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Optimising the feasibility of CO2 storage during EOR

A new paper, led by a STRATEGY CCUS researcher, has outlined optimal conditions for the use of CO2 in Enhanced Oil Recovery (EOR), at present the biggest commercial market for the utilisation of CO2. Importantly, the research examines the process from three perspectives: the possibilities for retention (or storage) of CO2, the levels of additional oil recovery and the economic costs/value created.

A well-established process accounting for around 20% of global oil production, EOR involves injecting CO2 into existing oil reservoirs to boost recovery rates. Some of the injected CO2 is retained while the CO2 that returns to the surface can be separated and reinjected for permanent storage.

To prove that CO2-EOR is a feasible and clean option for carbon, capture utilisation and storage (CCUS), it is crucial to single out the most economically favourable options that can be applied considering the parameters needed for CO2-EOR operations.

The paper, published in the journal Energies, highlights the need for changes to the legal framework in Europe to encourage CO2-EOR – incentives that exist in parts of the USA.  Globally, CO2-EOR has been the main driver to date of large-scale, commercial CCUS, which STRATEGY CCUS is helping deliver in eight European regions. At present, the CO2 retained via EOR is not included in the EU’s Emissions Trading Scheme (ETS), meaning companies get no credit for the avoided CO2 emissions.

“It is clear that including CO2-EOR in EU ETS for CCUS would make a big difference in terms of CO2 storage, providing more assets for the application of advanced methods of monitoring and tracking CO2 over the entire process,” the paper says.

Exclusion from the ETS means the cost of CO2-EOR has to be covered by the additional oil production, ruling out some more marginal operations, says Maja Arnaut, lead author of the paper and a STRATEGY CCUS researcher at the University of Zagreb, our Croatian partner.

“We hope that, in future, in Europe also that if you permanently store CO2 that can be accounted for as avoided CO2,” she says.

The paper determines the influence of parameters, such as permeability, depth, well distance and injection strategies, on the cost-effectiveness of the best economic and environmental outcomes, if the stored CO2 could be valued under the ETS. The research is based on water alternating gas (WAG) injection, the most widely used of the three main forms of CO2-EOR.

It finds that CO2 retention and overall costs are affected by the distance between the point of CO2 injection and the production well. The impact on costs was significant, with greater economic benefits for smaller distances and cases with a rock permeability of 50 mD. The most optimal case – the one with the highest economic value and CO2 retention and lowest utilisation of CO2 – had a permeability of 50 mD, depth ranges between 1,545m and 1,845m (from near miscible to miscible conditions) and WAG ratios of 1:2 and 1:1. Additional recovery and retention were opposed parameters – in other words, maximum retention does not always bring maximum addition recovery, hence the need to optimise the process.

“We calculated different oil compositions matched to reservoir depths/pressures, developed conceptual numerical reservoir simulation models, and finally, performed an extensive economic analysis for the obtained results,” says Maja.

“Usually, when such a comprehensive analysis is performed, it is based on one project/reservoir and does not result in such a large number of final scenarios. Here, we look at more than three thousand.”

Even when emissions from the eventual combustion of the additional oil recovered is included, the amounts of CO2 that can be stored via EOR mean that, overall, CO2 emissions are lower, she adds. Moreover, as additional oil is removed from the reservoir during EOR, it is possible to store more CO2 after EOR ends.

Photo: Ole Jørgen Bratland/Equinor