacity. The filling capacity of CO2 is 1.121–2.087 (an average of 1.473) times that of CH4. From the perspective of pore multifractality, higher pore heterogeneity results in stronger CH4 and CO2 storage capacities. The gas filling density in subnano-scale micropores changes with varying pore sizes, which differs from a constant value of the adsorbed gas density for mono-/multilayer adsorption in mesopores. Our findings can provide new geometrical constraints on gas storage behavior in shale reservoirs, which contributes to understanding the gas storage capacity and adsorbed-phase gas density.