Experimental Study on the Effect of Multiple CO2 Extractions on Wax Deposition in High-Wax Shale Oil and Prevention Recommendations

Abstract

Abstract CO2 fracturing and flooding have become key technologies for increasing shale oil recovery. However, for shale reservoirs with high wax content, the production data from Daqing oilfield indicate lower later-stage recovery rates and a significant occurrence of wax deposition during production employing CO2 injection. Therefore, it is crucial to clarify the applicability of CO2 in high-wax shale reservoirs. This study focuses on high-wax shale oil from the Qingshankou Formation in the Songliao Basin, China to explore the wax precipitation temperature and composition changes before and after CO2 extraction. Initially, the shale oil underwent repeated CO2 extractions and the variations in wax precipitation temperature and crystallization characteristics were analyzed. Subsequently, chromatographic column analysis was employed to examine the crude oil compound composition before and after CO2 extraction. Finally, the wax precipitation mechanism was explored, and corresponding suggestions were provided. Research suggests that after multiple CO2 extractions in the high-wax shale oil, light hydrocarbons decreased, the wax precipitation temperature and wax yield increased, and the wax crystal morphology became denser. After three cycles of CO2 extraction, the saturated hydrocarbon content in shale oil decreased by 21%. This may be attributed to the reduced solubility of crude oil in wax crystals due to CO2 extraction of light hydrocarbons. As a result, the content of heavy hydrocarbons such as asphaltene increased, providing more nucleation sites and accelerating wax precipitation. The deposition of organic solids such as paraffin causes damage to the micro-nano flow channels in the reservoir, increasing the flow resistance of the crude oil. Therefore, the development of high-wax shale oil reservoirs can be addressed by employing chemical inhibitors, optimizing CO2 flooding technology, and changing extraction methods. The comprehensive application of various technological approaches and effective reservoir management can mitigate the adverse effects of wax precipitation on the reservoir. This study investigates the applicability of CO2 to high-wax shale oil reservoirs, and challenges traditional perceptions of CO2 fracturing and flooding techniques benefiting all shale oil reservoirs. The corresponding recommendations provided in this paper serve as technical insights for the development of such reservoirs.