| CO2 is a kind of active gas that can be dissolved into water and then form acid fluid. The acid fluid can cause dissolution of soluble mineral such as feldspar and deposition of some new carbonate minerals. The transformation of CO2 charging on the reservoir sandstone is a new question of reservoir research. And laboratory simulation under high temperature and high pressure is one of useful methods to answer the question. At present, injecting CO2 into the subsurface to form stable carbonate minerals is a feasible method to reduce CO2 releasing into atmosphere. If the fixed carbonate minerals by carbon dioxide can exist stably in reservoir is another question that scholars at home and aboard concerned all along. The Yingcheng Formation Debris arkosic sandstone from well SN78-1 in southern Songliao Basin, dawsonite-bearing sandstone from well Tong6 of Wuerxun sag in Hailaer basin, quartz sandstone,monomineralic quartz and feldspar from outcrop were used to carry out hydro-thermal experiments on sandstone- CO2- H2O system at different temperatures(100℃,200℃,300℃). The process of sandstone- CO2 interaction was described semiquantitatively. The solution was prepared according to the features of stratum water in dawsonite-bearing sandstone of Wuerxun sag, then hydro-thermal experiments at different temperature(100℃,200℃,300℃) were made to verify the stability of dawsonite under the stratum water condition since dawsonite had formed. In addition, the features of stratum water, diagenetic paragenesis and reservoir features of dawsonite-bearing sandstone are analyzed in Wuerxun sag, Hailaer basin in order to study CO2-sandstone interaction in CO2 gas reservoir. Through SEM and analysis of liquid chemical constitution of 6 series 20 hydro-thermal experiments of carbon dioxide fluid and sandstones, and the hydro-thermal experiment in stratum water-dawsonite-bearing sandstone system such conclusions are made.(1)It is indicated that the higher the temperature rise, and the harder the samples are dissolved. Analysis the change of total salinity and the weight of the samples show that the sandstones dissolved harder as the temperature rise, through the series experiments of carbon dioxide fluid and feldspar,quartz,carbon dioxide fluid and arkosic sandstone,quartz sandstone, carbon dioxide fluid and dawsonite sandstone, stratum water and dawsonite-bearing sandstone.(2)As the temperature rise, feldspar, quartz, calcite and dawsonite dissolved harder. Through the hydro-thermal experiment of sandstone and monomineralic, found that feldspar, quartz, calcite and dawsonite dissolved harder as the temperature rise. At the same condition, calcium feldspar dissolved harder than sodium feldspar, sodium feldspar dissolved harder than potassium feldspar. The stability of dawsonite weaken. Dawsonite was severely dissolved at 200℃of carbon dioxide fluid and dawsonite sandstone hydro-thermal experiment, and was dissolved harder at 150℃of stratum water and dawsonite sandstone hydro-thermal experiment.(3)After the hydro-thermal experiment , gibbsite of any shape deposited at the surface of the grains. When the feldspar and arkosic sandstone reacted with the carbon dioxide fluid, gibbsite deposited at 200℃and 300℃at the surface of the grains, its shape include spherulitic, claviform, needle-leaved, efflorescent, radiate, and the higher the temperature was, the better the gibbsite grown. Gibbsite deposited at 200℃when the quartz sandstone reacted with carbon dioxide fluid. Gibbsite deposited at 100℃and 200℃when the dawsonite sandstone reacted with carbon dioxide fluid. The concentration of Al3+ ion was very low even below the detection limit, because the deposition of gibbsite.(4)Siderite, iron oxide and some unknown carbonate minerals formed after the hydro-thermal experiment which indicated that some minerals were dissolved by carbon dioxide fluid, but at the mean time they were fixed as carbonate minerals. At 200℃, feldspar(potassium feldspar and plagioclase), quartz and dawsonite sandstone reacted with carbon dioxide fluid , siderite was formed at the surface of the grains. At 200℃, iron oxide was formed when quartz sandstone reacted with carbon dioxide fluid. At 300℃, iron oxide was also formed when plagioclase reacted with carbon dioxide fluid. At 150℃and 200℃, some unknown carbonate mineral was found through the analysis of X-ray diffraction when dawsonite-bearing sandstone reacted with stratum water.(5)After the hydro-thermal experiment , new minerals appeared such as chlorite and albite. Through the experiment of carbon dioxide fluid and dawsonite-bearing sandstone at 300℃, foliated chlorite deposited in the surface of the grains. Through the experiment of dawsonite-bearing sandstone and stratum water at 200℃, foliated chlorite also deposited in the surface of the grains, and a large mount of albite formed as cubic monomer or spherulitic aggregation.By studying on the formation condition and stabilization of dawsonite through the combine of hydro-thermal experiment and geological example, it is found that dawsonite was formed at high pressure of carbon dioxide and alkalinity condition, and it is easy to identification, can exist stability in present stratum. Dawsonite is a kind of white carbonate mineral with sodium and aluminum, which grows in sandstone, dolomite, oil shale and volcanic ash of continental facies stratum, some were also found in marine facies and coal measures. Dawsonite grows in the pores of sandstone or replaces feldspars and rock debris, and it has generally a shape of radial, fascicular, trichoid, platelike. In Wuerxun sag ,the dawsonite-bearing sandstone distributes between 1300m and 2000m. Through hydro-thermal experiment at 100℃, it is found that dawsonite wasn't dissolved or only dissolved slightly. So under current stratum condition, dawsonite can exist stably even generating the phenomenon of carbon dioxide charging and interacting with stratum water, if a small amount of dawsonite is dissolved that siderite or other carbonate minerals can be formed, this made it possible to store carbon dioxide as carbonate minerals forever.The comparison of dawnsonite-bearing sandstone and sandstone showed that the stratum water of Wuerxun sag are characterized by high salinity and high concentration of Na+,K+ +Na+,HCO3-,CO32-. The value of pH is about 8.36, showing the alkalescent fluid. The total salinity is 22.832g/L. The water-type is NaHCO3, the concentratin Na+ of is 6.466g/L,the concentration of HCO3- is 14.533g/L, with the characteristics of high salinity and high concentration of Na+,K+ +Na+,HCO3-,CO32-. The origin of this is the carbonic acid made the content of HCO3- increase, which forming from igneous magma of Yanshan Stage carrying the carbon dioxide to Nantun formation along the deep fracture. At the mean time , feldspar was replaced by acid, made the content of Na+,K+,Ca2+ increase, and some of the Ca2+ combine with the CO32- forming cement of carbonate.And other minerals formed earlier than dawsonite in dawsonite-bearing sandstone from Wuerxun sag. The diagenetic paragenesis include clay mineral coating-calcite-feldspar erosion, secondary growth of quartz, kaolinite-injection of oil and gas-injection of carbon dioxide-dawsonite-ankerite. The combination of authigenic minerals before carbon dioxide filling are calcite,secondary growth of quartz and kaolinite , and these mineals are all dissolved. The authigenic minerals are dawsonite and ankerite after CO2 charging.Because the solution of calcite, quartz overgrowth and kaolinite by the CO2 acid fluid, the pore types of dawsonite-bearing sandstone are mainly secondary porosity. And the cementation of authigenic carbonate minerals such as dawsonite made the reservoir condition poorer. The average porosity of dawsonite-bearing sandstone in Wuerxun sag is 10.73%, permeability is 3.94×10-3μm2. So poor reservoir condition was caused by that abundant carbonate minerals such as dawsonite formed as CO2-sandstone interaction, and no dissolution happened.
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