Haracteristics And Mechanisms Of Interactions Between CO₂Fluids And Mudstones

Geological sequestration of industrial CO₂is an effective way to slow down theglobal warming. Demonstration projects are done by some countries in succession,along with the further study of CO₂geologic sequestration. It becomes one of themost important problems that how to decrease the risks of CO₂leakage. The integrityand stability was brought up naturally as the leading topic of the safty for CO₂geologic sequestration. The main scientific questions are the characteristics ofCO₂-mudstone caprock interaction and its impact on the capability of CO₂sequestration during the geologic period. Natural CO₂system is the natural analoguesof CO₂geologic sequestration. And dawsonite is the traced mineral of CO₂migrationand accumulation. Therefore, mudstone interlayers in dawsonite-bearing sandstoneformations are the most ideal research objects for the study of CO₂-mudstoneinteraction. The achievements of this research would provide proofs for geologicalsafety evaluation of CO₂sequestration, which makes the research having academicvalue and actual applied value.Comparative petrology, CO₂fluid-mudstone interaction experiments andnumerical simulation are used in the study. The object of comparative petrology is themudstone interlayers and the matched cores’ rocks in dawsonite-bearing sandstoneformation in South Songliao Basin. Matched cores’s rocks are the mudstones occurrednear the objective interlayers which show the similar sedimentary and diageneticfeatures without impact from CO₂fluid. FXY autoclave is used in the experiments of CO₂-mudstone interaction. The simulative software is TOUGHREACT.In the natural CO₂system in Honggang anticline and Huazijing district, themudstone interlayers are mainly consist of mudstone and siltstone. Thickness of theseinterlayers focus on0.5-2m, no more than11m. Plagioclase and orthoclase in themudstone interlayers are corroded, while dawsonite and siderite are precipitated aspart of authigenic minerals. The dissolution grade of orthoclase and plagioclase in theinterlayers are obviously higher than in matched cores’s rocks. So the development ofdissolution in the interlayers could relate to the CO₂-mudstone interaction. Theauthigenic dawsonite is a typical “trace” of CO₂injection in the Honggang anticline.Authigenic siderite is developed in the Huazijing district, which could due to the CO₂injection, either. The contained Fe²⁺in the CO₂fluid combined HCO₃^2-, and formedsiderite during the fluid flooding the mudstone.Contents of the main oxides change regularly upwards near the interface betweenmudstone interlayers and underlain dawsonite-bearing sandstones. In Huazijingterrace, contents of K₂O, Al₂O₃, Fe₂O₃, CaO and MgO in mudstones display afluctuating increase from the interface up while contents of Na₂O and FeO display theopposite trend. Possible explanations responsible for this change may attribute to thedissolution process of K-feldspar, calcite, dolomite and Fe³⁺compound during CO₂leakage towards overlying mudstones and the migration of related ions. Thisphenomenon is the most obvious at the interface between dawsonite-bearingsandstones and mudstones. The decrease in the content of FeO is probably owing tothe reduction from Fe³⁺to Fe²⁺, as for Na₂O, reasons for the decrease is yet unclear. InHonggang terrace, in the depth of1481.2¹481.22m, contentsof FeO, MgO, K₂O andAl₂O₃in mudstones tend to increase upwards near the interface between sandstonesand mudstones while content of Na₂O shows an opposite change. This change istriggered by the same mechanism related to CO₂leakage as which in Huazijingterrace. Content variation of Na₂O may be a result of dawsonite precipitation. Inmudstones interval of1477.5¹477.53m, the maim oxides display no verticallyregular change. One possible explanation is that thin layers of mudstones andsandstones sink in CO₂fluid, thus no regular petrographic and geochemical changes can be observed.Experiments of CO₂-mudstone interactions have verified petrographicobservations. Results show that total salinity, pH values decrease and concentrationsof K⁺, Na⁺and SiO₂increase as the increase of reaction temperature. TheConcentration of Ca²⁺、Mg²⁺has increased in low temperatures, while decreased inhigh temperature. Deoxidation from Fe³⁺to Fe²⁺with the increasing temperaturesleads to the colour fading of mudstone surface. Feldspar, quartz has dissolved, whileCalcite, siderite and chlorite mainly dissolve under relatively low temperatures（100℃、130℃）; indissolvable carbonates occur under higher temperatures（160℃、190℃、220℃）.Geochemical simulation by TOUGHREACT has reestablished thedissolution-precipitation process of minerals. The influential extent of CO₂influx is1500m horizontally and17m vertically from sand-shale interface. Results reveal thatdissolved minerals include K-feldspar, albite, chlorite and semctite; precipitatedauthigenetic carbonate minerals are calcite, ankerite, magnesite and dawsonite, othersinclude kaolinite, illite and micro-quartz. As time goes by, concentration of HCO﹣3、K⁺、Ca²⁺has increased, while Na⁺、Mg²⁺、Fe²⁺has increased in short time, decreasingin long time.With natural analogy, CO₂fuild and mudrock interation and numericalsimulation on the process of dissolution-percipitation, building a model of CO₂–murock interation. As thin layers of mudstone and sandstone soaking in CO₂fluidPetrological and geochemical characteristics should not display regularly change onthe longthways. CO₂was filled in sandstone leak to mudstone which is a thick layer inthe form of diffusionThe innovation points in this study mainly embodied in the method of naturalanalog, which interprets the petrology and geochemistry records of CO₂leakage inmudstone interlayers in natural CO₂system of the South Songliao Basin. And thepetrology results are complemented and verified by CO₂-mudstone interaction andnumerical simulation. CO₂-mudstone interaction model are built according to all thestudy results mentioned above. The achievements in this study could provide basic geologic information, ensure the possibility of permanent CO₂sequestration, andenhance the security of CO₂geologic sequestration.