Numerical Modelling of CO2 Injection and Storage in Low Porosity and Low Permeability Saline Aquifers: A Design for the Permian Shiqianfeng Formation in the Yulin Area, Ordos Basin

The geological storage of CO2 in saline aquifers is a crucial method for achieving large-scale carbon storage in the future. The saline aquifers with low porosity and permeability in the Ordos Basin exhibit high irreducible water saturation and restricted fluid mobility, necessitating further investigation of their injectivity and storage safety. The fifth member of the Shiqianfeng Formation (P3sh5) in the Ordos Basin serves as a key layer for geological CO2 storage (GCS). The numerical simulation of CO2 injection in this reservoir is an indispensable process for characterizing the migration and storage of CO2. Injection pressure and well type (vertical well or horizontal well) are critical factors affecting GCS. The results of the numerical simulation are important preliminary preparations for promoting the GCS in the saline aquifer of the Shiqianfeng Formation in the future. This paper focuses on P3sh5 in the Yulin area as a case study. It investigates the injectivity and CO2 migration characteristics of these low porosity and low permeability saline aquifers in the Ordos Basin. Relatively high-quality distributary channel sandstone bodies in integrally low porosity and permeability strata were identified for injection. As CO2 is injected, the formation pressure gradually increases. It is essential to maintain it below the fracture pressure during CO2 injection to ensure safety. High-pressure (8 MPa) injection could achieve volumes 2.9 times greater than those in the low-pressure scenario (4 MPa) of 2 km horizontal branch well. Under the three injection well types, the injection rate of vertical wells is the lowest. Employing a “horizontal branch well injection” strategy could potentially amplify the injection volume by 2.87 times. CO2 predominantly migrates vertically near the horizontal interval of interest, while horizontally, the area near the interval of interest experiences a higher CO2 saturation, with the maximum saturation reaching about 50%. Overall, CO2 is migrated in the distributary channel sandstone bodies, indicating a higher storage safety and lower leakage risk. It is recommended that the number of drilling wells be increased and multiple horizontal branch wells implemented to enhance the injection efficiency. Overall, this study provides a geological foundation for the previous design and construction of the GCS project in the Ordos Basin’s saline aquifer. It also provides a reference for GCS in low permeability saline layers in similar regions worldwide.