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A numerical study of EGS heat extraction process based on a thermal non-equilibrium model for heat transfer in subsurface porous heat reservoir

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Abstract

With a previously developed numerical model, we perform a detailed study of the heat extraction process in enhanced or engineered geothermal system (EGS). This model takes the EGS subsurface heat reservoir as an equivalent porous medium while it considers local thermal non-equilibrium between the rock matrix and the fluid flowing in the fractured rock mass. The application of local thermal non-equilibrium model highlights the temperature-difference heat exchange process occurring in EGS reservoirs, enabling a better understanding of the involved heat extraction process. The simulation results unravel the mechanism of preferential flow or short-circuit flow forming in homogeneously fractured reservoirs of different permeability values. EGS performance, e.g. production temperature and lifetime, is found to be tightly related to the flow pattern in the reservoir. Thermal compensation from rocks surrounding the reservoir contributes little heat to the heat transmission fluid if the operation time of an EGS is shorter than 15 years. We find as well the local thermal equilibrium model generally overestimates EGS performance and for an EGS with better heat exchange conditions in the heat reservoir, the heat extraction process acts more like the local thermal equilibrium process.

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Abbreviations

a :

Specific surface area of aperture network, 1/m

c p :

Heat capacity, J/(kg K)

g :

Acceleration of gravity, m/s2

h :

Convective heat transfer coefficient, W/(m2 K)

k :

Thermal conductivity, W/(m K)

k eff s :

Thermal conductivity of rock, W/(m K)

k eff l :

Thermal conductivity of heat transmission fluid, W/(m K)

K :

Permeability, m2

P :

Pressure, Pa

t :

Time, s

T :

Temperature, K

T f :

Liquid temperature, K

T f,in :

Injection temperature, K

T f,out :

Production temperature, K

T s :

Rock temperature, K

u :

Velocity vector, m/s

x :

Horizontal axis in Cartesian coordinates

y :

Vertical axis in Cartesian coordinates

z :

Horizontal axis in Cartesian coordinates

ρ :

Density, kg/m3

ε :

Porosity

μ :

Viscosity, m2/s

τ :

EGS lifetime, years

γ :

Heat extraction ratio

γ L :

Local heat extraction ratio

eff :

Effective

f :

Fluid

s :

Solid or rock

L :

Local

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Acknowledgments

Financial support received from the China National “863” Project (2012AA052802), the China National Science Foundation and Guangdong-Province Joint Project (U1401232), the Key Scientific Development Project of Guangdong Province (2014A030308001), the CAS “100 talents” Program (FJ), and the China National Science Foundation (51406213, 51206174) is gratefully acknowledged.

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Authors and Affiliations

  1. Laboratory of Advanced Energy Systems, CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS), Guangzhou, 510640, China

    Jiliang Chen & Fangming Jiang

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  1. Jiliang Chen
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Correspondence to Fangming Jiang.

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Chen, J., Jiang, F. A numerical study of EGS heat extraction process based on a thermal non-equilibrium model for heat transfer in subsurface porous heat reservoir. Heat Mass Transfer 52, 255–267 (2016). https://doi.org/10.1007/s00231-015-1554-y

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