In situ diamond anvil cell (DAC) experiments on equilibrated carbonate–CO₂–H₂O interaction under elevated temperature and pressure

Abstract

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 ${\text{H}}_{2} {\text{CO}}_{3}$ solution at acidic pH (≈ 5.6), 20–260 °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.