Thermoporoelastic Analysis of Artificially Fractured Geothermal Reservoirs: A Multiphysics Problem

Abstract

Geothermal systems are identified as either open-loop system (OLGS) or closed-loop systems (CLGS). In OLGS, fluid is produced from the subsurface, while there might be a concurrent fluid injection into the reservoir. The loss of working fluid, surface subsidence, formation compaction, and induced seismicity are major challenges in OLGS. To address the indicated challenges, closed-loop geothermal systems can be considered as an alternative option. In this method, a working fluid with low-boiling point is circulated through the coaxial sealed pipes to harvest heat from the formation of rock and fluid. Induced seismicity is essentially caused by the drastic quick changes in pore pressure. Thereafter, seismic risk assessment is expected for any new geothermal technology before starting the field implementation phase. To improve the heat recovery from closed-loop wells, we suggest highly conductive hydraulic fractures for CLGS to improve the heat generation rate. In conventional hydraulic fracturing treatments, fractures facilitate fluid flow; however, in the proposed configuration, induced fractures enhance heat flux into the wellbore. Considering the multiphysics nature of CLGS, a comprehensive analysis of this problem requires simultaneous modeling of fluid flow, energy transfer (heat), and rock deformation. A thermoporoelastic model is developed in finite element methods to simulate this problem. The numerical results suggest that fractures significantly improve thermal power and cumulatively produced heat in CLGS. The thermal conductivity of the proppants is the key parameter enhancing heat generation. The level of surface subsidence in the proposed technique is negligible due to the lack of geofluid production from the reservoir. Significant numbers of abandoned oil or gas wells exist around the globe which can be converted into the geothermal wells to produce electricity. This study shows the feasibility of electricity production from CLGS with minimum environmental hazards.