Investigation of caprock integrity during high-volume injection into the Utsira formation

Abstract:

The Utsira formation is a large offshore saline aquifer in the North Sea that is considered a likely candidate for storage of CO₂ emissions. Currently, the Utsira is host to the longest operating CO₂ storage project, the Sleipner project, which has injected 1 Mt CO₂/y since 1996. The entire Utsira formation has an estimated storage capacity of 15 Gt, which is equal to 300 Sleipner-sized projects in simultaneous operation for the next 50 years. Injectivity into the Utsira is exceptionally good, and no pressurization has been observed at Sleipner. The formation is over 100-m thick and comprised of unconsolidated sand with high porosity and permeability (30-40% and 1-3 Darcy). The Nordland shale has been characterized as a high-quality seal that is regionally thick, extensive and absent of significant faults.

Significant scale-up of CO₂ injection into the Utsira is required to increase storage of regional CO₂ emissions well beyond what is currently stored today. Full utilization of the Utsira storage capacity would result in injection rates >100 Mt/y, significantly larger than Sleipner. Despite the lack of pressure effects at Sleipner, higher injection rates will likely lead to pressure build-up in the Utsira. Relatively little is known about the magnitude of pressure build-up and resulting impact on caprock integrity with high-volume injection. The problem is complex, involving multiphase flow and mechanical deformation of the storage reservoir and the surrounding formations, and covers large spatial scales, ranging several hundred kilometers in lateral extent. There are significant challenges in applying fully coupled hydromechanical simulators to problems of this scale. The computational effort required to solve a resolved is significant, and efforts to reduce the complexity of the model are needed. In this study, simplified modeling approaches are investigated. A reduced order multiphase flow model coupled with a geomechanical model results in greater efficiency. Similarly, fully coupled hydromechanical models can be applied with single-phase flow conditions. Both approaches are applied to the Utsira and compared under different injection and boundary conditions. Large-scale pressure build-up and subsequent impact on caprock deformation are analyzed in the Utsira for high-volume injection rates.