Due to the severe natural gas imbalance in Iran and the significant reduction in injected gas volume from 220 to 60 MMscf/day in the South Carbonate Field (KJ) in southwestern Iran, this study evaluates the technical and economic feasibility of replacing gas injection with water injection for pressure maintenance. Technical screening was performed using EOR-GUI software and reservoir data (API ~28°, pressure 2050 psi, viscosity 0.8–1 cP). Although gas injection ranked as the preferred option in the screening priority, water injection was identified as the practical and necessary alternative under the current gas imbalance conditions. Reservoir pressure analysis showed that the current reservoir pressure (2050 psi) remains only slightly above the bubble-point pressure (2019 psi). Considering that the reservoir has been characterized as an initially saturated two-phase system, the current pressure condition highlights the importance of reservoir pressure maintenance. Furthermore, the estimated minimum miscibility pressure (MMP) of approximately 3500 psi is substantially higher than the current reservoir pressure, indicating that gas injection would operate under immiscible conditions. Based on the Voidage Replacement Ratio (VRR) principle, the equivalent water injection rate required to replace the current gas injection was estimated at approximately 10,000 STB/day. Economic evaluation of two scenarios—drilling new wells versus converting existing wells—indicated that the second scenario reduces initial capital expenditure (CAPEX) by about 60% (from €57.3 to €22.8 million) and yields a positive net present value (NPV) of €14.2 million, an internal rate of return (IRR) of 23%, and a payback period of 3.2 years. More importantly, this scheme frees approximately 160 MMscf/day of natural gas for domestic and industrial consumption, with the ratio of the value of freed gas to water injection cost estimated at approximately 3.2. It is concluded that under gas imbalance conditions, water injection using existing wells is an economically viable and strategically sound alternative to gas injection. It should be emphasized, however, that this study represents a screening-level technical and economic assessment. Therefore, detailed dynamic reservoir simulation, injection pattern optimization, and comprehensive engineering design are required prior to field implementation.