In situ diamond anvil cell (DAC) experiments on equilibrated carbonate<bold>-</bold>CO2<bold>-</bold>H2O interaction under elevated temperature and pressure

The carbonate-CO2-H2O interaction in a closed system under burial conditions was simulated in diamond anvil cell (DAC) with an in situ Raman spectroscopy. Experiments were performed in H2CO3 solution at acidic pH (approximate to 5.6), 20-260 degrees C, and atmospheric pressure to about 400 MPa. Pressure was first solely increased to separately evaluate the pressure effect on carbonate dissolution/precipitation. Then temperature and pressure were increased together alike in the burial process, to monitor the dissolution/precipitation tendency of carbonate during burial. A continuous dissolution was observed for both limestone and dolostone when solely increasing pressure. A rapid dissolution and then continuous precipitation were observed when increasing temperatures and pressures simultaneously. These observations indicate that pressure has a positive effect (prograde solubility), while temperature has a negative effect (retrograde solubility) on CO2 solubility and carbonate dissolution. In case of low initial CO2 content as our experiments revealed, the increase in solid carbonate volume and a counterbalance decrease in porosity caused merely by increasing burial depth are minor (<0.23%). The porosity decrease of dolostone was likely less than that of limestone at smaller burial depth (<1.5 km), and notable at bigger burial depth. This work may help to improve our understanding on carbonate reservoir porosity evolution during burial.