GRC Transactions, Vol. 38, 2014

549

Test-Stand for Geothermal Borehole Probes

Benedict Holbein1, Jörg Isele2, and Luigi Spatafora3

1M.Sc., Responsible for Cooling System Development 2Dr. Eng., Team Leader Geothermal Group at IAI: ZWERG Project

3M.Sc., Material Expert of ZWERG ProjectInstitute of Applied Computer Science IAI, Karlsruhe Institute of Technology KIT,

Herrmann-von-Helmholtz-Platz-1, Germany

KeywordsDeep geothermal energy, borehole tools, investigation & exploration, geothermal research, engineering development

ABSTRACT

The Test-Stand for geothermal borehole probes is an engineer-ing project, aiming at the support of a fast and target-oriented development of new borehole tools, usable in the Geothermal Energy field. Therefore it provides the possibility to test single components and complete systems under realistic conditions.

It is part of the ZWERG project which aims at the develop-ment of a system-platform for the standardized and affordable engineering of different borehole-tools.

The project is time-consuming, cost-intensive and challeng-ing in scientific and technical way. One problem is to identify the borehole conditions in the right way, since the geology and thermal condition of wells varies strongly between different regions and depths. This takes effect especially for the chemical composition of thermal waters.

The Test-Stand is therefor designed in a modular way, which allows the exchange of different components and a step for step enhancement of the complete setup.

In a first step the surrounding temperature of boreholes up to 250°C can be simulated to test a borehole cooling-machine, for the cooling of standard electronics below 70°C without time-limitation. In addition to the special cooling-system components, basic modules like casings and borehole-sensors for pressure- and temperature measurement can be tested. The components are heated up, using heating bands and jackets. A big deep fry allows testing components at a hot liquid environment. Like this, an operation in an environment with up to 250°C can be simulated, to test the functionality and performance of the cooling-machine under extreme conditions.

In a next step the borehole surrounding pressure shall be simulated as well. For that reason an autoclave with adequate di-mensions to receive complete borehole-tools, which can generate pressure up to 600 Bar at temperatures up to 250°C is planned.

Thus it will also be possible to test the chemical influence of the borehole water on different materials and functions.

Introduction

The Geothermal Energy sector has a high potential, which is often rarely used. Besides the economic difficulties linked to this new technology, there are also problems with the social accep-tance, especially in Europe. In Germany the Geothermal Energy has a hard time competing with wind-, solar- and the growing carbon-based energy. Although it provides the important aspects for a base-load-supplier, it´s missing the needed political and social support to take an essential role in the energy supply of the future. One basic reason is the lack of knowledge about the conditions and processes in and around the boreholes, deep in the earth crust. This lack is responsible for a big insecurity on the side of investors

Figure 1. Structure of the ZWERG project.

550

Holbein, et al.

and fears and concerns on the side of the citizens, which block the expansion of this energy supplier. To improve the situation it is evident to realize widespread and affordable possibilities for the investigation of geothermal boreholes. The project ZWERG is committed to provide a way of developing various investigation and interaction devices in a time- and money-saving way.

The Test-Stand, described in this paper is an important element for further development efforts. It will make the designing pro-cess more effective and increase the reliability of the engineered components.

The ZWERG project

The work for ZWERG began in 2011. It follows a system-platform strategy with standardized basic modules and a storage system for the knowledge about engineering solutions and suppli-ers [1]. The target conditions for the developed probes are based on the borehole conditions in a well in Soultz-sous-fôret, France at a depth of approximately 5000 m [2].

As first tools, a development project for the video inspection system GeoKam is currently running, the COBOLD project for the development of a borehole cooling-machine will start soon and a project for taking and conserving samples under original conditions is planned. One important factor of ZWERG is the acquisition of required materials. For the casing i.e. Inconel 718, a nickel-based alloy with yield strength (Rp0.2%) above 1000 MPa at 200 °C and high corrosion resistance is needed. Table 1 shows the prices of different suppliers for the dimensions used in ZWERG probes.

GeoKam

The first probe “GeoKam”, a video inspection tool for deep boreholes, is currently being developed within a BMU (Federal Ministry for the Environment, Nature Conservation and Nuclear Safety) -project, in cooperation with the company BRG, which provides well inspection services. The first prototype has been exhibited at the Trade Fair “Geotherm” in Offenburg, Germany in February 2014. Figure 2 shows the exhibited system connected to a Wireline. GeoKam allows producing life-videos with high

solutions inside boreholes with surrounding temperatures up to 200°C. Different cameras inside, in combination with a specific light-management can film in front and in radial direction. It is possible to focus interesting spots to detect i.e. flaws in the borehole-casing. This could be a starting point for further devel-opments of casing-repair-tools for operations on the spot. The probe-casing, including trans-parent ceramic windows withstands pressures up to 600 Bar [4]. The maximal outer diameter is 95 mm (~3.74 inch), so the probe can be used in 4 ½ inch boreholes. A PCM (Phase Change Material) based cooling-system, including heat-pipes, avoiding the danger of overheated components, allows operation times of 6-8 hours in a 200°C environment [5]. The project will be completed with a field test near Mu-nich, Germany in summer 2015.

Borehole Cooling-Machine

The borehole cooling-machine will allow the usage of standard electronic components even in hot environments with temperatures above 200°C without time limitation. To realize this, it conducts a thermodynamic cycle process. The principle is similar to the one used in most refrigerators but at

Table 1. Inconel 718 pre-product prices in Germany (Supplier I – III) and China (Supplier IV – VI) [3].

type L (m)

OD (mm)

t (mm)

ID (mm)

m (kg)

I (€)

II (€)

III (€)

IV (€)

V (€)

VI (€)

Pipe

170 6 168.3 14.2 139.9 324 74 220 26 602 26 603 21 639

170 6 168.3 19.05 130.2 43 860

95 6 101.6 11 79.6 147 36 600 12 115 12 582 10 623

95 6 101.6 9.5 82.6 20 850

95 6 101.6 17.5 66.6 218 36 600 17 894 17 894 15 691

95 6 101.6 19.05 63.5 29 520

15 9 16 2 12 6 27 000 32 400 525 846 442

5 90 6 1 4 11 31 500 36 900 14 720 936 2 641 811

Rod

170 6 168.3 1064 69 000 35 101 25 427 33 111

170 6 180 47 800

95 5 95.25 284 15 000

95 5 101.6 400.6 12 791 10 135 12 066

95 5 105 13 221

15 9 16 14 990 680 476 463 494

Sum 290 910 225 231 106 439 96 591 94 931

OD=Outer diameter, t=Wall thickness, ID=Inner diameter, L=Length, m=Weight

Figure 2. Photo of the exhibited GeoKam.

551

Holbein, et al.

completely different temperature and pressure levels and in an extreme environment. That´s why the used components have to be custom built with appropriate dimensions for the use within borehole probes. The main components, shown in Figure 3 are the inner heat-exchanger (evaporator) where heat from the cooled components and the cooled room is transferred to a refrigerant, the compressor which compresses the gaseous refrigerant to a higher pressure where it condenses in the outer heat-exchanger (condenser), at a temperature above the environment temperature and finally the throttle, where the condensed refrigerant expands back to start -pressure and -temperature. All of them have to fulfill the constraints of mechanical strength, corrosion and temperature -resistance and frame-size corresponding to the borehole-probe operation conditions.

In Figure 1 the main components can be seen as they are designed for a borehole probe with an outer diameter of 170 mm (~6.69 inch), which shall be usable in boreholes with 8 ½ inch diameters. The casings and outer components like the condenser are made of Inconel 718. Some electronic boards, mounted on the evaporator surface, as example for an application are illustrated too. The cycle process is shown in a logarithmic pressure-enthalpy diagram in Figure 4.

The example process (Fig. 4) is based on a prognosis program for the setting and validation of the Test-Stand Experiments [6]. It shows the process with acetone as refrigerant conducted at a surrounding temperature of 150 °C. The sub-processes are:

• Isothermal evaporation at ~56.5 °C• Polytropic compression to ~15 Bar• Isothermal condensation at ~170 °C• Isenthalpic expansion to ~1 BarThe cooling capacity, Qc, and the needed compression-work

effort, Pn, can be estimated graphically, using the enthalpy dif-ferences readable from the diagram.

Qc =dmdt* ∆hevaporation (1)

Pn =dmdt* ∆hcompression (2)

In the equations, dm/dt is the mass-flow controlled by the com-pressor frequency and Δh are the particular enthalpy-differences. A more exact way to calculate the compressor work is to use the formulation for a polytropic transformation, based on the idealistic gas approach:

Pn =dmdt* R*T1*

nn−1

⎛⎝⎜

⎞⎠⎟* p2

p1⎛⎝⎜

⎞⎠⎟

n−1n−1

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥

(3)

Therefore the specific gas constant R, the start temperature T1, the pressures p1 and p2 and the polytropic-exponent, n, are used.

An alternative way to calculate the usable cooling-capacity is to assume a liquid ratio after expansion x and to combine it with an empirical estimation for the evaporation-enthalpy of the refrigerant (acetone):

Qc =dmdt* x* A* 1− Te

Tc⎛⎝⎜

⎞⎠⎟

B+C* TeTc

⎛⎝⎜

⎞⎠⎟+D* Te

Tc⎛⎝⎜

⎞⎠⎟

2

+E* TeTc

⎛⎝⎜

⎞⎠⎟

3⎡

⎣⎢⎢

⎤

⎦⎥⎥

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥

(4)

The critical temperature, Tc, and the evaporation temperature, Te, have to be known as well as the substance coefficients A, B, C, D and E, listed in Table 2 [7].

Table 2. Substance coefficients for evaporation enthalpy of acetone.

A B C D E

726524 0.34430 -0,007167 0,001877 0,001354

Depending on the used equations and parameters the process-values which can be calculated, vary slightly. Table 4 shows the values for the named approaches for two different surrounding temperatures based on the parameters listed in Table 3.

Figure 3. Scheme of the Borehole Cooling System COBOLD.

Figure 4. log. P-h diagram of the cooling process with acetone at 150°C.

Evaporation-temperature Tc (°C) 56.5Gas-constant R (J/kg-K) 143.15Start-temperature T1 (°C) 56.5Start-pressure p1 (Bar) 1Polytropic-exponent n (-) 1.13Critical-Temperature Tc (°C) 235.15Mass-flow dm/dt (kg/h) 2

Table 3. Parameters for acetone process.

552

Holbein, et al.

The Test-Stand shall generate widespread process data to get reliable process-values for validation and the engineering of the systems.

Structure of the Test-Stand

Based on the described cooling system, the structure of the Test-stand provides a separation of the high-pressure from the low pressure level. This is practical for the experiment-construction and observation. Figure 5 shows a photo of the Test-Stand [6].

The pressure level separation is realized by a laboratory platform. At its top, a big deep fry can be seen where a liquid environment with high temperatures can be simulated to test the condenser installed inside. At the bottom in the middle, the cool-housing covered by a heating-jacket is placed. Inside, the

evaporator and the electrical loads which have to be cooled are mounted. At the right hand is the laboratory compressor, which is connected to the evaporator outlet. It is pneumati-cally driven and can be adjusted to different pressure ratios and mass-flows. The throttle, placed below the floor of the 2nd level connects condenser and evaporator.

The components and pipes are heated up by heating-bands, which can simulate different outer temperature-profiles. The temperatures, inside and outside the components are logged with a huge number of thermocouples. Additionally pressure

sensors and manometers are included which generate pressure data. The mass-flow is measured indirectly at the moment, using the compressor frequency or the calculated cooling-capacity. Ap-propriate sensors for a concrete flow measurement aren´t available so far, but their integration is planned.

The whole system can simulate surrounding temperatures of 20-250 °C.

With the heating-management, the operation of the system in another depth with higher temperature can be tested. Higher ambient-temperatures are not only challenging for the components functionality but also change the thermodynamic process deci-sively, like Table 4 and Figure 6 show. Experiments with variations of these parameters are important to prove the functionality of the systems under extreme conditions and to detect potentials for possible optimizations.

The Test-stand is constructed in a modular way, so compo-nents can be tested separately as well. It can be enhanced by new measurement devices and test-tracks.

An autoclave, which will allow managing the simultaneous simulation of pressures and temperatures in high depths with realistic ambient-fluids, is in planning phase.

First Test Results

The first experiments conducted with the help of the Test-stand infrastructure, are addressed to the throttle of the cooling-system.

Table 4. Process values of acetone process with different approaches.

Surrounding-temperature

(°C) Approach

End- pressurep2 (Bar)

Compression-work

Pn (W)

Cooling-capacityQc (W)

Heat-output

(W)

150graphical 14.9 125.4 186.2 311.6

empirical/polytropic 14.9 91.4 191.9 206.7

210graphical 44.9 196.7 127.5 324.2

empirical/polytropic 44.9 132.9 131.3 36.2

Figure 5. Photo of the Test-stand.

Figure 6. Log. p-h diagram of the cooling process with acetone at 210°C.

553

Holbein, et al.

To realize the expansion in the needed way, through reducing the pressure and the temperature, the Joule-Thomson-effect is used. While the fluid expands, the attractive forces between the fluid parts have to be overcome. Assuming an adiabatic process, where no energy-exchange with the surrounding is possible, the energy-difference has to be balanced by the inner-energy of the fluid itself (Fig. 7). This leads to a decrease of the kinetic energy of the fluid parts, which means the fluid cools down. For the throttle engineering, two different options to expand the fluid are consid-ered. The first option is a capillary which reduces the pressure by pipe-friction. The second option is an expansion valve, adjustable via a jet needle. Prototypes for both options are constructed and get tested in the Test-stand.

On way of estimation the reachable pressure reduction Δp is a combination of an incompressible pipe-friction approach and a compressive approach for ideal gases [8]:

∆ p = X * (Closs *ρ*u2

2)+ 1− X( )*

p1 − p12 − p1

2* C* u2* TmR*T1

2

⎛

⎝⎜

⎞

⎠⎟

(5)

In equation 5, X is the amount of fluid regarded as incom-pressible. Tm is the middle temperature before and after the expansion. C is a coefficient reflecting the pipe-friction, u is the estimated fluid speed. R is the gas-constant, ρ is the density of the fluid and p1 and T1 are the start pressure and temperature. Figure 8 shows an experiment result conducted with acetone in a capillary with an inner-diameter of 0.4 mm and a length of 3 m. The temperature-time profiles inside and outside the throttle-track and the pressure drop are detected by sensors.

It can be seen how that the expansion strongly decreases the fluid temperature. Although there is a heat-exchange with the environment, clearly shown by the increase

of the throttle-environment temperature (green curve), the time period of the temperature decrease is the short time period definitively shows the throttle-effect. An interesting point is the very low outlet temperature which is below the laboratory ambient temperature (20°C), although it started at above 180 °C. An explanation for this effect is the heat-transfer to the environment where energy gets lost, so the expansion causes a higher cooling. In the borehole cooling-process, the environ-ment temperature is almost as high as the start-temperature of the expansion-process, thus the transformation there is nearly adiabatic. For this reason, the expanded fluid still contains the whole energy and the temperature after expansion is equal to the evaporation temperature 56.5 °C.

Table 5 shows the measured and calculated expansion values.

Outlook

In the next months, further experiments of sub-processes as well as component test will be conducted. The complete cycle process will be running, while parameters like surrounding tem-perature, inner load input and mass-flow get varied.

Simultaneously the ZWERG work, including material acquisi-tion, component engineering and the database expansion goes on.

If everything goes right, the official start of COBOLD is expected for summer 2014. This will offer more support for the project efforts and push the development of the borehole cooling-machine strongly forward.

Table 5. Comparison of measured and calculated capillary expansion-process values with acetone.

Inner-diameter

(mm)Length

(m)

Start- temperature

T1 (°C)Density

ρ (kg/m^3)

Liquid amount X (%)

Mass-flow

(kg/h)

Fluid-speed

u (m/s)

End- temperature

(°C)

Pressure- reductionΔp (Bar)

Calculated

0.4 3 186.8 541.89 61.7 2.12 8.64 18.3 51.5

0.4 3 186.8 513.65 52.6 2.12 8.64 71.5 45

Measured

0.4 3 186.8 548.86 84 2.12 8.5 18.3 45

Figure 7. Scheme of the Throttle-Effect.

Figure 8. Experiment results of capillary throttle experiment.

554

Holbein, et al.

The ZWERG-tools will give further insights in the boreholes and deliver new possibilities of data recovery. By doing so it shall help advancing the Geothermal Energy sector in general.

References

[1] Karlsruhe Institute of Technology KIT Geothermal Website, Dec.13, 2013 “http://geothermiewiki.iai.kit.edu/index.php” (accessed February 10, 2014)

[2] Bresee J. et al, 1992. “Geothermal Energy in Europe - The Soultz Hot Dry Rock Project”, Gordon and Breach Science Publishers, Montreux, v.4, p.17.

[3] Isele J. et al, May 2014. “Standardisierte Bausteine für Geothermieson-den (Engl. Standardized Bricks for Geothermal Probes)”, Technic: Geothermal Energy, BBR Journal for Well and Pipeline Contruction and Geothermal Energy.

[4] Spatafora L. et al, February 2014. “Video Inspection Probe for deep Geothermal Boreholes”, 39th Geothermal Workshop Feb.24-26 2014, Stanford California.

[5] Holbein B. et al, February 2014. “Cooling System for Borehole Tools”, 39th Geothermal Workshop Feb.24-26 2014, Stanford California.

[6] Holbein B., April 2014. “Entwurf, Aufbau und Instrumentierung eines Test-Stands für die Entwicklung einer Bohrloch-Kältemaschine (Engl. Draft, Contruction and Instrumentation of a Test-stand for the deveopment of a Borehole - Cooling-machine)”, Master-Thesis, Karlsruhe Institute of Technology KIT.

[7] VDI - Heat Atlas 2006, Society of German Engineers VDI – Association for Process Technology and Chemical Engineering GVC (Ger. VDI-Wärmeatlas, Verein Deutscher Ingenieure VDI, GVC, Gesellschaft Verfahrenstechnik und Chemieingenieurwesen), Springer Publisher, Heidelberg, Germany, 2006.

[8] Windisch H., 2006. “Thermodynamik - Ein Lehrbuch für Ingenieure (Thermodynamics - A Textbook for Engineers”, Oldenburg Scientific Publisher, Munich, v.2, p.110ff.