Unconventional Storage

Brownfield laboratory characterization showing oil bleeding, and a halo of calcite cement along a fracture. Photo courtesy of the University of Illinois DE-FE0031700.

Unconventional Storage Overview:

The United States Department of Energy (DOE) has previously supported unconventional storage projects to work towards achieving low-, net-zero-, and net-negative-carbon oil, focusing on optimizing carbon dioxide (CO₂) storage associated with enhanced oil recovery (EOR), residual oil zones (ROZs), and tight oil formations (TOFs). DOE is not currently supporting any hydrocarbon production including those used in unconventional storage.

Storage of CO₂ in association with EOR offers a means to help offset capture costs and thereby accelerate the implementation of geologic storage. Carbon dioxide-EOR is commercially active, whereas ROZs and TOFs present a new opportunity for CO₂ injection for oil production and associated storage. Primary production of these reservoirs may have as low as 5% of original oil-in-place.

Associated storage field laboratory projects focused on unconventional ROZs and TOFs will demonstrate their potential as an additional type of carbon storage resource and support deployment of unconventional storage technology by industry. Field laboratories will also provide data to improve estimates for storage capacity of CO₂ in conjunction with EOR and will provide validation of modeling, simulation, and monitoring tools specific for associated storage in unconventional ROZs and TOFs.

Long-term CO₂ storage can be achieved as part of an EOR operation (Image not to scale, taken from Klapperich and others, 2013a).

Carbon Dioxide Enhanced Oil Recovery

EOR involves injecting CO₂ into existing oil fields. As CO₂ dissolves in oil, it acts as a solvent and swells the oil and reduces its viscosity. These effects help to improve the efficiency of the oil displacement process. Reginal Carbon Sequestration Partnership (RCSP) projects have injected more than 10 million metric tons of CO₂ in conjunction with EOR projects. Carbon dioxide-EOR can add value by offering a bridge to a reduced carbon emissions future. Carbon dioxide-EOR effectively reduces the cost of storing CO₂ by earning revenues for the CO₂ emitter from sales of CO₂ to oil producers.

Residual Oil Zones

ROZs are immobile areas of oil located below the oil-water contact of a reservoir, commonly located at the base of oil reservoirs. Carbon dioxide flooding mobilizes the residual oil, allowing it to be produced. Removal of the oil results in additional CO₂ storage capacity. Some studies have proposed that ROZs may also occur laterally beyond the boundaries of existing oil reservoirs and are termed “fairways.”

Tight Oil Formations

TOFs are oil formations with very low permeability (less than 10 millidarcies) that resist the flow of fluids without assistance from advanced drilling and completion. Tight oil formations, such as Bakken, Eagle Ford, and other shales, also offer potential opportunities for associated storage in conjunction with EOR. Current research is underway to enhance the understanding of "fairways," including the extent of the deposit and amount of hydrocarbon in the resource, and to characterize and determine storage capacity for CO₂ associated with EOR in TOFs.

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