Gas-Cap Impact on CO2 Plume Migration for Long-Term Storage in Saline Aquifers

Injection of CO2 in a hydrocarbon-rich area significantly impacts CO2 plume be­ havior. Migration of CO2 injected into the reservoir is of special concern because of as­ sociated liability and safety issues. This study assesses CO2 plume properties in saline formations typical of the Gulf Coast area under the presence of a gas cap and its conse­ quences for long-term storage. A synthetic reservoir model is used to perform a sensitiv­ ity analysis by means of an advanced compositional equation-of-state reservoir simula­ tor. Multiphase flow modeling ensures better understanding of CO2 plume distribution under different operational and reservoir conditions. Changes in gas-cap volume, com­ position, residual-gas saturation, and injector-gas-cap distance are studied, among oth­ ers. Under typical pressure and temperature for CO2 storage, brine is nearly incom­ pressible, causing extent and shape of the CO2 plume to be determined by CO2-brine density difference and gas-cap compressibility. The CO2 plume extends farther as gas­ cap volume increases and distance to the gas cap decreases. Residual-gas-saturation conditions in a depleted gas cap are not expected to affect the plume extent as much as in cases where water is residual. Pressure changes within the formation affect gas-cap compressibility and, consequently, plume maximum lateral extent. Appropriate assess­ ment of gas-cap impact on CO2 plume distribution and on aquifer-pressure buildup is fundamental to determining a regulatory area of review, project technical risks, and economics.