Numerical Simulation Of Multiple Factors Influence On CO₂ Storage Process

The content of greenhouse gases,mainly CO₂,is increasing year by year,leading to the increasingly severe global warming phenomenon,which has attracted widespread attention.And CO₂ geological storage is an effective means of mitigating global warming,China has adopted a series of policies and measures to deal with climate change.In 2010,China deployed and constructed the first CCS demonstration project in Ordos Basin.Numerical simulation plays an important role in CO₂ geological storage site selection,storage capacity and security.TOUGH2-ECO2N is widely used as a specialized numerical simulation software for CO₂ geological storage,and its simulation results are accurate and reliable.And the simulation results are accurate and reliable.However,due to the original intention of the TOUGH2-ECO2N simulator development is to be used as a research tool,which does not include the pre-processing and post-processing interface.Moreover,the simulation scale of the simulator is limited,which greatly restricts its application convenience,application promotion and application fields.Optimization of injection,migration safety and storage capacity are the most critical issues in the process of CO₂ geological storage.And the main factors affecting these problems include the complex structure of the actual formation（formation slope,fault）,the error of non in situ test results（salinity）,and the controllable external conditions（injection temperature,etc.）.Most scholars mainly focused on the single factor in the past.However,there are many and complex influencing factors in the actual storage site,it is urgent to combine them for in-depth research,which is of great significance to guide the selection of service storage site and injection optimization.Ordos Basin is a typical craton basin,or one of the largest land sedimentary basins.The main reservoir of Shiqianfeng Formation is characterized by low porosity and low permeability.Based on the comprehensive analysis of the geological conditions of the reservoir site,the conceptual model of the target study layer Shiqianfeng Formation is established,carries out spatial discretization,and constructs the field level three-dimensional numerical model.The improved TOUGH2-ECO2N simulator is used for calculation and analysis,and the visualization software-Tecplot is used for model result analysis.The effects of salinity,fault and injection temperature on CO₂ migration,distribution and storage capacity in sloping formation were investigated.Through these researches,the main achievements are as follows:1.TOUGH2-ECO2N simulator improvementBased on TOUGH2-ECO2N simulator,fine description and simulation scale are greatly limited.This paper proposes to try to improve TOUGH2-ECO2N simulator,which is suitable for medium scale numerical simulation,and build and serve the model research platform.By borrowing the characteristics of F90 module,the memory mode of variables in TOUGH2-ECO2N original program is optimized and improved,and the dynamic memory allocation of the simulator is carried out.By comparing the correctness and efficiency of the simulator before and after the improvement,the accuracy and efficiency of the improved simulator should be proved.2.Development of postprocessing interface for TOUGH2 simulatorIn view of the difficulty of post-processing in TOUGH simulator,based on Tecplot software features,Through the analysis of grid subdivision,simulation results and target data of TOUGH2-ECO2N simulator,as well as the key analysis of the plt file format of Tecplot software.VC++ language is used to compile the program,the storage space and analysis time of the simulation results are greatly reduced.At the same time,it also provides theoretical support for the seamless coupling of other simulation software to Tecplot software.3.Impact of salinity on CO₂ storage safety in sloping formationThe Shiqianfeng Formation is not horizontal and the inaccuracy of saline salinity test,which will directly affect the CO₂ storage and conversion.In order to clarify the influence degree,this paper is based on the characteristics of formation slope,stratigraphic lithology and formation fluid salinity of Shiqianfeng Formation,a total of24 sets of models were established for simulation study.The simulation results showed that formation slope and salinity had significant effects on CO₂ storage and conversion.The impact of salinity on the migration and distribution of dissolved phase CO₂ and storage amount are more obvious than that of formation slope.The greater the salinity,the higher the formation pressure near the injection well,the smaller the dissolved phase CO₂ mass fraction,CO2 gas phase,dissolved phase and total storage amount,the worse the storage safety.Salinity are S,3/4S,1/2S,1/4S and 0.03,in which the ratio of dissolved-phase storage amount accounts for the salt-free strata are 26.2%,38.0%,54.3%,74.5% and 88.1% respectively at CO₂ migration 200 years.Formation slope has a significant impact on the distribution of formation pressure and CO₂ migration safety.Larger formation slope results in the farther CO₂ migration distance and the worse migration safety.The farthest CO₂ migration distance of slope degree 0°,5°,10° formations are the 60%,73.3%and 86.7% of slope degree 15° formation at the CO₂ migration 200 th year.Larger formation slope and salinity result in the worse CO₂ geological storage safety.Therefore,when selecting the target reservoir for CO₂ storage,the formation with small formation slope and lower saline salinity should be selected.4.Impact of fault on CO₂ migration safety in the sloping formationCO2 storage security is a key issue in CO₂ geological storage（CGS）.Faults have great influence on the spatial migration of CO₂.Faults are conducive to the rapid balance of pressure and increase the storage potential of CO₂,but they also bring risks to CO₂ leakage.Based on the geophysical interpretation data of Shiqianfeng Formation,a three-dimensional fault numerical model of carbon storage is constructed,to investigate the impact of reservoir formation slope and fault on CO₂ storage and migration security.The simulation results showed that formation slope and fault had a significant impact on CO₂ migration and storage security.Increasing the formation slope increased CO₂ migration distance,decreased the total volume of CO₂ safely stored in the formation and resulted in increased maximum gas saturation and dissolved CO₂ mass fraction.The presence of fault provided a channel for CO₂ leakage and caused an irregular distribution of formation pressure.The onset time of leakage through the fault proved to be a function of formation slope degree,occurring at 465,230,and 160 years following commencement of CO₂ injection for formation slopes of 5°,10°,and 15°respectively.The lateral extent of both the high saturation CO₂ plume and the plume of dissolved CO₂ was greater in the steeply dipping faulted reservoir model.Therefore,when choosing CO₂ reservoir site,we need to comprehensively consider the formation slope and fault.5.Impact of injection temperature on CO₂ storage capacity in the sloping formationIn order to optimize CCS injection schemes and improve CO₂ storage capacity of reservoirs in Ordos Basin,injection temperature as a controllable factor was used to explore the influence of injection temperature on CO₂ storage capacity of Shiqianfeng Formation reservoir.Injection optimization is an important reason to evaluate CO₂ storage capacity.and the injection temperature is a controllable factor.For the sloping formation with fault,the influence of injection temperature on CO₂ storage capacity is worth discussing.By comparing the simulation results,it is found that:Higher injection temperature and larger formation slope is more likely to cause CO₂ leakage.When the injection temperatures are 11,31.5,51 and 71℃ in the 15°slope formation,the time points of CO₂ leakage are 200,170,150 and 140 years,respectively.When the injection temperature increases from 11℃ to 71 ℃ and formation slope increases from 0 to 15°,the migration distance of dissolved CO₂ becomes farther.The impact of formation slope on the migration distance of dissolved CO₂ is greater than that of injection temperature.With the increase of formation slope,CO2 migration distance increases significantly.The higher injection temperature results in a greater gas phase,dissolved phase,and total CO₂ storage amount in the whole formation.The larger the formation slope is,the smaller the CO₂ storage capacity will be for CO₂ injected over 20 years.However,the larger formation slope resulted in a smaller gas phase and larger storage amount of the dissolved phase CO₂ for CO₂ migration after 140 years.The impact of injection temperature on CO₂ storage capacity is much greater than that of formation slope.Therefore,for the weak undulating Shiqianfeng Formation,the CO₂ storage capacity can be enhanced by increasing the CO₂ injection temperature.