Modeling Asphaltene Precipitation and Implementation of Group Contribution Equation of State Into Utcomp
Creators: name: Qin, Xiangjun
Publisher: name: The University of Texas at Austin
Year: 1998
Subjects: Utcomp
Oil production

description: The main objective of this research was to improve the accuracy of the UTCOMP simulator for predicting oil production. This work includes two parts. The first part is to simulate the asphaltene precipitation during oil recovery, and the second part is to implement group contribution equation-of-state for the phase equilibrium calculations in reservoir simulations. Asphaltene precipitation is a serious problem in oil recovery, and it may damage the oil production. Many models have been developed to predict the asphaltene precipitation behavior. In this study, Nghiem's model for asphaltene precipitation was selected and implemented into the UTCOMP simulator. In order to simplify the phase equilibrium calculation, a pseudo-three-phase algorithm was used to obtain the amount of precipitation during the production process. The precipitated solid that resides in the reservoir may affect the porosity, permeability and wettability of the reservoir rock. To derive the effect of precipitation on porosity, the precipitated asphaltene was treated as part of the reservoir rock. This assumption makes it possible to compute the absolute permeability using a power-law model. The relative permeability was calculated using a resistance-factor model. However, permeability is a very strong function of pore structure. To relate the effect of asphaltene precipitation on both permeability and relative permeability to pore structure of the reservoir rock, a rock-fabric classification model was implemented into the simulator. This model combines the permeability and relative permeability models using a rock-fabric classification for carbonate reservoir rocks. To further improve the computational efficiency, a simplified algorithm was implemented to compute the amount of asphaltene precipitation. In this algorithm, the equilibrium calculation used to obtain the amount of precipitation was decoupled from the phase equilibrium computation. Simulation results indicate that asphaltene precipitation may damage the oil production in most cases. But if the rock permeability is high and the rock classification number is low, precipitation may actually result in a slight increase in oil production. The results also show that the new simplified algorithm to compute the amount of precipitation was successful in reproducing the coupled results while saving much of the computer time. The second part of this research was to implement the group contribution equation-of-state into the simulator. There were two choices of equations-of-state in the UTCOMP simulator. They were the Peng-Robinson equation-of-state and a modified Redlich-Kwong equation-of-state. However, both equations are not good in representing the phase behavior for mixtures containing polar components such as water or alcohol. In order to improve the accuracy of phase equilibrium, the Wong-Sandler group contribution equation-of-state was selected and implemented into the simulator. The model is a combination of the UNIFAC group contribution method, which is good in computing the activity coefficients for both polar and nonpolar mixtures, and the Peng-Robinson equation-of-state, which is good for phase equilibrium at high pressure. Thus, the group contribution equation-of-state takes advantage of both methods and results in more accurate predictions of phase equilibrium for mixtures containing polar components. This is illustrated for some binary polar mixtures. The applicability of the group contribution equation-of-state extends the capability of the simulator to some special reservoir conditions and problems.