A new study published in Engineering explores the potential of cyclic CO₂ injection in unconventional reservoirs. This research focuses on the complex processes of CO₂ utilization and geological storage, aiming to enhance oil recovery while reducing carbon emissions.

As the world seeks ways to mitigate climate change, carbon capture, utilization, and storage (CCUS) technologies are becoming increasingly important. Among them, CO₂-enhanced oil recovery (CO₂-EOR) shows great potential. However, in unconventional reservoirs like tight and shale oil, the mechanisms of CO₂-EOR and storage are distinct from traditional ones.

The research teams, led by Bing Wei and Valeriy Kadet, established a reservoir model using CMG-GEM software. They integrated various factors such as CO₂ solubility, molecular diffusion, geochemical reactions, and rock stress sensitivity into the model. The model was calibrated with experimental data to ensure its accuracy.

The study focused on the Triassic Baikouquan formation in the Mahu Sag, a typical tight reservoir. Through numerical simulations, the researchers investigated the performance of CO₂ utilization and geological storage during cyclic injection. They compared two cases: one only considering CO₂–crude oil interactions, and the other incorporating all CO₂ storage mechanisms.

The results show that after ten cycles of CO₂ injection, the oil recovery of the reservoir increased by 3.4% of the original oil in place (OOIP), and 48.3% of the injected CO₂ was stored underground. However, the CO₂ storage process reduced the CO₂–oil interactions, leading to a 25.9% reduction of the CO₂–oil mixing zone and a 2.2% decline in cumulative oil production.

The researchers also analyzed the migration and transformation of CO₂ in the reservoir. They found that different forms of CO₂ storage, such as dissolved in oil, water, and mineralized carbonate, were affected by the cyclic injection process. For example, the dissolved CO₂ in oil accounted for over half of the total storage but had the potential to be released during production.

The study investigated the geochemical reactions and porosity/permeability changes in the reservoir. The dissolution and precipitation of minerals were found to have a significant impact on the reservoir properties. After ten cycles, the porosity decreased by 0.86% and 0.81% in the non-fractured and fractured zones, respectively, while the permeability decreased by 2.51% and 2.39%.

Although the research provides valuable insights into the dynamic CO₂ transport and transformation processes, it also has limitations. The simplification of the geomechanical module and the uncertainty of the reactive surface area (RSA) parameter need to be addressed in future studies.

This research offers a more comprehensive and precise framework for assessing CO₂ capture, utilization, and storage with enhanced oil recovery (CCUS-EOR) performance in unconventional reservoirs after fracturing. It provides support for the design and optimization of injection–production schemes, and paves the way for further research in this field.

The paper “CO₂ Utilization and Geological Storage in Unconventional Reservoirs After Fracturing,” authored by Jinzhou Zhao, Lele Wang, Bing Wei, Valeriy Kadet. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.01.005.