Numerical Study of Multi-Fluid and Multi-Level Geothermal System Performance

Abstract

We introduce the idea of combining multi-fluid and multi-level geothermal systems with two reservoirs at depths of 3 and 5 km. In the base case, for comparison, the two reservoirs are operated independently, each as a multi-fluid (brine and carbon dioxide) reservoir that uses a number of horizontal, concentric injection and production well rings. When the shallow and the deep reservoirs are operated in an integrated fashion, in the shallow reservoir, power is produced only from the carbon dioxide (CO2), while the brine is geothermally preheated in the shallow multi-fluid reservoir, produced, and then reinjected at the deeper reservoir’s brine injectors. The integrated reservoir scenarios are further subdivided into two cases: In one scenario, both brine (preheated in the shallow reservoir) and CO2 (from the surface) are injected separately into the deeper reservoir’s appropriate injectors and both fluids are produced from their respective deep reservoir producers to generate electricity. In the other scenario, only preheated brine is injected into, and produced from, the deep reservoir for electric power generation. We find that integrated, vertically stacked, multi-fluid geothermal systems can result in improved system efficiency when power plant lifespans exceed ~30 years. In addition, preheating of brine before deep injection reduces brine overpressurization in the deep reservoir, reducing the risk of fluid-induced seismicity. Furthermore, CO2-Plume Geothermal (CPG) power plants in general, and the multi-fluid, multi-level geothermal system described here in particular, assign a value to CO2, which in turn may partially or fully offset the high costs of carbon capture at fossil-energy power plants and of CO2 injection, thereby facilitating economically feasible carbon capture and storage (CCS) operations that render fossil-energy power plants green. From a geothermal power plant perspective, the system results in a CO2 sequestering geothermal power plant with a negative carbon footprint. Finally, energy return on well costs and operational flexibility can be greater for integrated geothermal reservoirs, providing additional options for bulk and thermal energy storage, compared to equivalent, but separately operated reservoirs. System economics can be enhanced by revenues related to efficient delivery of large-scale bulk energy storage and ancillary services products (frequency regulation, load following, and spinning reserve), which are essential for electric grid integration of intermittently available renewable energy sources, such as wind and solar. These capabilities serve to stabilize the electric grid and promote development of all renewable energies, beyond geothermal energy.