Phase Behavior of Hydrocarbon Mixture in Shale Nanopores Considering the Effect of Adsorption and Its Induced Critical Shifts

Phase behavior of shale gas or oil in nanopores is not yet well understood. One complexity comes from the fact that fluid adsorption can be significant in densely developed nanopores. A prediction process for the behavior of a ternary mixture (CH4, n-C4H10, and n-C8H18) and an actual Bakken oil in nanopore formation are performed by applying an adsorption-dependent Peng-Robinson equation of state, and the effect of adsorption and its induced critical properties shifts is discussed. Results indicate that the presence of adsorption could decrease the vapor-liquid equilibrium coefficient (K-value) and the vapor phase molar fraction for the confined ternary mixture, especially in the nanopores with a few nanometers. The shift in the saturation pressures of the ternary mixture and Bakken oil shows that the bubble point pressure and upper dew point pressure are depressed and the lower dew point pressure is increased with the synthetic effect of adsorption and critical shifts. The suppressed bubble point pressure well explains the production performance of a long-term flat producing gas/oil ratio in the Bakken reservoir. It should be noted that adsorption is relatively more dominant on bubble point pressure and critical shifts exhibit a greater influence on dew point pressure. For Bakken oil, the presence of adsorption increases the oil density and viscosity; however, critical shifts present conflicting impacts on oil density and viscosity, and the critical shifts are more dominant compared with fluid adsorption. This emphasizes the importance of considering both adsorption and critical shifts when describing phase behavior in shale nanopores.