Low-carbon treatment and remediation of oil sludge in mid-to-high latitude regions: A coupled approach of freeze-thaw and supercritical CO₂ extraction

The oil sludge produced while extracting large oil and gas fields in the middle and high latitude regions has caused serious pollution to the surrounding soil. The key to solving this problem in the future is to unify the remediation of soil and the treatment of oil sludge. This study uses supercritical carbon dioxide(scCO₂) technology to construct a low-carbon method, providing a new approach to achieve this goal. The study determines the optimal extraction conditions for black calcareous soil with 15% oil content to be 55 °C, 25 MPa, and 90 min through single factor and response surface experiments. Experiments on the scCO₂ extraction coupled with freeze-thaw cycles show that oil sludge with a water content of 10% can improve the extraction efficiency of scCO₂ by about 2.69% after less than five freeze-thaw cycles. The study also compares the extraction efficiency of the four soils, with a difference of 6.03% observed under the same conditions. Additionally, we analyze the impact of the extraction process on changes in the properties of the oil and soil in the oil sludge. Comprehensive tests, including scanning electron microscope (SEM), nutrient detection, X-ray powder diffractometer (XRD), fourier transform infrared spectroscopy (FTIR), and Gas Chromatography (GC), have been conducted. Results show that standalone scCO₂ extraction can remove up to 98.2% of petroleum hydrocarbons from the oil sludge, while simultaneously causing small changes to the soil microstructure and the crystal structure of the oil sludge. Furthermore, this process does not lead to a significant depletion of key nutrients or the generation of new pollutants.