Numerical Modeling of CO2-Plume Geothermal (CPG) Systems

Video on CO2-Plume Geothermal (CPG) power plants combine geologic CO2 storage with geothermal energy extraction.© Shannon Gilley

Introduction

COPlume Geothermal systems (CPG) (Randolph and Saar, 2011a, b; Saar et al., 2012; Buscheck et al., 2013), involve injection of COthat has been captured at a CO emitter (e.g. cement manufacturer, biofuel refinery, fossil-fuelled power plant) as a working fluid into an underground reservoir (deep saline formation or oil/gas reservoir) to extract heat and pressure energy (enthalpy) from naturally high-permeability sedimentary basins. The injected COforms a large subsurface CO plume and permanently sequesters the CO underground. As more CO is injected, the CO plume continues to grow. The subsurface CO absorbs heat from the reservoir rocks. Given a sufficient pore-space CO saturation at a given reservoir location, the geothermal heated CO can be produced from that location to the land surface, used as direct heat or converted to electricity, cooled and condensed, and the reinjected into the original reservoir, closing the loop and ensuring that 100% of the originally injected CO is eventually still permanently sequestered underground (Randolph and Saar, 2011a; Saar et al., 2012; Adams et al., 2014, 2015). 

geophysics

The heat density of sedimentary basins is typically relatively low. However, this drawback can be counteracted by the large accessible pore-space volume of natural reservoirs, compared to the relatively small reservoirs developed artificially by hydraulic stimulation (Dezayes et al., 2005). Furthermore, supercritical COhas a high mobility (i.e., low kinematic viscosity) and high thermal expansibility, compared to water, resulting in the formation of a strong thermosiphon, which eliminates the need for parasitic pumping power requirements. 

geophysics

The thermosiphon effect and in particular the high mobility of CO significantly increase the electric power production efficiency of the geothermal system, compared to water-based geothermal systems. The produced electric power can then, for example, be used to drive the CO injection pumps and/or the CO capture facility if geologic CO capture and storage is ongoing or and/it could be supplied to the electric power grid to generate revenue. Similarly, produced heat could be directly used, if a heat consumer exists nearby, for example the COcapture facility. Because the produced CO is reinjected into the original geothermal reservoir with the main CO sequestration stream, coming from the CO emitter, all of the CO is ultimately geologically sequestered resulting in a CO sequestering geothermal power plant. Alternatively, when geologic CO storage is uneconomic, CPG systems could also be operated with a limited, finite amount of CO, stored underground, and thereafter run with little or no additional makeup CO.