The Effect of Asphaltene Precipitation on Carbonate Rock Permeability: An Experimental and Numerical Approach
Authors: Ali M.A. , Islam M.R. 
Published: December 2007

Abstract
Asphaltene precipitation can have profound effects on oil production during miscible flooding, heavy oil recovery, or even primary depletion. Even though asphaltene precipitation has been known to have strong effects on permeability reduction, no systematic results have been reported on the mechanism of permeability reduction due to the presence of asphaltenes In this paper, a series of experiments is carried out in order to determine the effect of asphaltene precipitation on the intrinsic permeability of carbonate rocks. The asphaltene concentration of the effluent as compared to that at the inlet shows that asphaltene is deposited in the pore channels. This deposition was found to be a function of permeability, flow rate and concentration. Two distinct mechanisms were identified, namely, deposition and adsorption. Deposition could be explained in terms of trapping and mechanical plugging. This was sharply dependent on the flow rate. Adsorption can be explained in terms of irreversible thermodynamics. Interestingly, the coupling between the two mechanisms into a unified model has not been reported in the past.
A mathematical model is finally reported. The model builds on the two mechanisms described above. The coupled mathematical model predicts permeability damage due to mechanical trapping and adsorption accurately, as evidenced when compared with experimental results. The model results agree favorably with experimental results for which different flow rates were used. Three different plugging regimes were identified and were verified with the numerical model.

Introduction
Asphaltene and its deposition is considered to be one of most difficult problems encountered during the exploitation of the oil reservoirs. Miscible and immiscible flooding operations exhibit suitable environments for such precipitation. In some cases, asphaltene precipitation can occur during natural depletion and oil transportation and, more commonly, during well stimulation activities.
Asphaletene is defined as the portion insoluble in heptane but soluble in toluene. Particles of asphaltenes are kept in suspension by the resins under normal conditions. The association of asphaltenes with resins can be altered by changing the prevalent pressure, temperature and crude oil composition 4. The presence of asphaltene in a crude oil has been shown to affect rock properties. Most of these effects have been recognized to be the result of asphaltene adsorption and precipitation on the rock surface - Previous studies indicated that asphaltene adsorption leads to alteration of rock wettability. Piro et al.conducted a series of adsorption studies to determine the isotherms of asphaltene on sandstone rocks. They observed that asphaltene adsorption is acontinuous phenomenon and does not reach saturation similar to conventional surface active materials. However, they did not report the effect of adsorption on rock permeability. Kamath et al. conducted a series of dynamic displacement tests in order to evaluate the effect of asphaltene deposition on waterflooding in both consolidated and unconsolidated sandpacks. They concluded that asphaltene depostion has impacts on reservoir rock permeability and end-pointsaturations. In this work, a series of experiments was conducted in order to determine the effect of asphaltene adsorption and precipitation in low-permeability carbonate rocks. A new theoretical model is proposed in order to couple asphaltene adsorption with mechanical plugging due to deposition.

Experimental Set-up and Procedure
The crude oil sample used in this work was a stock tank oil from one of themajor oil producing formations in the United Arab Emirates. The oil gravity was 29.29 API with a viscosity of 5.27 cP. The asphaltene content was 3% by weight. This was measured using the IP 143 method.
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