PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering

Stanford University, Stanford, California, February 11-13, 2013

SGP-TR-198

LABORATORY INVESTIGATION OF SUPERCRITICAL CO2 USE IN GEOTHERMAL

SYSTEMS

Mario Magliocco1,2

, Steven Glaser1,2

, and Timothy J. Kneafsey2

1Department Civil and Environmental Engineering

University of California, Berkeley, CA 94720 2Lawrence Berkeley National Laboratory

Berkeley, CA 94720

e-mail: mag@berkeley.edu

ABSTRACT

The use of carbon dioxide (CO2) as a heat transfer

fluid has been proposed as an alternative to water in

enhanced geothermal systems (EGS). Numerical

simulations have shown that under expected EGS

operating conditions, CO2 could achieve more

efficient heat extraction performance than water.

The simulations indicate that the performance

advantage of CO2 over water is a function of

operating parameters, reservoir temperature, and flow

geometry. Since the pore space in formations that

would be likely utilized for EGS are initially

saturated with water or brine, the creation of a CO2

based EGS reservoir would involve a development

period where the resident water would be removed

over time as CO2 is injected into the formation.

Using a specially constructed laboratory apparatus

that is capable of flowing temperature-controlled

supercritical CO2 through a heated porous sample, we

have investigated the behavior of CO2 as a working

fluid in EGS system. In our laboratory based

experiments we have investigated the injection of

CO2 into a heated water saturated sample and have

found that preferential flow paths develop which

bypassed much of the sample pore space.

Additionally we have also explored the behavior of

dry CO2 based heat extraction from a heated sample

and found that the behavior is highly dependent on

the operating conditions. Finally we have explored

the relative performance of dry CO2 versus single-

phase water based heat extraction and found that

under the operating conditions studied that water and

CO2 exhibit similar heat extraction rates.

These experiments are intended create a data set that

can then be used to validate numerical modeling

results and provide insight into the planning,

development, and operation of a CO2 based EGS

reservoir.

INTRODUCTION

The novel concept of using supercritical CO2 (scCO2)

as the working fluid in an enhanced geothermal

system (EGS) for both reservoir creation and heat

extraction was first proposed by Brown (2000). The

advantages of using CO2 instead of water as the

process fluid in a closed loop EGS system include a

much lower viscosity of CO2 resulting in

substantially larger mass flow rates for a given

pressure drop between injection and production

points, and a much larger density difference between

cold fluid in the injection well and hot fluid in the

producer providing increased buoyancy forces for

CO2. As an ancillary benefit, practical operation of a

CO2 system would result in some de facto carbon

sequestration due to fluid loss into the surrounding

formations (Brown, 2000, Pruess 2006). Numerical

simulations of a five-spot well pattern in a hot dry

rock system indicate that (1) CO2 achieves larger heat

extraction rates than water, and (2) the relative

advantage of CO2 increases with decreasing reservoir

temperature, ranging from approximately 50% larger

heat extraction rates at T = 240oC to about 80%

larger rates at T = 120oC (Pruess personal

communication 2009).

The increasing heat extraction efficiency of CO2 at

lower reservoir temperatures can be better understood

by considering the effects of pressure and

temperature on the “fluid mobility” m = ρ/μ (ρ is the

fluid density, μ is the fluid viscosity) (Figure 1, Press

2006).

Figure 1 - Fluid mobility of CO2 (Preuss 2006)

The contour lines in Figure 1 indicate the fluid

mobility for the pressure and temperature shown on

the axes. According to Darcy’s law, for a given

pressure gradient, fluid flux is proportional to the

fluid mobility. CO2 mobility exhibits a much more

dynamic dependence on temperature and pressure

than water, and under the conditions expected to be

present in a geothermal field, the CO2 mobility is

much higher than that of water.

In a water-based 5-spot EGS system the majority of

the driving head loss occurs near the injection well.

This is because the lower mobility resulting from the

colder conditions coupled with the high Darcy flux

due to the radial flow pattern around the well require

a high driving pressure gradient. This is in contrast

to the higher mobility CO2-based system where the

head loss is much more evenly distributed across the

entire flow path.

Typically, the pore space in formations that would be

likely utilized for EGS is initially saturated with

water or brine. The creation of a CO2 based EGS

reservoir in an initially brine-saturated reservoir

would involve a development period where the

resident water would be removed over time as CO2 is

injected into the formation. Simulations have shown

that water removal would occur through two

principal processes: (1) immiscible displacement of

aqueous phase by the CO2-rich phase, and (2)

dissolution of water into the flowing CO2 stream

(Pruess 2010). During the operation of a CO2 based

EGS reservoir, it is expected that the initial

production fluid would be composed of the native

brine until the breakthrough of CO2 occurs, after

which a brine-CO2 mixture would be produced.

In order to verify the results of the numerical

simulations, we have performed laboratory

experiments that have injected cooled CO2 into a

heated water saturated sample to study the reservoir

development stage. We have also injected cooled

CO2 into a heated CO2-saturated sample to study the

behavior of a fully developed, dry reservoir. We

have also injected cooled water into a heated water

saturated sample in order to compare the behavior of

a water-based system to a CO2 based system.

EXPERIMENT DESCRIPTION

Experimental Apparatus

The apparatus consists of a temperature-controlled

pressure vessel filled with porous media through

which temperature controlled fluid can be introduced

by means of high-pressure, high-flow rate pumps

(Figure 2). The pumps can be operated to provide a

constant fluid injection rate, or a constant differential

pressure. The fluid was delivered by a pair of Quizix

C-6000-5K pumps, capable of 5,000psi (345 bar) and

400 ml/min fluid delivery rate. The pumps are

capable of precisely controlled continuous and pulse-

Figure 2 - Simplified schematic of the experimental apparatus.

free flow with a resolution of 27.2 nanoliters. To

ensure that the pumps are filled with high-density

liquid CO2, the injection fluid is passed through a

chiller before entering the pumps. After leaving the

pumps, the fluid can be either chilled by a second

fluid chiller, or heated depending on the desired

experimental parameters. Before entering the vessel,

the injection fluid passes through a Siemens coriolis-

style mass flow meter. The meter could also be

placed at the outlet and used in conjunction with the

pump flow readings to measure fluid accumulation in

the vessel.

The pressure vessel is a hollow stainless steel

cylinder with an inside diameter of 9.1 cm, outside

diameter of 12.7 cm, 50.8 cm distance between the

end caps, and a pressure safety rating of 34.5 MPa

(345 bar, 5000 psi). Instrumentation access to the

interior of the vessel is through three axial passages

through one end cap, and one passage through the

other. The central passages through the end caps are

used as the injection and production ports and the

remaining two passages are used exclusively to pass

thermocouples through. The vessel is oriented

vertically such that the central axis is in line with

gravity. It has been shown that even for small length

scales, buoyant effects of scCO2 can have a large

effect on the dynamics of a scCO2-based system

(Liao and Zhao 2002). For a horizontal flow

arrangement, buoyant forces can result in pressure

gradients that are oriented perpendicular to the vessel

axis, complicating the dynamics. For modeling and

comparison purposes, the experiments were operated

such that the flow path was in the same orientation as

the gravity-induced pressure gradient.

Temperature measurements within the sample were

made with 23 stainless-steel clad type-T

thermocouples, which have a small diameter

(0.79mm) in order to increase the sensor response

time and to minimize disturbance to fluid flow. The

thermocouples are arranged at various elevations and

radii in the sample such that each successive vertical

level is offset angularly to minimize vertical sensor

shadowing (Figure 3). The offset angle we used is

based on the “Golden Angle” (137.5 degrees), found

in plant phyllotaxis that has been shown to minimize

shadowing (King 2004).

At one elevation in the porous medium, two

thermocouples were mirrored so that they were both

at the same radial distance from the central axis of

the vessel. The radially mirrored temperature

measurement was designed to test our assumption of

a radial symmetry in the heat transfer process.

Because the end caps are large, the injection port of

the vessel was lined with a length of nylon tubing

through the end cap in order to provide thermal

insulation for the injected fluid as it passed through

the relatively massive end cap. The injection port

was also fitted with a single thermocouple mounted

where the injected fluid enters the sample space (not

shown in Figure 2).

The sand used in the test sample was prepared from

F95 Ottawa silica sand (U.S. Silica). Sieving and

washing resulted in a narrow grain size distribution

(Figure 4). The mean grain size falls between 147

and 105 microns with no measurable portion below a

grain size of 45 microns.

Figure 3 - Orthographic diagram of thermocouple

placement inside the vessel. Axis units

are in meters. Colored dots index the

different elevations and correspond to the

colors in Figures 5, 6, & 8

Figure 4- Porous medium grain size distribution

The sand was dry placed in the vessel in multiple lifts

with vibratory compaction between lifts. This

method produced a relative density of 84 percent.

The porous core sample properties are listed in Table

1.

Table 1: Design case system properties

The vessel was wrapped with heat tape that extended

around the exterior of the cylinder and both end caps.

The heat tape thermal output was controlled by either

a PID controller using a thermocouple secured on the

vessel exterior or with a pulse width modulated

signal. This allows the vessel boundary to maintain a

constant temperature or a constant heat flux. Finally

the vessel was wrapped in an aerogel insulation

jacket and sealed. The current supplied to the heat

tape was monitored with a true RMS current sensor.

The pressure at the outlet of the vessel was controlled

by a pair of digital backpressure regulators in series.

The fluid exiting the backpressure regulators was

vented to the atmosphere at a safe location outside of

the building. A differential pressure sensor

connecting the inlet and outlet of the vessel was

located at the base of the vessel. The tubing that

connected the differential pressure sensor to the inlet

is encased in a constant temperature water bath so

that the state of the fluid column in the tube could be

determined.

We developed software that incorporates

experimental control and data acquisition. All sensor

readings were collected by a single Labview-based

program that allows for accurate time

synchronization of experimental data. The program

is capable of controlling the pumps, vessel heat input,

and the backpressure regulators. Combining these

functions allows for a tightly integrated experimental

setup, faster data processing, faster experimental

turnaround time, and less chance of experimental

errors.

Table 2 lists the range of operating parameters that

we have used in our experiments. Our current

apparatus is capable of achieving temperatures up to

200C and operating pressure of up to 275 bar.

Table 2: Range of operating parameters

Parameter Min Value Max Value

back pressure 87 bar 138 bar

initial core

temperature 50 C 100C

injection rate 50 ml/min 200 ml/min

Experimental Procedure

The sand-filled vessel was filled with fluid and

pressurized to the experiment pressure, and the vessel

was heated. For the single-phase CO2 experiments,

the vessel was filled with CO2, for the two phase

CO2/water and the single phase water experiment the

vessel was initially filled with water. The pumps and

tubing were then emptied of water if present, filled

with CO2, and pressurized to the vessel pressure. The

backpressure regulators were set to the desired

pressure, and CO2 injected into the bottom of the

vessel at a prescribed volumetric flow rate. The

pumps maintain a constant flow over multiple

injector volumes through computer controlled pumps

switching.

Experimental Challenges

During the course of our experiments we have met

with several practical challenges that are related to

CO2. The density of CO2 can change significantly

with relatively small changes in pressure or

temperature. For example, the density of the CO2 in

the vessel could change by more than a factor of two

within the conditions of a single experimental run.

Practically, this behavior made it difficult to measure

the mass of the CO2 without knowledge of the current

state of the CO2.

When using the first iteration of our apparatus we

inferred the mass flow rate of fluid entering the

vessel by recording the volumetric flow rate of the

0102030405060708090

0.147 0.105 0.075 0.053 0.045

% R

eta

ine

d

Mesh Opening Size (mm)

Porous Media Grain Size Distribution

Porous Core Properties

total core length L = 50.8 cm

cross sectional area A = 6.54x10-3

m2

grain density ρR = 2650 kg/m3

grain specific heat CR = 20 J/ g/ C

rock thermal

conductivity K = 2.51 /m/ C

permeability k = 9.3x10-13

m2

porosity ϕ = 41%

mean grain size d50 d50 ≈ 0.105 mm

pump, measuring the pressure and temperature of the

CO2 exiting the pump, using a lookup table to find

the density of CO2, and finally calculating the mass

flow rate. This method proved difficult as the

pressure and temperature of the fluid in the pump

changed throughout the course of a single

experimental run. To solve this problem we

employed a Siemens corriolis based mass flow meter

near the inlet of the pressure vessel. This device was

capable of measuring the mass flow of the fluid

without requiring knowledge of the current state of

the fluid being measured.

The dynamic density of the CO2 also made measuring

the pressure differential across the vessel difficult.

The differential-pressure sensor we used in our

apparatus required a tubing connection between the

inlet and outlet of the vessel so that the relative

pressure difference could be measured. Since our

vessel was mounted vertically, in line with gravity,

the weight of the fluid in the tubing connecting the

delta-pressure sensor with the vessel outlet resulted in

a hydrostatic pressure component on one side of the

delta pressure sensor. With fluids such as water that

don’t exhibit drastic density changes, the hydrostatic

pressure due to the fluid column in the connection

tube is relatively constant and can simply be

subtracted from the reading. With CO2 we found that

small changes in pressure and temperature in the

column produced relatively large changes in density

that caused the hydrostatic pressure to change

depending on the state of the CO2 in the connection

tube.

In the second iteration of our experimental apparatus

we attempted to impose a constant temperature on the

connection tube by using a small diameter tube and

placing it in a relatively large temperature controlled

water bath. The temperature of the bath was recorded

by a thermocouple throughout the experiment, and a

pressure sensor was located at the top end of the tube.

Using these measurements, the state of the fluid in

the column could be determined and used to calculate

the hydrostatic pressure component and subtract it

from the differential pressure measurement.

The final difficulty we experienced due to the

behavior of CO2 was in our backpressure regulation.

We used a pair of backpressure regulators driven by a

PID control system. As the state of the CO2 exiting

the vessel changed, the density and the viscosity of

the CO2 passing through the backpressure regulator

valve changed. This made it very difficult to identify

PID coefficient values that would provide a

sufficiently stable control tune that would work over

the course of a single experiment. To alleviate this

problem on our final apparatus iteration we placed

the back pressure regulator downstream of a heat

exchangers in order to keep the CO2 passing through

the regulator valve at a more constant temperature.

RESULTS

Single-Phase CO2

The temperature data from twenty-two

thermocouples from a representative single phase

CO2 experimental run is shown in Figure 5. The

thermocouples are numbered primarily in order

increasing radii and secondarily by increasing

elevation in the vessel. Thermocouple one for

example is located on the central axis at the bottom

of the vessel, while thermocouple number twenty-two

is located near the vessel wall at the top of the vessel.

It can be seen from the plot that there is an initial

temperature gradient present in the saturated medium

with a lower temperature at the base. This gradient is

most likely due to gravity and thermal convection.

The temperature front can be seen in the plot as it

passes axially through the sample past the

measurement locations. After the initial sharp

temperature drop, the temperatures gradually

approach equilibrium, and a radial temperature

gradient then develops, indicated by the grouped

lines spreading out. The exterior thermocouple

locations trend towards a higher temperature than

those that are more central (solid lines).

The large spikes can be seen in temperature data for

TC1 at the vessel inlet are due to short interruptions

in flow as the pumps stop during the pump switch

over event. During the brief time when flow was

stopped, heat from the steel end cap increased the

temperature of the cold fluid that was located within

the injection passage

The interplay between convective and conductive

transport can be seen in the shape of the temperature

vs. time curves. A purely convective process would

feature sharp thermal fronts and a near-vertical slope

at the time when the cold fluid slug reached the

thermocouple. A purely conductive process would

generate a gentler slope with smooth transitions. The

experimental run shown in Figure 5 was at a

relatively high flow rate with a calculated bulk Peclet

number of around 1500. The steep temperature front

corresponds to convectively dominated behavior.

Figure 6 shows temperature data from a lower flow

rate experimental run that was less convectively

dominated (only temperature data from the central

axis of the vessel are shown for clarity). The

calculated bulk Peclet number of this experimental

run was approximately 550, or one third that of the

run in Figure 5.

Water & CO2 Compared

In order to compare the performance of CO2 heat

extraction with that of water, we performed a single

phase water run at the same temperature and pressure

as a previous CO2 run (Figure 7). Both experiments

were run with a volumetric fluid injection rate of 150

ml/min, an initial core temperature of 75C, and a

backpressure of approximately 1450 psi. The CO2

and water experiments had approximately the same

pressure differential across the sample for both water

and CO2 (2 bar), and a similar mass flow rate (2.51

g/s for water, and a range of 2.3 to 2.04 g/s for CO2)

which allowed a more straightforward comparison of

the performance of the two fluids. The injected water

and CO2 increased in temperature after the first two

pump volumes due to the fact that we were recycling

the injection fluid. The CO2 plot shows a much

steeper temperature front indicating a more advection

dominated flow than the water experiment.

0 500 1000 1500 200010

20

30

40

50

60

70

80

90

100

Temperature History, 100 (ml/min) Flow Rate, 100C Vessel Temperature, 138 bar Back Pressure

Time (s)

Tem

pe

ratu

re (

C)

TC1

TC4

TC8

TC12

TC13

TC17

TC21

Figure 6 - Temperature vs time from a

experimental run with a CO2 flow rate of

100ml/min, and a bulk Peclet number of

556.

0 100 200 300 400 500 600 700 800 9000

10

20

30

40

50

60

Temperature History, 200 (ml/min) Flow Rate, 60C Vessel Temperature, 83 bar Back Pressure

Time (s)

Tem

pe

ratu

re (

C)

TC1TC2TC3TC4TC5

TC6TC7TC8TC9TC10

TC11TC12TC13TC14TC15

TC16TC17TC18TC19TC20

TC21TC22

Figure 5 - Temperature vs. time data from twenty-two thermocouples from a representative single phase CO2

experimental run. Plot line colors indicate the thermocouple elevation and correspond to the colored

markers shown in the thermocouple diagram (Figure 2)

In order to compare the heat extraction of the two

fluids, we used the experimental data to calculate the

heat extraction rates of the two fluids. Figure 8

shows the heat extraction rate of water (blue line) and

the heat extraction rate of CO2 (red line). Initially the

heat extraction rates of the two fluids are similar,

with a higher rate for water at the start of the

experiment and CO2 overtaking it in as the

experiment progresses. The performance of CO2

stays somewhat stable despite the fact that the mass

Figure 7 - Comparison of temperature history data of water and CO2 as working fluids. Top plot shows a water

run and the bottom plot is a CO2 run. (Thermocouple labeling is not consistent with other figures in

this paper)

0 200 400 600 800 1000 12000

50

100

150

200

250

300Heat Extraction Rate

Time (s)

He

at E

xtr

actio

n R

ate

(W

atts)

H2O

CO2

Figure 8 - Comparison of the heat extraction rate of water and CO2 under similar experimental conditions.

flow rate is decreasing during the experiment due to

the increase in injection temperature. The water heat

extraction rate decreases after the first two pump

volumes due to the increased injection temperature.

Since both the water and CO2 were operated at the

similar pressure and volumetric flow rate, it can be

assumed that the work performed by the pumps in

both experiments was comparable. A complete

thermodynamic characterization of the runs would

require measurements of the fluid accumulation

inside the vessel which was not recorded for these

experiments.

Two Phase Experiments

We performed an experimental run in which cold

CO2 was injected into a heated water-saturated

sample, analogous to the initial development phase of

a CO2 geothermal reservoir. Figure 9 shows the

temperature history.

The temperature drop at the bottom of the vessel,

where the cold CO2 was injected, was very

pronounced, but temperature trends at the other

thermocouple locations were very smooth and

gradual. The temperature drop at the top of the

vessel was not significantly greater than the

temperature drop that would be expected due to

passive cooling to the lab atmosphere. The

temperature spikes seen at the injection location (e.g.

at 1200 seconds) were due to a temporary cessation

of injection flow during the pump switchovers. The

overall temperature change shown in this experiment

is not as drastic as the single phase experiments.

The presence of viscous fingering was indicated by

multiple pieces of experimental evidence. After the

CO2 was vented to the atmosphere, it was found that

approximately 74 percent of the pore volume still

contained the original water. More than one pore

volume of CO2 was injected into the vessel, and if no

fingering occurred the CO2 would act as a solid slug

displacing all of the water originally present in the

vessel. Further evidence of fingering is indicated by

the CO2 breakthrough occurring at approximately 500

seconds after injection began. Breakthrough

occurred after approximately 251 mL of CO2 was

injected into the vessel, corresponding to

approximately 20 percent of the sample pore space.

The volume of CO2 injected at breakthrough, and the

remaining water in the pore space indicate that very

little water was lost after the initial water was

displaced during the CO2 flow path formation.

Compared to our previous results with an initially

CO2 saturated sample, our new results show much

less change in temperature in the vessel during the

course of the experiment. Besides the effects of

0 500 1000 1500 2000 2500 3000 3500 400020

25

30

35

40

45

50

Time (s)

Tem

pe

ratu

re (

C)

TC1TC2TC3TC4TC5

TC6TC7TC8TC9TC10

TC11TC12TC13TC14TC15

TC16TC17TC18TC19TC20

TC21TC22

Figure 9 - Temperature history of cold CO2 injection into a heated water-saturated sample, 45C initial

temperature, 25 ml/min flowrate, 17 MPa (2500 psi) outlet pressure.

viscous fingering, this smaller temperature drop is

most likely the result of the higher heat capacity of

water compared to CO2, the lower flow rate of the

injected CO2, and the higher temperature of the

injected CO2 when compared to our earlier single

phase experiments. At the initial conditions, the

water-saturated sample contained 234 kJ of heat

energy in the pore space compared to 123 kJ for CO2

under the same conditions. The thermal energy of

the water in the vessel was so great and the cooling

capacity of our injected fluid was so low, that we

only saw evidence of the cooling effects of the CO2

flow at the injection end of the sample.

DISCUSSION

The design of our experiment was primarily intended

to produce a data set that could then be used to

validate numerical modeling of the use of CO2 as the

working fluid in an EGS reservoir, and was not

intended to be directly applicable to full-scale EGS

systems. Specifically, our porous medium sample

was not designed to replicate the characteristics of

the flow paths that would be expected in a field-scale

geothermal system. Despite this, our results can be

used to gain insights into the behavior of CO2 as an

EGS working fluid.

Our single-phase CO2 experiments have shown that

the heat extraction behavior of CO2 is sensitive to the

initial and operating conditions; injection rate,

operating pressure, and initial reservoir temperature

(Magliocco 2011). The Peclet number and the shape

of the temperature histories vary greatly depending

on the initial and operating conditions.

We have also shown experimentally that CO2 and

water have comparable behavior under one particular

set of initial and operating conditions. This result

could be viewed as discouraging in the context of

CO2 based EGS, but was in line with previous

modeling results (Preuss 2007). The greater benefits

of CO2 over water are expected to occur in the 5-spot

well geometry that is dominated by radial flow. In

our apparatus, the flow is predominately linear along

the length of the vessel with small portions of radial

flow near the inlet and outlet boundaries. In addition,

the thermal energy stored in the large end caps used

in our vessel may retard the development of cold

zones near the fluid injection point.

Experimental results for the more realistic conditions

of CO2 injection into a water-saturated sample show

convincing evidence of preferential flow of CO2 into

the saturated sample. Much of the sample pore space

appears to have been bypassed by the flowing CO2,

as indicated by the temperature data, the early CO2

breakthrough, and the remaining water in the sample

at the conclusion of the experiment. The effect of

CO2 bypassing large regions of the sample (or

reservoir) will reduce the effective heat transfer to the

working fluid, thus this is an important consideration

warranting further investigation. In future

experiments, a much clearer picture of the system

could be realized by analyzing the produced fluid

over the course of the experiment to determine the

ratio of CO2 to water.

CONCLUSION

Using our laboratory apparatus, we have now

successfully explored three key aspects of using CO2

as an alternative to water in geothermal systems.

During the initial injection of CO2 into a previously

water-saturated sample, we found that preferential

flow patterns strongly affect the heat production of

the system. Our single-phase experiments have

shown that heat production and the temperature

profile of the system is strongly affected by the

injection rate, back pressure, and initial system

temperature. We have also demonstrated that under a

particular set of operation conditions that water and

CO2 have comparable heat extraction rates. We are

currently working on expanding the temperature and

pressure range of our experiments, in order to create

a well curated body of data that can be used to

validate numerical modeling software.

REFERENCES

Brown D. . (2000), “A Hot Dry Roc Geothermal

Energy Concept Utilizing Supercritical CO2

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Fifth Workshop on Geothermal Reservoir

Engineering, Stanford Univ, 233-238

King, S., F. Beck, and U. Lüttge. "On the mystery of

the golden angle in phyllotaxis." Plant, Cell &

Environment 27.6 (2004): 685-695.

Liao S.M. and Zhao T.S. (2002), “An experimental

investigation of convection heat transfer to

supercritical carbon dioxide in miniature tubes”,

Int. J. Heat Mass Transfer, 45, 5025–5034

Magliocco, M., Kneafsey, T., Pruess, K., Glaser S.,

(2011) Laboratory experimental study of heat

extraction from porous media by means of CO2

Proceedings of the Thirty-sixth Workshop on

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National Laboratory, Berkeley, California,

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Pruess, K. and Spycher, N. (2010) Enhanced

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