Abstract
Rapid implementation of global scale carbon capture and storage is required to limit temperature rises to 1.5 °C this century. Depleted oilfields provide an immediate option for storage, since injection infrastructure is in place and there is an economic benefit from enhanced oil recovery. To design secure storage, we need to understand how the fluids are configured in the microscopic pore spaces of the reservoir rock. We use high-resolution X-ray imaging to study the flow of oil, water and CO2 in an oil-wet rock at subsurface conditions of high temperature and pressure. We show that contrary to conventional understanding, CO2 does not reside in the largest pores, which would facilitate its escape, but instead occupies smaller pores or is present in layers in the corners of the pore space. The CO2 flow is restricted by a factor of ten, compared to if it occupied the larger pores. This shows that CO2 injection in oilfields provides secure storage with limited recycling of gas; the injection of large amounts of water to capillary trap the CO2 is unnecessary.
Highlights
Rapid implementation of global scale carbon capture and storage is required to limit temperature rises to 1.5 °C this century
There are abundant CO2 storage sites in deep saline aquifers[10], given the short time frame to implement the technology at a global scale, geological storage of CO2 in the decade is most practical in depleted oil and gas reservoirs, where the infrastructure including facilities, pipelines, and injection wells, as well as detailed knowledge of the fields already exists, combined with an immediate financial incentive from enhanced oil recovery (EOR)[11,12,13,14], see Fig. 1
In CO2-EOR projects, the injection of CO2 into depleted oil and gas reservoirs can result in an additional hydrocarbon recovery, which may offset some of the cost of CO2 capture and storage[15,16,17]
Summary
Rapid implementation of global scale carbon capture and storage is required to limit temperature rises to 1.5 °C this century. CO2 is often injected at conditions designed to be nearly, but not completely, miscible with the oil, to improve oil recovery[22,23,24,25,26] In this case, it is conventionally assumed that CO2 is the most non-wetting phase and flows rapidly, preferentially filling the larger pore spaces, while oil and water occupy the smaller pores[18,27,28]. It is believed that CO2 movement can be prevented through capillary trapping by both oil and water regardless of the rock wettability[28,29,30], see Fig. 1C To maximize both oil recovery and CO2 storage, it is considered that CO2 and water should be injected together to restrict the flow of CO2 to oil production wells[31,32]
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