Criteria for Depleted Reservoirs to be Developed for CO₂ Storage

Publication Overview

In this review we consider advantages and disadvantages of using depleted fields in comparison to deep saline reservoirs as carbon dioxide (CO₂) storage sites. The study consists of three parts. The first looks at ten case studies with operational experience and the insights they offer. The second presents original research on three factors that may impact evaluation of depleted field storage opportunities: 1) the impact of reservoir pressure depletion on storage capacity prediction; 2) the effect of residual hydrocarbons on capacity and injectivity; and 3) the net economic benefit of inherited hydrocarbon infrastructure, including elements that are reusable and those that are not. The third section is a discussion of criteria for evaluating depleted fields for CO₂ storage.

Publication Summary

• Depleted hydrocarbon fields offer many attractive advantages for CO₂ storage, including extensive reservoir data, proven geologic containment and potentially re-usable infrastructure.
• As the term is used in the published literature, “depleted field storage” encompasses a wide range of scenarios including post-production storage in a former hydrocarbon reservoir, syn-production storage in the water leg of a hydrocarbon reservoir, and CO₂-EOR.
• Pure storage operations and CO₂-EOR operations have some operation similarity, but have substantially different motivations and different regulatory maturity in different regions. In the US, CO₂-EOR offers more mature regulation and easier permitting. In the EU, post-production storage has greater regulatory maturity and has been the primary choice of EU depleted field storage projects.
• In all projects, outreach and public relations are important, even critical. Early stakeholder engagement is key.
• Storage projects completed to date have proven and extended the science and developed a wide range of monitoring tools to assure storage security. All have their merits but their cost effectiveness varies depending on project specifics, particularly the surface environment, the presence of hydrocarbons above the storage reservoir and project-specific concerns.
• Sub-hydrostatic reservoir pressure is a sign of closed or semi-closed reservoir boundaries. Such reservoirs may offer greater storage security but also place sharp limits on capacity by limiting the propagation of injection pressure
• The presence of remaining hydrocarbon gas in place does not negatively affect the CO₂ storage capacity of the depleted dry gas reservoirs, other than occupying pore space.
• The large density and viscosity contrast between CO₂ and methane limits the mixing of the two, leading the CO₂ plume to be mainly accumulated at the bottom of the reservoir at the post-injection stage. The gas (methane) layer forming below the reservoir top may act as a barrier, lowering the risk of CO₂ plume migration towards the top seal and hence improving the CO₂ storage integrity in a depleted dry gas reservoir.
• While storing CO₂ in a depleted dry gas reservoir, the majority of CO₂ plume remains mobile, while capillary and dissolution trapping mechanisms play minor roles in trapping the CO₂
• Beyond occupying pore space, the amount of remaining gas (methane) in place does not significantly affect the capillary and dissolution trapping efficiency of CO₂ plume in a depleted dry gas reservoir.
• It should not be taken as guaranteed that infrastructure reuse will always result in lower costs for CCS projects.
• For a hypothetical base-case CO₂ storage project size of 1 Mtpa and a 50-kilometer pipeline length, the total project cost is most sensitive to pipeline costs. Reuse of existing pipelines can save significant cost but depends on a number of variables, chiefly capacity, pressure rating and condition.
• In contrast to hydrocarbon exploration where the goal is to maximize discovered reserves, current and planned storage projects are typically driven by the need to abate specific emissions sources and therefore have well defined requirements that can be met in a variety of ways. As long as a storage site meets project requirements, bigger is not necessarily better.
• Published criteria for evaluating depleted fields identify many of the key factors but definition of favorable and unfavorable ranges for all variables creates a long list that fails to recognize both the relationships between them and the potential for excellent overall results from a variety of combinations that may include individually sub-optimal parameters.
• We recommend that screening focus on five overarching factors: capacity, injectivity, containment security, reusable infrastructure and public/regulatory acceptance. These factors have multiple inputs which combine to create flexibility in how to meet project requirements.
• The report details the criteria to efficiently screen for needed capacity, injectivity, containment security, reusable infrastructure and suitability for CO₂-EOR. Capacity and injectivity are presented as graphs, offering efficient order-of-magnitude evaluation.