Background to the Study

The benefit of documenting and evaluating key criteria for depleted oil and gas reservoirs to be developed for CO₂ storage would be to provide a more refined estimation of the storage capacity of this type of site in the different areas of the world, particularly where there are high concentrations of large-scale industrial CO₂ emissions.

Introduction

The long-term, secure storage of CO₂ depends on injection and retention within well characterised geological reservoirs, such as saline aquifers or depleted oil and gas fields.  Depleted oil and gas reservoirs are often selected for first-generation CO₂ storage sites because they have been characterised from their discovery date, during the whole production phase and possibly during post-production observations.  The potential CO₂ storage capacity in saline formations is well understood, and so the objective of this study is to specifically focus on a set of storage conditions that apply to depleted oil and gas fields.  The study is split into three main sections: a review of case studies for CO₂ storage in depleted hydrocarbon fields; original research looking into reservoir pressure depletion, boundary conditions, the effect of residual hydrocarbons on injectivity and capacity; and the economics of infrastructure reuse for CO₂ storage sites.  The third section discusses and integrates the lessons learned to facilitate evaluation of future depleted field storage opportunities.

Key messages

Depleted hydrocarbon fields are valuable and advantageous sites for the storage of CO_2.

Site evaluation when considering depleted fields for storage should be project-specific and should consider the storage requirements and the operators’ metrics for success and views of acceptable risk.

Sub-hydrostatic reservoir pressure is a sign of closed or semi-closed reservoir boundaries and such reservoirs may offer greater storage security but also place limits on capacity.

The presence of remaining hydrocarbon gas in place does not necessarily affect the CO₂ storage capacity of the depleted dry gas reservoirs, other than occupying pore space.

The majority of a CO₂ plume in a depleted dry gas reservoir remains mobile, while capillary and dissolution trapping mechanisms play minor roles in trapping.

Other than occupying pore space, the amount of remaining gas in place does not significantly affect the capillary and dissolution trapping efficiency of CO₂ plume in a depleted dry gas reservoir.

Infrastructure reuse, based on a comparison of modelled examples, will not always result in lower costs for CCS projects.

In all projects, outreach and public relations are crucial for reassurance.

The best scenarios for CO₂ storage in depleted fields may be ‘hybrid’ situations, such as CO₂-EOR or injection into the water leg down-dip of a depleted reservoir.

The report includes key guidance on Site Evaluation and Desirable Characteristics.

This report is available to download.