Thermodynamically-consistent modeling of adsorption in liquid-rich shales

Reservoir simulation and material balance techniques require accurate estimation of fluid storage and transportation mechanisms. In dry-gas shale reservoirs, it is widely acknowledged that gas adsorption is one of the most important storage mechanisms, and that it accounts for close to 45% of initial gas storage. In case of liquid-rich shales however, typically, adsorption is not considered as a storage mechanism. This paper proposes a method to estimate fluid adsorption from liquid phase in shale reservoirs. The proposed adsorption modeling formalism is based on the thermodynamically-consistent Ideal Adsorbed Solution (IAS) theory. The main development in this work is that adsorption from liquid phase can be calculated from its unsaturated vapor phase at the limiting saturation condition. Once the composition of this unsaturated phase is determined, the use of IAS theory would result in the amount and composition of the adsorbed phase being calculated. Initial results suggest that, given a liquid phase mixture, typically about 5-13% of the fluid is adsorbed. This indicates that a significant amount of the liquid mixture initially exists in the form of adsorbed phase. However, the amount of each component in the adsorbed form is strongly dependent upon the total adsorption capacity of the particular shale, the amount of adsorbent present, as well as the adsorption affinity exhibited by the shale towards each component. Negligence of accounting for adsorption from liquid phase would result in erroneous calculation of total recoverable reserves from a particular liquid-rich field. This would also affect the enhanced recovery calculations, as thermodynamics would play an important role in the sorption of hydrocarbon components. This paper presents a direct and thermodynamically-consistent method of calculating the amount and composition of the adsorbed phase from liquid-rich shale reservoirs.