Abstract: Repurposing in-service petroleum product pipelines for gaseous CO₂ transport is an important approach for large-scale and efficient CO₂ delivery, yet its economic feasibility is influenced by multiple factors such as repurposing techniques and carbon subsidy mechanisms, with currently no reliable method for economic evaluation and operational optimization. This study aims to develop an economic assessment and optimization methodology for gaseous CO₂ repurposing that considers pipeline and station modifications, carbon subsidies, and phase-state constraints, providing guidance for process design and economic evaluation. Based on the net present value method, an economic assessment and optimization model was established for repurposing in-service petroleum product pipelines to transport gaseous CO₂, with pipeline throughput, discharge pressure and temperature at the initial station as optimization variables, while incorporating pipeline modification costs, station retrofit costs, operation maintenance and energy consumption expenses, and CO₂ transportation revenue. The model was solved using an adaptive genetic algorithm. A case study of a petroleum products pipeline in eastern China optimized the repurposing process parameters and investigated the impact of pipeline throughput, service life after repurposing, and subsidy mechanisms on annual net cash flow. For the case pipeline, with a transportation fee of 0.5 CNY/(t·km) and a sequestration subsidy of 65 CNY/t, the optimal pipeline throughput was 1383 t/d, with a discharge pressure of 5 MPa and a temperature of 292 K at the initial station, achieving an annual profit of three million yuan. The factors affecting the economics of the CO₂ repurposing project, in descending order of influence, are pipeline throughput, carbon subsidy mechanism, and service life after repurposing. Within the range of 30-2500 t/d, annual net cash flow initially increases and then decreases with rising throughput. Under a fixed subsidy mechanism, when the subsidy is below 57 CNY/t, the combined costs of pipeline and station modifications, operational energy consumption, and maintenance exceed the transportation revenue; as the sequestration subsidy increases from 60 to 80 CNY/t, the payback period shortens from 11 to 4 years. Under a floating subsidy mechanism, break-even can be maintained with initial subsidies of 54, 52, and 50 CNY/t and annual growth rates of 1%, 1.5%, and 2%, respectively. Extending the service life after repurposing from 10 to 25 years increases the average annual profit by 69.7%. For the studied petroleum pipeline, with sequestration subsidies ranging from 57 to 80 CNY/t, the optimal throughput after repurposing ranges from 1127 to 1581 t/d, with a discharge pressure of 4-5 MPa and a temperature around 295 K at the initial station. It is recommended that the fixed subsidy be no less than 57 CNY/t and the floating subsidy be no less than 47 CNY/t. These findings provide important references for pipeline and station modifications, process design, and carbon subsidy policy formulation for repurposing petroleum product pipelines for CO₂ transport.