Case Studies of CO₂ Storage in Depleted Oil and Gas Fields

Publication Overview

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. The aim of this study is to highlight 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. Fundamental differences in the reservoir pressure and risk profiles between the different storage sites have been explored. These studies have also allowed a comparative assessment to be made of the requirements for CO₂ storage projects from different geographic and regulatory regimes. The work was led by the British Geological Survey with support from the Gulf Coast Carbon Center based at the University of Texas at Austin.

Publication Summary

• The aim of this study is to highlight key factors that influence CO₂ storage in depleted oil and gas fields, drawing on four case studies.
• 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 might 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 might 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.
• IEAGHG should review monitoring techniques for tracking CO₂ mixed with other reservoir gases. Progress with research on the use of AZMI pressure monitoring and wellbore integrity should also be regularly appraised.