Effects of Nano-Confinement and Heat Transfer on Phase Transition and Multi-Component Diffusion of CO2-Hydrocarbons During the Flowback and Early-Production Stages: A Field Example from a Liquid-Rich Shale Volatile Oil Reservoir

Phase transitions of CO2-Hydrocarbons in liquid rich shale (LRS) volatile oil reservoirs after the CO2 pre-pad energized fracturing is quite obvious, particularly due to the impact of temperature changes and nano-confinement. In this paper, the impact of phase transitions caused by heat transfer and nano-confinement effects on the CO2 effective diffusion coefficient (CO2-EDC) after CO2 pre-pad energized fracturing was investigated. A novel multi-component diffusion model incorporating both heat transfer and nano-confinement effects was proposed to accurately evaluate CO2-EDC in the Gulong LRS volatile oil reservoir located in the Songliao Basin, China, which provides valuable insights into fracturing design and CO2-EOR in shale oil reservoirs. Firstly, the nano-pore network model (PNM) was constructed based on focused ion beam scanning electron microscopy (FIB-SEM). Secondly, components of oil samples were analyzed by chromatographic experiments. Then, the temperature in each pore-throat was calculated using Fourier heat transfer equations. In addition, phase states (liquid or vapor) of CO2-hydrocarbons in each pore-throat were determined by the modified PR-EOS considering nano-confinement effects, and diffusion mechanisms (Knudsen, Transition, Maxwell-Stefan diffusion) were determined by the Knudsen number. Finally, the novel PNM with multi-scale diffusion equations was established to calculate the molar flow rate, which is used to obtain CO2-EDC by solving Fick's law. The phase behavior of CO2-hydrocarbons in the nano-confined pores was investigated, and the CO2-EDC was calculated under reservoir conditions (137.5 ℃, 37 MPa), and at varying injection temperatures. The results show that three distinct phase behaviors considering nano-confinement effects were observed under reservoir conditions: volatile oil in pore-throats larger than 33nm, condensate gas in pore-throats ranging from 5nm to 33nm, and wet gas or dry gas in pores/throats smaller than 5nm. However, it is only liquid in each pore-throat without considering the nano-confinement effects. As temperature increased, the phase behavior of CO2-hydrocarbons underwent a gradual transformation from a liquid state to a state of vapor-liquid coexistence, and finally to a vapor state. The phase transition is proved by the observation of a 2-month single gas production period prior to oil-gas production and a rapid decline in GOR (from 3559.7 m3/m3 to 318.5 m3/m3) followed by a period of stability in the Gulong LRS volatile oil reservoir. It is worth noting that the CO2-EDC increased significantly with the nano-confinement effects, rising by 896.96% from 0 ℃ to 300 ℃ compared to an increase of 10.31% without the nano-confinement effects. Specifically, the CO2-EDC increased slowly in the liquid-dominated stage (< 180 ℃) and rapidly rose in the vapor-dominated stage (> 180 ℃).

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