| As CO2 gas reservoir is a natural analogue for the long-term geological sequestration of industrial CO2, studying on the geological characteristic for sandstones in CO2 reservoir may provide a useful insight into CO2-fluid-sandstone reaction during CO2 injected into reservoirs. Thermodynamic analysis indicated that the partial pressures of CO2 for dawsonite forming equal to, or should be greater than,10-2bar. Dawsonite, as a function of the role on 'CO2 trace mineral', with its existence can record that a large scale of CO2 injected or resided in geological history.The Cretaceous reservoir in Honggang, southern Songliao Basin, is abundant in dawsonite-bearing sandstones, and in well 73 and 77, inorganic CO2 is reported, which means this reservoir could provide idealized, natural laboratory for studying on CO2-fluid-sandstones interaction.Dawsonite-bearing sandstones are poorly to medium sorted, lithic arkoses to feldspathic litharenites. The mineral assemblages after CO2 flooding are dawsonite, late generation quartz overgrowth, late generation kaolinite, late generation calcite and ankerite.δ13C PDB values for dawsonite are from -4.77 to 3.29‰and the average value is-0.574‰, showing remarkably consistent with the values which have been confirmed to be formed under inorganic CO2 background, and have an intimate relationship with the magmatic activity; C isotope of CO2 in isotopic equilibrium with dawsonite (δ13 CCO2) is in the range of -11.13～-4.14‰(PDB), with the average value of -7.23‰(PDB). It is suggested that, CO2 for dawsonite formation in researching area was mainly inorganic, with a bit of mixture with inorganic and organic CO2.δ13C PDB values for calcite is -2.16‰, and for ankerite are -6.35～2.12‰, which are similar with the values for dawsonite (-4.77 to 3.29‰). Temperatures for mineral growth have been bracketed by reference to the fluid inclusion data and burial history: for dawsonite, late generation calcite and ankerite are 100～120℃,100～120℃and 90～120℃, respectively. The formation water in the late Cretaceous during dawsonite, calcite and ankerite growth has theδ18O values of -6.46～2.75‰,-6.74～-4.77‰and-7.07～1.98‰. Taking the formation temperature of carbonate minerals and the oxygen isotope values of the formation water present during carbonate minerals growth event as the two identification indices, it is implied that the late generation calcite and ankerite had evolved progressively. It is by no means to conclude that the C source for dawsonite, late generation calcite and ankerite were the mantle-magmatic CO2, and partly of this natural CO2 had been locked up as these carbonate minerals.The simulation with TOUGHREACT shows that, as CO2 injected into the sandstone reservoir, plagioclase, k-feldspar and chlorite turn to dissolve, while four minerals deposite, including dawsonite, ankerite, calcite and quartz. After 10,000 years, owing to the mineral precipitation, the porosity of the injection area turns from the original 10% to 5%. The contents of dawsonite, ankerite and calcite are 7.7%,4.6% and 4.89% after 10,000 years. There are great comparability between the numerical simulation and the field observation in the changes of pH values, the types and contents of the mineral dissolution and precipitation.The capacity for mineral trapping of industrial CO2 in the studying area is gigantic. For natural analogue, a formula is designed by using the conservation of mass and the transformation relationship of mole among CO2 and trapping minerals. The total quantity of natural CO2 captured by dawsonite-bearing sandstones is calculated to be an average of 99.51kg per cubic meter, and by dawsonite is 12.48～118.57 kg/m3, which occupying about 42.2% of the total mineral trapping quantities. As the simulation shows, after 10,000 years, about an average of 89.73～98.5kg CO2 per cubic meter is trapped into the solid phase in the radial distances within 200 meters from the injecting well.It is suggested that the carbon capture ability is restricted by the types of the sandstones, the values of the compositional maturities and the contents of feldspar. A great mineral trapping ability for sandstones is characteristics with the rock types of arkose, litharenite or greywacke, and with relatively lower compositional maturaities and higher contents of feldspar.Constraints... |
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