Implications of Sub-Hydrostatic Pressures in the Bravo Dome Natural CO2 Reservoir for the Long-Term Security of Geological Carbon Dioxide Storage

The Bravo Dome field in northeast New Mexico is one of the largest gas accumulations worldwide and the largest natural CO₂ accumulation in North America. The field is only 580-900 m deep and located in the Permian Tubb sandstone that unconformably overlies the granitic basement. Sathaye et al. (2014) estimated that 1.3 Gt of CO₂ is stored at the reservoir. A major increase in the pore pressure relative to the hydrostatic pressure is expected due to the large amount of CO₂ injected into the reservoir. However, the pre-production gas pressures indicate that most parts of the reservoir are approximately 5 MPa below hydrostatic pressure. Three processes could explain the under pressure in the Bravo Dome reservoir; 1) erosional unloading, 2) CO₂ dissolution into the ambient brine, 3) cooling of CO₂after injection.

Analytical solutions suggest that an erosion rate of 180 m/Ma is required to reduce the pore pressures to the values observed at Bravo Dome. Given that the current erosion rate is only 5 m/Ma (Nereson et al. 2013); the sub-hydrostatic pressures at Bravo Dome are likely due to CO₂dissolution and cooling.

To investigate the impact of CO₂ dissolution on the pore pressure we have developed new analytical solutions and conducted laboratory experiments. We assume that gaseous CO₂ was confined to sandstones during emplacement due to the high entry pressure of the siltstones. After emplacement the CO₂ dissolves in to the brine contained in the siltstones and the pressure in the sandstones declines. Assuming the sandstone-siltstone system is closed, the pressure decline due to CO₂ dissolution is controlled by a single dimensionless number, η = K_HRTV_w /Vg. Herein, K_H is Henry’s constant, R is ideal gas constant, T is temperature, V_w is water volume, and V_g is CO₂ volume. The pressure drop is controlled by the ratio of water volume to CO₂ volume and η varies between 0.1 to 8 at Bravo Dome. This corresponds to pressure drops between 0.8-7.5 MPa and can therefore account for the observed 5 MPa drop in pore pressures at Bravo Dome. This is consistent with geochemical observation suggesting significant dissolution of CO₂ at Bravo Dome (Gilfillan 2009). The observation of sub-hydrostatic pressures in CO₂ reservoirs is important because they illustrate that CO₂ dissolution may mitigate problems due to injection induced overpressure in the long-term.