CARBON DIOXIDE SEQUESTRATION IN SALINE AQUIFERS

Determination of the carbon dioxide sequestration capacity of saline formations by solubility and mineral trapping processes is best accomplished by the use of geochemical simulations. However, predictions made using many commonly used geochemical models are only reliable if the data and reaction scheme on which they are based are accurate. The extreme pressures and salinities encountered in many aquifers targeted for sequestration make modeling of carbon dioxide sequestration capacities difficult. To provide meaningful predictions, theoretical simulations must be verified against experimental data.

In order to test theoretical predictions, carbon dioxide solubility experiments were conducted in a Dickson-type flexible-cell system in pure water and natural brine containing Na-Ca-Mg-K and Cl equivalent to a 20 weight percent NaCl fluid. Experiments were conducted under conditions that bracket the range of temperatures and pressures expected for typical sequestration scenarios. The Dickson-type reaction cell allows for gas-saturated fluid samples to be taken into gas-tight syringes without changing in-situ conditions during sampling. As expected, experimental results indicate a decrease in carbon dioxide solubility in the brine compared to that for pure water at the same temperature and pressure. The experimental results are in good agreement with theoretical predictions provided that equilibrium constants are adjusted to reflect the elevated pressures of the system and appropriate activity-concentration relations are accounted for with elevated salinities. Failure to properly account for elevated pressures and salinities in the theoretical models can result in large errors in solubility estimates.

Absence of experimental data on carbonate mineral and carbon dioxide solubility in natural brines under elevated carbon dioxide pressures makes it extremely difficult to verify modeling results, especially when considering rock/water interaction in the subsurface after injection of carbon dioxide. Although experimental results indicate carbon dioxide solubility estimates may be reliable, overall, it is difficult to confidently place quantitative constraints on the ultimate sequestration capacity of deep saline aquifers.