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

Abstract

Knowledge of CH 4 and CO 2 storage in pore systems of organic-rich shale can provide valuable perspectives on gasbearing properties and CO 2 sequestration in shale reservoirs. To finely characterize the angstrom-scale pores and investigate their role in CH 4 /CO 2 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, fieldemission scanning electron microscopy (FE-SEM), low-pressure CO 2 adsorption (LPGA-CO 2 ), 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-CO 2 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 CO 2 has a higher storage volume in the angstrom scale than CH 4 , whereas CH 4 has a higher ratio of the filling volume to the maximum adsorbed capacity. The filling capacity of CO 2 is 1.121−2.087 (an average of 1.473) times that of CH 4 . From the perspective of pore multifractality, higher pore heterogeneity results in stronger CH 4 and CO 2 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.

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