Presented at SPE Project and Facilities Challenges Conference at METS, Doha, Qatar, February 2011.
Sulfate Precipitation during CO2 Sequestration in Carbonate Rock
Carbon capture and storage (CCS) has been proposed to mitigate the accumulation of CO2 in the atmosphere. Water alternating gas (WAG) technique is used in the industry to inject CO2 into underground formations either in sequestration or in EOR. CO2 dissolves in water, generating carbonic acid, which dissolves carbonate rock. The composition of the water is a critical factor that affects the rock permeability during sequestration. Sulfate content of the injected water can cause precipitation of calcium sulfate, which can negatively affect the rock permeability. This paper addresses potential formation damage due to sulfate precipitation during CO2 sequestration. A core flood study was conducted using limestone cores. CO2 was injected at pressure greater than 1300 psi and temperatures (70 and 200°F). CO2 and brines were injected in WAG cycles under injection rates of 2 and 5 cm3/min. Seawater and formation brine were examined in this study. Core effluent samples were collected and concentrations of calcium and sulfate ions were measured to assess calcium sulfate precipitation. Core permeability was measured before and after the experiment. The results show that temperature is the main parameter that affects sulfate precipitation during CO2 sequestration. Injection rate doesn’t have a significant effect on the core permeability. At high salinity calcium sulfate precipitation occurs even with low sulfate concentration present.