Influence of solvents on pore structure and methane adsorption capacity of lacustrine shales: An example from a Chang 7 shale sample in the Ordos Basin, China

The dissolution of soluble organic matter by solvents can increase the porosity and pore connectivity of shale, which is conducive to the exploitation of shale gas. To explore the influence of solvents on the pore structure and methane adsorption capacity of extracted shale, a sample of the Chang 7 shales from the Ordos Basin, China is extracted using acetone, tetrahydrofuran (THF), carbon disulfide (CS2), and benzene, respectively. Low pressure gas (CO2 and N-2) adsorptions and methane adsorption experiments were carried out on the studied sample before and after extraction. The results indicate that the extraction yields of THF, CS2, benzene and acetone were 0.84%, 0.63%, 0.34%, and 0.31%, respectively, which is jointly influenced by the molecular dynamics diameter, aromaticity, boiling point, and polarity. The extraction yield of acetone (0.31%) is the lowest, which may be attributed to its low boiling point. The extraction yield of CS2 is about 2 times higher than acetone. It may be related to that the CS2 can destroy ionic crosslinks in organic matter, improving the extraction yield significantly. The highest extraction yield (0.84%) of THF is consistent with its aromatic structure, high boiling point, and strong polarity. The extraction yield (0.34%) of benzene is relatively low among the used solvents, which may be aroused by its larger dynamic diameter and weak polarity. Interestingly, the size ranges of pores that increased after extraction are strongly controlled by solvents. Acetone is easier to enter the micropores due to its smaller dynamics diameter. Therefore, the organic matters in micropores were more easily dissolved in acetone, which results in the significant increases in micropore volume and surface area. Comparably, mesopore volume and surface area increased greatly after CS2 extraction. One possibility is that CS2 mainly dissolved the organic matters in mesopores. Another possibility is that a part of micropores were enlarged to mesopores by extraction. The macropore volumes of the sample extracted by benzene and THF obviously increased due to their larger dynamics diameters. The methane adsorption capacity of the extracted sample is stronger than the raw sample. The type of adsorption isotherm was not changed. The acetone extracted sample has the highest methane adsorption capacity, followed by THF, CS2, and benzene, which is positively related with their micropore surface areas.