Quantitative characterization of fluid occurrence in shale reservoirs

Adsorption of hydrocarbons may significantly affect hydrocarbon migration in unconventional reservoirs. Clay minerals form the primary adsorbent surfaces for hydrocarbons adsorbed in mudstone/shale. To study the adsorption properties of hydrocarbons (n-decane (C₁₀H₂₂), methyl cyclohexane (C₇H₁₄) and toluene (C₇H₈)) on clay minerals (i.e., cookeite, ripidolite, kaolinite, illite, illite/smectite mixed-layer, Na-montmorillonite and Ca-montmorillonite), hydrocarbon vapor adsorption (HVA) tests were conducted at 298.15 K. The results showed that (i) the adsorption amounts of C₁₀H₂₂, C₇H₁₄ and C₇H₈ ranged from 0.45–1.03 mg/m², 0.28–0.90 mg/m² and 0.16–0.53 mg/m², respectively; (ii) for cookeite, ripidolite and kaolinite, the adsorption capacity of C₁₀H₂₂ was less than C₇H₁₄, which was less than C₇H₈; (iii) for illite, Na-montmorillonite and Ca-montmorillonite, the adsorption capacity of C₁₀H₂₂ was greater than that of C₇H₈, and the adsorption capacity of C₇H₁₄ was the lowest; (iv) for an illite/smectite mixed-layer, C₇H₁₄ had the highest adsorption capacity, followed by C₁₀H₂₂, and C₇H₈ had the lowest capacity. Adsorption properties were correlated with the microscopic parameters of pores in clay minerals and with experimental pressure. Finally, the weighted average method was applied to evaluate the adsorption properties of C₁₀H₂₂, C₇H₁₄ and C₇H₈ on clay minerals in oil-bearing shale from the Shahejie Formation of Dongying Sag in the Bohai Bay Basin, China. For these samples, the adsorbed amounts of C₇H₁₄ ranged from 18.03–28.02 mg/g (mean 23.33 mg/g), which is larger than that of C₁₀H₂₂, which ranges from 15.40–21.72 mg/g (mean 18.82 mg/g). The adsorption capacity of C₇H₈ was slightly low, ranging from 10.51–14.60 mg/g (mean 12.78 mg/g).