Evaluation of CO2 Injectivity and Geological Storage Scenarios Using Nodal Analysis and Tubing Injectivity Index in a Depleted Gas Field in Malaysia

This study presents a CO2 injectivity analysis for the depleted gas field Z offshore Malaysia using nodal analysis and sensitivity analysis. Reservoir permeability was estimated from the appraisal well DST report, which recorded an absolute open flow (AOF) of 253 MMscfd, and sensitivity analyses were conducted for injection pressure, tubing diameter, reservoir pressure, permeability, and thickness. The base-case nodal analysis resulted in an optimal CO2 injection rate of 52.3 MMscfd. Injection pressure, permeability, and thickness were linearly proportional to injection rate, whereas reservoir pressure showed an inverse relationship. The analysis of injection rate per tubing diameter indicated that 4.548-inch tubing, with 15.11 MMscfd per inch, provided the highest efficiency. A total CO2 injection volume of 5 Tcf was distributed among five wells, and four injection period scenarios (20, 15, 10, 5 years) were designed based on flow efficiency. In the 5-year scenario, the bottomhole pressure of all wells exceeded the formation parting pressure at a reservoir pressure of approximately 1000 psia, indicating that the target injection rate of 2739 MMscfd could not be achieved. Tubing injectivity index (TII) analysis showed that higher TII values represented greater injection efficiency from a vertical flow perspective.