A comparison of electric power output of CO_2 Plume Geothermal (CPG) and brine geothermal systems for varying reservoir conditions

摘要

In contrast to conventional hydrothermal systems or enhanced geothermal systems, CO_2 Plume Geothermal (CPG) systems generate electricity by using CO_2 that has been geothermally heated due to sequestration in a sedimentary basin. Four CPG and two brine-based geothermal systems are modeled to estimate their power production for sedimentary basin reservoir depths between 1 and 5 km, geothermal temperature gradients from 20 to 50 ℃ km~(-1), reservoir permeabilities from 1 × 10~(15) to 1 × 10~(12) m~2 and well casing inner diameters from 0.14 m to 0.41 m. Results show that CPG direct-type systems produce more electricity than brine-based geothermal systems at depths between 2 and 3 km, and at permeabilities between 10~(-14) and 10~(-13) m~2, often by a factor of two. This better performance of CPG is due to the low kinematic viscosity of CO_2, relative to brine at those depths, and the strong thermosiphon effect generated by CO_2. When CO_2 is used instead of R245fa as the secondary working fluid in an organic Rankine cycle (ORC), the power production of both the CPG and the brine-reservoir system increases substantially; for example, by 22% and 20% for subsurface brine and CO_2 systems, respectively, with a 35 ℃ km~(-1) thermal gradient, 0.27 m production and 0.41 m injection well diameters, and 5 × 10~(-14)m~2 reservoir permeability.