Coke Formation and Coupled Effects on Pore Structure and Permeability Change during Crude Oil in Situ Combustion

In situ combustion (ISC) is an important thermal recovery technique. Significant open ISC questions include the effect of coke formation on the pore structure and permeability. In the study, an experimental apparatus was constructed to not only physically simulate coke formation similar to the crude oil ISC process but also to in situ measure postdeposition permeability. Effects on coke deposition with the Xinjiang crude oil were studied, including reaction atmosphere, temperature, and time. The results indicate that the critical coking temperature differs significantly by at least 200 °C between low-temperature oxidation (LTO) runs with air flow and coking runs with nitrogen flow for the Xinjiang crude oil. The coke generation promoted by LTO and the coke consumption via high-temperature oxidation (HTO) result in a maximum coke production with temperature in the LTO runs. In addition, the study found that many resins and the small amount of asphaltenes in the Xinjiang crude oil prolonged the induction coking period in the coking runs. This understanding of the coke deposition process led to the production of core samples with different amounts of coke deposition for selected reaction conditions. The pore structure of the coked grain clusters was viewed with a scanning electron microscopy (SEM) and mercury porosimeters. The results showed the complicated pore structure and increasing number of micropores with increasing coke deposition, which not only reduced the permeability rapidly so that it deviated from the Kozeny–Carman relationship at the Darcy scale but also further promoted the Klinkenberg effect. In addition, the global permeability damage would be further underestimated regardless of the coke concentration heterogeneity in the core samples. The permeability change was then correlated with coke deposition for numerical simulations of ISC or ToeHeel Air Injection (THAI) processes in sandstone reservoirs.