Enhanced Gas Recovery By CO₂ Injection And Storage Study In The Shallow Gas Reservoir

The shallow gas reservoir has the characteristics of shallow burial,low cost and quick effect,and it has broad prospects in exploration and development.China's shallow gas resources are abundant,the research on shallow gas reservoir development has made significant progress.With the development of gas reservoirs,it is into the middle and late time.At this time,the pressure is dropping,the production is decline.In order to efficiently develop residual natural gas remaining in the gas reservoir,injecting CO2 into the formation not only can improve the natural gas recovery rate,but also can achieve stable C02 storage.This measure will reduce CO2 emissions and achieve the use of C02.At present,the development and utilization of shallow gas injection in China is still in the initial stage.It is lack of targeted theoretical basis and the mature foreign technology.Therefore,this paper has great practical significance.In this paper,shallow gas reservoirs are the main research objects.The temperature and pressure of shallow gas reservoirs are consistent with the temperature and pressure of supercritical CO2,so CO2 can be buried at high density in supercritical state.Based on the successful projects of CO2 displacing CH4 in foreign countries,the influencing factors of gas purging effect are analyzed and the standard system of suitability rating is established.Combined with the parameters of gas reservoirs in Sichuan Basin,the evaluation of the suitability of shallow CO2 injection in Sichuan Basin was carried out.Finally,the appropriate blocks were selected for later research.In addition,considering the physical and chemical changes of the CO2-CH4-formation water three-phase system after CO2 injection into the formation.Based on the diffusion dispersion theory,this paper carried out the diffusion experiment of CO2 and formation water,and the thin tube experiment of CO2 flooding CH4.On the basis of the experimental understanding,the numerical simulation sensitivity analysis of CO2 injection mechanism(gas injection parameters)was carried out finally.Through the above research,this paper has obtained the following research results and understanding:1)Further studies on the phase change characteristics of C02,the phase of carbon dioxide in shallow gas reservoirs is supercritical,and the high density and low viscosity of supercritical CO2 make the diffusion less weak and easier to realize gas injection C02 to improve oil recovery.2)Based on the investigation of carbon sequestration with enhanced gas recovery,the factors influencing CO2 displacement natural gas are selected,and a set of index system for evaluating the suitability of gas injection is established.The fuzzy comprehensive evaluation method is used to evaluate the indexes of shallow gas reservoirs,which provides a theoretical basis for the evaluation of C02 fitness in shallow gas reservoirs.3)Establish the dissolution and diffusion model of C02 and formation water,and carry out the dynamic dissolution and diffusion experiment of C02 and formation water.The theoretical values were calculated by PVTSim software and compared with the experimental results.Finally,the diffusion law of CO2 and formation water was obtained.4)The physical experiments of CO2 displacing natural gas show that the diffusion of CO2 is obvious at the critical temperature and pressure.This will lead to CO2 breakthrough earlier than CH4,causing the lowest recovery rate of CH4.The supercritical CO2 can achieve the effect of enhanced gas recovery.5)CMG numerical simulation results show that the factors such as injection pressure,heterogeneity,well spacing and injection rate have a great influence on CO2 purgping effect.Through the simulation results of the section analysis,the gas storage capacity and the natural gas recovery degree of the carbon dioxide were analyzed.The optimized gas recovery efficiency and the storage effect were better,the recovery efficiency and the buried effect were the best,and the reasonable well spacing was 600m,the best injection rate of 30000m3/d,the amount of CO2 storage was 2.478×107m3.