Experimental Study Of CO₂Fluid On The Geological Transformation Of Reservoir Sandstone

CO₂is a kind of gas that can form acid fluid and stably exist in the atmosphere.But when the temperature is higher than31℃and the pressure is greater than7.38MPa, CO₂will exist in supercritical state, that is, it existed between the gas and liquidphase. The acid fluid can cause dissolution of soluble mineral such as feldspar, quartzand deposition of some new carbonate minerals. Therefore, the injection ofsupercritical CO₂on the transformation of the reservoir sandstone and the feasibilityof CO₂underground storage are the main subjects in this study. Using sandstone （coresamples） in CO₂geological storage sites in China, the hydro-thermal experiments atdifferent temperatures in CO₂-H₂O-reservoir sandstone system were carried out in thisstudy. Furthermore, the process and mechanism of the interaction between CO₂fluidreservoir sandstone were discussed. According to reservoir formation water qualitytest report, to prepare the formation water solution, then to inject of CO₂withformation water and reservoir sandstone under simulated geological conditions and todetermine the stability of the formation of new mineral.Through SEM and analysis of liquid chemical constitution in differentsolid-liquid ratios, the disturbance turbulent condition experiments, formationwater-CO₂fluid interaction experiment and the hydro-thermal experiments, the resultsare as following:（1） In solid-liquid ratio experiment, it was found that the experiment reachedequilibrium when the solid-liquid ratio was0.06:1. While the HCO₃^-, pH and ionconcentration was in a slightly smaller fluctuation, illustrating little impact on thefollow-up experiment. Therefore, the optimal solid-liquid ratio in this experimental is0.06:1.（2） In the disturbance turbulent condition experiment, the disturbance of thereaction solution in a turbulent state can accelerate the reaction rate, thus greatlyreducing the experimental time and providing a good prerequisite for the conduct offollow-up experiments.（3） In Hydro-thermal experiments, it was found that the dissolution of the sampleincreased gradually with increasing temperature. Through CO₂-H₂O （distilled water）-feldspar quartz sandstone experiment, the results showed that the experimentalequilibrium time was18-24d.The siderite intermediate state and the aluminum silicate minerals precipitated on the surface at70℃and100℃, respectively, illustrating thefeasibility of CO₂storage underground.（4） In the CO₂-H2O （formation water） and CO₂-H2O （formation water）-feldsparquartz sandstone interaction experiments, the SEM of feldspar dissolution, thereaction solution ion-exchanges of concentration and the loss of sample after thereaction were investigated in this study. The results showed that the dissolution offeldspar quartz sandstone samples increased with the increasing temperature. Thecalcite middle state and unknown aluminum silicate minerals precipitated on thesandstone surface at85℃and100℃, respectively, illustrating that when the, CO₂was stable fixed in the new mineral at the meantime of mineral re-dissolution.（5） In the high-temperature hydro-thermal experiments, there are somespherical-rich "C" K-feldspar on the sample surface at100℃, this rich "C" indicatedthat if the reaction time is long enough, CO₂will be fixed in the form of carbonate.Some containing Fe aluminum silicate was formed on the surface of the sample at150℃. Moreover, there were some spherical kaolinite on the surface of the sample inthe interaction of feldspar quartz sandstone and CO₂at180℃.