| CO2geologic storage projects could decrease the increasing concentration ofCO2in the atmosphere. The mineral trapping of CO2is one of the most stable waysduring the long-term underground storage of CO2. The main factor that affects thepotential of carbon sequestration is the amount and dissolution rate of the host rock.Pyroclastic rock contains a large amount of mental elements and is easier to dissolute,thus pyroclastic rock could be the target of CO2sequestration. The thesis is carryingon the study of the potential capacity of CO2mineral trapping on two aspects. One istaking the dawsonite-bearing pyroclastic rock in Hailar Basin as the natural analogueof CO2geologic storage. Another aspect is studying the geochemistry mechanism ofinteraction between tuff reservoir, mudstone caprock and water in different fluids’condition.The petrologic study of the natural analogue of CO2geologic storage shows thatthe rock type of dawsonite-bearing reservoir, which was charged by CO2, istuffaceous sandstone and tuff. The content of pyroclasts is about53~81%, whichmainly consist of quartz, feldspar crystal fragments, igneous rock fragments andmatrix. The matrix was mainly kaolinite formed from the alteration of volcanic ash.The content of terrigenous fragments are3~10%, that contains monocrystal andpolycrystalline quartz, and sedimentary rock fragments. Authigenic minerals aremainly dawsonite, ankerite, siderite and kaolinite, and the content of calcite,overgrowth quartz and micro quartz are small. There are two periods of petroleum charging. The first period of petroleum inclusions appeared during the overgrowthingof quartz and dawsonite forming, with a high abundance of about5%±. The firstperiod of petroleum inclusions shows strong beige red under fluorescence, and thetemperature of fluid when the inclusion’s forming is about70~120C. The secondperiod of petroleum inclusions generated after dawsonite with a low abundance of lessthan5%±. The occurrence of the inclusions are along with the fracture that cutting thequartz and its overgrowth quartz, showing yellow and light yellow under singlepolarized light and tawny under fluorescence. The forming temperature is about140~160oC. the proposed paragenetic sequence of authigenic minerals, from early tolate, are siderite Ⅰ, kaolinite Ⅰ/overgrowth quartz/micro quartz, the first period ofpetroleum charging, dawsonite/micro quartz/kaolinite Ⅱ, calcite, ankerite, kaoliniteⅡsiderite, and the secondary period of petroleum charging.Dawsonite, known as the trace mineral of CO2, occurs as radiate andflower-shape in the study area. Feldspar, quartz, rock fragments and matrix areslightly to totally replaced by dawsonite. The grains that are totally replaced bydawsonite present pseudomorph of original grains. The content of dawsonite is about5~15%. The study of the chemical composition of dawsonite shows that, theroot/nuclear of dawsonite aggregates contains impurities composition of K2O andSiO2and the content of the impurities is up to1.415wt.%. Along with the growthdirection of dawsonite, the content of Na2O and Al2O3are gradually increasing, whichmeans the purity of dawsonite is increasing. The contents of dawsonite’s maincomposition are increasing with the degree of replacement by dawsonite. The contentsof Na2O and Al2O3are higher in the dawsonite that replaced feldspars, than in thedawsonite formed in the pore. The most important reason for this phenomenon is thatNa2O and Al2O3in the original feldspars could be provided to the dawsonite directly.The fluids inclusion found in the dawsonite present the formation temperature ofdawsonite is about98~118oC. The main source of mental elements for dawsoniteformation is from the process of replacing feldspars, fragments and kaolinite.The carbon isotope of dawsonite is from-4.1to-2.2‰PDB, and the carbon isotope of calcite, siderite and ankerite are distributed between-4.3and-6.7‰PDB.The carbon source of the4kinds of carbonate minerals are the CO2from the mantlemagma. The dawsonite and CO2gas in the adjacent Wuerxun Sag have the samerange of carbon isotope and origin.The content of CO2mineral trapping in study area is102.7Kg/m3averagely.Compared with other natural analogues of CCS (sandstone reservoirs) at home andabroad, the content of dawsonite, carbonate minerals and CO2trapped by minerals aremuch higher in pyroclastic rock than in sandstone. The pyroclasts are easier todissolute and release more mental elements in carbon acid fluids, which could providea larger amount of material for the formation of carbonate minerals. Therefore,pyroclastic rock contains a large capacity of CO2mineral trapping.A detailed study was carried on the mudstone caprock which overlie thedawsonite-bearing pyroclastic rock. The results present that the mudstone layers lessthan10m are unable to stop the leakage of CO2. An effective caprock will be thosemudstone layers that are thicker than10m. The clay minerals in the mudstoneinterlayers are dominated by kaolinite and illite, with the absence of smectite. Thereason is that smectite reacted with CO2-rich fluids or K+-rich fluids, and generateddawsonite, kaolinite, illite or quartz.There are totally22experiments for tuff and mudstone interacting with fluids.The main elements of the two rocks are turned out to be released mismatched with theelement’s ratio in the average chemical formula of these two rock. It is because of themulti composition of minerals for each of these two rocks. Minerals that haveunstable thermodynamic properties, take calcite as example in these experiments,would dissolute fast, thus the high release rate of Ca is obtained. While the quartz andclay minerals have relatively stable thermodynamic properties, and they dissoluteslowly. The release rete of silicon and magnesium are relatively slow. Both the tuffand mudstone have the highest dissolution rate in the experiments of100oC,50barCO2. The logarithmic value of dissolution rate of tuff is-12.02, and the mudstone is-11.59. In the100oC experiment without CO2attend, the dissolution rate of tuff is-13.24, and the mudstone is-11.96. Tuff has lower dissolution rate compared with mudstone under the same conditions of experiments. The main reason is that all thedissolution rate showed above are calculated by the concentration of Si. The mainmineral that contains Si in tuff is quartz, while it is clay minerals that are the mainminerals contain Si. Clay minerals are known as dissolute faster than quartz. Thedissolution rate of tuff is decreased with the increasing of pH from3to9, and so doesthe mudstone. In the experiment of25oC and pH3, the dissolution rate of tuff is-12.84, and the dissolution rate of mudstone is-14.22. The dissolution rate of tuff is-13.80when the pH is9, and mudstone’s is-14.93.The precipitation of gibbsite appeared frequently in the experiments of25oC andpH3~5. Gibbsite is normally a transition mineral in the natural geologic conditions.After long-term interaction with acid fluid, the gibbsite tend to transform to kaoliniteand albite, while in the carbon acid-rich fluid, it will transform to dawsonite.At the early stage of CO2geologic storage, CO2-rich fluid is acid, which willdissolute the tuff reservoir and mudstone caprock strongly. Carbonate mineralsdissolute firstly and release a certain amount of mental ions. Feldspar and clayminerals dissolute slowly. The pH value of the formation fluids turns to medium andalkaline, when the hydrogen ions are consumed totally. Then the dissolution rates oftuff and mudstone decrease, accompany with gibbsite precipitation. The fluids isbeneficial for the precipitation of carbonate minerals. The mental ions are easilycombined with carbonate and bicarbonate and turn to carbonate solid eventually. Theprecipitated carbonate minerals fix the carbon by the solid in the tuff reservoir, andthey are not only trapping carbon in mudstone, but also decrease the porosity andpermeability, as well as improve the sealing ability of mudstone caprock. |
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