Study On Fluids Transportation And Water-rock Interactions Of CO₂ Geological Storage In Sandstone Reservoirs In Continental Sedimentary Basins

The technology of CO₂ geological storage can effectively reduce industrial CO₂emission,mitigate the greenhouse effect and meet the need of low-carbon economic development.To date,sandstone reservoirs are most common sites for CO₂ storage.In China,continental sedimentary basins widely develop,in which the sandstone reservoir shows quite different characteristics in petrology and tectonics when comparing with the reservoir in marine sedimentary basins.Generally,the minerals maturity in continental sedimentary basins keep low,with complex fault systems developed in multiple spatial and temporal scales.Therefore,special attention should be paid on the effects of petrological and tectonic characteristics on CO₂ migration,accumulation and CO2-water-rock interactions when conducting CO2 geological storage in continental sedimentary basins.The CO₂-water-rock interaction is one of the most important topics when studying CO₂ geological storage,and to be fully understood of the differences of minerals hydro-chemical quality,as well as the similarity of same mineral shown in different reservoir and water chemical conditions,is the priority task before studying the more complex CO₂-water-rock interaction in real felid worlds.With usage of mineral thermodynamics and kinetics datum,we first analyze and compare the mineral sensitivity with CO₂ fluids by PHREEQC and Excel calculation,in terms of mineral solubility,mineral dissolution rate,the fastest time to approach mineral dissolution equilibrium and mineral sensitive index.The results show that all of sulfate minerals,most of carbonate minerals?except for dawsonite?,minor clay minerals such as chlorite and the detrital mineral as anorthite have both the high mineral solubility and the fast dissolution rate,which are sensitive to CO₂ fluids and become the key factors in controlling the CO₂-water-rock interaction in sandstone reservoir especially at early stage of CO₂ injection.In contrast,the detrital minerals including the quartz and alkali feldspar are of medium sensitivity towards CO₂ fluids,dissolve sluggishly and work as the main dissolution minerals at the mid or late stage of CO2 storage.Given that the high CO₂-sensitive minerals in sandstone reservoir are also the cement minerals in most cases,and that the high spatial heterogeneity of cement mineral distribution,we design and conduct a series of hydrothermal experiments in autoclave to further investigate the effects of cement mineral component changes on CO₂-water-rock interaction.During reactions,we record the water chemistry of effluent fluids,surface morphology and elemental composition changes of reactant minerals,and further analyze the ionic activity diagrams and minerals saturation indexes by PHREEQC.The PHREEQC is also used to set up the kinetics models corresponding to the experiment configuration,to reveal the mineral alteration pathway.The experiment results indicate that the differences of cement mineral components can significantly influence the water chemistry and mineral alterations from different cemented groups,including the pH value of solution,the concentrations of master species in water,primary minerals dissolution extent and reaction paths,as well as the secondary minerals precipitation.For instance,the calcareous cemented groups?which contain calcite?show the highest pH value,TDS and Ca²⁺concentration,which suppress the albite dissolution and facilitate the transition of K-feldspar dissolution form kaolinite to be K-mica.In terms of secondary mineral precipitations,we observe the chlorite deposit in dolomite-cemented group in experiment and predict the dolomite precipitation at the expense of calcite and chlorite dissolution in mix-cemented group by kinetics model.In macro view,since the fault systems widely develop in continental sedimentary basin,we base on the geological conditions of Songliao basin and combine the usage of TOUGH2,T2Well and TOUGHREACT to investigate the effect of fault growing feature,including the extent of fracture development,fault physical conditions and the differences of fault structure,on CO₂ leakage along regional fault towards to the shallow aquifers,and the CO₂ fluids migration,accumulation and CO₂-water-rock interaction inside the sandstone reservoir with local small-size faults developed.In the case of CO₂ leakage along reginal fault between sandstone reservoirs,the simulation results suggest that at the constant injection mode,the size of fracture aperture can significantly affect the pressure responses and fluids flow regime in fault zone,fluids dynamics at the interface between fault and each aquifer and the final CO₂ mass fraction distribution in aquifers.For specifics,when aperture size reduces to be 1mm,the friction of fault wall increases to slow down the mixture velocity,which makes the laminar flow prevails.At this condition,the velocity of gas phase CO₂ also increases to significantly enhance the pressure responses and gas expansion effect in fault zone,and finally leads to a higher breakthrough rate of CO₂ into the aquifers nearby.As results,the amount of CO₂ leakage into shallow aquifers also increase.However,this increase will become more and more neglectable as time going.On the contrary,when the aperture size increase to be 5cm,the friction of fault sidewall decreases and the turbulence flow occurs.Interestingly,the gaseous CO₂ velocity decreases due to the constant injection system,and there are less influences on pressure,fluids dynamics,e.g.,compared with the small aperture case.At this moment,the equivalent porous media model shares similar results as the momentum model,suggesting the Darcy's law may still be functional in dealing with the CO₂ leakage problems along the fractured-fault when the constant injection strategy adopted at long time scale.Similarly,the local fracture systems developed in sandstone reservoir also play significant roles in controlling the CO₂ fluids migration,accumulation and CO₂-water-rock interactions.For specific,the simulation results of CO₂ fluids migration on the typical oil and gas profile in Sanzhao depression indicates that under the interbedded stratigraphic combinations,the high-permeability faults developed in the sandstone reservoir can work as the main channel for CO₂ fluid migration and accumulatiuon.As results,the CO₂ fluid distribute at the vicinity of the fault zone.A reduction of one-tenth of the fault permeability is not sufficient to affect the migration process of CO₂ fluid along the fault to adjacent reservoirs;however,the development of the fault dual structure has significantly influence the fluid transportation on both sides of the fault core.When the fault core is developed,the CO₂ fluid is trapped in its own independent area surrounded by two-side faults.Consequently,much more pressure builds up in the area,which in turn is beneficial to the fluid exchange between sandstone reservoirs and the maximum usage of sandstone reservoir space.In addition,the occurrence of sandstone formations,their physical conditions,and the distance from the fault also have an importatant impact on the CO₂ migration process.When fault occurrence is consistent with sandstone reservoirs,CO₂ fluids are readily to be entered into faults and then leak to shallow aquifers.On the contrary,with dip direction different between falut and sandstone reservoir,the CO₂ tends to migrate upwards in the sandstone reservoir,rather than flow towards the fault.This secinario ends when some barrier occurs along the pathway of CO₂ migration,such as the pinching-out of strata.The simulation results with extra consideration of chemical reactions show that both the minerals assemblage and the fluids flow rate influence the CO₂-water-rock interaction,in which the fluids flow rate is controlled by stratigraphic occurrence,CO₂injection strategy and fault development characteristics.In the initial stage of CO₂injection,the groundwater flow rate in sandstone reservoirs is relatively fast,chemical reactions mainly occur in form of calcite dissolution.While in the shallow part of fault zone,the flow velocity is relatively slow and there is no calcite in initial mineral assamblage,oligoclase and k-feldspar become the main dissolvable minerals with the precipitation of quartz,illite and kaolinite.Due to the accumulation of Fe²⁺,the dissolution of chlorite is surpressed.In the interlayer of mudstone which has the slowest fluid velocity,the chlorite dissolution and the precipitation of Ca²⁺-montmorillonite and ankerite prevails.After CO₂ injection stops,the groundwater flow field changes and accordingly,the active areas of CO₂-water-rock interaction switch.Foe details,at the down-dip direction of the stratum,the fluid migration velocity is relatively slow to favor the accumulation of gaseous CO₂ here,which results in a lower pH distribution,more chlorite and calcite dissolution,and siderite,ankerite,and Ca²⁺montmorillonite precipitation.At the end of 300 years after 10yr's CO₂ injection,a considerable amount of carbonate mineral deposits can be formed in sandstone reservoirs,mainly in form of siderite and ankerite with a volume fraction up to 10.Besides,there is also a little calcite can be formed in the shallow part of fault zone.At last,the extent of CO₂-water-rock interactions in the faults are quite limited to change its hydralic parameters and further influence the self-closing or opening trendency of fault.This study contributes to reveal the possible mechanisms and effects of the petrology and tectonic characteristics of sandstone reservoirs in continental sedimentary basins on CO₂ fluids migration,accumulation and CO₂-water-rock interaction during CO₂ geological storage,which have some theoretical and practical implications in guiding the large-scale CCS project in China around the corner.