Chemical structure changes of marine shale caused by supercritical carbon dioxide

Abstract

The interaction between shale and CO 2 is of great significance to CO 2 storage and shale gas production; however, there are few reports on chemical changes after CO 2 injection into shale reservoirs. We used Fourier transform infrared spectroscopy to study the chemical structure changes of shale samples before and after CO 2 injection. In this paper, self-developed high-temperature and high-pressure vessels were used to examine CO 2 treatment of shale to simulate the stratigraphic environment and infrared was used to study the influence of changes in pressure and temperature on the chemical interaction between shale and CO 2 . These results showed that: (1) after shale CO 2 treatment, the number of functional groups remained unchanged, though the content and structure of the shale did change; (2) pressure increases reduced hydroxyl and aliphatic group levels in the shale; the branched-chain and aromaticity indexes of the shale were reduced by 81.5% and 53.8%, respectively, relative to untreated samples; (3) below 60°C, free water in the shale increased with temperature increases and dissolution of the shale increased; however, above 60°C, dissolution was inhibited which resulted in precipitation; (4) the dissolution degree of shale carbonate minerals positively correlated with CO 2 pressure and temperature, while silicate minerals were unchanged CO 2 pressure and temperature. Finally, the change of CO 2 pressure had a more significant influence on the chemical structure of shale. These results provide a basis for the continuing study of the influence of chemical structure change of shale caused by CO 2 on shale wettability and enhance the theoretical level of CO 2 shale mining.