Simulating the Permeability Reduction due to Asphaltene Deposition in Porous Media

Abstract

Abstract A static to dynamic approach to modeling Asphaltenes has been developed and validated. A new algorithm for static asphaltene modeling uses a multi-solid thermodynamics approach where the equality of fugacity for each component and phase is applied at equilibrium conditions. This is required for minimizing the Gibbs free energy. The fractal distribution function used for the splitting and characterization of heavy components provides accurate results. The precipitation and re-dissolution of asphaltenes are investigated for a relatively heavy crude oil from an Iranian field. A series of experiments are designed and conducte quantitatively to obtain the permeability reduction in a slim tube. Using a dynamic reservoir simulator, a 3-dimensional asphaltene model is developed to simulate the precipitation, flocculation, deposition and its impact on permeability in a slim tube. With this approach, the asphaltene is defined as a set of component(s) that can precipitate depending on their molar percentage weight in solution. The simulated permeability reduction due to asphaltene deposition shows good agreement with our experimental data. 1. Introduction Asphaltenes can reduce the hydrocarbon effective mobility by:a) Blocking pore throats thus reducing the rock permeability.b) Adsorbing onto the rock and altering the formation wettability from water-wet to oil-wet, hence diminishing the effective permeability to oil and increasing the irreducible oil saturation.c) Increasing the reservoir fluid viscosity by forming a colloidal solution in the oil phase. The most frequently encountered scenario of asphaltene-induced formation damage is when under saturated oil is being produced without water. The most dominant damage mechanism is blockage of pore throats by asphaltene precipitation and thus causing a reduction in rock permeability. In comparison most wax-related formation damage is caused by cooling due to excessive draw down across the perforations or due to hot oiling operations or organic solids entering the formation due to overbalance conditions. In all cases, the damage area is restricted to the wellbore zone (several inches to 1 feet). In the case of asphaltenes, the formation damage may occur many feet from the wellbore depending on the drawdown [1,2]. Several authors have studied different models for asphaltene deposition in core tests. Civan et al. [3, 4] assumed that the flow channels in porous media are grouped into plugging and non-plugging pathways, and for the deposition model they use surface adsorption, pore throat plugging, and entrainment of deposits. Ali and Islam [5] assumed that Asphaltene is suspended in crude oil and ready to deposit, and for the deposition model they consider surface adsorption and entrainment of deposits. Wang et al. [6,7] considered asphaltene precipitation could be simulated by ideal solution theory, and for the deposition model they used surface adsorption, plugging, and entrainment.