A preliminary site selection system for a CO₂-AGES project and its application in China

Because of the high costs involved in CO₂ sequestration in deep saline formations, it has been difficult for its widespread application in the short term. Recently, however, a new technology called “combined geothermal production–CO₂ sequestration technology” has made the technique more attractive not only by increasing the CO₂ storage capacity, but also by decreasing the operational costs, through the utilization of geothermal energy, either directly or for the purpose of producing electricity. A CO₂-aided (or CO₂-involved) geothermal extraction system (CO₂-AGES) is presented, based on existing technologies and the new ideas of “combined geothermal production-CO₂ sequestration” introduced in this paper. This system can be used to extract geothermal energy from sedimentary aquifer formations (low–medium temperature) at different stages. The purpose of this paper is to set up an evaluation system for selecting a suitable site for this CO₂-AGES system. There are both similarities and differences in the procedure needed for selecting a site for a conventional pure CO₂ sequestration and for this CO₂-AGES system. Both are carried out at different scales, including basin, region and target formation scales. The biggest difference is the temperature gradient, which plays an opposite role in the two systems. By using the preliminary ranking and screening method presented in this paper, it can be shown that the Bohaiwan, Songliao and Qiangtang basins have the highest potential for the application of this CO₂-AGES technology. However, if CO₂ sequestration is considered alone, the Ordos, Tarim and Bohaiwan basins should be selected as the best three operational sites in China. While preliminary evaluation methods can provide some useful information on the selection of the best sites for the “combined geothermal production–CO₂ sequestration technology”, more detailed work is still required because of the strong uncertainties that exist in the determination of the upper and lower boundaries for each indicator.