Modeling Thermal Fracturing During Operation of Enhanced Geothermal Systems: Improved Heat-Transfer Area and Reservoir Sustainability


Quanlin ZHOU, Bin CHEN

Key Words:

EGS, thermal fracturing, thermal stress, cooling, reservoir sustainability


Stanford Geothermal Workshop




Enhanced Geothermal Systems



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1586 KB

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In the current concept of an enhanced geothermal system (EGS), a pair of horizontal wells are used to circulate fluid through stimulated hydraulic fractures; reservoir sustainability and improvement during operation are critical for long-term power generation. Injecting cold fluid into a geothermal reservoir will cause significant cooling and thermal stress, possibly resulting in secondary thermal fractures perpendicular to primary hydraulic fractures and forming a well-connected fracture network for heat transfer. First, we derived analytical solutions of dimensionless fracture length L, aperture profile Ω, and spacing D (as well as pattern) as functions of time τ and dimensionless effective confining stress T using a plane strain model and the displacement discontinuity method (Chen & Zhou, 2022, 10.1029/2021JB022964). It was observed that fracture length increases nonlinearly with √τ and then transitions to scaling law L=f(T) √τ, indicating that late-time fracture length increases linearly with the square root of cooling time. The scaling coefficient f(T) shows the effects of inter-fracture stress interaction and fracture arrest. The solutions and scaling law provide fast predictions for all reservoir and cooling conditions using (single) model parameter T. Application to the Utah FORGE EGS site with T=0.11 demonstrates that thermal fractures reach 0.67, 6.25, and 78.00 m in length, 0.49, 2.30, and 13.00 m in spacing, and 0.43, 2.09, and 12.19 mm in surface aperture at 1, 100 and 10,000 days. Second, numerical modeling of thermal fractures from two parallel hydraulic fractures was conducted and the modeling results show that it is effective for thermal fractures from different hydraulic fractures to eventually merge and connect, thus enhancing fracture connectivity and permeability. For a cooling of 150 °C at the Utah FORGE site, thermal fractures will connect two neighboring hydraulic fractures with 20 m spacing in ~150 days of EGS operation.

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