| CO2-Enhanced Gas Recovery(CO2-EGR)is a win-win technology that can enhance natural gas recovery and achieve CO2 mitigation by geological storage,and attracts much attition.In CO2-EGR implementation,the displacement behavior between CO2 and natural gas,thermodynamic properties of CO2-natural gas mixture,and dispersion characteristics have profound effects on the CO2 migration rule,which is of great significance for the efficient production and purity of natural gas.Therefore,it is a key scientific problem of CO2-EGR to study the thermodynamic properties,miscible displacement behavior and dispersion characteristics of CO2-natural gas in porous media.In this paper,the density characteristics of the CO2-CH4 mixtures,observation of miscible-phase displacement bebavior,in situ measurement of dispersion characteristics,and apparent dispersion characteristics of horizontal and vertical CO2-CH4 displacements in porous media were studied,and the function mechanism of gravity effect and various factors on miscible displacement were elucidated by experiments and modellings.The study provides data and theoretical support for the study of CO2 transport and the assessment of natural gas recovery in CO2-EGR.The main research content of this paper is as follows:A high-precision measurement system was built for gas density based on a magnetic suspension balance,and the densities of CO2-CH4 binary systems under gas reservoir geological conditions and critical conditions were systematically measured.The density change regulation of CO2-CH4 mixtures was analyzed under different temperatures,pressures,and CO2 concentrations.The optimization of the binary interaction parameter(kij)were proposed as a function of the CO2 concentration for CO2-CH4 mixtures,improving the prediction ability of PR EoS and PC-SAFT EoS,respectively.The density prediction accuracy and applicable conditions of the different themodynamic models were also evaluated.In this paper,an experiment system and mothed of visualization and in situ dispersion measurement was established for CO2-CH4 displacement in porous media by the microfocus X-ray CT.Based on CT characteristics of non-destructively detecting material density,the sturucture and porosity distribution of sandpacks were obtained.Moreover,it was visually verified that the liquid/supercritical CO2 displacing CH4 in sandpack is a miscible-phase displacement.The dispersion coefficient of this miscible-phase displacement in sandpack was in situ measured simultaneously.It was proved that the entry/exit effect increased the dispersion coefficient by 14%?29%,and the residual water suppressed the dispersion by dissolving CO2.The changes of apparent dispersion of CO2-CH4 horizontal and vertical displacements in sandpack were measurd by the dynamic column breakthough curves.The mechanism of gravity effect on the vertical dispersion and the horizontal dispersion was revealed that the gravitational differentiation inhibites the vertical CO2 diffusion and migration and indirectly accelerates horizontal migration.The influence of injection parameters,porosity and permeability of sandpack,gas impurity and formation depth on dispersion were analyzed.The dispersion process was dominated by convection and diffusion in this study.The vertical dispersivities of the BZ04 and BZ01 sandpacks were estimated as 0.0023 m and 0.0019 m,respectively.A homogeneous gas reservoir model and a vertical heterogeneous gas reservoir model were constructed to carry out CO2-CH4 displacement simulation studies.The thermodynamic property calculation module in TOUGH2 was modified by employing the improved PR EoS in this paper.The role of gravity effect was analyzed by the contrastive simulation cases with or without gravity.The optimized injection pressure was obtained by simulating CO2 injection into reservoir at different exploitation periods,provided some support for the implementation of CO2-EGR. |
PDF Full Text Request