A Compositional Hindered Transport Model for Primary and Enhanced Oil Recovery from Fractured Wells in Unconventional Tight-Oil Plays

Abstract The fact that produced oils mainly consist of the light end of the hydrocarbons indicates that heavier hydrocarbons may be filtered, hindered, and retained in the reservoir. Hindered transport has been observed in experiments and attributed to various mechanisms, such as molecular sieving and adsorption on the pore surfaces of the tight matrix. This work targets to convert experimental observations to mechanistic model, develop a numerical model for hindered transport in naturally fractured nanoporous media, provide deeper understanding of physical phenomena contributing to hinderance, and assess the importance of hindrance on field scale by numerical simulations. While the effects of compositional changes on fluxes and contribution of adsorption to the accumulation of components are regularly accounted for, accumulation due to sieving and flux alterations caused by adsorption, sieving, and crowding are not considered in the conventional models. Therefore, the central contribution of this work is to delineate major retention mechanisms and propose a mechanistic model for hindered transport of hydrocarbons and CO2 in unconventionals. We have constructed a one-dimensional, compositional numerical model, which considers fluid transfer between fracture and matrix with the effects of molecular interactions and steric and dynamic effects on flow and transport. Hindered transport values and ranges provided in the literature for our objective to demonstrate the utility of our model and highlight the hindrance transport significance. This paper demonstrates the importance of molecular level phenomena and diffusion-driven exchange between matrix and fracture. The importance of the matrix-block sizes and studying transient matrix diffusion in numerical models are shown. Results emphasize the use of appropriate multicomponent diffusive flux relations and highlight the inadequacy of the bulk and binary diffusion coefficients to model multicomponent diffusion in crowded media. It is shown that hinderance causes considerable changes in compositional distributions. Pressure dependency of the diffusion, adsorption, and sieving parameters is more superefficient at lower pressure ranges. The largest impact of hindered transport was observed for injection- soaking-production cases. Considerable changes were observed in the pressure and composition profiles, and magnitude of production and composition of a produced fluid. This research was motivated by the effect of hindrance becoming important and causes significant compositional changes in unconventionals. Under these conditions, steric effects were expected to dominate the molecular-level phenomena in the nanoporous matrix and create significant compositional changes. In addition, we recognized that the existence of fractures in the flow domain would create complex compositional interactions between matrix and fracture. The key aspect of the developed model is that hindrance of heavier hydrocarbons caused by adsorption and sieving does not only affect the accumulation terms but also alters the advective and diffusive flux components of the mass balance equation.