Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 1 Numerical Study of Multi-Fluid and Multi-Level Geothermal System Performance Martin O. Saar 1, 2 , Thomas A. Buscheck 3 , Patrick Jenny 4 , Nagasree Garapati 2 , Jimmy B. Randolph 2 , Dimitrios. C. Karvounis 5 , Mingjie Chen 3 , Yunwei Sun 3 , and Jeffrey M. Bielicki 6,7 1 Geothermal Energy and Geofluids Group, Department of Earth Sciences, ETH-Zürich, Zürich, CH 2 Department of Earth Sciences, University of Minnesota, Minneapolis, MN, USA 3 Atmospheric, Earth, and Energy Division, Lawrence Livermore National Laboratory, Livermore, CA, USA 4 Institute of Fluid Dynamics, ETH-Zürich, Zürich , CH 5 Swiss Seismological Service, ETH-Zürich, Zürich, CH 6 Department of Civil and Geodetic Engineering, The Ohio State University, Columbus, OH, USA 7 John Glenn School of Public Affairs, The Ohio State University, Columbus, OH, USA saar@umn.edu Keywords: Geothermal energy, horizontal wells, parasitic load, working fluid, bulk energy storage, brine, water, CO 2 , N 2 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 (CO 2 ), 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 CO 2 (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. 1. INTRODUCTION Previous studies have numerically investigated the performance of geothermal energy production systems that use carbon dioxide (CO 2 ) as the subsurface heat extraction fluid (Randolph and Saar, 2011a; 2011b; 2011c; Saar et al., 2012), multiple subsurface heat extraction fluids such as CO 2 , nitrogen (N 2 ), and brine (Buscheck et al., 2013b; 2014a), and multi-level reservoirs that are vertically stacked, but separated (Karvounis and Jenny, 2012; Karvounis, 2013; Karvounis and Jenny, 2014). Here, we integrate these approaches and investigate how a combination of brine and CO 2 , and N 2 can be used in multi-level reservoirs to improve the overall performance of the geothermal energy production system on three dimensions: 1) energy extraction and conversion efficiency, 2) reservoir lifetime, and 3) economic performance. In the following, we describe in Section 2.1 using a subsurface working fluid other than brine, namely CO 2 , to extract geothermal energy. We then introduce the concept of using multiple subsurface working fluids in Section 2.2. Multi-level geothermal systems are described in Section 2.3. In Section 3 we introduce the idea of combining multi-fluid and multi-level geothermal systems, where one of the working fluids is CO 2 . The numerical methods are described in Section 4 and results are presented in Section 5. 2. BACKGROUND In this section, we provide some background information on the separate systems, CO 2 -Plume Geothermal (Section 2.1), multi-fluid geothermal energy systems (Section 2.2), and multi-level geothermal reservoirs (Section 2.3) that are then combined to one integrated system (Section 3). 2.1 CO 2 -Plume Geothermal (CPG) Brown (2000) was the first to propose using CO 2 as a geothermal working fluid, however only in Enhanced or Engineered Geothermal Systems (EGS) which are typically hydro-fractured or hydro-sheared, low-permeability but hot crystalline basement rocks. Using CO 2 has multiple advantages compared to brine, including: 1) it has a low kinematic viscosity, allowing for effective heat advection despite its relatively low heat capacity; and 2) the thermal expansibility of supercritical CO 2 is much larger than that of brine, generating a much stronger thermosiphon effect through the injection well, the reservoir, and the production well. These and other advantages of CO 2 over brine can reduce or eliminate the need for pumps circulating the underground working fluid through the reservoir (Atrens, et al., 2009; 2010; Adams et al., 2014). Furthermore, CO 2 typically exhibits diminished fluid-mineral