Temperature-Driven Interconversion of Shale Oil

Abstract

Understanding the occurrence state and thermal mobilization mechanism of shale oil is essential for improving recovery efficiency from nanoporous shale reservoirs. In this study, a series of thermal volatilization experiments (40–80 °C) combined with NMR (T2 and T1-T2) were conducted to quantitatively characterize the occurrence and dynamic evolution of shale oil within pores of different sizes. The results indicate that shale oil can be classified into adsorbed oil, bound oil, and free oil based on the linear relationship between oil loss and the square root of heating time. As the temperature increases, the total amount of oil volatilized increases significantly, accompanied by a dynamic interconversion among different occurrence states. This interconversion exhibits a clear pore-size-dependent response: free oil in macropores (P3) is rapidly released at low temperatures; mesopores (P2) serve as the primary domain for temperature-induced oil mobilization; and micropores (P1) remain dominated by adsorption-controlled and diffusion-limited processes. Notably, mesopores contribute most to the incremental oil recovery due to the effective conversion of bound oil into free oil under thermal stimulation. Based on these findings, a hierarchical mechanistic model is proposed that describes shale oil volatilization as a progressive interconversion pathway. The results provide new insights into pore-scale-controlled mechanisms and offer important guidance for optimizing thermal- and chemical-enhanced recovery strategies in shale reservoirs.