Abstract
A direct CO2-Plume Geothermal (CPG) system is a novel technology that uses captured and geologically stored CO2 as the subsurface working fluid in sedimentary basin reservoirs to extract geothermal energy. In such a CPG system, the CO2 that enters the production well is likely saturated with H2O from the geothermal reservoir. However, direct CPG models thus far have only considered energy production via pure (i.e. dry) CO2 in the production well and its direct conversion in power generation equipment. Therefore, we analyze here, how the wellhead fluid pressure, temperature, liquid water fraction, and the resultant CPG turbine power output are impacted by the production of CO2 saturated with H2O for reservoir depths ranging from 2.5 km to 5.0 km and geothermal temperature gradients between 20 °C/km and 50 °C/km. We demonstrate that the H2O in solution is exothermically exsolved in the vertical well, increasing the fluid temperature relative to dry CO2, resulting in the production of liquid H2O at the wellhead. The increased wellhead fluid temperature increases the turbine power output on average by 15% to 25% and up to a maximum of 41%, when the water enthalpy of exsolution is considered and the water is (conservatively) removed before the turbine, which decreases the fluid mass flow rate through the turbine and thus power output. We show that the enthalpy of exsolution and the CO2-H2O solution density are fundamental components in the calculation of CPG power generation and thus should not be neglected or substituted with the properties of dry CO2.
Highlights
The emission of carbon dioxide (CO2) to, and its accumulation in, the atmosphere is a strong driver of climate change and the associated increase in the global mean surface temperature of Earth
We show in particular, how the consideration of water, first dissolved in CO2 at the bottom of a production well and later partially, exothermally exsolving from the CO2 as the fluid rises in the production well, can result in a substantial increase in the electric power generation capacity of direct CO2-Plume Geothermal (CPG) power plants
The results of our numerical simulations, for reservoir depths of 2.5 km to 5.0 km and geothermal temperature gradients of 20 °C/km to 50 °C/km, suggest the following: The inclusion of H2O in solution with CO2 leads to the production of freephase liquid H2O at the production wellhead, due to the exsolution of the initially dissolved H2O during the upwards flow of the production fluid as it experiences a decrease in fluid pressure and temperature, reducing the solubility of H2O in CO2
Summary
The emission of carbon dioxide (CO2) to, and its accumulation in, the atmosphere is a strong driver of climate change and the associated increase in the global mean surface temperature of Earth. The Intergovernmental Panel on Climate Change (IPCC) has unequivocally concluded that the generation of electricity and heat by the combustion of fossil fuels is responsible for at least 25% of the total amount of CO2 that has been emitted by human activities (IPCC, 2014). Renewable energy sources, such as wind, solar, bio-energy, and geothermal energy, have been developed and used to generate power with few, if any, operational CO2 emissions, thereby reducing the overall emission of CO2 for electricity generation. This process can isolate CO2 underground in saline reservoirs or (partially) depleted oil or gas reservoirs, which are overlain by a low- to almost zero-permeability caprock, such that the leakage risk of the buoyant CO2 is minimal (Bielicki et al, 2016, 2015, 2014; IPCC, 2005)
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