Studies on Geothermal Power Generation Using Abandoned Oil Wells

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Energy xxx (2013) 1e7

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Energy

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Studies on geothermal power generation using abandoned oil wells

Wen-Long Cheng*, Tong-Tong Li, Yong-Le Nian, Chang-Long WangDepartment of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei, Anhui 230027, China

a r t i c l e i n f o

Article history:Received 5 December 2012Received in revised form25 June 2013Accepted 2 July 2013Available online xxx

Keywords:Geothermal power generationAbandoned oil wellsFormation heat transferIsobutane

* Corresponding author. Tel./fax: þ86 551 6360030E-mail address: [email protected] (W.-L. Chen

0360-5442/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.energy.2013.07.008

Please cite this article in press as: Cheng W-dx.doi.org/10.1016/j.energy.2013.07.008

a b s t r a c t

In this paper, abandoned “dry hole” oil wells are simulated as a source of geothermal power forgenerating electricity. Isobutane is chosen as the working fluid. It is heated by the geothermal energyfrom geologic formation and drives the turbine to generate electricity. A model based on transient for-mation heat transfer is presented. Furthermore, a numerical simulation for an abandoned well with adepth of 6000 m is performed. The result shows that the outlet temperature of fluid leaving the recoverywell gradually decreases with the system operating time increasing and ultimately approaches stability.The stabilized time of the system can be shortened by either the increasing formation thermal con-ductivity or the decreasing inlet velocity of fluid entering the injection well but is not significantlyinfluenced by the formation heat capacity. The net power depends on the total obtained heat and theoutlet temperature of fluid leaving the recovery well. And increasing the inlet velocity can increase thetotal obtained heat but lower the fluid outlet temperature. Consequently there is an optimal inlet velocityof fluid entering the injection well to maximize the net power for a specified geothermal heat source.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The current development of human civilization has been madepossible by the extensive use of fossil fuels. However, fossil fuels arebecoming increasingly scarce and burning fossil fuels generatescarbon dioxide, which is a greenhouse gas that contributes towarming of the atmosphere. Geothermal energy is abundant andclean, i.e., it emits no greenhouse gases. Several American re-searchers [1] did the Los Alamos experiment in New Mexico andexploited the geothermal energy in hot dry rock to generate elec-tricity in the 1970s. Then Japan [2] and several European countries[3] carried out the experimental study on the hot dry rock in the1980s. However the total cost of geothermal power plants isincreased significantly and the wide application is restricted by thehuge cost of drilling [4]. Meanwhile there are many abandoned oilwells all round the world. These oil wells are dry holes that neverstruck oil but have a large amount of heat energy. Therefore thedrilling costs can be saved and safety problems of abandoned oilwells will be solved through transforming those abandoned wellsinto geothermal wells. And it may become a new way to utilizegeothermal energy.

A number of scholars began to study obtaining geothermal en-ergy from abandoned oil wells. Kujawa et al. [5] proposed a double-

5.g).

All rights reserved.

L, et al., Studies on geotherma

pipe heat exchanger for the acquisition of geothermal energy usingwater as the working fluid and established a heat transfer modelbetween the heat exchanger and the formation. The influences ofdifferent insulation materials and mass flow on the performance ofdouble-pipe heat exchanger were analyzed. Bu et al. [6] suggestedthat water was utilized as the cycle working fluid for obtaininggeothermal power produced from abandoned oil wells andconcluded that the flow rate of fluid and the geothermal gradientwere two main factors in obtaining geothermal energy.

Geothermal energy obtained from the abandoned oil wells is themedium and low quality sources. Feng et al. [7] presented the ad-vantages of organic Rankine cycle in the use of the medium and lowquality sources. Organic fluid due to its low boiling point convertsinto high temperature steam. Hettiarachchia et al. [8] presented acost-effective optimum design criterion for organic Rankine cycleutilizing low-temperature geothermal heat sources. Yamamotoet al. [9] estimated the optimum operating conditions of organicRankine cycle through a numerical simulation and an experimentalapparatus. Wei et al. [10] presented the performance analysis andoptimization of an organic Rankine cycle system using HFC-245faas working fluid for waste heat recovery. The results show thatchoosing a proper nominal state is a good idea for improving thesystem output net power and efficiency according to the runningenvironment. The effects of different working fluids on organicRankine cycle were investigated and analyzed for improvingthe thermal performance [11e14]. Davis et al. [15] suggestedgeothermal power production from abandoned oil wells using

l power generation using abandoned oil wells, Energy (2013), http://

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Nomenclature

VariablesAinj flow area of injection well, m2

a geothermal gradient, K m�1

cp specific heat capacity of fluid, J kg�1 K�1

de hydraulic diameter of injection well, md _Q=dz rate of heat flow over dz, W m�1

dz length of well segment, mf friction factor, Eq (6)f(t) transient heat-conduction time function,

dimensionlessh well depth, mh1 specific enthalpy of fluid in the state 1, J kg�1

h2 specific enthalpy of fluid in the state 2, J kg�1

hw convective heat transfer coefficient, W m�2 K�1

J0 zero-order Bessel function of the first kindJ1 first-order Bessel function of the first kindM mass flow rate of fluid, kg s�1

Q total obtained heat from well, WP actual power generated by system, WPnet net power, WPpump electricity consumed by pump, WPr Prandtl numberpin inlet pressure of fluid, MPaRe Reynolds numberRinj external radius of injection well, mRrec external radius of recovery well, mrinj internal radius of injection well, mrrec internal radius of recovery well, mT0 surface temperature of formation, K

Tc condensing temperature, KTei formation temperature at the infinite distance from

well axis, KTf fluid temperature, KTw heat exchanger/formation interface temperature, KTin temperature of fluid entering injection well, KTout temperature of fluid leaving recovery well, Ku dummy variable for integrationuf fluid flow velocity, m s�1

uin inlet velocity of fluid entering injection well, m s�1

Y0 zero-order Bessel function of the second kindY1 first-order Bessel function of the second kindz variable well depth from surface, m

Greek lettersae thermal diffusivity of formation, m�2 s�1

hg generator efficiencyhm turbine mechanical efficiencyhoi turbine relative internal efficiencyhpump pump efficiencyhth overall efficiency of systemle thermal conductivity of formation, W m�1 K�1

lf thermal conductivity of fluid, W m�1 K�1

mf viscosity coefficient of fluid, N s m�2

rf density of the fluid, kg m�3

(rc)e formation heat capacity, J m�3 K�1

(rc)w wellbore heat capacity, J m�3 K�1

u ratio of formation heat capacity and wellbore heatcapacity, dimensionless

s operating time, dsD dimensionless time ð¼ aes=r2injÞsf friction-loss gradient, Pa/m

1 Turbine

Generator

2 Condenser

Cooling water

1 4 Pump 3

Organic Rankine Cycle

Recovery well Injection well

A double-pipe heat exchanger

Fig. 1. Structure diagram of geothermal power generation system.

W.-L. Cheng et al. / Energy xxx (2013) 1e72

isobutane as working fluid and concluded that the operation of thesystems attains a maximum power that depends on the tempera-ture of the well bottom and the injection pressure.

However, models established in these studies at present weresimplified to facilitate the simulation, in some of which the for-mation heat transfer or the momentum transport was ignored. Thefocus of this article is the study on the effects of formation heattransfer and the theoretical analysis of geothermal power genera-tion from abandoned wells using isobutane as the cycle fluid. Acomprehensive model including transient formation heat transfer,fluid momentum and energy equation is presented. At the sametime the factors that affect the system achieving stable state andthermal performance are investigated.

2. Geothermal power generation system

Fig. 1 shows the structure diagram of geothermal power gener-ation system which consists of a double-pipe heat exchanger andorganic Rankine cycle. Isobutane is chosen as the cycle working fluidto acquire geothermal energy and generate power. Firstly, isobutane(state 4) boosted by the pump flows into the double-pipe heatexchanger. Isobutane (state 4) is heated by geothermal energy andbecomes high-temperature and high-pressure fluid (state 1) at theoutlet. Then the fluid (state 1) enters the turbine and expands towork for power generation. Isobutane (state 2) whose pressure andtemperature declines after working is cooled by the cooling water inthe condenser and becomes subcooled liquid (state 3). The liquid(state 3) is boosted by the pump to become high-pressure subcooledliquid (state 4). The high-pressure liquid (state 4) is sent into the heatexchanger again. Thereby the entire power cycle can be completed.

Please cite this article in press as: Cheng W-L, et al., Studies on geothermadx.doi.org/10.1016/j.energy.2013.07.008

Isobutane is a flammable liquid and a solvent. Leaks aboveground in the piping systems or around the gas compressor can befire hazards. Hence the piping systems are equipped with intelli-gent alarm detectors of gas leaks and polyester lubricants are usedfor preventing isobutane dissolving in lubricants.


W.-L. Cheng et al. / Energy xxx (2013) 1e7 3

3. Theory

3.1. A double-pipe heat exchanger

The double-pipe heat exchanger is composed of twoconcentric pipes, as shown in Fig. 2. The outer wall of the innerpipe is wrapped with insulation and the bottom of the well issealed. The borehole is all lined with a steel casing to preventleakage into the formation. The diameter of the steel pipe is 28/25 cm with the length of 6000 m and the wall thickness is1.5 cm. Isobutane is injected into the ring-shaped channelnamed injection well and flows down. The pressure of isobutaneflowing downward in the injection well increases rapidly andexceeds the critical pressure of isobutane due to the gravity. Thetemperature of isobutane can be heated above its critical tem-perature in the downward course through adjusting the inletvelocity of isobutane entering the injection well. Supercriticalisobutane can be reversed to flow upward along the insidechannel named recovery well. The external diameter of the re-covery well is 12 cm and the internal diameter is 10 cm. Thedifference between vapor and liquid disappears when the fluidis in the supercritical state. Isobutane leaves the top of the re-covery well at a high temperature and supercritical pressurebecause the loss of heat from the recovery well to the injectionwell is prevented by the insulation.

3.1.1. Transient formation heat transferUsing Ramey’s definition the radial heat flow from the forma-

tion at the heat exchanger/formation interface is expressed as fol-lows [16]:

Extract

fluid

Injected fluid

Heat flow

from the formation

Insulation

Fig. 2. Structure diagram of a d


d _Q ¼ 2pleðTei � TwÞ (1)
dz f ðtÞ
where Tei is the formation temperature at infinite distance fromwell axis, K; Tw is the heat exchanger/formation interface temper-ature, K; le is the thermal conductivity of the formation,Wm�1 K�1; d _Q=dz is the rate of heat flow over dz, Wm�1; f(t) is thetransient heat conduction function.

The traditional f(t)just consider the effect of time and ignore thewellbore’s heat capacity, however the heat capacity has obviouseffect on the heat transfer in wellbore especially in short time. Thenovel transient heat conduction function considering the effect ofthe heat capacity of wellbore is written as [17,18]:

f ðtÞ ¼ 16u2

p2 $

ZN

0

1� exp��sDu2

�u3Dðu;uÞ du (2)

where sD is defined as dimensionless time, sD ¼ aes=r2inj; rinj is theinner radius of the injectionwell, m; s is the operating time, d; ae isthe thermal diffusivity of the formation, m�2 s�1; u is the ratio ofthe formation heat capacity and the wellbore heat capacity,u ¼ (rc)e/(rc)w; (rc)e is the formation heat capacity, J m�3 K�1;(rc)w is the wellbore heat capacity, J m�3 K�1; u is the dummyvariable for integration, and [17,18]:

Dðu;uÞ ¼ ½uY0ðuÞ � uY1ðuÞ�2 þ ½uJ0ðuÞ � uJ1ðuÞ�2 (3)

where J0 and J1 are the zero-order Bessel function of the first kindand the first-order Bessel function of the first kind, respectively; Y0

ed

Injected fluid

Formation

Sealing bottom

of the well

ouble-pipe heat exchanger.


T

1

4’

4

3 2 2’

s

Fig. 3. Temperature-entropy diagram of supercritical organic Rankine cycle.


and Y1 are the zero-order Bessel function of the second kind and thefirst-order Bessel function of the second kind, respectively.

Assuming that the formation temperature at the infinite dis-tance from well axis along the vertical direction changes linearly,that is:

Tei ¼ T0 þ a,z (4)

where T0 is the surface temperature of the formation, K; a is thegeothermal gradient, K m�1; z is the variable well depth fromsurface, m.

3.1.2. The fluid momentum equationThe total pressure changes of fluid in the double-pipe heat

exchanger results from momentum changes, friction and gravity.According to the momentum balance principle, the total pressuregradient can be determined as follows [19]:

dpdz

¼ rfg � sf � rfufdufdz

(5)

where dp/dz is the total pressure change of injected fluid over dz,Pa m�1; rf is the density of the fluid, kg m�3; uf is the fluid flowvelocity, m s�1; g is the gravitational acceleration, m s�2; sf is thefriction-loss gradient, which can be acquired as the followingequation:

sf ¼ f rfu2f

2de(6)

where de is the hydraulic diameter of the injection well,de ¼ 2 � (rinj�Rrec), m; f is the friction factor, that is [20]:

1ffiffiffif

p ¼ �1:8log10

��D=de3:7

�1:11

þ 6:9Re

�(7)

where D is the equivalent absolute roughness, m.

3.1.3. The fluid energy equationDue to the temperature difference between the fluid and the

formation, the process of fluid flowing downwards involves heattransfer from the surrounding formation to the fluid in the injectionwell. And the heat transfer is significantly influenced by fluidproperties as well as the dynamics of fluid flow. The pressure of thefluid flowing downward in the injection well rapidly increases andexceeds the critical pressure of isobutane. Therefore the liquidevapor phase transition of fluid heated constantly by geothermalenergy is performed at a continuously variable temperature [21].The equation of heat transfer between the fluid and the formationcan be written as:

2prinjhwTw � Tf

¼ d _Q

dz(8)

where Tf is the fluid temperature, K; hw is the convective heattransfer coefficient, which can be calculated as follows [22]:

hw ¼ 0:23lfRe0:8Pr0:4=de (9)

where lf is the thermal conductivity of the fluid, W m�1 K�1;Re ¼ rfufde=m; Pr ¼ cpmf=lf ; mf is the viscosity coefficient of thefluid, N s m�2.

Assuming that the inside pipe is perfectly insulated, heattransfer between the fluid in the injection well and the counter-current fluid in the recovery well does not occur. The heat flow to


the fluid in the injectionwell is equated to the radial heat flow fromthe formation at the heat exchanger/formation interface. Then theenergy equation of the fluid in the injectionwell can be acquired asthe following equation:

vrfAinjcpTf

vs

þvrfAinjufcpTf

vz

¼ d _Qdz

(10)

where cp is the specific heat capacity of the fluid, J kg�1 K�1; Ainj isthe flow area of the injection well, m2.

3.2. Organic Rankine cycle

The geothermal power generation using abandoned oil wells isdifferent from a classic Rankine cycle. The boiler and pump arereplaced by the double-pipe heat exchanger and isobutane iscompressed and heated during flowing downward. Isobutaneleaves the top of the recovery well at a high temperature and su-percritical pressure. Fig. 3 shows the supercritical organic Rankinecycle clearly that results from this operation. The actual powergenerating of the system is obtained [23,24]:

P ¼ Mðh1 � h2Þhoihmhg (11)

where P is the actual power generated by the system, W; M is themass flow rate of the fluid, kg s�1; h1 is the specific enthalpy of thefluid in the state 1, J kg�1; h2 is the specific enthalpy of the fluid inthe state 2, J kg�1; hm is the turbine mechanical efficiency; hg is thegenerator efficiency; hoi is the turbine relative internal efficiency.

The net output power can be expressed as [23,24]:

Pnet ¼ P � Ppump ¼ Mhðh1 � h2Þhoihmhg � ðh4 � h3Þ=hpump

i

(12)

where Pnet is the net power,W; Ppump is the electricity consumed bypump, W; hpump is the pump efficiency.

The overall efficiency of the system can be expressed as [23]:

hth ¼ PnetQ

(13)

where hth is the overall efficiency; Q is the total obtained heat fromthe well.


Table 1Basic parameters of the double-pipe heat exchanger and the fluid.

Parameter Value

External radius of injection well Rinj (m) 0.14Internal radius of injection well rinj (m) 0.125External radius of recovery well Rrec (m) 0.06Internal radius of recovery well rrec (m) 0.05Well depth h (m) 6000Inlet pressure of fluid pin (MPa) 2.0Surface temperature of formation T0 (K) 288.15Condensing temperature Tc (K) 298.15Equivalent absolute roughness D (m) 0.00026

0 200 400 600 800 10000

100

200

300

400

Operating time (d)

Tem

pera

ture

of i

sobu

tane

leav

ing

reco

very

wel

l (K) λe=1.5W/(m•K)

λe=1.8W/(m•K)λe=2.0W/(m•K)

Fig. 4. Temperature of isobutane leaving recovery well change with the running timefor different formation thermal conductivity.

200

300

400

of i

sobu

tane

leav

ing

very

wel

l (K) (ρc)e=1.8×106J/(m3•Κ)

(ρc)e=2.3×106J/(m3•Κ)

(ρc)e=2.5×106J/(m3•Κ)


4. Results and discussions

The basic parameters of the double-pipe heat exchanger and thefluid are presented in Table 1 and Table 2 shows the efficiency of theturbine and the power generation equipment [25]. The abandonedoil wells in our simulation are in northern China and the annualaverage temperature of water is 293.15 K. It is assumed that thecondensing temperature is at 298.15 K and the temperature dif-ference of heat transfer is 5 K. The properties of isobutane are notconstant but change with the pressure and temperature in thesimulation and all the properties can be obtained from Ref. [26].

Figs. 4e6 describe the influence of formation thermal conduc-tivity, formation heat capacity and the inlet velocity of isobutaneentering the injectionwell respectively on the stabilized time of thesystem for a ¼ 0.040 K m�1. Fig. 4 shows the change of outlettemperature of isobutane leaving the recovery well as a function ofthe operating time and the formation thermal conductivity foruin ¼ 0.2 m s�1 and (rc)e ¼ 2.3 � 106 J m�3 K�1. It can be observedthat Tout decreases with the increase of operating time. In the 100days of the initial operation, the fluid outlet temperature sharplyreduced from 458 K to 410 K at le ¼ 1.8 W m�1 K�1. After running300 days, the fluid outlet temperature is 403 K and then changesslowly. And the system approaches a stable state. At the sameoperation time, the fluid outlet temperature increases with theincreasing formation thermal conductivity. Meanwhile for thehigher formation thermal conductivity, the curve is more gentlyand the shorter stabilized time is spent by the system. As shown inFig. 5, the outlet temperature of isobutane leaving the recovery wellincreases slightly with the increasing formation heat capacity forle ¼ 1.8 W m�1 K�1 and uin ¼ 0.2 m s�1. The tendency of the fluidoutlet temperature changing with operating time for differentformation heat capacity keeps fairly consistent, therefore the for-mation heat capacity has no measurable effect on the stabilizedtime of the system. Fig. 6 shows the change of the outlet temper-ature of isobutane leaving the recovery well with the inlet velocityof isobutane entering the injectionwell as a function of the runningtime for le ¼ 1.8 W m�1 K�1 and (rc)e ¼ 2.3 � 106 J m�3 K�1. Asillustrated in Fig. 6, the stabilized time is affected greatly by thefluid inlet velocity. The fluid mass flow increases and the fluidoutlet temperature declines due to the increasing of the fluid inletvelocity. Meanwhile the tendency of the fluid outlet temperaturechange with operating time increases. Therefore stabilized time of

Table 2The efficiency of the turbine and the power generation equipment.

Parameter Value

Turbine relative internal efficiency hoi 0.80Turbine mechanical efficiency hm 0.97Generator efficiency hg 0.98Pump efficiency hpump 0.80


the system can be shortened by lowering the inlet velocity ofisobutane entering the injection well.

Fig. 7 illustrates the variation of the fluid temperature andpressure in the injection well as a function of well depth after 300days operation for uin ¼ 0.2 m s�1 and a ¼ 0.040 K m�1. The fluidtemperature decreases slightly at the first 200 m of the entrancebecause the inlet temperature of isobutane entering the injectionwell is greater than the formation temperature. The formationtemperature increases constantly with increasing the well depth.And while it is higher than the fluid temperature, the isobutane isheated by the geothermal energy. The fluid temperature reaches amaximum of 403 K at the bottom of the well and the formationtemperature at the infinite distance from well axis is 528 K. Thetemperature difference between the fluid and the formation for theheat transfer always exists due to the thermal resistance of theformation. At the same time the fluid pressure increases withincreasing the well depth and reaches a maximum of 34 MPa at thebottom.

Fig. 8 shows the variation of the fluid temperature in the in-jection well with geothermal gradients as a function of well depthafter 300 days operation for uin ¼ 0.2 m s�1, le ¼ 1.8Wm�1 K�1 and(rc)e¼ 2.3�106 J m�3 K�1. The three curves are for a¼ 0.030 Km�1,

0 200 400 600 800 10000

100

Tem

pera

ture

reco

Operating time (d)

Fig. 5. Temperature of isobutane leaving recovery well change with the running timefor different formation heat capacity.


200 400 600 800 10000

100

200

300

400

500

Tem

pera

ture

of i

sobu

tane

leav

ing

reco

very

wel

l (K)

Operating time (d)

vm=0.10m/s vm=0.20m/s vm=0.30m/s

Fig. 6. Temperature of isobutane leaving recovery well change with the running timefor different inlet velocity of isobutane entering the injection well.

0 1000 2000 3000 4000 5000 6000280

300

320

340

360

380

400

420

440

Well depth (m)

Flui

d te

mpe

ratu

re (K

)

a=0.030K/m a=0.040K/m a=0.050K/m

Fig. 8. Temperature of fluid in the injection well change with well depth for differentgeothermal gradient after 300 days operation.

0.1 0.2 0.3 0.4 0.5

360

380

400

420

440

3.0

4.5

6.0

7.5

9.0

Out

let p

ress

ure

(MPa

)

Flow rate (m/s)

temperature pressure

Tem

pera

ture

of i

sobu

tane

leav

ing

reco

very

wel

l (K)

Fig. 9. Temperature and pressure of isobutane leaving recovery well temperaturechange with the fluid inlet velocity after 300 days operation.


a ¼ 0.040 K m�1 and a ¼ 0.050 K m�1, respectively. The fluidtemperature increases with the increase of geothermal gradient atthe same height in the injection well. The temperature of fluid atthe bottom is 377 K, 403 K and 429 K, respectively. Meanwhile, thetotal obtained heat from formation increases with the increase ofgeothermal gradient and is 873 kW,1201 kWand 1535 kW for threedifferent geothermal gradients, respectively.

Fig. 9 shows that the outlet temperature and pressure ofisobutane leaving the recovery well vary with the fluid inlet ve-locity after 300 days operation for a ¼ 0.040 K m�1,le ¼ 1.8 Wm�1 K�1 and (rc)e ¼ 2.3 � 106 J m�3 K�1. Increasing fluidinlet velocity will cause the mass flow rate and the friction lossincreasing. Then the fluid outlet temperature and pressure willdecrease.

Fig. 10 depicts the change of the total obtained heat and the netpower as a function of the inlet velocity of isobutane entering theinjection well after 300 days operation for a ¼ 0.040 K m�1,le ¼ 1.8 W m�1 K�1 and (rc)e ¼ 2.3 � 106 J m�3 K�1. The fluid inletvelocity varies between 0.1 m s�1 and 0.5 m s�1. It can be observedin Fig. 10 that the total heat obtained from the formation increaseswith the increase of the fluid inlet velocity. The total heat obtainedfrom the well is 876 kW and 1531 kW for uin ¼ 0.1 m s�1 and

0 1000 2000 3000 4000 5000 6000280

300

320

340

360

380

400

420

0

5

10

15

20

25

30

35

Tem

pera

ture

(K)

Pres

sure

(MPa

)

Well depth (m)

temperature pressure

Fig. 7. Temperature and pressure of fluid in the injection well change with well depthafter 300 days operation.


0.5 m s�1, respectively. The fluid outlet temperature and thegeothermal heat obtained from the well determine the amount ofpower generation. However the two factors are constraints. Thegreater obtained heat means the lower outlet temperature, which

0.1 0.2 0.3 0.4 0.5800

1000

1200

1400

1600

110

120

130

140

150

160

170 Rate of heat flow P

Rat

e o

f he

at f

low

(W)

Flow rate (m/s)

P (k

W)

Fig. 10. Heat flow rate and the net power change with inlet velocity of isobutaneentering injection well after 300 days operation.


Table 3The temperature, pressure, heat flow and state of isobutane at the points on Fig. 1.

Point Temperature (K) Pressure (MPa) Heat flow (kW) State

1 409.15 6.75 1493 Supercritical2 298.15 0.35 1323 Mixture3 298.15 0.35 324 Liquid4 299.15 2.0 340 Liquid


leads to lowering the evaporating temperature of the organicRankine cycle and reducing the power cycle efficiency. Therefore,there is an optimal inlet velocity making the net power to achievethe maximum. And the maximum net power of 154 kW can beobtained at uin ¼ 0.18 m s�1. The temperature, pressure, heat flowand state of isobutane for uin ¼ 0.18 m s�1 at the points on Fig. 1 areshown in Table 3. The overall efficiency of the system hth is 13.3%

5. Conclusions

In this study a double-pipe heat exchanger to acquiregeothermal energy from abandoned oil wells is created and poweris generated through organic Rankine cycle. Isobutane is chosen asthe cycle working fluid. A model based on the transient formationheat transfer, the fluid momentum and energy equation is estab-lished. Meanwhile, a numerical simulation for an abandoned wellwith a depth of 6000 m is performed. The formation heat transfer,formation properties and the inlet velocity of isobutane enteringthe injection well which impact on stabilized time and thermalperformance of the system are discussed. The conclusions aresummarized as follows:

(1) The geothermal power generation is largely influenced by theformation heat transfer. The outlet temperature of isobutaneleaving the recovery well decreases gradually with theincreasing operating time and eventually approaches a steadystate. Due to that the fluid is constantly heated by thegeothermal energy from formation and affected by the gravityduring flowing downward in the injection well, the tempera-ture and pressure rise gradually and the temperature differencebetween the fluid and the formation for the heat transfer al-ways exists because of the thermal resistance of the formation.

(2) The stabilized time is shortened and the outlet temperature ofisobutane leaving the recovery well is improved by theincreasing formation thermal conductivity. The stabilized timeis not significantly affected by the formation heat capacity. Andthe fluid temperature and the total obtained heat increase withthe increasing geothermal gradient.

(3) The stabilized time is shortened by decreasing the inlet velocityof isobutane entering the injectionwell. The heat obtained fromabandoned well increases but the fluid outlet temperature de-creases with the increasing fluid inlet velocity. And the gener-ating power depends on the fluid outlet temperature and theobtainedheat, therefore there is anoptimal inlet velocityoffluidentering the injection well to maximize the net power for aspecified geothermal heat source.

(4) The outlet temperature of isobutane leaving the recovery wellchanges slowly and the system is close to the stability after 300days operation for a ¼ 0.040 K.m�1, le ¼ 1.8 W m�1 K�1 and(rc)e ¼ 2.3 � 106 J m�3 K�1. And the isobutane outlet temper-ature and the total obtained heat can be controlled by adjustingthe inlet velocity of the isobutane. When the inlet velocity ofisobutane entering the injection well is 0.18 m s�1, themaximum net power of 154 kW is achieved.


The electricity per abandoned oil well is little because of lowheat transfer rates through the rock. However, abandoned oil wellscluster together in the oil field and geothermal power output willbe improved significantly if groups of abandoned oil wells areconnected together. Hence it is feasible to transform abandoned oilwells in geothermal wells for geothermal power production.

Acknowledgments

The authors would like to thank the National Natural ScienceFoundation of China (grant no. 51176182) for the financial support.

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Studies on Geothermal Power Generation Using Abandoned Oil Wells