Post on 06-Mar-2021
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.
TABLE OF SYMBOLS