In order to improve the quality of natural gas exported from offshore gas fields, reduce the corrosion of long-distance pipelines caused by CO2 in natural gas, and decrease carbon emissions, a CO2 in-situ capture and reservoir reinjection project was developed for the first time, using CO2 removal membrane based CO2 capture technology on an offshore platform of a high CO2 gas field in the South China Sea. Based on on-site experiments and numerical simulation models, Carbon Capture & Storage(CCS) engineering practice and operation optimization were carried out from the aspects of carbon sequestration feasibility analysis, optimization of hydrocarbon loss in the CO2 membrane capture system, transformation of the natural gas commodity rate improvement process, and energy consumption optimization. The feasibility of carbon sequestration in reinjection wells was analyzed from three aspects: wellbore integrity, inhalation capacity, and reservoir pressure variation trends. The research results indicate that when the injection rate is 0.15×106 m3/d, the bottom-hole flowing pressure increases by approximately 1.0 MPa, and the reservoir pressure is expected to increase by 2.4 MPa by 2030, with a pressure coefficient of 1.02. Based on flow relationship experiments, adding a Joule-Thomson(J-T) valve at the inlet of the CO2 removal membrane system can effectively enhance the CO2 removal performance under high processing capacity. By controlling the opening of the JT valve in the CO2 removal membrane system and the PV valve in the subsea pipeline, the issue of controlling the injection volume of CO2 fixed-frequency compressors can be resolved. After optimizing the energy consumption of the main energy-consuming equipment in the complex offshore CCS project, the energy consumption can be reduced by up to 8.2%. Additionally carbon emission prediction has been carried out, indicating that carbon emissions after 20 years will be 20 575 556 t/a. This study has accumulated experience in the capture and storage of carbon dioxide in offshore oil and gas fields in China, and holds important engineering value as a reference for the long-term efficient operation of similar projects.