Unlocking the potentials of depleted gas fields: A numerical evaluation of underground CO2 storage and geothermal energy harvesting

Using partial underground CO₂ storage as a working agent to harvest geothermal energy is a promising carbon capture, utilization, and storage (CCUS) method. It is particularly economically feasible to use or retrofit the existing infrastructure of a hydrocarbon field. Although technical advantages of integrated CO₂ sequestration and CO₂-circulated geothermal harvest using depleted hydrocarbon reservoirs have been reported, quantitative evaluations of economic benefits using existing wells of realistic reservoirs are rare. In this study, a 3-D hydrothermal flow model is built for the Triassic Argilo-Gréseux Supérieur (TAGS) Formation of the Toual gas field, Algeria using Schlumberger Petrel and CMG-STARS software. A three-phase operational scheme is proposed for sequential CO₂ sequestration and CO₂-circulated geothermal extraction over 100 years. The first phase is injecting CO₂ for 30 years, followed by concurrent cold CO₂ injection and hot CO₂ extraction in the developed CO₂ plume (circulation) for 40 years as the second phase. In the third phase, producing wells in the second phase are converted to injection wells while outer wells start to extract hot CO₂ for another 30 years. Scenario 1 is simulated using the selected nine existing wells of the field, while an optimized Scenario 2 is designed and simulated by adding seven newly drilled wells in addition to the existing wells. Scenario 3 shares the same numerical simulation of Scenario 1, but assumes the selected nine existing wells are newly drilled for the economic evaluation. Levelized Cost of Energy (LCOE), Net Present Value (NPV), and Return on Investment (ROI) are used as economic indicators. The results demonstrate that Scenario 2, which combines the use of existing and newly drilled wells, yields improved economic metrics compared to Scenario 1: 0.97 USD/MWh vs. 1.54 USD/MWh for LCOE and $2.9M vs. $1.1M for NPV. Both scenarios represent profitable endeavors, with ROI values of 1.3 % and 1.5 %, respectively. In contrast, Scenario 3 represents the worst-case scenario, with the highest LCOE at 2.90 USD/MWh and the lowest NPV and ROI at -$0.4M and -0.2 %, respectively. The negative NPV and ROI in Scenario 3 indicates that CO₂-circulated geothermal harvesting in aquifers or giant depleted hydrocarbon fields, without leveraging existing infrastructure, is economically infeasible.