QUANTITATIVE EVALUATION OF ADSORBED/FREE SHALE OIL: EXPERIMENTS AND MODEL STUDIES

Shale oil is primarily present in shale pores as adsorbed oil and free oil (movable oil and irreducible oil). The shale oil at three occurrence states has different thermodynamic characteristics. Movable oil is retained in a free status in larger pores and has a normal vapor pressure, while oil adsorbed on pore surface has a lower vapor pressure than normal. Thus, the three states of oil in shale pores can be quantitatively characterized by evaporation experiments. In this study, evaporation experiments simultaneously with NMR tests were conducted for 13 oil-saturated (n-dodecane) shales. The adsorbed oil contents of 13 shales range from 2.75 mg/g to 17.61 mg/g (mean 8.40 mg/g). The irreducible oil content is between 1.69 mg/g and 6.43 mg/g (mean 4.25 mg/g) and the movable oil contents vary from 0 mg/g to 20.61 mg/g (5.89 mg/g). The average value of adsorption proportion (r_a) is 0.497, ranging from 0.237 to 0.699. NMR T₂ spectra show that movable oil is mainly retained in meso- (0.1 μm~1 μm) and macropores (>0.1 μm); irreducible oil is mainly confined in mesopores; and adsorbed oil is predominant in micro- (<0.1 μm) and mesopores. According to the quantitative evaluation model for hydrocarbon adsorbed and free oil in the authors’ previous work, the shale oil r_a evaluation model can be determined as: 1000V/S=d_m=(ρ_a/ρ_f)H(1/r_a-1)+H (where V and S are the pore volume and surface area, respectively; d_m is the equivalent pore diameter; ρ_a and ρ_f are the densities of adsorbed and free oil, respectively; H is the thickness of the oil adsorbed on the pore surface). The ρ_a/ρ_f and H values can be obtained as 2.0802 and 0.723 nm, respectively, based on the measured r_a values. The adsorbed and free oil contents can be obtained from NMR T₂ spectra and the calculated r_a values correspond well with measured values with a high correlation coefficient of 0.92. And the calculated adsorbed oil T₂ spectra are well consistent with the measured T₂ spectra, indicating that r_a evaluation model combined with NMR T₂ spectrum can effectively evaluate the adsorbed oil content of shale oil and the r_a for pores at different scales. In this study, the quantitative evaluation method for adsorbed and free shale oil was established, which can be used to determine the adsorbed and free shale oil contents by combining with NMR T₂ spectrum.