Sensitivity analysis of carbon dioxide storage in saline aquifers in the presence of a gas cap
Creators: name: Solano, Silvia Veronica
Publisher: name: The University of Texas at Austin
Year: 2010
Subjects: Carbon dioxide
Saline aquifers
Gas caps
Plume distribution
Gulf Coast area

description: Deep brine-bearing formations contain a significant CO₂ storage potential as they are usually permeable sandstones at depths in which pressure and temperature conditions assure supercritical state for the injected CO₂. When injecting CO₂ in a hydrocarbon-rich area, presence of a gas cap significantly impacts the CO₂ plume behavior. This study focuses on the assessment of the CO₂ plume properties in formations typical of the Gulf Coast area, under the presence of a gas cap and its consequences for long-term storage. The study is prompted by the presence of a large depleted gas cap at Cranfield, Mississippi where CO₂ is being injected for long-term storage. Presence of the gas cap, even depleted, near the injection site provides an exceptional opportunity to investigate an area made of higher compressibility fluids and its impact on reservoir and operational parameters, particularly CO₂ plume behavior. Enhanced gas recovery is not planned within this area. Considerable volumes of native brine are displaced when large amounts of CO₂ are injected, and when this displacement occurs in a closed system, the amount of stored CO₂ will depend solely on the additional pore space available owing to compressibility of the pore structure and fluids. As a result, presence of a gas cap is expected to impact plume characteristics, as well as operational conditions, because of its larger compressibility. A multi-parameter sensitivity analysis, based on a generic reservoir model, was performed to appreciate relevant factors to CO₂ migration under the influence of the nearby gas cap. It was achieved using the compositional reservoir simulator CMG-GEM and allied modules. Main parameters taken into account for the sensitivity analysis included variation in gas cap properties such as: volume, gas composition and gas residual saturation. Additionally, other parameters have been included in this study such as reservoir dip, injector-gas-cap distance, injection pressure, plume asymmetry and horizontal centroid location. The CO₂ plume extends farther as the gas cap volume increases and the distance to the gas cap decreases. Gas residual saturation conditions in the gas cap region are not expected to affect the maximum lateral plume extent as much as the existent volume of gas. The effect of gas cap composition in CO₂ migration is dominated by pressure changes within the formation which subsequently affects the gas cap compressibility and in consequence the plume maximum lateral extent. For example, contamination of a methane-rich gas cap by injected CO₂ has a strong effect on the plume maximum lateral extent due to compressibility changes. This, in turn, affects regulatory Area of Review, project technical risks, and economics. In another part of the study, a dimensional analysis was performed to identify and assess dominant forces relevant to CO₂ plume distribution in the presence of a gas cap. Dimensionless groups were used to express the relationship between centroid location and the ratio of gravity and viscous forces given by the gravity number. Appropriate assessment of gas cap impact on CO₂ plume distribution and on aquifer pressure build-up is fundamental for developing an accurate economic outlook as well as for taking into account regulatory constraints (including a monitoring plan addressing leakage risk and possible aquifer contamination)