A global review of deep geothermal energy exploration: from a ......2.2 Rosemanowes The Rosemanowes HDR project (1977–1991, UK) by the Camborne School of Mines with the purposes

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A global review of deep geothermal energy exploration: from a view of rock

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DOI: 10.1007/s40948-019-00126-z

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ORIGINAL ARTICLE

A global review of deep geothermal energy exploration:from a view of rock mechanics and engineering

Yuliang Zhang . Gao-Feng Zhao

Received: 25 July 2019 / Accepted: 18 October 2019

� Springer Nature Switzerland AG 2019

Abstract Deep geothermal energy exploitation has

gained a lot of attentions in energy field due to its large

reserve. Enhanced geothermal system (EGS) is the

only one mode to explore hot dry rock (HDR) with real

engineering practice throughout the world. To date, it

is also a topic facing several key issues among

sustainable and renewable energies because no com-

mercialized mode has been made. Rock plays an

important role in deep geothermal energy exploitation

because it is not only a carrier of heat energy but also

provides artificial paths for heat exchange after

stimulation. There are some practices and researches

about rock drilling, stimulation and stability of

fractures in deep geothermal engineering. This paper

collected the results of the previous deep geothermal

exploitation researches that relate to rock mechanics

and engineering and summarized the experiences and

lessons of deep geothermal energy exploitation from

literatures. Some new thoughts like the mode based on

EGS and pipe were also collected. Finally, the

knowledge about deep geothermal energy exploitation

has been derived. Despite of many works on HDR,

some key issues still face challenges and need

breakthrough. Therefore, deficiencies and prospection

in future research were pointed out. The review results

showed that the evolution and mechanism of HDR

such as weathering, damage, cracking, failure and

stability under multi-field coupling with the interac-

tion of fluid is an important issue to be further studied

and addressed.

Keywords EGS � HDR � Deep geothermal energy �Rock mechanics � Rock engineering

List of symbols

t Temperature

t0 Initial temperature

tin Injection temperature

s Times0 Cohesive strength of the sliding surfacescrit Critical shear stressl Coefficient of frictionrn Normal stresskR Thermal conductivity of rockkW Thermal conductivity of watercR Specific heat capacity of rock

cW Specific heat capacity of water

cF Specific heat capacity of fluid

qR Density of rockqF Density of fluidqW Density of waterlW Viscosity of waterr Confining stressre Effective stressP Pore pressure or fluid pressure

Y. Zhang � G.-F. Zhao (&)State Key Laboratory of Hydraulic Engineering

Simulation and Safety, School of Civil Engineering,

Tianjin University, Tianjin 300354, China

e-mail: [email protected]

123

Geomech. Geophys. Geo-energ. Geo-resour. (2020) 6:4

https://doi.org/10.1007/s40948-019-00126-z(0123456789().,-volV)(0123456789().,-volV)

http://orcid.org/0000-0001-9962-8743http://crossmark.crossref.org/dialog/?doi=10.1007/s40948-019-00126-z&domain=pdfhttps://doi.org/10.1007/s40948-019-00126-z

q Unit-width discharge

b Relative efficiencies of confining stress versusfluid pressure

k Permeability

W Fracture aperture

W0 Maximum aperture

i Dimensionless material constant

c Material constantEf Effective modulus of the fracture asperities

1 Introduction

The increasing demand of clean energy with little

environmental impact promotes the development of

sustainable and renewable energy. Deep geothermal

energy, generally at more than 3 km depth and with

more than 150 �C (Wang et al. 2012), has a largereserve and wide distribution, which is one of a

sustainable energy presenting a potentially exploita-

tion capability (Tomac and Sauter 2018). The geother-

mal energy in the upper 10 km crust is * 1.3 9 1027

J which could supply the global use for * 217 millionyears (Lu 2018). Exploring deep geothermal energy is

always a topic in academic and engineering fields.

In the earlier 1970s, a small group of researchers at

Los Alamos National Laboratory put forward an

audacious proposal to extract heat energy from those

vast regions of the earth’s crust that contain no fluids

and is characterized by far the largest part of the

earth’s drilling-accessible geothermal resource. This

opened a new chapter in human exploration of deep

geothermal resources. There are four parts at least in

EGS: injection well, stimulated rock volume (SRV),

production well and energy conversion plant (see for

e.g. Heidinger 2010). After that, several enhanced

geothermal system (EGS) projects has been conducted

through the world (Feng et al. 2012; Olasolo et al.

2016). A lot of improvements have been made in deep

geothermal exploitation such as the Los Alamos HDR

Geothermal Energy Project (Kelkar et al. 2016) and

the Hijiori test site (Kuriyagawa and Tenma 1999),

however, up to now, there is no commercialized

exploitation project (Xu et al. 2016).

Deep geothermal exploitation is a complex process

that includes geological survey, generating plant

installation, drilling, reservoir establishing, fluid

circulation, operation, energy utilization. The biggest

challenge or obstruct of deep geothermal energy is to

extract acceptable heat energy form HDR. In EGS,

cold fluid was injected to HDR and was heated to high

temperature so that we can extract heat energy by heat

exchange. Nevertheless, this process is so difficult that

a lot of problems occurred such as wellbore failure

(Zhao et al. 2015b) and water loss (Batchelor 1983),

which are closely relative to rock mechanics and rock

engineering. Conducting the research of rock mechan-

ics and engineering that is suitable for deep geother-

mal exploitation is necessary and will lay a good

foundation for further exploration.

Deep geothermal energy exploitation is a type of

underground engineering. First of all, more than one

deep-drilling must be penetrated into HDR. Compared

with other drilling, high temperature and high pressure

should be considered. Secondly, reservoir establishing

is one of the key technologies in EGS, which provide

paths for flowing and heat exchange between rock and

fluid. Thirdly, fluid circulation experiments would be

conducted to form smooth flow and wide heat

exchange. However, fractures will change in such a

complex condition, which may cause unpredicted

results such as the reduce in effective heat exchange.

Fundamental rock physical and mechanical properties

are important and have been researched by many

scholars including thermal properties (Vosteen and

Schellschmidt 2003), cracking when hydraulic stim-

ulation (Zimmermann and Reinicke 2010; Troiano

et al. 2017), weathering under acid solution (Schmidt

et al. 2017) and deformation under multi field coupling

coupling (Chen et al. 2018c; Zhao et al. 2015a; Cao

et al. 2016a). These fundamental researches are

valuable for deep geothermal research, but they lack

collection.

With the development of EGS, some researchers

also proposed new thoughts to extract heat from HDR

such as the models revised from EGS and the pipe

models. And, carbon dioxide was proposed as fluid to

inject into reservoir. Relative research results show

that new model or thoughts has some advantages over

EGS.

In view of the researches of deep geothermal energy

exploitation, especially at the view of rock engineer-

ing, this paper collected the results of previous studies

and summarized the knowledge (experience and

lessons) about deep geothermal energy exploitation.

Finally, conclusions and prospection were derived.

123

4 Page 2 of 26 Geomech. Geophys. Geo-energ. Geo-resour. (2020) 6:4

2 Typical EGS practices

Practical EGS projects demonstrated that EGS is

theoretically feasible. EGS is a man-made geothermal

system where we can control production temperature

by drilling depth, size of reservoir by hydraulic

fracturing, injection conditions e.g. fluid pressure

and flow rate by the SRV flow situation and the

near-wellbore outlet impedance, amount of reservoir

growth by applying over-pressure that is above the

joint-extension pressure, production well backpres-

sure and number and placement of production wells

(Brown 2009).

2.1 Fenton Hill

Fenton Hill is the first site (1970–1995) to exploiting

deep geothermal energy funded by the Atomic Energy

Commission (Fig. 1a). In the first phase (1972–1980),

the injection well (GT-2) was drilled to 2932 m depth,

and the production well (EE-1) was drilled to 3064 m

depth. Hydraulic stimulation tests were conducted

separately, but hydraulic conductivity was not made.

Finally, GT-2B was drilled from GT-2 and hydraulic

conductivity was made between EE-1 and GT-2B

(Tester and Albright 1979). In the second phase

(1979–2000), EE-2 and EE-3 were drilled to over

4000 m depth with 380 m well distance. Hydraulic

stimulation tests were conducted at different depth in

EE-2. Micro-seismic events showed that fractures did

not extend in the predicted direction. Adequate

hydraulic conductivity was not made (Whetten et al.

1987). EE-3A was drilled by altering EE-3 at * 2830m in 1985 to obtained accepted hydraulic conductiv-

ity. Results show that it is nearly impossible to

produce satisfied hydraulic conductivity between two

or more wells if the wells were beforehand drilled and

-600 -500 -400 -300 -200 -100 0 100 2004000

3800

3600

3400

3200

3000

(d)(c)

(b)

Ver

tical

dep

th (m

)

Horizental distance (m)

EE-2 EE-3 EE-3A

Packer

Brown et al. 1987

RH11 RH12 RH15 B

View from NE

200 m

(a)

Parker et al. 1999

-2000 -1500 -1000 -500 0 5006000

5000

4000

3000

2000

1000

0

Ver

tical

dep

th (m

)

Northing (m)

EPS1 GPK1 GPK2 GPK3 GPK4

Genter et al. 2010

-400 -300 -200 -100 0 100 200 300 4001600

1400

1200

1000

800

600

400

200

0

Dep

th (m

)

East (m)

OGC-1 OGC-2 OGC-3

Kaieda et al. 2005

Fig. 1 Well layout of longitudinal section of the real EGS practices. a Fenton Hill, b Rosemanowes, c Soultz and d Hijiori

123

Geomech. Geophys. Geo-energ. Geo-resour. (2020) 6:4 Page 3 of 26 4

stimulated separately due to the orientation of frac-

tures in a specific geological setting (Brown 2009).

While Fenton Hill isn’t a commercial-scale project

for exploiting deep geothermal energy, it developed

several innovated technologies, devices and measur-

ing methods (Ziagos et al. 2010). It is feasible to drill a

deep wellbore in granitic rock which has a high

temperature. Hydraulic stimulation could generate or

activate fracture network in low-permeability crys-

talline rock in a large scale of over 1 km3, which can

be detected by acoustic emission technology. Fracture

extension is unpredictable due to complex geological

settings, so in engineering reality, the production well

should be drilled after the installation and stimulation

of the injection well and referring to the acoustic

emission location to confirm that the production well

can drilled into the stimulated region. Flowing

impendence can be reduced at elevated pressure.

2.2 Rosemanowes

The Rosemanowes HDR project (1977–1991, UK) by

the Camborne School of Mines with the purposes of

developing equipment and techniques that could later

be used for heat extraction (Ziagos et al. 2010) can be

divided into three phases (Fig. 1b). In phase 1

(1977–1980), 300-m deep wellbores were drilled,

and water circulation test was conducted after

hydraulic stimulation which was conducted in the

reservoir of natural fractures. In phase 2A

(1980–1988), reservoir development was investigated

at * 2000 m with the expectation of 1 �C/year ther-mal drawdown, 75 kg/s flow rate, 0.1 MPa/(kg/s)

impendence and 10%maximum water loss. RH11 and

RH12 were drilled and were deviated at an angle of

30� (to the NW) from vertical in the lower sections. Anextensive reservoir development program was con-

ducted by injecting 30,000 m3water into RH12. In this

phase, high water loss is related to the downward

growth of the reservoir at around 3 - 3.5 km (Batchelor

1983; CSM 1984, 1985). In phase 2B, RH-15 was

drilled under RH12 into the microseismicity region. A

medium viscosity gel stimulation was conducted from

RH15 which form a tube-shaped zone between RH15

and RH12. In phase 2C, long-term circulation was

conducted between RH12 and RH15. The main

purpose of phase 3 is to establish a commercial system

for generating electricity, but it was terminated in

1991.

The most important issue of Rosemanowes is the

fluid short circuit. In 1989, 55 tons of proppant (sand)

were successfully injected into the fractured reservoir

at wellhead pressures of 24 MPa, as a result impen-

dence and water loss were reduced. However, gel

stimulation with high pressure led to fluid short circuit,

resulting in production temperature drawdown. Seis-

mic events showed fractures expand in downward

direction, revealing that the failure of rock belongs to

shear failure (Pine and Batchelor 1984). This project

also demonstrates that production well should be

installed before simulation because the orientation of

fractures cannot be controlled.

2.3 Soultz

The Soultz project is so far the most successful

demonstration (Fig. 1c). The granitic rock locates

under the * 1-km sedimentary cover whose thermalgradient is over 110 �C/km. After the installation andstimulation of GPK2, GPK3 was drilled to the

fractured reservoir which was stimulated from

GPK2. Therefore, GPK2 and GPK3 have a good

hydraulic conductivity. In order to expand fractured

reservoir, hydraulic stimulation as well as acidizing

treatment were used at the bottom of GPR4, but good

well connectivity was not made due to a linear non-

seismic zone. Circulation experiment of GPK2 (pro-

duction well), GPK3 (injection well) and GPK4

(production well) showed that the production wells

have different production temperatures, revealing the

complexity and inhomogeneity of the flow path

(Genter et al. 2010).

2.4 Hijiori

The Hijiori site is the first EGS project of Japan, which

locates the southern edge of the two-kilometer diam-

eter Hijiori caldera in Okura Village in the Yamagata

Prefecture (Fig. 1d). This project consists of two

reservoirs: the shallow reservoir at * 1800 m and thedeep reservoir at * 2200 m with fine natural frac-tures (Tenma et al. 2008) and four man-made

wellbores (SKG-2, HDR-1, HDR-2, HDR-3). In first

phase (1985–1991), water loss reached over 70% after

stimulation from SKG-2 at * 1800 m. With theprocess of circulation, water loss reduced to * 22%.In second phase (1992-), circulation experiment was

conducted setting SKG-2 and HDR-1 as the injection

123


well and HDR-2 and HDR-3 as the production well.

Nevertheless, lots of water loss existed, and produc-

tion temperature of HDR-2 reduced a lot. Finally,

circulation experiment was terminated (Yamaguchi

et al. 2000).

2.5 Ogachi

The Ogachi HDR Project (1989-) locates in Akita

Prefecture, northeast Japan inheriting the Akinomiya

Project (1986–1988). This project consists of three

wellbores (OGC-1, OGC-2, OGC-3) with the largest

depth of 1300 m. OGC-1 was drilled first and stim-

ulated at 1017–1027 m (bottom) and 710–720 m

intervals to form the upper and lower fractured

reservoirs (Kaieda et al. 2005; Kitano et al. 2000).

OGC-2 was drilled to 1100 m in 1992. OGC-1 and

OGC-2 did not have a good connection, and high-

water-loss existed though multiple stimulations were

conducted. In 1999, OGC-3 was drilled to 1300 m

with good connection with other wellbores. However,

water loss was also high.

2.6 Cooper Basin

The Geodynamics EGS Project at Innamincka, South

Australia (2003-) consists of three wellbores (Ha-

banero-1, Habanero-2, Habanero-3). Habanero-1 was

drilled in 2003 to 4421 m with 250 bottom tempera-

ture. Habanero-2 was drilled in 2004 to 4358 m.

Habanero-3 was drilled in 2008 to 4221 m with good

connection with Habanero-1.

2.7 Knowledge from EGS projects

The EGS demonstration projects have significant

importance to further researches and provide impor-

tant lesson, from which valuable information has been

got. Now, technologies applicable to deep geothermal

exploiting are obtained:

(1) Deep drilling could be conducted in HDR

(fractured or intact granitic rock) which is more

than 5000 m depth and more than 300 �C.(2) Hydraulic stimulation can be used to extend or

generate fractures in fractured or intact granitic

rock, which is an effective method to connect

wells by generating flow paths. The stimulated

fracture may extend to a large scale (200 m

thick and about 500 m wide, propagating

1000 m (Kitano et al. 2000)). Stimulation

technology at different elevation intervals in

one wellbore is feasible (Kitano et al. 2000;

Kaieda et al. 2005). Directional drilling can be

controlled in hard crystalline rock mass (Brown

2009). Increasing wellhead pressure could pro-

mote large fractures in reservoir.

(3) Fracture distribution can be obtained by acous-

tic emission technology, chemical tracer and

other geophysical technologies.

(4) Water circulation can also stimulate new frac-

tures in reservoir and reduce flow impendence

and water loss.

(5) Acid treatment or gel is also a method to

generate fractures in reservoir, which con-

tributes to resolving pressure drawdown at

nearby wellbore.

Despite of many valuable acquisitions about EGS,

much other unknown knowledge also exists. Some

problems in EGS practice need to further investigate.

(1) Fracture extension is unpredictable. Due to the

complexity of geological setting, the directions

of principle stress would change with location,

which addition to the nature fissures and other

conditions may lead to the fracture extension

direction. Therefore, it is hard to get good

hydraulic conductivity between wellbores by

hydraulic stimulations that is conducted before

the entire installation of all the wellbores. The

recommended step is that the injection well was

drilled first, then stimulations should be con-

ducted. Finally, production well can be drilled in

terms of the fractured reservoir zone, in which

the monitor of fractures is critical for well

connectivity.

(2) Acoustic emission technology is an effective

method to evaluate the stimulated fractures in

reservoir in fracturing and circulation process.

This method can provide a 3D distribution of the

fractures by spacing location technology, but it

still needs improve. Brown (2009) pointed that

there is more to do in this field to understand the

123


effective portion of seismicity that is really

related to opening fractures. Indeed, the induced

fractures without enough aperture or without

connection may not be the flow path, but it can

be monitored by acoustic emission instrument

when it generates by rock breaking. Precise

positioning technology is a promised direction.

As known, geological setting up the target

reservoir is complex, which consist of all kinds

of stratum, fissures, faults and other geological

structures. Those heterogeneous and anisotropy

structures could lead to the refraction, reflection,

attenuation and superposition of wave. Seismic-

ity positioning under complex geological setting

faces lots of challenges and need much work.

Waveform processing is also a promised direc-

tion. it is necessary to acquire the effective wave

characters to evaluate the fractures such as by

using waveform separation, spectrum analysis

and amplitude information. The ultimate pur-

pose of acoustic emission technology is not only

evaluating the distribution of fractures, but also

evaluating the effective fractures that is a real

path for fluid flowing. Other geophysical and

chemical monitor methods should also be

researched in this process.

(3) Though acid treatment and gel proppant can

effectively increase connectivity between wells,

its consequences may be negative if inappropri-

ate. It is hard to ensure the relatively same

enlargement of fracture aperture when stimula-

tion. Excessive stimulation especially with

chemical of gel proppant with high wellhead

pressure possibly results in partial extension of

fracture aperture which may cause a critical

issue—fluid short circuit. New stimulation

methods including chemic should be

strengthened.

(4) Water loss is another issue in EGS practice. For

a specific reservoir volume, the relationship of

groundwater recharge, runoff and drainage

determines the water loss mass, in which the

connection of fractures in reservoir and that out

of reservoir is important. This puts forward

higher requirements for the investigation of

geology setting before drilling and stimulation

as well as water plugging technology if massive

water loss occurs. Of course, other methods to

solve this issue is also welcomed.

Figure 2 shows an EGS flow chart concerning the

knowledge and lessons from the real EGS practices. A

site selection should be first conducted with sufficient

geological survey before design (e.g. Wan et al. 2005).

The geological survey should include engineering

geology, hydrogeology, tectonic geology. Under-

standing the geological setting is the prerequisite.

Stratum, fault and fissure condition are important

because they relate well connection, water loss and

fluid short circuit. The thermal properties of stratum

are also needed such as thermal conductivity, thermal

diffusivity and heat capacity. Foundation thermal

parameters are important such as thermal gradient and

heat flux which are useful to evaluate heat distribution

of underground. It is better to obtain the geo-stress of

reservoir which has significant impact on induced

fracture orientation. After necessary information has

been got, a total evaluation should be made consid-

ering technology and economic analyses. Then, a

rigorous scheme can be designed, and drilling

followed.

Fracturing and circulation are the key processes of

EGS. Figure 2 also show a recommended step of

drilling and fracturing. In practice, those steps are not

restricted, but adjustable according to field monitored

data. For example, if impendence is high after

fracturing, refracturing even redesign should be con-

ducted. Therefore, multi fracturing or circulation

experiment may be made in terms of the evaluation

of reservoir quality.

All the processes are finished and acceptant

production temperature is made without high draw-

down, sustainable geothermal energy can be extracted.

3 Continuous research on EGS

3.1 Analytical and numerical model

Water is a common fluid in circulation process due to

its easy availability. Physical and thermal properties of

water are variables of pressure and temperature. The

density and viscosity of water can be written as the

variable of temperature (Holzbecher 1998):

123


Yes

Site selection

Design

Feasibility analysis

No

Drilling

Circulation

Fracturing Drilling

Connection

Yes

No

Multi process

Evaluation ProductionYesNo

Depth, Diameter, Layout, Fracturing, …

Geological setting, Thermo properties, Economic analysis, …

=?AE Fracture

Injection well Production well

Fracturing

Geo-survey+

Fig. 2 Flow chart of EGS

qW ¼1000� 1� ðt � 3:98Þ

2

503570� t þ 283t þ 67:26

!; 0 �C� t� 20 �C

996:9� ð1� 3:17� 10�4 � ðt � 25Þ � 2:56� 10�6 � ðt � 25Þ2Þ; 20 �C� t� 250 �C1758:4þ 10�3tð�4:8434� 10�3 þ tð1:0917� 10�5 � t � 9:8467� 10�9ÞÞ; 250 �C� t� 300 �C

8>>><>>>:

ð1Þ

lW ¼10�3 � ð1þ 0:015512� ðt � 20ÞÞ�1:572; 0 �C� t� 100 �C0:2414� 10ð247:8=ðtþ133:15ÞÞ; 100 �C� t� 300 �C

�ð2Þ

123


The effect of pressure on the heat capacity and

thermal conductivity of water is little when no phase

transition occurs, so the effect of pressure can be

ignored, and the heat capacity and thermal conductiv-

ity under different temperature is calculated by the

following empirical equation (Cui et al. 2017):

cW ¼ 4:1712� 5:8� 10�4t þ 10�5t2 � 5� 10�8t3

ð3Þ

kW ¼ 5087:8þ 138:2t � 0:864t2 þ 0:001728t3 ð4Þ

The most advantages of analytical model are low-

investment, fast calculation and convenience for

substantial parameter analyses. Adoption of analytical

model in deep geothermal exploitation mainly has two

aspects.

One aspect is about the change of permeability or

reservoir during the process of flow and heat exchange

between cool fluid and hot rock matrix. Changes in

fracture aperture is the base of permeability alter.

Deformation would occur in rock matrix considering

thermal stress, according to which a one-dimensional

analytical solution of displacement with temperature

can be derived (Jaeger et al. 2009). A semi-analytical

correlation was employed to analyze the change of

fracture aperture under thermal and mechanical cou-

pling (Wang et al. 2016). Due to thermal stress (shrink

stress when cooling), fracture aperture would enlarge

(Fig. 3). The poroelastic theory was often employed to

calculate reservoir rock deformation with the action of

fluid flow assuming reservoir rock as a porous medium

(Zimmerman et al. 1986). Through this theory,

injection pressure tends to increase at initial stage

due the poroelastic stress, then it decreases when rock

temperature decreases and fracture aperture increases

(Ghassemi and Tao 2016).

The relationship between permeability and effec-

tive stress has been studied and analyzed for fluid

flowing in a single fracture. Permeability is known to

be a function of effective stress re:

k ¼ kðreÞ ð5Þ

where re ¼ r� bP, r is the confining pressure orstress, P is the fluid pressure. b determines the relativeefficiencies of confining stress versus fluid pressure in

changing permeability. Experiment demonstrated that

b can be substantially greater than 1 or less than 1(Zoback and Byerlee 1975; Warpinski and Teufel

1992). Generally, b equals to 1.Gangi (1978) proposed that fracture aperture and

permeability should follow a relation of the forms:

W ¼ W0 1� ðreEfÞ1=n

� �ð6Þ

k ¼ k1½1� ðcreÞm�3 ð7Þ

where c is a material constant, andm is around 0.25,Wis fracture aperture, W0 is the maximum aperture

possible (corresponding to zero effective stress), Ef is

the effective modulus of the fracture asperities, and

n is a parameter related to the distribution of nail

heights.

Fig. 3 Change of fractureaperture increase vs. time

due to thermal–mechanical

coupling after cold fluid

action (Wang et al. 2016).

Young’s modulus = 66

GPa; Poission’s

ratio = 0.25; thermal

expansion

coefficient = 7.9 9 10-6 m/

(mK); heat

capacity = 1000 J/(kgK)

123


Walsh (1981) proposed that permeability should

follow a relation of the form:

k ¼ k0 1� i lnrer

h i3ð8Þ

where i is a dimensionless material constant.

A simple inverse-power relation between perme-

ability and effective stress was proposed by Nathenson

(1999):

k ¼ k0are

ð9Þ

where k0 is the initial permeability when fluid pressure

equal to zero where ar = 1. This equation is valid atlow effective pressure but less sensitive at high

effective pressure.

The second aspect is the analysis of production

temperature. The analytical solution for the normal-

ized fracture water temperature was proposed by

Lauwerier (1955).

In an infinite rectangular fracture:

t � t0tin � t0

¼ erfc kRxqWcWq

þW4

� � ffiffiffiffiffiffiffiffiffiffiqRcRkRs

r� �ð10Þ

In an infinite radial fracture:

t � t0tin � t0

¼ erfc pkRx2

qWcWQinþW

4

� � ffiffiffiffiffiffiffiffiffiffiqRcRkRs

r� �ð11Þ

where x is the perpendicular distance from the interest

point to the injection well, q is the unit-width

discharge, s is time, t0 is initial temperature, and tinis injection temperature.

A one-dimensional analytical solution of rock

temperature with temperature difference or thermal

diffusivity was given by Jaeger et al. (2009), while a

three-dimensional analytical solution of the tempera-

ture in fractures of a semi-infinite single fracture under

a specific condition by Barends (2010). Wu et al.

(2016) used the exact results of the semi-analytical

method as the benchmark of numerical analysis and

predicted the variation of output temperature with

mining time under different numbers of fracture and

spacing. Under the condition of appropriate fracture

spacing, the increase of number of fractures can

effectively increase the fluid–solid heat transfer area,

thereby increasing the output temperature. When the

fracture spacing decreases, especially when the frac-

ture spacing is too small. The output temperature of

single fracture model is equal to that of single fracture

model, providing no advantage of the multiple fracture

model. A temporal semi-analytical method is used by

Liu and Xiang (2019) considering finite-scale frac-

tures and three-dimensional conduction in the rock

matrix, which showed a coincident result with that of

Wu et al. (2016). Besides, increasing number of

fractures is more efficient than that of increasing

fracture spacing.

With the development of computer science, numer-

ical simulation developed rapidly. Compared with

theoretical analysis, it can simulate complex boundary

conditions. Compared with physical experiment, it has

less investment and shorter period, and can carry out

large-scale parametric design. At present, the numer-

ical calculation methods of rock mechanics can be

divided into two categories: continuum-based

mechanics and discontinuous-based mechanics.

Among them, the finite element method is the

representative of the continuum mechanics method.

Finite element method is widely used in deep

geothermal mining research, which mainly involves

rock mechanics, fluid mechanics, heat conduction

(Hadgu et al. 2016), permeability (Durlofsky 1991).

The main types of numerical software are FRACTure

(Kohl and Hopkirk 1995), ANSYS/CFX (Xu et al.

2015a), TOUGH2 (Pruess et al. 1999) and COMSOL.

Discrete method is also used in deep geothermal

mining, the most common one is Itasca’s PFCD.

Thermal reservoir is a complex network with multi-

fractures. Fracture network provides a channel for

fluid migration, which is essential for further stimu-

lation and fluid flow studies (Tran and Rahman 2007).

The numerical calculation of complex fractures is

particularly important. However, the existing com-

mercial software has some deficiencies in the ability to

simulate the real three-dimensional complex fracture

network of rocks. In order to solve this limitation,

Chen et al. (2018a) established a three-dimensional

complex fracture network using pipeline model to

simulate the heat exchange between reservoir rock and

fluid. The numerical analysis method plays an impor-

tant role in the research and exploration of deep

geothermal mining. Previous studies have made

fruitful contributions to multi-field coupled deep

geothermal research. In the study of EGS, fracture

network provides the main space for fluid heating and

is the main research content of numerical simulation.

Fluid flow simulation in reservoir can be divided into

three categories: porous mediummodel (e.g. Luo et al.

123


2014; Salimzadeh et al. 2018), commonly used

fracture model (Xu et al. 2015a; Pandey et al. 2017)

and the newly developed pipe-base model (Chen et al.

2018a, b, c). Fracture opening is an important

parameter affecting fluid flow rate and heat exchange.

It is generally used in literatures in the range of

0.1–1 mm (e.g. Wang et al. 2016). In fractured model,

fractures with single aperture were usually used, but

some complex fracture network with discrete distri-

bution apertures are also used shown in Fig. 4a.

Production temperature is one of the most impor-

tant parameters of deep geothermal mining, which

determines the success or failure of a project.

Gholizadeh Doonechaly et al. (2016) introduced the

shear displacement model to study the variation of the

production temperature showing that the production

temperature decreases with time, and it is lower than

that without thermal stress considering thermal stress

(about 5 degrees). The variation of production tem-

perature with mining time was studied by using a

three-dimensional fracture model under the condition

of thermal–hydraulic-mechanical coupling (Pandey

et al. 2017; Zhao et al. 2015a).

3.2 Laboratory experiment

Compared with demonstration project, laboratory

experiment has less investment, which plays an

important role in breakthrough of key technologies

and in understanding mechanism in deep geothermal

(a)

(b)

Fig. 4 Fracture distributionused in a simulation andtested by b laboratoryexperiment

123


exploitation. Laboratory experimental studies on deep

geothermal mining mainly focus on fracture network

establishing, fracture permeability evolution, wellbore

or fracture stability and fluid–solid heat exchange.

HDR properties are essential for underground reser-

voir characterization. A valuable data was tested on

granodiorite rock cores obtained from 4.2-km-deep

geothermal well at the Pohang Enhanced Geothermal

System site in Korea such as X-ray CT images,

deformation and strength properties and thermal

properties (Kwon et al. 2018).

Establishing good hydraulic connections in reser-

voirs is one of the core technologies of EGS. Good

hydraulic connections can not only provide paths for

fluid flow but also fluid–solid heat transfer. Hydraulic

fracturing is the most commonly used method to

establish hydraulic connection in EGS. This technol-

ogy originated in the field of oil and natural gas. Its

principle is to use fracturing equipment (mainly high-

pressure water pump) to squeeze high-viscosity fluid

into the stratum to generate or extend cracks. In this

process, hard proppant particles are usually used to

prevent crack closure (see for e.g. Zhang et al. 2015).

Frash et al. (2014) developed a true triaxial device,

which can simulate the process of drilling and

hydraulic fracturing in EGS, and can also carry out

acoustic emission testing and strain testing. Xu et al.

(2015b) carried out hydrofracturing experiments, and

simulated the occurrence, propagation and develop-

ment of rock fractures under different temperatures

and confining pressures. The mechanism of crack

initiation and propagation pressure by hydrofracturing

in rock mass at different temperatures under triaxial

stress state was systematically analyzed (Zhou 2017).

Increasing rock temperature can efficiently reduce

breakdown pressure in hydraulic process, while high

confining pressure is disadvantageous (Zhang et al.

2019). Besides, the original fracture in reservoir has

significant influence on hydraulic fracture. Outcomes

from the interaction of a hydraulic fracture (HF) with a

natural fracture (NF) are a long-standing research

direction and can be divided into four types consid-

ering two dimensions: (1) the HF crosses the NF, (2)

the HF is diverted into the NF, (3) the HF is blunted at

the NF and the HF diverts into the NF and then

reinitiates leaving an offset in its path (details see for

Thiercelin et al. 1987). The type of interaction will

affect the hydraulic fracture growth past the junction

and upstream of that junction because of the difference

in pressure (Jeffrey et al. 2017).

Substantially using the data of acoustic emission or

microearthquake during stimulation is an efficient

method to describe the distribution of factures (Fang

et al. 2018; Lu and Ghassemi 2019; Xing et al. 2019).

Acoustic emission would occur with rock breaking,

which was monitored when supercritical carbon

dioxide was injected into 8 m deep granite boreholes

by hydrofracturing. Supercritical carbon dioxide

migrated more easily in primary fractures and

enhanced the generation of acoustic emission signals

(Ishida et al. 2017). Of course, some other methods

were used to evaluate the fractures such as resistivity

(Li et al. 2015). In addition, some scholars have also

studied chemical method to assist in establishing

fracture networks. A mixture of hydrochloric acid and

hydrofluoric acid was used to percolate through

granite fractures, showing that the permeability of

rocks was significantly improved (Luo et al. 2018). A

high concentration sodium chloride solution on the

fracture of feldspar sandstone can dissolve some

minerals, which has a strong effect on the fracture

surface morphology (Schmidt et al. 2017).

Fracture aperture is important for evaluating per-

meability, whose distribution inhomogeneity was

shown by experiment result (Fig. 4b). Fracture per-

meability is not a simple function with effective stress.

Indeed, it can be influenced by temperature, pressure,

roughness (Huang et al. 2019) cyclic stress, mineral

dissolution and so on. Increasing temperature or

effective stress can reduce fracture aperture and

permeability in rock (Kamali-Asl et al. 2018; Shu

et al. 2019). Caulk et al. (2016) experimentally tested

the effect of fluid flow on the fracture permeability

(opening) of granite under near-in situ state. The

actions of dissolution of propping asperities and

subsequent rock creep under effective (confining)

pressure will lead to the decrease of permeability

(fracture aperture) (Fig. 5a). For a cylinder rock

sample with splitting fracture in principle direction,

increasing effective (confining) pressure or pressure

cycles can reduce permeability (Fig. 5b). Temperature

may also change rock permeability. The drastic

increase of permeability of granite occurs when the

temperature rises to about 630 K (Fig. 5c). Besides,

chemical reactions with rock minerals can increase

permeability (Fig. 5d).

123


In the aspect of wellbore or fracture stability, Zhao

et al. (2015) designed and carried out triaxial high-

temperature and high-pressure tests of granite samples

to simulate the stability of borehole wall in deep

geothermal mining, which found that the deformation

of the samples obeyed the generalized Kelvin model

under hydrostatic pressure of less than 100 MPa and

temperature of 400 �C. Zhou et al. (2018) found thatthe cooling effect of fluids can cause thermal shock

and tensile stress in rocks. After drilling, borehole

breakout may occur. Zheng et al. (1989) analyzed the

breakout shape. Borehole breakouts propagate into the

rock until they reach a stable state. The key for the

stabilization of a breakout is that the ever-sharpening

breakout point functions as a stress concentrator. The

stresses around the original borehole boundary are

greatly reduced and the rock in these low-stress

regions becomes stable. Their results showed that

moderate breakouts will not significantly change the

results of hydraulic fracturing measurements. Lee

et al. (2012) developed a now model in which the

anisotropic rock strength characteristic is incorpo-

rated. Both bedding plane and in situ stress field have

significant effects on the extent of failure region

around the wellbore and the safe mud weights. A two-

dimensional discrete element model was used to better

understand the mechanisms controlling the initiation,

propagation, and ultimate pattern of borehole break-

outs in shale formation when drilled parallel with and

perpendicular to beddings (Duan and Kwok 2016).

Borehole breakout was influenced by particle size

distribution, borehole diameter and rock anisotropy.

Far-field stress anisotropy plays a dominant role in the

shape of borehole breakout when drilled perpendicular

to beddings.

(a) (b)

(d)(c)

Fig. 5 Evolution of fracture permeability with a time under confining pressure, b effective pressure, c temperature and d chemicalreaction time

123


In the aspect of fluid–solid heat exchange, the heat

transfer between fluid and solid in single-fracture

granite under different temperatures and pressures was

studied by Bai et al. (2017). They proposed the

equation of total heat exchange coefficient compared

with other equations, effectively and accurately pre-

dicting the results of fluid–solid heat transfer in single-

fracture granite. Jiang et al. (2017) experimentally

studied the convective heat transfer behavior of

supercritical carbon dioxide in rock fissures.

The effect of supercritical carbon dioxide on

minerals in granite under high temperature and

pressure was also experimentally studied. Na, Si, K,

Ca, Mg, Fe and Al could dissolve into supercritical

carbon dioxide at the rates of 4.5, 2.7, 1.6, 0.5, 0.3, 0.2

and 0.1 ppm/day, respectively (Zhou et al. 2016).

Carbon dioxide could be absorbed by kaolinite in

sandstone, resulting in a decrease in rock stiffness

(Delle Piane and Sarout 2016). Salt precipitation

caused by pore water evaporation and chemical

reaction of supercritical carbon dioxide with rock

have destructive effects on reservoir rock properties.

The existence of pore water can lead to a 27% decrease

in heat extracted (Zhang et al. 2016). Lebedev et al.

(2014) measured the P-wave and S-wave velocities of

salt-saturated sandstone injected with supercritical

carbon dioxide. P-wave velocities decreased by 3.5%

after injecting supercritical carbon dioxide. The

P-wave velocities can be used to estimate the carbon

dioxide saturation in rocks. Suekane et al. (2009)

measured the distribution of supercritical carbon

dioxide injected into porous rocks by using magnetic

resonance imaging (MRI). About 7% of the compres-

sion wave vibration occurred during the process of

supercritical carbon dioxide injected into sandstone by

monitoring the wave velocity response characteristics

(Lebedev et al. 2013).

3.3 Induced earthquake

Both hydraulic stimulation and circulation experiment

would induce microseism (Sasaki 1998; Bommer et al.

2006) which is shown in Tables 1 and 2. Microseism

must occur when crack generation and expansion, but

its intensity is generally weak. There are many debates

on that if deep geotherm could induce sufficient

earthquake. To date, all the seism is not enough to

cause destructive earthquake. Big earthquake can not

be excluded due to pore pressure increase in fault

whose mechanism is shown by Kang et al. (2019). In a

zone with a fault, a fault is stable if shear stress is less

than the strength of the contact. The failure condition

to initiate rupture is usually expressed in terms of the

effective stress:

scrit ¼ lðrn�PÞþs0 ð12Þ

where scrit is critical shear stress, l is the coefficient offriction, rn is normal stress, P is the pore pressure, s0 isthe cohesive strength of the sliding surface.

In hydraulic stimulation or circulation process, pore

pressure would increase duce to injection, thus

resulting in the decrease in critical shear stress. As a

result, induced earthquakes occur. Ellsworth (2013)

reviewed the seismic activity that may be associated

with industrial activity, with a focus on the disposal of

wastewater by injection in deep wells such as the cases

of Rocky Mountain Arsenal, Rangely and Paradox

Valley. The moment magnitude (Mw) 5.5 earthquake

struck South Korea in November 2017. An enhanced

geothermal system site, where high-pressure hydraulic

injection had been performed during the previous

2 years, was considered to raises the possibility of this

earthquake (Grigoli et al. 2018). Though the effective-

stress model [Eq. (12)] provides straightforward guid-

ance for avoiding induced earthquakes, it is difficult to

know initial stress and pore-pressure conditions and

how perturbations to those conditions due to injection.

4 New thoughts for deep geothermal energy

Despite of lots of work on EGS, some researchers

proposed novel models or methods for exploiting deep

geothermal energy, which can be generally divided

into three categories. Table 3 lists the fundamental

geological and operation parameters used in new deep

geothermal exploitation researches.

4.1 New thoughts based on EGS

Figure 6 shows the EGS researches with novel

conceptual models and production temperature. Well-

bore layout has substantial impact on economic

investment of EGS. A long-term heat extraction

process of EGSs was numerical simulated by Chen

and Jiang (2015), which includes a standard double-

well layout, two triplet-well layouts and a quintuplet-

well layout. Results showed that increasing production

123


Table

1Inform

ationofthereal

EGSproject

inhydraulicstim

ulationprocess

Project

Site

Year

Stimulation

method

No.of

wellbore

Fracturing

depth

(m)

Flow

rate

(kg/

s)

Injection

volume

(m3)

Seism

ic

volume

(m3)

Wellhead

pressure

(MPa)

References

TheLosAlamosHDR

Geothermal

Energy

Project

TheJemez

Mountains,

FentonHill,US

1974

Water

GT-2

2932

Brown

(1995,2009)

1975

Water

EE-1

3064

19

107

1982–1984

Water

EE-2

Multi-

depth

3.3

9108

TheRosemanowes

HDR

project

Rosemanowes

Quarry,

Cornwall,UK

1977–1980

Water

300

Pearson(1980),

Ziagoset

al.

(2010)

1980

Water

RH12

2100

100

30,000

14

1983

Water,gel

RH15

2600

theSoultzproject

Soultz,

Alsace,

France

1991

Water

GPK1

1402–2002

10,000

Genteret

al.

(2010),Xuet

al.

(2016)

1992

Water

GPK1

2850–3590

1995

Water

GPK2

3211–3876

10

1996

Water

GPK2

58,000

2000

Water

GPK2

5000

23,400

1.1

9109

14.5

2001

Water

GPK3

5093

2003

Water,acid

GPK4

5105

TheHijioriProject

1985–1987

Water

SKG-2

1802

2000

Yam

aguchiet

al.

(2000)

1988

Water

HDR-1

2150–2200

2115

TheOgachiHDRProject

AkitaPrefecture,

northeast

Japan

1991

OGC-1

990

640–710

10,000

59

105

18–18.5

Kitanoet

al.

(2000)

1992

OGC-1

711

500–700

5500

39

105

22–23.5

1994

OCG-2

1100

750

3000

89

104

13

1995

OGC-1

1000

1500

4000

29

104

16

1995

OCG-2

1100

2200

4000

89

104

18

TheGeodynam

icsEGS

Project

Innam

incka,

South

Australia

2003

Water

Habanero-

1

4421

13.5–26

20,000

79

108

35–65

Wyborn

(2010)

2005

Water

Habanero-

1

4421

20,000

49

109

2005

Water

Habanero-

2

4358

2008

Water

Habanero-

3

4221

123


Table

2Inform

ationofthetypical

EGSproject

inmaincirculationexperim

ents

Project

Year

Depth

(km)

Reservoir

temp.(�C)

Injection/back

pressure

(MPa)

Injection/

productionflow

rate

(kg/s)

Water

loss

(kg/s)

Flow

impendence

(MPa/(L/s))

Production

temp.(�C)

Thermal

power

(MW)

References

TheLosAlamosHDR

Geothermal

Energy

Project

1980

*2.8

195

9.7/1.4

6.3/5.9

0.2

13

158

3–5

Brown(1995,2009)

1986

*3.5

235

26.9/3.3

6.3/5.9

0.15

19

190

4

TheRosemanowes

HDR

project

1980–1983

*0.2

79

14/-

\100/

*70%

Parker

(1999)

1985–1987

2.6

100

8–10

\25

[0.5

1987–1988

2.6

100

25.1

TheSoultzproject

1995–1996

*3.8

168

21

136

9Genteret

al.(2010),

Xuet

al.(2016)

1997

*3.8

168

25

010

2005

*5

202

15

0120,160

2008

*5

202

6–7

25

TheHijioriProject

1988

1800

253

70%

Yam

aguchiet

al.

(2000),Oikaw

a

etal.(2001)

1991

1800

253

22%

150–180

8.5

1995

2200

270

16.7–33.4

60%

180

2000

2200

270

9–13

16.7–20

*75%

163,172

8

2001

2200

270

8.1/1.29

15.8

*50%

179

TheOgachiHDR

Project

1993

1000

228

17–19

750–1200/12–30

98%

109

Kitanoet

al.2000

1994

1100

240

13–16

500–750/50–65

90%

160

1995

1100

240

7–9

500–700/

125–150

75%

170

1997

1100

240

13

400–75

85%

116

123


well has little influence on heat extraction perfor-

mance. The double-well layout is better than the

triplet-well layout and the quintuplet-well layout.

Therefore, decreasing number of injection and pro-

duction well should be first considered in EGS design.

The most important segment of EGS is the instal-

lation of fracture network which is the main heating

process when fluid flowing. Zeng et al. (2013a)

proposed a novel model which consists of an injection

well, a production well and a vertical fracture by

Table 3 Main researches of new thoughts including fundamental geological and operation parameters used in deep geothermalexploitation

References Rock parameter Fluid parameter Thermal

gradient (K/

km)

Fracture

aperture

(mm)

Reservoir

depth (m)

Injection

temperature

(�C)kR(W/

(m�K))

cR (J/

(kg�K))qR(kg/

m3)

Type cF (J/

(kg�K))qF (kg/m

3)

Yang and

Yeh

(2009)

2.6 1046.4 2650 H2O 4185.5 1000 3 –

Zeng et al.

(2013a)

2.395 1100 2850 H2O 0

(reservoir)

2 \ 1619 60

Zeng et al.

(2013b)

2.395 1100 2850 H2O 0

(reservoir)

0.05–2 1219–1619 60

Xu et al.

2014

2.51 920 2650 CO2 4.26 – 3800 20

Biagi et al.

(2015)

2.1 1000 2650 CO2 20

Chen and

Jiang

(2015)

2.1 1000 2650 H2O 4200 1000 4 – 4250 27

Jiang et al.

(2016)

3.9 700 2240 H2O 4182 998.2 5.75 – 4000 70

Cui et al.

(2017)

2.25 2112 H2O Equation (3) 2.3 – 4600 25

Xia et al.

(2017)

1.5, 3 790 2700 H2O 4818 Cooper and

Dooley

(2008)

6.5 3000

Huang

et al.

(2018)

2.5 1000 2650 CO2 Cao et al.

(2016)

Cao et al.

(2016)

3.5 – 4000 30

Levy et al.

(2018)

2.51 920 CO2 30

Song et al.

(2018)

2.8 1000 2700 H2O 4200 Holzbecher

(1998)

4 – 4500 * 5500 50

Sun et al.

(2018)

1.75 CO2 Change

with t and

p

Wang

et al.

(2018)

2.7 900 2700 CO2 4 40

Tang tea l.

(2019)

850 2600 H2O –

123


(a)

(b)

(c)

(d)

Fig. 6 The EGS researches with novel conceptual models and production temperature. a Conceptual model of Zeng et al. (2013a),b the horizontal well model (Zeng et al. 2013b), c conceptual model and d the multilateral-well EGS model (Song et al. 2018)

123


injecting water at the bottom of injection well. The

results showed that this model at Desert Peak

geothermal field can provide more than 420 K

production temperature at least 20 years. Besides,

Zeng et al. (2013b) also proposed a horizontal well

model to investigated the heat production potential at

Desert Peak geothermal field by simulation research.

This model consists of two horizontal wells and

fractures between them. Results showed that accep-

tance production, as well as water production rate,

water flow impedance and electricity production

power, can be obtained with proper conditions.

Certainly, increasing fractures properly can increase

production temperature. A pair of parallel injection

and production wells connected by a set of single large

wing fracture model was designed by Xia et al. (2017).

This model highlights the multi-paralleled wing

fracture to connect injection and production well.

Acceptant production temperature and temperature

drawdown had made.

A multilateral-well EGS model was proposed be

Song et al. (2018). This model mainly consists of a

main wellbore, two horizontal injection wells, two

horizontal production wells and three vertical stimu-

lated fractures connecting injection well and produc-

tion well. The cool water was injected from main

wellbore into the injection well. Water could be heated

in the manmade fractures and flow to production well.

Finally, hot water flows to surface through the main

wellbore which is thermal insolated. This model has

advantages of output thermal power, production

temperature, heat extraction ratio and accumulative

thermal energy than conventional double well EGS.

Shi et al. (2018) using this model to study the heat

extraction of carbon dioxide. The results indicate that

multilateral-well CO2-EGS has a greater heat extrac-

tion performance than conventional double-well CO2-

EGS. Due to the complexity of fracture network, it is

hard to simulate the fractured reservoir under real

conditions. Asai et al. (2019) reduced the simulation

times significantly by 1.5–14.5 times without com-

promising the accuracy of the results by proposing an

efficient workflow for simulation of multi-fractured

enhanced geothermal systems.

4.2 Pipe and other models

Some researchers also proposed other novel models to

exploit deep geothermal energy due to the lack of

commercialization of EGS. Figure 7 shows the pipe

models and production temperature. The model based

on pipe has drawn a lot of concentration. The main

difference of the pipe model and EGS is that the pipe

model does not need fracturing process, which

allowed the fluid circulation only in an artificially

pipe (wellbore). Compared with fracture, fluid in pipe

need more time to be heated, the pipe model therefore

needs longer heating path. Sometimes, only one

wellbore is needed in the pipe model such as the work

of Cui et al. (2017). A wellbore consisting of a vertical

segment and a horizontal segment was drilled first.

Then, a pipe with low thermal conductivity and

smaller diameter was set in the wellbore. Cold fluid

was injected from the gap between the wellbore and

pipe into heat reservoir and been heated. Hot fluid can

be recovered through the pipe. In order to substantially

extract the reservoir heat energy, a multi-well model

was proposed. This type of model consists injection

well, horizontal well and production well. Jiang et al.

(2016) proposed a multi-horizontal-well system and

an annular-well system (Fig. 7b). The multi-horizon-

tal-well system includes several injection wells and a

production well. And, the an annular-well system

includes several injection wells and production wells.

Each injection well, horizontal well and production

well compose a relatively individual subdomain. The

comparison of this model and the EGS model of

European EGS site at Grob Schonebekc, Germany was

conducted. Results showed a significant application

potential of the multi-well system. The production

temperature and pressure of eight horizontal wells

system with CO2 were respectively 38.9 K higher and

10.9 MPa higher than that of the fractured reservoir

system with CO2 after 20 years at a flow rate of 20 kg/

s, an injection temperature of 20 K and an injection

pressure of 10 MPa when the horizontal well length of

well pattern system was about 10 times to the fractured

reservoir.

Recently, a new EGS-E model based on excavation

technology is proposed by Tang et al. (2019). This

model includes (1) a super large shaft, (2) heat source

with large volume and high permeability due to crack

and fragmentation formation through drilling and

blasting and (3) heat storage with a large capacity and

high conductivity. Through numerical simulation, the

EGS-E may be another potential model to extract heat

from HDR.

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4.3 Supercritical carbon dioxide in deep

geothermal energy

Deep geothermal power generation, combined with

supercritical carbon dioxide thermal conductivity

technology and coal power generation (Mohan et al.

2013), forming a new deep geothermal development

technology of coal plus geothermal, has dual strategies

for mining deep geothermal energy and reduce carbon

dioxide. If the installed capacity of deep geothermal

power generation can reach one-third of the installed

capacity of thermal power generation, the supercritical

carbon dioxide needed for deep geothermal exploita-

tion can basically consume the carbon dioxide gener-

ated by thermal power generation (Pruess 2006).

Supercritical carbon dioxide deep geothermal

exploitation is a very complex project, which involves

many factors such as temperature field, mechanical

field, flow field, chemical field, etc. These physical

quantities have a great impact on fluid movement, heat

transfer, reservoir deformation and so on. In particular,

the dynamic interaction between supercritical carbon

dioxide and fractured rock is a complex process,

including physics, chemistry, mechanics, failure,

wave propagation, acoustic emission and so on. To

date, there is no effective numerical method and

software for rock mechanics to describe the process

perfectly. The thermal and physical properties of

supercritical carbon dioxide are variables of pressure

and temperature including density, viscosity, specific

heat capacity and thermal conductivity (details refer to

Span and Wagner 1996; Heidaryan et al. 2011;

Jarrahian and Heidaryan 2012; Cao et al. 2016b).

Deep geothermal mining is a project with a large

scale of time and space. The numerical analysis

overcomes the shortcomings of short physical test

cycle and small model size, and can simulate large-

scale models for decades or even longer, which is one

(a)

(b)

Fig. 7 The pipe models and production temperature. a the single well system (Cui et al. 2017), b the multi-horizontal-well system(Jiang et al. 2016)

123


of the most advantages of the numerical simulation.

For example, Pruess’s (2006) numerical analysis on

the Soultz demonstration project showed that the

thermal extraction ratio using supercritical carbon

dioxide as a fluid is 50% higher than that using water

as a fluid. Pan et al. (2016) used TOUGH2 software to

simulate geothermal extraction ratio of three repre-

sentative thermal reservoirs (Acoculco, Purundiro,

Agua Caliente Comond). The results showed that the

thermal extraction ratio of supercritical carbon dioxide

is 1.6 times than that of water. The results of Cao et al.

(2016b) showed that the power generated by EGS is

positively correlated with the density and specific heat

of supercritical carbon dioxide, and negatively corre-

lated with the viscosity, and less correlated with the

thermal conductivity. Pruess (2008) studied the influ-

ence of gravity and density of supercritical carbon

dioxide on thermal extraction rate. The results showed

that supercritical carbon dioxide with low temperature

and high density is easy to flow in the lower part of

reservoir under gravity, which leads to premature

thermal breakthrough in the lower part of the reser-

voir, leading to excessive temperature reduction in

production wells. Sun et al. (2018) studied the

geothermal extraction rate of supercritical carbon

dioxide flowing in horizontal wells. The results

showed that increasing injection flow rate or decreas-

ing injection temperature can improve the geothermal

extraction rate. Wang et al. (2018) studied supercrit-

ical carbon dioxide geothermal exploitation based on

the change of permeability of surrounding rock. The

results showed that the increase of permeability of

surrounding rock resulted in a small decrease in

thermal extraction rate and a significant increase in

supercritical carbon dioxide storage capacity. Jiang

et al. (2017) experimentally studied the convective

heat transfer of carbon dioxide at supercritical

pressure in a horizontal rock fracture. These literatures

reveal a potential of supercritical carbon dioxide, but

some knowledge should be clear for evaluation. For

example, the mineralogical structure of Harcourt

granite rock specimens was mainly changed by the

dissolution of silicate minerals and plagioclase phase

feldspar minerals (Isaka et al. 2019).

Besides supercritical carbon dioxide, Olasolo et al.

(2018) evaluated the use of nitrous oxide in enhanced

geothermal system which concluded that single

supercritical phase nitrous oxide seems to be an

alternative to the two working fluids (water and carbon

dioxide) used to date.

5 Discussion

Though substantial improvement has been made in

deep geothermal energy exploitation, lots of efforts

need to be made. The key purpose of deep geothermal

energy exploitation is to extract acceptant heat energy

for utilization such as supply heating and electric

generation. Therefore, the technologies relating to

deep underground engineering have got a lot of focus

and are reviewed in this paper. Now, the technologies

were grasped e.g. deep and directed drilling at high

temperature and pressure in hard HDR, kilometer-

scale hydraulic stimulation, long-term circulation and

location technology of rock failure. These technolo-

gies could support in establishing an industrial grade

geothermal factory for the utilization of deep geother-

mal energy. Nevertheless, critical problems are still to

be solved, of which two aspects should be concen-

trated throughout fracturing and circulation process.

Figure 8 shows the rock mechanics and rock engi-

neering tasks in deep geothermal energy exploitation.

Fig. 8 Rock mechanics androck engineering tasks in

deep geothermal energy

exploitation

123


In the circle the main tasks were listed, while the

relative research targets were listed out the circle.

5.1 Properties of HDR including physical,

thermal, chemical and mechanical parameters

Physical and thermal parameters are the foundation of

thermal exchange between rock and fluid as well rock

interior. Three fundamental thermal parameters –

thermal conductivity, thermal diffusivity and heat

capacity – as well as other parameters of HDR such as

density determine the heat conduction rate in HDR.

Rock physical and thermal parameters also are the

base of digital model for evaluation such as the

temperature–pressure dependence of these parame-

ters. Strengthening the research of these fundamental

parameters especially at different conditions of pres-

sure and temperature contributes to accurate evalua-

tion of critical parameters in deep geothermal

exploitation process such as production temperature

and flow rate design.

Chemical properties of HDR is critical due to their

relevance to rock weathering which mainly influences

on fluid flow path and the corrosion of machine and

casing. In the processes of fracturing and circulation,

some chemical solution may be added to the fractured

reservoir to reduce impendence between injection and

production well. The choice of chemical reagents,

concentration, of solution, technological process and

time control are variables of rock weathering extent.

The characteristic of chemical stimulation on large

scale rock fractures is also unknow. Besides, unsuit-

able chemical stimulation might cause fluid short

circuit.

Rock mechanical properties relate to deformation,

damage, crack initiation, growth, propagation, frac-

ture wall or wellbore stability. These properties are

much importance because they, as well as geological

settings, influence the quality of fracture network. The

main problems– fluid short circuit, high impedance

and water loss – easily occur in deep geothermal

engineering, which are considered closely relating to

rock mechanical properties. The evaluation of funda-

mental mechanical properties of rock under hydro-

thermo-mechanical-chemical coupling still need

strengthen. Rock cracking should be especially

focused. One aspect is the hydraulic stimulation. The

hydraulic stimulation in geotherm is different with

those in oil and gas field. The main purpose of

hydraulic stimulation in geotherm is to establish

sufficient fracture network which should have these

characters: enough space scale and number for extract

volume reservoir energy, enough fracture aperture for

smooth flow of fluid, good connection for low

impedance flow, good sealing property to reduce

water loss, without large aperture path to avoid fluid

short circuit. Obviously, the current stimulated frac-

ture network is far away from these ideal character-

istics. Another aspect is rock cracking in operation

phase. For example, an EGS project usually undergoes

multiple stimulations and circulations with various

pressures which cause the cyclic stress on rock by fluid

pressure. the effects of non-uniform in situ stresses and

loading history on rock fracturing are not well

understood (Tomac and Sauter 2018). Fatigue damage

of rock especially in the aspects of subcritical and

critical cracking is importance to deep geothermal

exploitation, while less literature reported it. Study on

the morphology and evolution of fracture should be

enhanced under real geological settings especially on

the nonuniformity of weathering.

Permeability is a critical parameter in fractured

reservoir, whose evolution should be focused in

fracturing and circulation phase. Rock permeability

enhancement still faces many challenges (Tomac and

Sauter 2018). A reality must be kept in mind that in

reservoir fracture aperture is inhomogeneous which

may cause flow differences in different zone (Zhang

et al. 2017). In multi-field coupling (HTMC) condi-

tion, fracture evolution is also inhomogeneous which

need to further research. Scale effect on application of

laboratory results to real practice should be

considered.

5.2 Stimulation methods

Tradition stimulation of HDRwere developed from oil

and gas field. There is a big difference between HDR

exploitation and gas extraction. In gas field, natural

gas can transmit if fracture occurs due to its internal

pressure, while fluid flows in fractures in heat reservoir

must be driven by wellhead pressure. In some fractures

with low aperture, fluid flow may be slow, which may

cause less heat extraction in these areas. Stimulation

combined other technologies such as chemical reagent

or gel forming an appropriate technology for deep

geothermal exploitation are needed. However, partial

fractures with large aperture may cause fluid short

123


circuit. Efficient porous plugging material and con-

struction technique may be a good solution.

5.3 New exploiting model

Exploiting deep geothermal method may not be only

one model. New and applicable methods would be

welcomed. Fortunately, several new methods have

been proposed such as the revised EGS pipe model.

Other new thoughts like the method combined with

mining was also proposed. These methods were only

recommended without real practice examination.

They must face many potential problems. Much works

will be needed in these methods.

5.4 Model establishing in simulation

Numerical simulation is an important tool in research-

ing deep geothermal energy exploitation. Applicable

and useful methods have been developed in recent

years and made valuable contributions. Nevertheless,

there is a great gap in modeling a real-like fractured

reservoir that limits the application and accuracy of

simulation results. A lot of work should be made to

establish a model that can realistically represent the

characteristics of reservoir such as permeability

according to field monitored data.

5.5 Highly efficient drilling

No matter what methods, drilling to deep reservoir

can’t be avoid. Highly efficient drilling with proper

economic cost is a necessary precondition to acceler-

ate the step of deep geothermal exploitation. Mechan-

ical drilling combined with laser or micro wave is a

potential technology.

5.6 Monitor technology

Using geophysical exploration such as active or

passive acoustic emission describes the effective

reservoir fracture distribution is a difficult work, but

we must go forward. Only in this way can we ascertain

the real reservoir condition to correctly guide further

step.

6 Conclusions

This paper reviews the history and research status of

deep geothermal energy exploration e.g. EGS and new

thoughts in a view of rock mechanics and rock

engineering. Some conclusions can be made. EGS has

a dominant position in deep geothermal energy

exploitation. The biggest issue of EGS is to establish

an applicable fractured reservoir and keep it in fine.

Current technologies such as hydraulic stimulation

have the ability to form an artificial fractured network

in HDR but with some deficiencies. And, reservoir

evolution is uncontrollable and not well understood.

Researches relating to rock mechanics and rock

engineering should be strengthened, of which funda-

mental rock properties, controllable stimulation tech-

nology, high-efficient drilling and fracture evolution

should be especially strengthened. The effects of non-

uniform in situ stresses and loading history on rock

fracturing are not well understood especially the effect

of fluid at variable temperatures and pressures.

Mechanism and prediction of fracturing in complex

geological settings were not well revealed. Knowledge

of long-term interaction between rock and fluid and its

alteration function on fractures is deficient.

Furthermore, mode of deep geothermal energy

exploration is not only one. Researches and problem

prediction of new thoughts can be further conducted.

Besides, monitor technologies have a great potential

improvement. With the improvements of these tech-

nologies, we can better utilize the deep geothermal

energy in the future.

Acknowledgements This research is financially supported bythe Natural Science Foundation of Tianjin, China (Grant No.

19JCZDJC39400).

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A global review of deep geothermal energy exploration: from a ......2.2 Rosemanowes The Rosemanowes HDR project (1977–1991, UK) by the Camborne School of Mines with the purposes