Study Makes Value Proposition for Carbon-Neutral Fuel From Light Tight Oil
Authors: Carpenter Chris 
Published: August 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper URTeC 2021-5437, “Carbon-Neutral Fuel From Light Tight Oil: A Value Proposition,” by Christine A. Ehlig-Economides, SPE, University of Houston. The paper has not been peer reviewed.

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Emissions for light tight oil often begin with associated gas. Rather than flare the stranded methane, an option may be to use it to power capture of CO2 from the air using direct air capture (DAC) technologies. This study catalogs global gas/oil ratio (GOR) data to identify currently produced light crude oils that could be rendered carbon-neutral through the DAC mechanism. It then considers operational aspects, including proximity to a suitable location for a DAC unit and proximity to subsurface utilization or storage opportunities, that favor or discourage this approach.

Introduction
Previous work by the author defined carbon-neutral crude oil (CNCO) as crude oil that has offset the CO2 to be emitted by its combustible products before its arrival at the sale point. Other authors refer to enhanced oil recovery (EOR) using anthropogenic CO2 and including monitoring, verification, and reporting as EOR+. They describe three EOR+ categories: conventional, advanced, and maximum storage. Conventional storage minimizes CO2 use to reduce cost, resulting in net usage of 0.3 t CO2/bbl. Advanced-storage miscible flooding follows current best practices for optimized oil recovery and also may involve some “second-generation” approaches that boost CO2 use, resulting in net usage of 0.6 t CO2/bbl. Maximized storage effectively uses the reservoir pore space for additional CO2 storage without additional oil production and may include removing water, resulting in net usage of 0.9 t CO2/bbl.

CO2 Capacity
The pore-space requirements for saline aquifer CO2 storage depend on whether primary or secondary storage is envisioned for the short term. Primary storage entails bulk CO2 injection into an aquifer storage volume effectively limited by interference with neighboring injection wells, while secondary storage entails producing the same volume from the reservoir as is injected as CO2. The bulk rock volume required to store CO2 dissolved in brine is approximately 200 times the original coal volume (Fig. 1). Limiting the injection pressure below the fracturing pressure to avoid breaching the caprock, however, further limits the CO2 storage capacity to approximately 6% of the pore space. This makes the multiplier nearly 120 times the original coal volume without CO2 dissolution. Generally, CO2 captured from stationary point sources and stored through multiwell bulk injection in a saline aquifer has a primary storage capacity of approximately 4.2 t CO2/acre-ft without increasing the original aquifer pressure. Three tables in the complete paper list inputs for these estimations and parameters derived from these inputs for cases corresponding to primary and secondary storage without aquifer pressurization.
If CO2 injection elevates the aquifer average pressure, this increases the risk of inducing earthquakes and fracturing through the caprock. Using a typical fracture gradient of 0.7 psi/ft, injectivity of 3.23 BOPD/ft/psi could enable elevating the aquifer average pressure to 6,297 psia, which would, in turn, enable increasing the primary storage capacity to 18 t CO2/acre-ft and secondary storage capacity to 135 t CO2/acre-ft, again without CO2 dissolution.
It is useful to compare maximized EOR+ with secondary CO2 storage in a saline aquifer. Effectively, maximized storage is like secondary saline aquifer storage in a sealed and trapped reservoir volume. Elevating pressure can increase the storage efficiency in either case, though with associated risks.