Improved Thermodynamic Modeling of Gas Hydrate Systems
Creators: Chen, Xin
Publisher: name: University of Alberta Library
Year: 2022

<p>Natural gas hydrates are solid crystalline mixtures of water and small gas molecules that typically form at relatively low temperatures and moderate pressures. As a promising energy resource, the natural gas hydrates are discovered in many offshore and permafrost geological formations. Besides, the natural gas hydrates are also found to form in the pipelines located in cold areas and in the wellbores used in offshore petroleum industry, causing the flow assurance problems. The decomposition of in-situ hydrates during the exploitation process and the formation of hydrates in pipelines or wellbores will lead to a series of changes on the number of equilibrium phases and the phase compositions. How to accurately describe the phase behavior of natural gas hydrates plays a fundamentally important role in the accurate modeling of multiphase flow involving gas hydrates in both reservoirs and wellbores/pipelines. This study will start from developing improved thermodynamic frameworks that can improve the accuracy in modeling the phase behavior of reservoir fluids and gas hydrates. Then based on the improved thermodynamic models, we provide a reliable multiphase equilibrium calculation algorithm for gas hydrate systems.<br>
As an efficient and reliable thermodynamic tool for modeling the multiphase behavior of reservoir fluids, cubic equation of state (CEOS) has been widely adopted in industrial simulators. However, most of the CEOS models cannot provide an accurate density prediction for the liquid phase. Although the temperature-volume-dependent volume translation (VT) is deemed as the most accurate method to correct the liquid density yielded by CEOS, the available VT-models do not fully exploit the potential of distance function and there is still a room for improving the prediction accuracy of saturated and single-phase liquid densities for water and hydrocarbons by VT-CEOS. Hence, this study proposes a series of improved VT-models to achieve more accurate volumetric calculations for water, hydrocarbons and their mixtures. The absolute percentage deviations of the liquid molar volumes yielded by the newly-proposed VT-CEOSs for different compounds are usually lower than 1%.<br>
In academia and industry, the van der Waals-Platteeuw (vdW-P) hydrate model is one of the most popular and classical hydrate-equilibrium calculation methods. Nevertheless, the hydrate equilibria of gas-mixture systems predicted by the vdW-P model are not as accurate as those predicted for pure-gas systems. In contrast to the previous studies that focused on the modifications of functional forms, the current study aims to provide new pragmatic strategies for tuning the gas-dependent parameters in the vdW-P hydrate model. A new procedure is developed for fitting the Kihara potential parameters in the vdW-P hydrate model using the experimental hydrate equilibrium data for both pure gases and binary-gas mixtures, considering the differences between hydrate structures I and II. As a result, the vdW-P model coupled with the newly fitted Kihara potential parameters performs well in gas hydrate equilibrium calculations and also properly detects the hydrate structure transition and cage occupancy behaviors.<br>
Lastly, on the basis of the improved thermodynamic models, we develop an algorithm for multiphase equilibrium calculations in the presence of gas hydrates. The number of equilibrium phases that can be detected by this algorithm is up to four phases, i.e., a vapor phase, a hydrocarbon-rich liquid phase, an aqueous phase, and a gas hydrate phase. In this algorithm, a new criterion for determining the onset of hydrate dissociation is proposed based on van der Waals-Platteeuw model. To calculate the phase fractions and phase compositions, this new algorithm provides a series of material-balance equations involving hydrates. Example calculations demonstrate that this algorithm is capable of robustly conducting hydrate-inclusive multiphase equilibrium calculations for a given fluid at specified temperature and pressure.</p>