A FOUR-DIMENSIONAL STUDY ON THE EVOLUTION OF IN-SITU STRESS IN FRACTURED AND FAULTED RESERVOIRS FOR CO₂-EOR TECHNOLOGY IN TIGHT OIL RESERVOIRS

To address the risk of reactivation of faults/fractures during the CO₂ flooding and storage (CO₂-EOR) process in tight oil reservoirs, this study innovatively constructed a four-dimensional dynamic evolution model of geostress for the “CO₂-EOR-storage” integrated system, revealing the spatio-temporal response mechanism of geological structures under the coupling effect of multiple physical fields. By integrating multiphase flow, thermodynamic effects, and dynamic geostress fields, and introducing the theories of fracture stretching failure and fault slip trend, a dual geological mechanics-flow coupling model considering fracture expansion and fault slip was established, enabling four-dimensional geostress evolution simulation throughout the injection-displacement-storage cycle. Taking the tight sandstone reservoir in the Yanchang Oilfield as an example, the study shows that the reactivation of fractures presents a four-stage characteristic: “dot-like expansion-extend on both sides-accelerated penetration-gradual stabilization”, and the expansion rate is nonlinearly positively correlated with the injection pressure; the reactivation of faults is controlled by the slip trend and permeability conduction coefficient. It is found that in the low-permeability reservoir during the CO₂ injection-production stage, the threshold of injected fluid is 12 000 ~ 16 000 m³/d. During the CO₂ injection-storage stage, when the injection rate exceeds 6 000 m³/d, point-like leakage channels appear on the faults. The research results provide theoretical support for the safe and efficient enhancement and long-term storage of CO₂-EOR, and have guided the formation of an engineering optimization scheme of “high-speed injection during production period-low-speed pressure stabilization during storage period”.