Development of Robust Multiphase Equilibrium Calculation Algorithms for Complex Reservoir Fluids Containing Asphaltene
Creators: Chen, Zhuo
Publisher: name: University of Alberta Library
Year: 2024

<p>Multiphase equilibria, including three-phase vapor-liquid-asphaltene (VLS) equilibria and four-phase vapor-liquid-aqueous-asphaltene (VLAS) equilibria, can appear during enhanced oil recovery (EOR) processes. Robust multiphase equilibrium calculation algorithms are important techniques in reservoir simulations to better simulate such EOR processes. However, developing such robust algorithms can be challenging since convergence problems are frequently encountered during multiphase equilibrium calculations. In this research, we intend to develop a suite of robust multiphase equilibrium calculation algorithms dedicated to VLS and VLAS equilibria. <br>
There are different thermodynamic models that have been proposed to model asphaltene precipitation. The simplest model is the pure solid model, in which asphaltene is considered as a solid phase that only contains asphaltene. With this assumption, we first develop a robust and efficient three-phase VLS equilibrium calculation algorithm based on the recent work by Li and Li (2019). The results show that our algorithm is able to predict asphaltene precipitation under different pressure/temperature conditions and the injection of different gases. However, asphaltene may behave like a highly-dense liquid phase, especially at high temperatures. Therefore, we subsequently modify our algorithm based on a so-called free-asphaltene assumption, in which the asphaltene phase is considered as a pure liquid phase. The findings indicate that, compared with the outcomes derived from the solid assumption, those obtained from the free-asphaltene assumption are more consistent with experimental observations.<br>
In fact, the asphaltene phase is not an entirely pure phase. To further increase the accuracy of the three-phase equilibrium calculations, we aim to extend the trust-region-based three-phase VLL equilibrium calculation algorithm to conduct three-phase VLS equilibrium calculations. In such an algorithm, the asphaltene phase is considered as a liquid phase containing both asphaltene and other oil components. However, it is a challenging task to properly identify different kinds of two-phase equilibria that can possibly appear in the three-phase equilibrium calculations. Since the composition of the asphaltene phase or the low-density liquid phase is dominated either by the asphaltene component or a gaseous solvent component, we further develop a VL/LL and VL/LA phase boundary tracking algorithm, in which the mole fraction of the most dominant component in such phases is used as an indicator to track the VL/LL and VL/LA phase boundaries. It can be concluded from the calculated results that the predicted VL/LS and VL/LL two-phase boundaries show smooth behavior and intersect with the VL1L2 three-phase region&#39;s apex, supporting the idea of an extended three-phase region. Besides, our prediction results align well with that predicted by Bennett and Schmidt (2017), validating the accuracy of this new method.<br>
In addition to three-phase VLS equilibria, four-phase VLAS equilibria can also occur when the reservoir fluid contains a substantial amount of water. Based on the proposed trust-region-based three-phase VLS equilibrium calculation algorithm, we further aim to develop a four-phase VLAS equilibrium calculation algorithm. The algorithm is validated by comparing the calculated asphaltene precipitation amount during CO2 injections with and without water and experimental data. Once validated, this method is applied to forecast asphaltene behavior under different pressures and temperatures, as well as to generate PT and Px phase diagrams. Observations reveal that the presence of water increases the area of asphaltene precipitation in the Px diagrams, implying that water may facilitate asphaltene precipitation during the CO2 flooding process. Nevertheless, water overall seems to diminish the peak amount of the precipitated asphaltene.</p>