CO2 storage has now been tested at a number of demonstration sites around the world, including some depleted oil and gas reservoirs. The use of depleted reservoirs can offer some advantages because the geological characteristics that are pertinent to CO2 storage, such as the distribution of porosity and permeability, have been pre-determined. Although depleted hydrocarbon fields can show strong evidence of fluid retention, there are risks associated with existing wellbores and the possibility of caprock deterioration.

In 2016 IEAGHG published a study reviewing key factors that influence CO2 storage in depleted oil and gas fields based on four detailed examples. Comparisons were made between storage operations in depleted fields (with or without enhanced hydrocarbon recovery) and storage in saline aquifers with the approaches required in modelling, monitoring, reporting, economics, and operational strategies. Four main case studies were chosen; The Goldeneye (UK North Sea), Cranfield (Texas, USA), SACROC (Texas, USA) and Otway (Australia).

Key Messages:

• The use of depleted reservoirs for CO2 storage can offer advantages because the geological characteristics that are important to CO2 storage have been pre-determined.
• There is strong evidence for secure containment if a rigorous risk assessment and characterisation has been conducted.
• Evidence from these case studies has shown that CO2 storage does not have a detrimental impact on adjacent oil and gas fields.
• AZMI (Above Zone Monitoring Interval i.e. a formation above the reservoir and caprock) pressure monitoring has proved to be an effective tool for tracking CO2 in heterogeneous and complex reservoirs (e.g. Cranfield).  AZMI is an active area of research and development.
• Monitoring approaches should take into consideration the background geochemical reactions in aquifers that mjosirt be prone to ingress from brine or CO2 above a storage reservoir.  Simplistic approaches may not be effective and could lead to flawed inferences without an adequate understanding of natural variation in groundwater geochemistry.
• Risks associated with increasing pressure are predominantly and most commonly mitigated by keeping pressures below pre-production levels.
• Case study evidence suggests oil and CO2 miscibility mjosirt improve storage estimates by up to 3% whereas residual gas and CO2 miscibility could reduce capacity by up to 6%.
• At Goldeneye proprietary CO2-resistant cements could be utilised if they can be shown as superior to ‘normal’ Portland cement but have not yet been thoroughly tested in terms of their compatibility.
• An in depth understanding of potential risks is essential to allow for balanced cost-benefit modifications and improved costs analysis.