Abstract
The energy efficiency of the enhanced geothermal system (EGS) measures the economic value of the heat production and electricity generation, and it is a key indicator of system production performance. Presently there is no systematic study on the influence of well layout on the system energy efficiency. In this work we numerically analyzed the main factors affecting the energy efficiency of EGS using the TOUGH2-EOS1 codes at Gonghe Basin geothermal field, Qinghai province. The results show that for the reservoirs of the same size, the electric power of the three horizontal well system is higher than that of the five vertical well system, and the electric power of the five vertical well system is higher than that of the three vertical well system. The energy efficiency of the three horizontal well system is higher than that of the five vertical well system and the three vertical well system. The reservoir impedance of the three horizontal well system is lower than that of the three vertical well system, and the reservoir impedance of the three vertical well system is lower than that of the five vertical system. The sensitivity analysis shows that well spacing has an obvious impact on the electricity production performance; decreasing well spacing will reduce the electric power, reduce the energy efficiency and only have very slight influence on the reservoir impedance. Fracture spacing has an obvious impact on the electricity production performance; increasing fracture spacing will reduce the electric power and reduce the energy efficiency. Fracture permeability has an obvious impact on the electricity production performance; increasing fracture permeability will improve the energy efficiency and reduce the reservoir impedance.
Highlights
In China, the total enhanced geothermal system (EGS) resource reserve within 3–10 km depths amounts to 20.90 M EJ; if 2% is taken as the recoverable fraction, the recoverable EGS resource amounts to 4400 times total annual energy consumption in 2010 [4]
We mainly investigate the energy efficiency of the three systems, and analyze the influence of well spacing, fracture spacing and permeability on the energy efficiency
According to the principle that during the production period Pinj < Pmax = 37.416 MPa and the production temperature drop is lower than 10%, we can determine that for the three vertical well system the water production rate is 32 kg/s in the simulated domain and the total water production rate is
Summary
Enhanced geothermal systems employ artificial circulating water or CO2 to extract heat from subsurface fractured hot dry rock (HDR) at depths of 3~10 km, and the produced heat is mainly used for electricity generation [1]. Sanyal et al used the DPM method to analyze the electricity production prospects from EGSs based on the geological data at Desert Peak geothermal field, and found that increasing permeability without increasing matrix-to-fracture heat transfer area has little effect in heat recovery [28]. Gelet et al established a thermo-hydro-mechanical coupled model based on the DPM method and analyzed the impacts of fracture spacing on the heat production performance of EGS [29,30,31,32]. Zeng et al used the MINC method to study the electricity generation potential from fractured granite reservoir at Yangbajing geothermal field [37,38]. Chen et al employed the LNTE model to analyze the heat production performance of EGS reservoirs, and they made many important progress [47,48,49]. These will provide guidance for future improving the energy efficiency of EGSs
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