A Quantitative Study on the Evolution of the Asphaltene Distribution during Gas Charge Processes
Authors: Zuo Julian Y. , Pan Shu , Wang Kang , Mullins Oliver C. , Elshahawi Hani , Canas Jesus A. , Chen Li , Dumont Hadrien , Mishira Vinay K. , Garcia German , Jackson Richard 
Published: October 2017

The Flory-Huggins-Zuo equation of state (FHZ EOS) was developed based on downhole fluid analysis (DFA) measurements and the Yen-Mullins model to delineate equilibrium asphaltene gradients and reservoir connectivity. However, dynamic processes are often observed in reservoirs, and these cause nonequilibrium fluid distributions. Gas charges into reservoirs can result in asphaltene flocculation, formation damage, and/or tar mat formation, which significantly impact reservoir architectures and field development planning. Therefore, it is important to understand and simulate reservoir fluid geodynamic processes. In this work, a new reservoir fluid geodynamic model is proposed to quantitatively study asphaltene distributions over geological time. The model has shown a great potential to bring an insightful understanding of history and architectures of petroleum reservoirs.

The diffusion model is developed for multicomponent systems in the framework of the generalized Maxwell-Stefan mass transfer theory. Moreover, to account for asphaltene migration, diffusion, Stokes falling, and advective currents are all considered. In addition, to take into account the fact that asphaltenes exist as nanoaggregates and clusters, an engineering approach is proposed to simplify the generalized Maxwell-Stefan theory by lumping two asphaltene gravitational terms. Advection is taken into account by buoyancy velocity induced by density inversion that is created upstructure in reservoirs during density stacking of gas charge into oil. A numerical solver is applied to solve the asphaltene migration equations with relevant boundary conditions.

This model has been applied to two case studies. The first case is a hypothetical reservoir in which a significant density inversion forms during the gas charge, which induces (rapid) gravity currents (advection). The evolution of the asphaltene migration and present day distribution in this reservoir is simulated by considering all these complexities. The second case study is based on an actual reservoir under active gas charging. In this case, no dominant density inversion was observed in simulation using the diffusion model either with or without the gravity term. The results from the new model with the Stokes sedimentation term for asphaltene clusters show an excellent agreement with the field observations and superior to the simulated results without gravitational forces.

In summary, this new reservoir fluid geodynamic model has quantitatively described the asphaltene migration driven by not only diffusion in a concentration gradient but also Stokes falling and advection in a gravitational field. The gravitational terms of two forms of asphaltenes are well approximated by a new lumping approach. This work quantifies asphaltene migration using diffusion, Stokes falling and advection, all with crucial contributions during gas or light hydrocarbon charge into oil reservoirs.