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Numerical investigation on the effects of the fracture network pattern on the heat extraction capacity for dual horizontal wells in enhanced geothermal systems

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Abstract

In an enhanced geothermal system (EGS), fractures and fracture networks are the predominant elements for fluid flow and heat transfer through the artificial reservoir. In this work, different conceptual discrete fracture networks were generated by characterizing the fracture number, fracture bifurcation and fracture connectivity of the fracturing area. A thermal–hydraulic (TH) coupled mathematical model was applied to evaluate the EGS thermal recovery process. Heat extraction capacity was appraised in terms of the temperature production, net power generation and thermal recovery rate. The results show that an interconnected fracture network with considerable bifurcation results in high heat production and power generation, however the energy efficiency is not optimized due to water loss. The effect of fracture connectivity is more significant than that of fracture spacing. The more fractures (bifurcations) the higher the overall and local heat recovery rates near the production well. For the case with connected fractures without bifurcation, the less characteristic length in the direction of flow results in a lower heat production and power generation.

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Abbreviations

d f :

Fracture width (m)

t :

Time (s)

\(\eta\) :

Dynamic viscosity of fluid (Pa s)

v :

Water flow rate (m/s)

k r :

Rock permeability (m2)

k f :

Fracture permeability (m2)

S :

Storage coefficient of rock mass (1/Pa)

S f :

Storage coefficient of fracture (1/Pa)

Q :

Source-sink term of the seepage process (1/s)

Q f :

Flow exchange between rock mass and fracture surface (m/s)

\(\rho_{s}\) :

Rock density (kg/m3)

\(\rho_{f}\) :

Water density (kg/m3)

\(\lambda_{s}\) :

Thermal conductivity of rock mass (W/m/K)

\(\lambda_{f}\) :

Thermal conductivity of water (W/m/K)

C s :

Heat capacity of rock block (J/kg/K)

C f :

Heat capacity of water (J/kg/K)

T s :

Rock temperature (K)

T f :

Water temperature in fracture (K)

W :

Heat source (W/m3)

W f :

Heat absorbed from matrix block on fracture surface (W/m2)

v f :

Water flow velocity in fracture (m/s)

h :

Surface heat transfer coefficient (W/m2/K)

\(\upsilon\) :

Kinematic viscosity coefficient

L :

Length integral of the output along fracture

\(\varGamma\) :

Surface integral of the output along the rock matrix block

h inj :

Injection specific enthalpy (kJ/kg)

h pro :

Production specific enthalpy (kJ/kg)

q :

Total production rate (kg/s)

W h :

Heat production (MWe)

W e :

Power generation (MWe)

W p1 :

Energy consumption of injection pump (MWe)

W p2 :

Energy consumption of suction pump (MWe)

W p :

Total energy consumption (MWe)

T o :

Reinjection temperature of injection well (K)

\(\eta_{p}\) :

Pump efficiency

dV :

Volume of injected water per second (m3/s)

h 1 :

Depth of injection well (m)

h 2 :

Depth of production well (m)

\(\eta_{h}\) :

Energy consumption based on heat production

\(\eta_{e}\) :

Energy consumption based on power generation

\(\gamma_{L}\) :

Local heat recovery rate

\(\gamma\) :

Overall heat recovery rate

T r :

Rock temperature at the initial time (K)

Ts(t):

Rock temperature at time t (K)

T inj :

Temperature of the injected fluid (K)

\(\rho_{s}\) :

Rock density (kg/m3)

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Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant No. 51774317), and National Science and Technology Major Project (Grant No. 2016ZX05011004-004).

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Authors and Affiliations

  1. School of Petroleum Engineering, China University of Petroleum (East China), Qingdao, 266580, China

    Ying Xin & Zhixue Sun

  2. Extreme Engineering Research Center, Korea Institute of Civil Engineering and Building Technology, 283, Goyang-daero, Ilsanseo-gu, Goyang-si, Gyeonggi-do, 10223, Republic of Korea

    Li Zhuang

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Xin, Y., Zhuang, L. & Sun, Z. Numerical investigation on the effects of the fracture network pattern on the heat extraction capacity for dual horizontal wells in enhanced geothermal systems. Geomech. Geophys. Geo-energ. Geo-resour. 6, 32 (2020). https://doi.org/10.1007/s40948-020-00151-3

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