Transport Modelling Of Impure CO₂ With N₂ And O₂ For Geological Storage

The energy consumption based on fossil fuels has produced a large amount of CO2,which has caused global warming.CO2 emission reduction has become a common problem and a hotspot of concern for mankind.In order to reduce greenhouse gas CO₂ emissions to the atmosphere,we need to adjust the energy structure,improve energy efficiency and develop low carbon technology.Meanwhile,we also need to achieve the effective reduction of CO₂ emission.Carbon dioxide capture and storage?CCS?is considered to be an important way to reduce CO₂ effectively.However,the CCS project is facing many difficulties in operation and technology extension due to its high cost and no direct economic benefits.Impure CO₂ geological sequestration can greatly reduce the cost of CO2 capture for CCS technology by reducing the requirements of the purity of CO2 capture.At present,the main ways of CO2 capture for CCS project are post-combustion,oxy-fuel combustion and pre-combustion.The CO₂ concentration in gas flue generated by oxy-fuel combustionoxy-fuel combustion is the highest,which could up to 80%⁹5%,and the impurity is only a small amount of N2 and O2.The lower amount and smaller proportion of the occupying ratio of impure gas in the CO2 gas,the lower the cost of CO2 capture.Therefore,carrying out the impure CO₂ geological storage with impurity N₂ and O₂ can achieve the economic efficiency of CCS technology,facilitate the scale implementation of the CCS project,and reduce CO₂ emission to mitigate the global warming problem effectively.For the impure CO₂ geological sequestration with impurity N₂ and O₂,only a few studies show that N2 and O2 will reduce the viscosity and density of injected fluid,increasing the gas migration rate to make more impure CO₂ gathered at the bottom of the caprock,which may increase the risk of CO₂ leakage.On the other hand,the decrease of aqueous density after the dissolution of injected liquid into aqueous phase will delay the generation and decay of convection.The dissolution rate and amount of CO2 will decrease,and the possibility of CO2 leakage will also increase.Therefore,it is of great significance to study the migration of CO₂-N₂-O₂ in the deep saline aquifers for the feasibility and safety assessment of the impure CO₂ geological sequestration.While the experimental conditions are difficult to restore the deep geological environment,numerical simulation can be used to characterize the migration and distribution of mixed fluids in geological media at different time and spatial scales.However,there is still lack of software and related studies to simulate the flow of CO₂-N₂-O₂-brine mixture under the condition of CO₂ geological sequestration.In this study,we have taken Shenhua CCS demonstration project in the Ordos basin as an example,developed CO₂-N₂-O₂ mixed gas geologic sequestration numerical simulator based on TOUGH2 framework for impure CO₂ geological sequestration with impurities N₂ and O₂.We have studied the migration of CO₂,N₂ and O₂ in deep saline aquifers,and discussed the impurity concentration of injected fluid and capillary pressure on the impure CO₂ migration,as well as the effect of the formation temperature,pressure and salinity on the dissolution of gas composition.The main conclusions are as follows:The precise characterization of CO₂-N₂-O₂-brine mixture is an important technical barrier in the study of the impure CO₂ geological sequestration.Based on the TOUGH2 computing framework,a flow calculation method for CO₂-N₂-O₂-brine water mixed fluid under geological storage has been established.And a CO₂-N₂-O₂ gas mixture geological storage numerical simulator has been developed.The following achievements and recognition have been made:?1?Based on the material composition,phase composition and phase equilibrium of CO₂-N₂-O₂-brine water mixed fluid under the conditions of geological storage,the mixed gas state equation and gas-liquid interaction solution model are used to construct the CO₂-N₂-O₂ mixed gas geological sequestration mathematical model and to calculate the thermodynamic properties of the geological storage of CO₂-N₂-O₂-under the condition of salt water mixed fluid.?2?Using the integral finite difference method to discrete CO₂-N₂-O₂ mixed gas geological storage model,using the first order implicit difference method and Newton iteration method to establish the residual equations and Jacobian matrix.So as to combined with linear equation solver CO₂-N₂-O₂ mixed gas geological sequestration numerical model in TOUGH2.?3?Combined with CO₂,N₂ and O₂ mixed gas geologic sequestration mathematical model and numerical solution of the model,using Fortan 90/95development language,a mixed gas state equation based on the model of the gas phase,liquid phase miscibility function module has developed.Coupling it in TOUGH2multicomponent multiphase flow calculation,to completed the CO₂,N₂ and O₂ mixed gas geologic sequestration numerical simulator development,which can provides numerical simulation tool for the study of the non-pure CO₂ deep salt water layer sealing technology with impurity N₂ and O₂.For the CO₂-N₂-O₂ mixture geologic storage model of impure CO₂ geological sequestration,we have compared the study results with the known analytic solution,the experimental data,and the calculation results of other similar software,so as to verify the reliability of the mixed gas geological sequestration numerical simulator.The following conclusions are obtained:?1?The comparison results of CO₂-N₂-O₂-brine mixed fluid state equation with known analytical solution of gas phase shows that the relative error of compression factor below 10%,the average absolute error of about 5%,and the calculation shows good consistency in the gas phase component that is easy to compress.Gas phase composition of fugacity factor relative error is under 20%,the average absolute deviation was 2²0%,and the calculation results are good when the temperature is45⁷0?.The relative error of the enthalpy of gas phase and the average absolute deviation of CO₂ is less than 10%,and the amount of N₂ and O₂ is about 5%,while the mean absolute deviation of CO₂ will decrease with the decrease of temperature.In general,the average absolute deviation of the calculation result of the fluid state equation and the analytical solution is below 20%.?2?For the mutual solubility of the gas component and pure water,the CO₂-H₂O and N₂-H₂O systems have experimental data points in a wide range of temperature and pressure conditions,and the calculation results of the fluid state equation show high consistency.The experimental data of O₂-H₂O system are limited,but the deviation from calculation results is very small.For gas composition and the miscibility of salt water,CO₂,H₂O-NaCl system experimental data of temperature,pressure and salty water salinity range can meet the needs of geologic storage condition,and the experimental data and calculated results of high consistency.The experimental data of N₂-H₂O-NaCl and O₂-H₂O-NaCl system are very few,but they are consistent with the calculation results.?3?On the basis of deep salt water layer of the CO₂ injection model,compared with the calculation result of TOUGH2-ECO2N,two results of the pressure accumulation caused by CO₂ injection,the migration distance of CO₂ in the aqueous phase,and the calculation of solid phase saturation in the evaporation area all show high consistency.However,the CO₂,N₂,O₂ mixed gas numerical simulator completely underestimated the CO₂ geological sequestration displacement speed of saline water,make the dry area have small radial range,and the mass fraction of CO₂ in aqueous phase is small,which relative error is about 10%.Both the TOUGH2-ECO2N and CO₂-N₂-O₂ mixed gas geologic storage numerical simulators require that the mass fraction of CO₂ or NaCl in the liquid phase set at zero when the aqueous phase saturation is zero,and further optimization is urgently needed.In general,the characterization of multicomponent multiphase flow under geologic sequestration is reasonable and reliable for CO₂-N₂-O₂ mixed gas geologic storage simulator.For the study of the migration of impure CO₂ with N₂ and O₂ in the deep saline aquifers,we takethe Shihezi Formation of Shenhua CCS demonstration project site in the Ordos Basin as the target reservoir.Using CO₂-N₂-O₂ mixed gas geological sequestration numerical simulator,the migration and distribution patterns of CO₂-N₂-O₂ in deep saline aquifers were investigated,and the effects of impurity concentration and capillary pressure on the migration of impure CO₂ plumes in the injected fluid were discussed.And the effects of formation temperature,pressure and salinity on the dissolution of gas components were also discussed.The main conclusions are as follows:?1?When the concentration of impurities of N₂ and O₂ were 6%and 2%,and CO₂injection at a constant rate of 3.17 kg/s into Shihezi Formation reservoir,the three components are mainly radial migration in the injection stage.After 2 years of injection,the migration distance is 300 m.The migration rate of the liquid components is larger than that of the gas phase.N₂ and O₂ are clustered in the front of the gas-liquid phase displacement,while CO₂ has lagged behind due to its larger solubility.After stopping injection,the accumulation of pressure in the formation disappeared,the formation water reflux,and the CO₂,N₂ and O₂ migrated upward by buoyancy.After50 years of stopping injection,the migration distance was 80 m.When the aqueous phase and the gas phase formed convection,the vertical distribution of CO₂,N₂ and O₂was more uniform.In the whole simulation process,CO₂,N₂ and O₂ are mainly in the form of gaseous state.After stopping injection 50 years,the proportion of dissolved amount is increased by 10%.?2?When the CO₂ injection rate is constant and the concentration of the injected impurities of N₂ or O₂ improved,it will increase the total injection rate of injected fluid,enhance the driving force for gas flooding.On the other hand,the gas viscosity will reduce and the migration rate of gas phase will increase,so the range of CO₂ and impurity gas in the gas and liquid phase will be expand.In the process of gas-liquid displacement,the contact time between injection fluid and formation water decreases,which decreases the amount of CO₂ and impurity gas dissolved in the liquid phase,and increases the gas storage.When the gas migration rate increases,it will also increase the contact area between the gas mixture and the salt water in the formation,and make the gas components with greater solubility more lags.The migration distance of the components with smaller solubility will be further,which lead to the more obvious separation of the gas components.?3?The lower pressure will reduce the gas solubility in the formation water,and it will also increase the driving pressure difference between the initial formation pressure and the injected gas.These will promote the displacement of gas relative aqueous phase,expand the gas distribution and increase the gas phase saturation,but also increases the contact area of gas components and formation water and the dissolution amount of CO₂,N₂ and O₂.?4?When the formation temperature is low,the solubility of gas components will increase,which will make the migration of CO₂ more lagged,the contact range of the formation water and the total dissolved amount of CO₂ reduced.On the other hand,the lag of the CO₂ migration will make the gas mass fraction of N₂ and O₂ in the front increases.The contact range of N₂ and O₂ and the formation water will increases,thereby improving the two components of their total dissolved quantity.Therefore,the lower formation temperature is beneficial to the dissolution of N₂ and O₂,but it is not conducive to the solubility trapping of CO₂.?5?When the salinity of the formation water is high,the solubility of CO₂,N₂ and O₂ will reduce and the viscosity and density of the aqueous phase increase,which is not conducive to the displacement of gas relative to the aqueous phase,thereby reducing the contact area between CO₂,N₂ and O₂ and formation water.Both of them reduce the total dissolution of CO₂,N₂ and O₂ in the reservoir.Therefore,the higher salinity of formation water is not conducive to the dissolving and collecting of CO₂,N₂and O₂ in the reservoir.?6?The effect of capillary pressure on impure CO₂ solubility trapping showed that high capillary pressure will reduce the gas flooding rate for formation water,hinder the migration of fluid in gas phase.And if the fluid is still injected into the reservoir at a constant rate,the pressure accumulation in the formation will be enhanced.The larger formation pressure will increase the solubility of CO₂,N₂ and O₂,reduce the gas saturation,and eventually increase the total dissolution of CO₂,N₂ and O₂ in the reservoir.Therefore,the higher capillary pressure is beneficial to the solubility trapping of the impure CO₂.