Phase Behavior Characterization For CO₂ Exchange Method Of Gas Hydrate Production

The reserve volumes of natural gas hydrate on the seafloor are enormous.Currently,researchers estimate the global accumulations of methane in the hydrate are 1.8×1016?1.39x1017 m3.As an important part of gas hydrate research,hydrate exploitation technologies has become the key research topics since 1990s.The traditional gas hydrate production methods are external excitations,which change the environment or state around the hydrate by means of heat injection,depressurization,and chemical injection.However,the hydrate decomposition induced by the external excitation methods would easily destroy the hydrate formation structure,leading the ocean floor slope disaster.Carbon dioxide sequestration coupled with hydrocarbon resource recovery is often economically attractive.Using CO2 to exchange CH4 in the hydrate phase can also reduce the risk of geological hazard because such a displacement process does not change the hydrate formation structure.On the other hand,replacing CH4 with CO2 obtained from the seafloor yields the formation of CO2 hydrate in reservoirs,which reduces the CO2 emission amount in the atmosphere,and related greenhouse effect.Therefore,applying CO2 to displace CH4 in the hydrate phase represents a growing concern in hydrate exploitation field.The process of displacing CH4 in hydrate with CO2 is essentially a multiphase equilibrium process involving CO2,CH4,H2O,and many other components.Aiming at this phase equilibrium problem,the researches of this paper are divided into experiment,thermodynamic theory and numerical simulation three aspects.The main contents are as follows:(1)Construct a model to describe the physical parameters of natural gas-water system based on the Peng-Robinson(PR)EoS combined with Huron-Vidal mixing rule.In order to enhance the accuracy of PR EoS for containing CO2 gas,a new correction parameter associated with three-parameters(the reduced temperature Tr,reduced pressure Pr and CO2 volume content)is introduced into the attractive term of PR EoS based on experimental data.The prediction accuracy of the corrected PR EoS is improved.A new and reliable model is developed for CO2,H2O and gas system physical parameters calculation.(2)Construct Parrish-Prausnitz hydrate thermodynamic model to predict hydrate formation conditions.In order to enhance the prediction accuracy of P-P model for containing CO2 gas,a new correction parameter associated with CO2 volume content is introduced into the water chemical potential term.The improved P-P model have a satisfactory prediction accuracy for containing CO2 gas.(3)Construct a multiphase flash model in presence of hydrates,based on the minimization of the Gibbs free energy,combined with PR-HV EoS and Parrish-Prausnitz model.This model can give predictions on hydrate formation conditions and components,phase distributions at equilibrium state,offering basic parameters for CO2 exchange CH4 phase behavior.(4)This model has been examined by total 61 groups of experimental data of CO2+CH4+H2O and CO2+N2+H2O systems.After comparing this model to PVTsim and CSMGEM,the results show this model provides prediction values which are in satisfactory agreement with the experimental data and this model also has good convergence.The research results provide a basis for CO2 exchange CH4 process,including CO2 injection volume,CH4 production volume,and even the recovery rate of hydrate reservoir.(5)A subsea natural gas reservoir has been taken as an example to show the variations of CO2 and CH4 contents in the vapor pahse and hyrate phase during the CO2 replacenemt process.The results demonstrate that achievements of this paper would provide an essential basis for the calculation of CO2 injeciton amount,the CH4 produciton amount,and the recovery rate of gas hyrate reservoirs.