| The CO2released from the burning of fossil fuels（oil, coal and natural gas）hasbeen consider as the the main warming gases leading to the greenhouse effect. The CO2enhanced oil recovery (CO2-EOR) could be the significant method for CO2Capture,Utilization and Storage engineering, because the CO2-EOR not only effectivelyenhances oil recovery but can also contribute to a reduction in greenhouse gasemissions. In terms of operational technology, the recovery efficiency of CO2-EOR is15%–25%(average,20%), indicating that despite this enhanced recovery, a significantamount of residual crude oil remains in the reservoir. The residual crude oil in thereservoir could affect the interactions between CO2and minerals, and further affect theprecipitate of carbon mineral. The reservoir coexisted crude oil and CO2gas and theinteraction experiment of CO2and minerals provide an efficient technique for the studyof short-term and long-term interactions between CO2, crude oil, water, and minerals,and predicted the security and ability for the CO2storaged underground.Abundant crude oil and CO2gas coexist in the the Cretaceous sandstone reservoirof the southern of Songliao basin, and a CO2-EOR demonstration project has beencarried out in this basin.These occurrences provide natural laboratories for the study ofinteractions between fluid and minerals after the termination of CO2-EOR projectsThe reservoir sandstone contains coexisting mantle-derived CO2gas and crude oilconsists of dawsonite-bearing lithic arkoses and feldspathic litharenites. Theauthigenetic minerals/material of dawsonite-bearing sandstones are dawsonite, calcite, ankerite, authigenetic quartz(quartz overgrowth and microcrystalline quartz) andclay.The clay minerals include, illite-smectite, ilite and kaolinite. Thedawsonite-bearing sandstones record a paragenetic sequence involving early formationof clay minerals to the later formation of quartz overgrowths, first-generation calcite,dawsonite, second-generation calcite, and the final formation of ankerite.The C isotopic composition of CO2in isotopic equilibrium with dawsonite(13CCO2), second-generation calcite, and ankerite consistent with the13C values ofCO2gas derived from wells in the study area, these values are indicative of CO2derived from an inorganic source. The δ13C (PDB) values for dawsonite are from-6.3to-0.1‰, for second-generation calcite are from-8to-1.7‰, for ankerite are from-8.3to-1.5‰. The C isotopic composition of CO2equilibrium with carbonate minerals are-10.68~-5.9‰(dawsonite) and-12.37~-8.09‰(calcite), respectively.According to the comprehensive analysis of paragenetic sequence, fluid inclusions,diagram of burial history (thermal history), tectonic evolution history and ages ofvolcanic activities, the earliest hydrocarbon filled into the Quantou-4member of thestudy area at85-65Ma. And the age of magmatic/mantle CO2charged at51-41Ma.Fluid and rock interaction experiment after termination of the CO2-EOR suggests thatwith low oil saturability, the CO2could be a reactive gas that occurred the dissolve-precipitate geochemical property. After the experiment, the feldspar and calcite couldbe dissoluted and kaolinite and an unknown “solid mineral” could be precipitated. Thepresent of oil could reduce superficial area of the mineral which exposed to theformation water in unit volume, and then reduce the rate of mineral dissolution. Theexposed superficial area are mainly controlled by the mineral wettability, content ofmineral and the oil saturability. Furthermore, The mineral wettability could control theselective dissolution between mineral and formation water. The component and contentof oil was not changed while the ion concentration of water was increased after theinjection of CO2. The mineral dissolution increased the ion concentration of water.However, the kerosene component was so light(mainly C7-C12) that the component was not changed. Otherwise, the injection of CO2would increases the porosity and reducesthe permeability. The mineral dissolution increase the core porosity, while theprecipitation of tiny kaolinite and unknown “solid mineral” could be the mainly reasonsthat decrease the permeability.According to the exploration results and the analysis for physical and chemicalproperties of crude oil, the physical and chemical properties of crude oil and the ionicconcentration of water were changed after the injection of CO2gas into the oil pool.These changes include as following, the crude oil would become lower density,viscosity, freezing point, initial boiling point, the content of resin and asphaltene; theconcentration of water would be more salinity, K++Na+'HCO3-. The change of crudeoil were recorded in the hydrocarbon and aqueous fluid inclusions of dawsonite-bearingsandstones. The lighter crude oil and higher concentration water were situated next tothe high concentration of CO2gas, and distributed consistent with the CO2gas. Thecrude oil become heavier and the water become less salinity extend outward from thecenter.The presence of prolate primary hydrocarbon inclusions within the dawsoniteindicates that these minerals precipitated from oil-bearing pore fluids, suggesting inturn that CO2could be stored as carbonate minerals after the termination of a CO2-EORproject. However, according to the relationship between the Quantitative grainfluorescence index (QGF index and QGF-E index) and XRD, the the growth ofdawsonite was inhibited by crude oil in the pore. These is consistent with the result thatthe content of dawsonite in CO2/H2O/oil/sandstone system was lower than that inCO2/H2O/sandstone system.The capacity for mineral trapping in sandstone contain oil was lower than that inreservoir without oil. According to the calculation results, the total quantity of naturalCO2trapped in sandstone reservoir contain crude oil is54.3kg/m3, and by dawsonite is6.72~56.49kg/m3and18.43kg/m3on average; the total quantity of natural CO2trappedin sandstone reservoir without oil is85.35kg/m3(36.4%higher than that within oil), and by dawsonite is6.72~70.61kg/m3and39.00kg/m3on average (52.8%higher thanthat within oil). As the simulation by TOUGHREACT shows, after3500years, thecapacity for mineral trapping within crude oil and without oil are1.1108kg and9.44107kg, respectively. |
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