CO2 Utilization and Geological Storage in Unconventional Reservoirs After Fracturing

Abstract

Cyclic injection holds great potential for CO 2 emission reduction coupled with enhanced unconventional oil recovery. There is, however, a lack of a thorough understanding of carbon distribution, migration, and transformation underground over time at the reservoir scale. To address this issue, we conducted a rigorous numerical simulation integrating microseismic events, multi-geomechanics, and multi-geochemistry to represent the complex fracture geometry, rock stress sensitivity, and CO 2 –oil–brine–rock interactions. The fluid model, reservoir model, and geochemical reaction kinetics were carefully validated and calibrated using experimental data. The performance of CO 2 utilization and geological storage was comprehensively investigated in terms of changes in oil production, CO 2 storage, carbon distribution, and petrophysical properties. The results indicate that 48.3% of the injected CO 2 was stored stably underground after ten cycles (ten years), with a 3.4% increase in oil recovery. The presence of multiple CO 2 storage forms, such as dissolved in water and mineralized carbonate, impeded CO 2 –oil interaction, leading to a 25.9% reduction in the volume of the CO 2 –oil mixing zone and a 2.2% decrease in cumulative oil production, albeit with a 7.7% increase in the storage rate. The cyclic injection mode had a significant impact on the migration and transformation of CO 2 in the reservoir. While dissolved CO 2 in oil accounted for over half of the total storage, it had the possibility of being released during production. After ten cycles, 20% of the injected CO 2 (approximately 12 000 t) reached long-term storage in four forms: mineralized carbonate (6%), water-dissolved CO 2 (6%), aqueous ions (4%), and trapped gas (4%). Notably, the non-fracture zone within the stimulated reservoir volume (SRV) served as the primary trapping area for residual gas. This work provides valuable insights into dynamic CO 2 transport and transformation processes under cyclic injection and presents a more comprehensive and precise framework for assessing CO 2 capture, utilization, and storage with enhanced oil recovery (CCUS-EOR) performance in unconventional reservoirs after fracturing.

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