CO2 STORAGE IN THE SUBSURFACE - TRAP, SEAL, AND GEOMECHANICS CONSIDERATIONS

CO2 capture and long-term geological storage is widely recognized as a potential mitigation of green house gas emissions. This presentation describes geological considerations for retention of supercritical CO2 injected into subsurface reservoirs.

Buoyancy-driven CO2 pressure impacts all aspects of retention. Optimal reservoir geometries and depths minimize plume buoyancy pressure and maintain supercritical CO2 phase. Ideal geometries include horizontal to very low dip monoclines and low relief structural closures at depths greater than 1000 m.

Faults and fractures commonly are considered as likely vertical CO2 leakage conduits. Fault dip-parallel CO2 migration, a common assumption, is strongly at odds with petroleum industry experience, which shows that cross-fault juxtapositions dominate the impact of faults on hydrocarbon storage. Apparent vertical leakage along faults typically is by stair-stepping upwards across successively shallower juxtapositions.

Much attention is given to leakage on critically-stressed faults and fractures, but this consideration is most relevant in the reservoir where the stress ratio is small, usually less than 0.4 (particularly in depleted hydrocarbon reservoirs). Seal rock stress ratios commonly are greater than 0.8. Because the failure envelopes of reservoir and seal rocks are significantly different, and because the stress ratios are different, the failure mechanisms are different – hydraulic fracturing in the seal and shearing in the reservoir.

Capillary seal envelopes require consideration of CO2 phase and rock properties. Supercritical CO2 maintains low solubility in brine (in contrast to gaseous CO2), which reduces buoyancy and maintains predictable capillary-dominated flow behavior. Favorable capillary seals are continuous at the scale of anticipated CO2 plume dimensions and have small pore throats. Maintenance of supercritical CO2 phase in the reservoir reduces CO2-water buoyancy (capillary pressure) and enhances capillary seal capacity.

Successful CO2 retention at a subsurface storage site requires confluence of reservoir geometry (trap), fault behavior, bed seal integrity, and CO2 phase maintenance. Integrated study of these factors provides full characterization of the chance that CO2 will be retained for a specified time period.