Huanquan Pan

Pressure and Composition Effect on Wax Precipitation: Experimental Data and Model Results

Wax precipitation is often studied using the stock tank oil. However, precipitation may be very different in well tubing and production facilities due to the effects of pressure and composition. As an example, the cloudpoint temperature may decrease as much as 15 K from atmospheric pressure to the saturation pressure of 100 bar mostly due to the dissolution of light gases into the oil (i.e. due to composition changes). It is also often assumed that the addition of solvents such as C 5 and C 6 decreases the cloudpoint temperature. On the contrary, from our modeling results, we have found that the mixing of a crude with a solvent increases the cloudpoint temperature (i.e., enhances the wax precipitation). In this study, the cloundpoint temperature at live oil conditions and the amount of the precipitated wax at stock tank oil conditions are provided for three crudes. A modified multisolid wax precipitation model is used to study the effects of pressure and composition on wax precipitation. The modeling results reveal that an increase in methane and CO 2 concentration decreases the cloudpoint temperature while an increase in C 5 concentration increases the cloud point temperature.

Fast and robust algorithm for compositional modeling : Part I : Stability analysis testing

Given pressure, temperature, and composition of a fluid, one desires to determine whether the single phase state is stable. This problem is, in principle, much simpler than phase-behavior calculations. For certain applications such as compositional-reservoir modeling, stability testing can be the most important item for efficient phase-behavior calculations. In this paper, we use the tangent-plane-distance (TPD) in the reduced space to perform stability analysis testing. The results reveal that there are major advantages in the reduced space. One interesting feature of the transformation is that the TPD surface becomes smooth and has one minimum. The combination of a single minimum and the surface smoothness contributes to a remarkable robustness in calculations.

Fast and Robust Algorithm for Compositional Modeling: Part I - Stability Analysis Testing

Given pressure, temperature, and composition of a fluid, one desires to determine whether the single phase state is stable. This problem is, in principle, much simpler than phase-behavior calculations. For certain applications, such as compositional-reservoir modeling, stability testing can be the most important item for efficient phase-behavior calculations. In this paper, we use the tangent-plane-distance (TPD) in the reduced space to perform stability analysis testing. The results reveal that there are major advantages in the reduced space. One interesting feature of the transformation is that the TPD surface becomes smooth and has one minimum. The combination of a single minimum and the surface smoothness contributes to a remarkable robustness in calculations.

Fast and Robust Algorithm for Compositional Modeling: Part II - Two-Phase Flash Computations

In Part I of our study, stability analysis testing in the reduced space was formulated, and its robustness and efficiency in comparison to the conventional approach was explored. In this paper, we present formulations including, first, direct solution of the nonlinear equations, and second, minimization of Gibbs free energy for twophase flash computations in the reduced space. We use various algorithms including the successive substitution (SS), Newton’s method, globally convergent modifications of Newton’s method (line searches and trust region), and the dominant eigenvalue method (DEM) for direct solution of the nonlinear equations defining two-phase flash and the minimization of Gibbs free energy. We also suggest a criterion based on the tangent-plane-distance (TPD) for the initialization from the equilibrium ratios. The proposed criterion has a significant effect on reducing the number of iterations. The results from various algorithms reveal that the direct solution of the nonlinear equations in the reduced space, combined with the use of the TPD criterion for initialization in the combined SS and Newton’s method, can make flash computations extremely efficient. The efficiency and robustness of flash computations in the critical region are especially remarkable.

Two-Phase Isentropic Compressibility and Two-Phase Sonic Velocity for Multicomponent-Hydrocarbon Mixtures

Two-phase compressibility and two-phase sonic velocity of hydrocarbon mixtures are needed for a variety of applications in well testing, metering, and seismic exploration. In this work, a thermodynamic model is presented to estimate the two-phase isentropic compressibility and two-phase sonic velocity. The model accounts for the mass transfer between the equilibrium phases and the effect of capillary pressure. The results reveal that isothermal and isentropic compressibilities can be different by a factor of 20 in the two-phase near the retrograde dewpoint. With the exception of the retrograde dewpoint, the difference between the isentropic compressibility in the single phase and two phase is less than the corresponding difference for the isothermal compressibility. The sonic velocity in the two phase can be either less or more than the single phase. For the hydrocarbon mixtures that the sonic velocity decreases in the two phase, the decrease is much less pronounced than in nonhydrocarbon systems such as water-steam and water-air.