PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 11-13, 2013

SGP-TR-198

DESIGN PARAMETER ACQUISITION OF AN UNDERGROUND HEAT STORAGE AND

EXTRACTION SYSTEM – A DEEP BHE ARRAY IN A KARSTIC ALPINE MARBLE

AQUIFER FOR 1 GWH POWER

Ingo Sass & Clemens Lehr

Technische Universität Darmstadt - Institute of Applied Geosciences

Chair of Geothermal Science and Technology

Darmstadt, Germany

e-mail: sass@geo.tu-darmstadt.de

ABSTRACT

Borehole Heat Exchangers (BHE) are useful for both:

underground heat extraction and underground heat

storage. Therefor such systems are applicable for

heating and cooling purposes and are valid to deliver

basic load into the infrastructure.

To dimension such a geothermal array it is necessary

to explore the geophysical and geological conditions

of the subsoil. At the following example the project

engineering of a prospective geothermal array is

shown from the investigation up to the execution

design. The investigation drilling was executed in the

following steps:

- Drilling and recording of the geologic

profile.

- Mounting of a Duplex BHE and fiberglass

hybrid cable into the borehole

- Measurement of the rock-physical

parameters by means of an enhanced GRT

with a spatial depth resolution of 0.5 m (1.64

ft.).

- Detection of ground water flow by analyzing

the measured geophysical parameters.

- Calculation of the Darcy flow in as ground

water-leading identified horizons by means

of Peclet number analysis.

- Use of the measured data in a simulation for

the conceptual design of the prospective

geothermal array.

The geothermal array should be installed in a

mountain region of the Austrian Alps. For the

geothermal investigation a 400m (1,312 ft.) deep

wellbore was drilled and equipped with 50 mm (1.97

in.) duplex BHE. With the mounting of the BHE a

fiberglass hybrid cable was inserted as a loop parallel

to the shanks of the BHE. In the following the

drilling was filled with thermally optimized grout.

INTRODUCTION

The energy demand of the Hotel Resort Expansion

was determined with 986 MWh, with heating base

load power at 22 MWh (summer) and cooling base

load 15 MWh. The installed power of three heat

pumps was added to 378 kW. The monthly heating

and cooling load data are given in Tab. 1. The 25 X

12.5 m outdoor swimming pool requires about 6.000

h/a heating power.

Table 1: Energy demands of the hotel expansion

The heat to be stored underground will come from

the hotel laundry and the saunas in the wellness area.

In a preliminary design a geothermal array with 12

BHE each 400 m deep was designed applying the line

source approach using conductive thermal

conductivities of 2,5 to 2,8 W/(m·K). With an

investigatory BHE drilling the geological setting and

with DTS (distributed thermal sensing) the thermo-

physical properties of the formation were explored.

OPTICAL-FREQUENCY-DOMAIN

REFLECTOMETRY

The investigation drilling was performed and

completed with Double-U-Type Polyethylen BHE

equipped with additional hybrid glasfiber-copper-

cable as known for distributed temperature sensing .

Figure 1 gives a scheme of the realized completion

design.

Figure 1: Schematic illustration of the realized

completion of the investigation BHE with

the hybrid glasfiber-copper-cable (black

circles in the cross sectional drawings)

The built in hybrid cable carries along a copper cable

as a heating wire beside the fiberglass. The copper

cable was connected to an electrical power source

and therefore a thermal impulse was generated. The

heating power is identical along the heating wire at

every place of the hybrid cable. A controller

measures the resistivity of the copper leader during

heating phase and holds by adaptation of the voltage

and the amperage the applied electric power steady.

The undisturbed temperature of the subsoil and the

temperature rise of the system are recorded by means

of Optical-Frequency-Domain-Reflectometry method

(OFDR). On this application, the fiberglass itself is

the temperature sensor. Temperatures of from -200 (-

328 °F/73.15 K) to 400 °C (752 °F/ 673.15 K) can be

measured with the OFDR. By use of a laser diode a

frequency-modulated optical signal is sent into the

fiberglass. The optical impulse is scattered and split

in a Raman and Raleigh part of the signal. The

impulses are reflected by the fiberglass

proportionally.

Figure 2: Temperature depending displacement of

the Raman spectra in a glass fiber cable.

A temperature depending phase shift of the optical

spectra of the Raman parts (Stokes and Anti-Stokes)

enables the calculation of the temperature at its place

of origin (fig. 2). An exactness of 0.02 K is

attainable. The run time analysis of the back scattered

light leads to a spatial resolution up to 10 cm (3.94

in.).

The presented installation was measured with a

spatial resolution of 0.5 m (1.64 ft.). The evaluation

of the recorded temperature curves follows Kelvin’s

line source theory. For every detecting point along

the hybrid cable the effective thermal conductivity of

the surrounding rock can be determined thus. A high

local resolution over the whole profile enables to

differentiate high conductive sections from

convectively influenced heat transfer zones.

Applying these parameters in a Peclet-Number-

Analysis (Zschocke, 2005) the groundwater flow

patterns adjacent to a particular BHE can be

calculated.

With the help of the ascertained geophysical and

hydraulic rock parameters solid rock, cleavages and

karst cavity could be identified. Also the undisturbed

ground temperature, the effective thermal

conductivity and areas with different geothermal

gradients and the groundwater velocity in cleaved

and caveated rocks were determined.

The measuring results lead into optimized design

procedures for the BHE array. The required number

of BHW each drilled to 400 m TVD was reduced by

3 to a total number of 9. The marble karst system

could then be explored by using downhole airlift

hammer technology which typically is not applied to

that particular depths and complicated geological

formations. Drill Path / trajectory

To ensure vertical boreholes at these depths heavy

drill pipes (etc.) must be used. The boreholes are

expected to show less deviation from the planned

trajectory than would be possible with shallower

down-the-hole-hammer bores. Employing a

directional drilling method is economically not

feasible. In order to minimize possible mutual

interference between the individual boreholes are

sunk slightly off-set from each other. The actual

borehole trajectory will primarily be deflected by the

stratification, joints and cleavage of the Hochstegen

series.

Due to the nature of the drilling process the

termination point of the borehole cannot be precisely

predicted. Since the exploratory borehole showed the

Hochstegen marble to be dense and mechanically

homogeneous, it can be assumed that deviation of the

other proposed boreholes should generally be in the

same direction.

GEOLOGICAL PROFILE

Geological analysis of the existing exploratory

borehole revealed the geological profile at the site.

A survey of the local hydrogeological conditions was

conducted by means of the information obtained by

the drilling of the exploratory borehole in

July/August 2012. Near-surface geological

information was continuously collected during

drilling, starting May 23rd 2012 through to mid-July.

The geographic position of the exploratory borehole

is: R: 0714107 H: 5226212, elevation 830 m. a. s. l.

The project area/site is situated at the lower end of

the Tuxertal/Tuxer Valley above Mayrhofen in the

Zillertal/ Ziller Valley in close proximity to the

Tuxbachklamm/Tuxbach Gorge (distance ca. 150 m,

depth about 100 m). The aquifer shows

characteristics of both a karst aquifer and a jointed

aquifer. Areas of intense karstification have been

detected/proven. Transfer of this knowledge to the

project site is generally possible due to the geological

information obtained so far. It appears that the

Hochstegen Marbles are steeply inclined, and the

exploratory borehole has not passed through the

dolomitic marbles. This makes it reasonable to

assume, that the other proposed drillholes will neither

encounter other strata of the schist envelope than

those of the Hochstegen Series or the Ahorn Gneiss

Core. With this exception the predicted geological

and hydrogeological conditions were confirmed by

the exploration.

Perennial groundwater flow follows the slope of the

valley down towards Mayrhofen (640 m a.s.l.).

Presumably the depth of the water table below the

ground surface is subject to great seasonal variations.

A continuously connected groundwater table can

only be expected to form at the level of the bottom of

the valley at Mayrhofen. The project site is situated at

ca. 845 m. a. s. l., resulting in continuously saturated

conditions to be expected between 170 to 200 m

below the ground surface. The evaluation of the

results of the EGRT suggest in all likelihood that

contact with the groundwater table of the

Zillertal/Ziller Valley occurred at 180 m depth (650

M. a. s. l.).

Due to additional inflow of groundwater from the

mountains the hydraulic head can be higher in certain

layers. Combined with the secondary porosity of the

rocks of the Hochstegen Formation it is possible for

localized slight artesian conditions to occur. For these

specific proposed boreholes the occurrence of such

slight artesian conditions is highly unlikely, as

several surface springs expose mainly the near

surface groundwater inflow and naturally reduce the

hydraulic head. The hydraulic head of the deep

aquifer in the valley is much too low to develop an

artesian head at the proposed drill sites.

To begin the measurements (fig. 3) the temperature

distribution in the completed borehole was recorded,

followed by an EGRT (Heidinger et al., 2004) with

temperatures determined by OFDR. On the basis of

these measurements the effective thermal

conductivity could be deduced as a function of depth.

Average effective thermal conductivity of the rocks

penetrated by the exploratory borehole is 4.86 W/

𝑃𝑒 =𝑞𝑎𝑞𝑐

= 𝜌𝑐𝑝𝑣𝑓∆𝑇

𝜆 𝛥𝑇𝑙

(m·K). This average value includes both the

conductive and convective thermal flow.

Figure 3: Geological profile and geophysical

properties determined with OFDR in the

investigation drilling

The average undisturbed underground temperature

was 15.2 °C. The near-surface temperature was 12.2

°C; the temperature at the final depth was 19.5 °C.

The average value was calculated using all measured

values from the ground surface to the final depth,

thereby precisely establishing the actual thermal

underground conditions. Therefore the calculations of

the geothermal array need not rely on estimated heat

flux data.

In the thermal conductivity log (fig. 4) some

sections/horizons of increased thermal conductivity

stand out which can be explained with increased

groundwater flow. From the ground surface to 200 m

depth two horizons with increased thermal

conductivity are detected, at which the horizon from

180 m to 198 m is in good agreement with the

expected depth of the hydraulic head of the lower

aquifer in the valley. Smaller peaks represent

conductive joints or sets of cleavage, which have not

been evaluated in particular. Also inhomogenities can

be observed, attributed to variable thermal properties

of the rock. Beyond 200 m depth the increasing

influence of secondary porosity is observed, resulting

in a rapid increase of effective thermal conductivity

from 280 m on. At 315 m, 332 m, 361 m and 375 m

very intense groundwater flow is detected. The

values lie within the range usually associated with

hydraulic karst systems. At 332 m the peak value of

about 14 m/d (1.6·10-4 m/s) for the filter velocity was

detected. This value is a strong indication for karst-

based groundwater flow in this area. The effective

conductive thermal conductivity increases from ca.

2.0 W/(m·K) to 3.4 W/(m·K) at 145 m depth. These

values provided the basis for the calculation of the

groundwater flow using Péclet analysis. Below 145

m depth the effective thermal conductivity is largely

dominated by conductive heat transfer.

The hydrogeological system can be regarded as stable

and sustainable regarding the proposed thermal

utilization. Judging from the depth of the karst

aquifer it is probably the main way of drainage of the

Tux Valley. Due to the overall geological situation it

is not expected that the aquifer is subject to a great

variability of hydraulic head, typical for karst

aquifers. Therefor it is reasonable to include the

convective heat transfer not only through the

increased thermal conductivity value, but also in the

long-term perspective.

Furthermore the EGRT proved that dimensioning the

BHE to the relatively great depth of 400 m is

thermally beneficial, as the technically possible heat

extraction would have been significantly less with

final depths of only 200 or 100 m. The geothermal

gradient increases with depth. As a result the number

of BHE was settled for 8 after 8 to 12 being stated in

a preliminary schematic design of the proposed

project. The exploratory borehole will be integrated

as a BHE for additional security and unforeseen

operating conditions of the geothermal array.

BOREHOLE PRESSURE CONDITIONS

The BHE pipes should be designed for pressures of

up to 12 bar. The maximum approvable pressure is

16 bar. These pressures are unproblematic with

regard to the pipe material at the temperature range of

geothermal heat extraction. The possible pressure

regimes require a continuous backfill of the annular

space and a specifically adapted approach to the

backfilling process. Additional instructions given in

Swiss standard SIA 384/6 should be observed, also

applying to the procedure described below, of which

modifications are permissible. The pressure-depth

tables stated therein can be extended linearly to

400 m depth.

The use of the thermally enhanced grouting material

is thermally necessary. Fully hardened it features a

thermal conductivity of 2.0 W/(m·K). This value is in

the range of the thermal conductivity of the matrix of

the surrounding rock. A lower thermal conductivity

could result in an over-dimensioning of the

geothermal installation.

A compressive strength of 6.0 N/mm and hydraulic

conductivity of less than 1 x 10-10

m/s is reached after

28 days. The system combination of borehole wall

and grout backfill fulfills the criteria of a permanent

seal against migration of gas or liquid. Special

pressures or squeezing rock are not expected after

assessing the findings of the exploratory borehole.

Also no swelling clays or other minerals were

detected. Therefor it is assumed that the selected

grouting material will provide a permanently firm

and impermeable backfill, satisfying the safety

requirements.

Figure 4: Determination and correlation of the

effective thermal conductivity with the

groundwater flow regime.

The slurry density of the grout is 1.460 kg/m³. At

400 m depth in the slurry-filled borehole 58.4 bar of

fluidstatic pressure act on the BHE tubes. This

situation is used as the rated value for subsequent

dimensioning until the grout slurry has partially set.

After 20 hours the partially set grout exhibits shear

strength of 4 kPa. This rate of hardening applies at

10 °C. This process will be accelerated in the

proposed boreholes, as the temperature between 200

and 400 m depth is 17.5 °C on average. Tests are

conducted by the manufacturer to determine the

applicable setting time for these temperatures. It can

be assumed to lie between 15 and 18 hours. This

implies that backfilling the borehole in two stages

leads to added safety against collapse, if a setting

time of 20 hours is observed between the two stages.

During the first stage 49.2 bar act on the BHE tubes

at the final depth. Inside the waterfilled BHE tubes

40 bar act as counter-pressure, thus meeting the

requirements of pressure rating (max. 16 bars)

without pressurizing the BHE tubes. In the top 200 m

of the second stage a maximum pressure of 29.2 bar

acts on the tubes by the grout slurry, compensated

largely by a counter-pressure of 20 bar of water

pressure inside the tubes without additional

pressurization. Therefore cementation will be

completed in two stages.

To increase the safety against collapse during

cementing the BHE tubes will be pressurized with 6

to 8 bar (calculation value 8 bar) to be established by

pressure gage and documented accordingly. This

results in 48 bar pressure at 400 m depth inside the

BHE tubes acting against an outer fluid static

pressure of 49.2 bar or 58.4 bar (if borehole is

completely filled with fluid slurry). With these

measures the required safety margins are fulfilled.

The continuous hydraulic head of the main aquifer is

located at ca. 170 m below the ground surface. Due

to joints and karstification there exists a certain

degree of hydraulic communication between the

aquifer in the valley and the aquifer on the slopes of

the valley. This leads to groundwater level rising to

10 to 25 m below ground surface in the borehole.

Taking seasonal fluctuation into account, a rated

value of 100 m below ground surface is on the safe

side.

Considering the eventuality of faulty cement grout at

400 m below ground surface resulting in a breach of

the seal would result in hydrostatic pressure of 30 bar

acting on the BHE tubes. The inner pressure of the

BHE tubes filled with a 30 % mixture of mono

ethylene glycol and water due to its slightly higher

density (1,100 kg/m³) is 41.3 bar. The pressure

difference lies within the safe operational range of

the HD-PE tubes.

Top of cement needs to be established either by

sounding or thermal log. If this is technically not

possible the guidelines given in SIA 384/6 regarding

section by section cementing are applicable.

SUMMARY

Geothermal heat extraction will be realized through

400 m deep boreholes, completed with double-U-

pipe borehole heat exchangers (BHE) using PE-HD

DA 50 mm pipes. The annular gap will be filled and

sealed with thermally enhanced grouting material by

grout injection pipe in two stages (ending at 200 and

400 depth).

The heat exchanger pipes will be filled with a

mixture of 30 % Mono-Ethylene glycol and water as

heat transfer fluid.

Dimensioning of the geothermal array was performed

using the analytic Earth Energy Designer (EED) 3.16

software. Analytic calculations are based on the

results from the EGRT performed on the exploratory

borehole and the information on the utilities of the

building. Several variations were examined in the

dimensioning process resulting in a preferred variant.

REFERENCES

Heidinger, G., Dornstädter, J., Fabritius, A.,Welter,

M., Wahl, G. and Zurek (2004): EGRT –

Enhanced Geothermal Response Test. Proc. 8.

Geothermische Fachtagung, 316-323.

Heldmann, C.D. (2013): Die hydrothermalen

Vorkommen im Zillertal (Hydrothermal springs

in Zillertal). - Unpublished master thesis,

Institute of Applied Geoscience, Technische

Universität Darmstadt

Sass, I., Lehr, C. (2011): Improvements on the

Thermal Response Test Evaluation Applying the

Cylinder Source Theory. Proc. Thirty-Sixth

Workshop on Geothermal Reservoir Engineering

Stanford University, Stanford, California.

Swiss Standard (Schweizer Norm) (2010): SIA 384/6

Erdwärmesonden (Borehole Heat Exchangers),

Schweizerischer Ingenieur- und

Architektenverein, Zürich.

A. Zschocke (2005): Correction of non-equilibrated

temperature logs and implications for geothermal

investigations. Journal of Geophysics and

Engeneering, 2, 364–371.

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