Experimental investigation on pyrolysis products and pore structure characteristics of organic-rich shale heated by supercritical carbon dioxide

The efficient pyrolysis and conversion of organic matter in organic-rich shale, as well as the effective recovery of pyrolysis shale oil and gas, play a vital role in alleviating energy pressure. The state of carbon dioxide (CO₂) in the pyrolysis environment of shale reservoirs is the supercritical state. Its unique supercritical fluid properties not only effectively heat organic matter, displace pyrolysis products and change shale pore structure, but also achieve carbon storage to a certain extent. Shale samples were made into powder and three sizes of cores, and nitrogen (N₂) and supercritical carbon dioxide (ScCO₂) pyrolysis experiments were performed at different final pyrolysis temperatures. The properties and mineral characteristics of the pyrolysis products were studied based on gas chromatography analysis, X-ray diffraction tests, and mass spectrometry analysis. Besides, the pore structure characteristics at different regions of cores before and after pyrolysis were analyzed using N₂ adsorption tests to clarify the impact of fracturing degree on the pyrolysis effect. The results indicate that the optimal pyrolysis temperature of Longkou shale is about 430 ℃. Compared with N₂, the oil yield of ScCO₂ pyrolysis is higher. The pyrolysis oil obtained by ScCO₂ extraction has more intermediate fractions and higher relative molecular weight. The ScCO₂ can effectively improve the pore diameter of shale and its effect is better than that of N₂. The micropores are produced in shale after pyrolysis, and the macropores only are generated in ScCO₂ pyrolysis environments with temperatures greater than 430 ℃. The pore structure has different development characteristics at different pyrolysis temperatures, which are mainly affected by the pressure holding of volatile matter and products blocking. Compared to the surface of the core, the pore development effect inside the core is better. With the decrease in core size, the pore diameter, specific surface area, and pore volume of cores all increase after pyrolysis.