Mechanism and mitigation strategies of wellbore wax plugging during CO₂ flooding in tight conglomerate reservoirs

To address the frequent wax deposition issues in wellbores, caused by lower water cut (≤10%) and higher CO₂ content in the produced fluid, during CO₂ flooding development of tight conglomerate reservoirs in the Upper Wuerhe Formation of Mahu 1 well area, Mahu Sag, single-stage flash and constant composition expansion experiments on live oil were conducted. Gas chromatography, differential scanning calorimetry, and polarized light microscopy were employed to characterize the composition of sediments, and wax and asphaltene deposition envelopes with different CO₂ contents were plotted. A four-stage flash experimental apparatus was set up to reproduce the temperature and pressure variation path in the wellbore, obtaining phase behavior and deposition data for the live oil injected with CO₂ at different wellbore positions. Finally, a one-dimensional wellbore wax deposition simulation framework was constructed by tight coupling among "bulk phase behavior model, a wellbore wax deposition model, and a physical property & heat transfer model," enabling the simulation of spatiotemporal evolution of pressure, temperature, flow regime, and wax layer thickness. The results show that the deposits consist of an nC₂₇-nC₃₅ microcrystalline wax skeleton cemented with asphaltenes, and that deposition is jointly driven by gas stripping extraction and Joule-Thomson cooling. CO₂ dissolution elevates the bubble-point pressure and shifts the wax deposition envelope (WDE) toward higher pressures. Deposition is strongly concentrated in the shallow section (0 to 1 000 m as high risk area, with a peak around 400 m), and when mole fraction of CO₂ in produced gas climbs to 50% from zero, the maximum wellhead deposition thickness increases by more than fivefold. Furthermore, when CO₂ mole fraction exceeds approximately 30%, the risk of deposition grows exponentially. Based on these findings, segmented and targeted control for the 0 to 1 000 m shallow interval and CO₂ mole fraction-based threshold early warning method are proposed to support flow assurance and stable production during CO₂ flooding.