Modeling Evaluates CO2 EOR, Storage Potential in Depleted Reservoirs
Authors: Feder Judy 
Published: August 2021

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 200560, “CO2-EOR and Storage Potentials in Depleted Reservoirs in the Norwegian Continental Shelf,” by Elhans Imanovs, SPE, and Samuel Krevor, SPE, Imperial College London, and Ali Mojaddam Zadeh, Equinor, prepared for the 2020 SPE Europec featured at the 82nd EAGE Conference and Exhibition, originally scheduled to be held in Amsterdam, 8–11 June. The paper has not been peer reviewed.
A combination of carbon dioxide (CO2) enhanced oil recovery (EOR) and storage schemes could offer an opportunity to produce additional oil from depleted reservoirs and permanently store CO2 in the subsurface in an economically efficient manner. The complete paper evaluates the effect of different injection methods on oil recovery and CO2 storage potential in a depleted sandstone reservoir in the Norwegian Continental Shelf (NCS). The methods include continuous gas injection (CGI), continuous water injection (CWI), water alternating gas (WAG), tapered WAG (TWAG), simultaneous water above gas coinjection (SWGCO), simultaneous water and gas injection (SWGI), and cyclic SWGI.

CO2 EOR and Storage in the NCS
In recent years, the number of newly explored fields in the NCS has decreased. Approximately 47% of total resources in the NCS have been produced, and approximately 20% of resources are estimated as recoverable reserves. To fill in the gap between energy demand and recoverable reserves, EOR methods could be employed. One of the most efficient EOR methods is CO2 injection, because complete microscopic sweep efficiency can be achieved, leading to a total depletion of the reservoir.
The three major types of CO2 EOR processes—miscible, near-miscible, and immiscible—are described and discussed in the full paper. Four primary CO2-trapping mechanisms are used in the subsurface: structural/stratigraphic, solubility, residual, and mineral trapping. The main locations for underground geological storage are depleted oil and gas reservoirs, coal formations, and saline aquifers.
Currently, underground CO2 storage is believed to be a major technology to dramatically reduce CO2 amounts in the atmosphere. According to the International Energy Agency, 54 major oil basins around the world have the potential to produce 75 Bsm3 of additional oil and store 140 Gt of CO2.
CO2 EOR and storage projects in the NCS could have several benefits. First, surface and subsea facility availability in the NCS region reduces capital expenditures. Second, in addition to the revenue from extra oil production, carbon credits could be awarded for the CO2 storage. The main challenges of CO2 EOR and storage offshore projects are high operational and capital expenditures. In depleted reservoirs, these include modification of offshore platform materials; additional power supply for CO2 compression and recycling; and replacement of the tubing because wet CO2 is highly corrosive, resulting in scale, asphaltene, and hydrates formation. Contamination of a gas cap with injected CO2 might lead to loss of hydrocarbon gas market value. Only one CO2 EOR project has been implemented offshore—the Lula field in Brazil’s Santos Basin—meaning that industry has very limited experience in such projects.