Analytical Model for Screening Potential Repositories for Subsurface Sequestration of CO2

We have developed a screening tool for assessing the suitability of candidate repositories for subsurface carbon dioxide (CO2) sequestration, using a combination of easy-to-use analytical equations. We considered the injection of CO2 at a constant rate into a confined homogeneous deep saline aquifer via a single vertical well. The analytical equations give predictions of (1) the location of the CO2-brine interface, (2) temporal and spatial variations of pressure in the formation, and (3) temporal pressure changes at the injection well. By estimating these few parameters, it may be possible to determine if a candidate repository is viable. We demonstrate the validity of the analytical model through comparison with the TOUGH2 numerical model. The formation conditions and assumptions in TOUGH2 were set similar to those of the analytical model. Estimates of fluid saturation and pressure profiles show excellent agreement between the two models. The analytical model neglects changes in fluid properties due to changes in pressure, but simulations in TOUGH2 indicate that little error is introduced by this simplification. Comparison with TOUGH2 also enables us to determine the likely importance of other assumptions or simplifications invoked in the analytical model. Generally, the analytical model predicts a slightly higher pressure and plume extent than the numerical model, but the differences are small. Therefore, the easy-to-use analytical model may be a suitably accurate and conservative tool for regulatory or screening use in assessing candidate CO2 repositories.