Occurrence states and transport behavior of crude oil in different permeability oil reservoirs during depletion development

The fluid occurrence states exert a significant influence on the crude oil development. Previous studies relied on unrealistic formation fluid models and assumptions, resulting in imprecise methods for determining fluid occurrence states. Moreover, limited research has addressed the different oil reservoir types on occurrence states and transport behaviors during the depletion development process. This study establishes a precise characterization method for three fluid occurrence states: free fluid (FF), capillary-bound fluid (CAF), and clay-bound fluid (CBF). The transport behavior of these occurrence states under various depletion development conditions is analyzed comprehensively. Results indicate that the accuracy of identifying different occurrence states improves by 11.5 %–34.0 % using the modified approach. Variations in pore diameter and clay mineral content result in a 4.5 %–24.3 % higher proportion of FF in low permeability reservoirs compared to ultra-low permeability and tight oil reservoirs, while CAF and CBF proportions are greater in the latter. During the depletion development process, FF is mobilized first, followed by a smaller amount of CAF, while CBF remains undeveloped. The factors influencing reservoir depletion development are ranked in order of significance as follows: production pressure difference, Z factor, fluid viscosity, pressure reduction rate, and pressure reduction pattern.