Thermoporoelastic Analysis of a Single-Well Closed-Loop Geothermal System

Geothermal systems are identified as either open-loop geothermal systems (OLGS) or closed-loop geothermal systems (CLGS). In OLGS, fluid is produced from the subsurface, while there might be concurrent fluid injection into the reservoir. The loss of working fluid, surface subsidence, and formation compaction are major challenges in OLGS. To address the indicated challenges, closed-loop geothermal systems can be considered as an alternative option. In CLGS, a working fluid with low boiling point is circulated through the coaxial sealed pipes to extract heat from the geothermal reservoir. Conduction is the main heat transfer mechanism in the CLGS; however, available mere-conduction configurations are not capable to quickly transfer heat from the reservoir to the wellbore. To improve conductive heat extraction from this system, we suggest induced thermal-conductive fractures for CLGS to enhance conductive heat transfer into the wellbore. Comprehensive analysis of this problem requires simultaneous modeling of fluid flow, heat transfer and rock deformation. A finite element thermo-poroelastic model represents CLGS. The numerical results suggest that fractures significantly improve thermal power and cumulative extracted heat for the proposed configuration. Thermal conductivity of the proppants filling the induced fractures is the key parameter affecting the heat extraction. Negligible surface subsidence in the proposed technique suggests this configuration is suitable for the areas where surface subsidence or induced seismicity restricts the application of OLGS.