Evolution of CO2 Storage Mechanisms in Low-Permeability Tight Sandstone Reservoirs

Understanding the storage mechanisms in CO2 flooding is crucial, as many carbon capture, utilization, and storage (CCUS) projects are related to enhanced oil recovery (EOR). CO2 storage in reservoirs across large timescales undergoes the two storage stages of oil displacement and well shut-in, which cover multiple replacement processes of injection–production synchronization, injection only with no production, and injection–production stoppage. Because the controlling mechanism of CO2 storage in different stages is unknown, the evolution of CO2 storage mechanisms over large timescales is not understood. A mathematical model for the evaluation of CO2 storage, including stratigraphic, residual, solubility, and mineral trapping in low-permeability tight sandstone reservoirs, was established using experimental and theoretical analyses. Based on a detailed geological model of the Huaziping oilfield, calibrated with reservoir permeability and fracture characteristic parameters obtained from well test results, a dynamic simulation of CO2 storage for the entire reservoir life cycle under two scenarios of continuous injection and water–gas alternation were considered. The results show that CO2 storage exhibits the significant stage characteristics of complete storage, dynamic storage, and stable storage. The CO2 storage capacity and storage rate under the continuous gas injection scenario (scenario 1) were 6.34 × 104 t and 61%, while those under the water–gas alternation scenario (scenario 2) were 4.62 × 104 t and 46%. The proportions of storage capacity under scenarios 1 and 2 for structural or stratigraphic, residual, solubility, and mineral trapping were 33.36%, 33.96%, 32.43%, and 0.25%; and 15.09%, 38.65%, 45.77%, and 0.49%, respectively. The evolution of the CO2 storage mechanism showed an overall trend: stratigraphic and residual trapping first increased and then decreased, whereas solubility trapping gradually decreased, and mineral trapping continuously increased. Based on these results, an evolution diagram of the CO2 storage mechanism of low-permeability tight sandstone reservoirs across large timescales was established.