O One of the main oil fields in PT “E” is “AH” oil field which contribute almost 10% of the PT “E” company’s daily production. The oil field is carbonate oil reservoir with current oil production is around 10,000 bopd decreased from it’s peak production + 45,000 bopd due highly decline rate and some field operation problem, such as poor cement bonding substantially due to high loss circulation rate during well drilling in the most of the well’s drill in this field.This study will aim at how to develop the best way to inject the CO2 into the reservoir in order to recover more oil and to optimize production.The methodology of this study is initially by review papers related to the study, gathering all updated data (reservoir, well, production, laboratory data include: MMP data, etc), and then modeling it and run history matching. Following the model matching, the pilot area is selected in the flooded zone, match it, and follow by forecasting it with several miscible CO2 injection scenarios, namely : continuous gas injection (CGI), water alternate gas injection (WAG), simultaneous water and gas injection (SWAG), and combination of this method and adding horizontal well(s) or vertical well(s) into the scenario, as well. The simulation study result by using t-Navigator (RFD) reveal that the best injection scenario is the SWAG-WAG method and it will recover of 54.97 MMstb of oil with 71.77% of Recovery (RF-additional = 12.61%).