Final report

ELFO:

Eavor Loop Feasibility for Tilburg (Amer network) and Outlook forapplication in the Netherlands

TKI reference

ELFO: Eavor Loop Feasibility for Tilburg (Amer network) and Outlook
for application in the Netherlands
1921406

Penvoerder

TNO

Consortium partners

Eavor, EnNatuurlijk, Huisman, TNO

Duration
Work package

01/11/2021 – 31/01/2023
WP6 – Final report

Dissemination level

PublicLead author(s)Maartje Koning (TNO)

Contributors

Hans Kol (Eavor), Stephen Longfield (Eavor), Harald Droog (ENN),
Arthur de Mul (Huisman), Jan-Diederik van Wees (TNO), Hans
Veldkamp (TNO)

Date

Date: 30/08/2023

Version

2.0 (revision 1)

Project name

The ELFO project has received funding through Topsector Energy (TKI Urban
Energy programme) and these financial contributions are gratefully
acknowledged. The contents of this publication reflect only the author’s viewand do not necessarily reflect the opinion of the funding agencies.
Het project is uitgevoerd met PPS-programmatoeslag subsidie van het
Ministerie van Economische Zaken en Klimaat voor TKI Urban Energy,
Topsector Energie. www.tki-urbanenergy.nl.

TKI reference 1921406 – Public

Executive summary
The energy generation and consumption in the Netherlands is slowly but steadily shifting away fromfossil fuels towards renewable energy sources, such as offshore wind, and solar, thereby reducing
greenhouse gas emissions. In order to improve renewable energy security and affordability, othersources of renewable energy, like geothermal energy, need to become part of the energy mix in thenear future. At present, the majority of geothermal wells in The Netherlands are shallow wells, but
deeper wells with high power output will be required to make a larger contribution to the overall
energy supply. In this light, the ELFO project assessed the feasibility and economic viability of a deep
well system: the Eavor Loop, consisting of 12 multi-laterals. The consortium of TNO (independent
research institution), Eavor (geothermal energy company), Huisman Geo BV (drilling equipment &research) and EnNatuurlijk (operator of district heating) evaluated the suitability of this technology as
primary heat source for city heating networks in the Tilburg area and for possible wider adaption in
the Netherlands.
The results of the project contribute to the objective of the TKI Urban Energy scheme for Sustainable
Heat Networks and Geothermal Energy (MMIP 4) by demonstrating that an Eavor Loop system for the
Tilburg area can be seen as a possible option for a renewable heat source for the existing Amer heat
network. Within the current market, deployment of the technology requires CAPEX subsidy and SDE++
guaranteed pricing. However, the study identified the possibility to further reduce drilling costs by
20%, making it commercially viable without CAPEX subsidy.
These conclusions are based on the following ELFO project components:
A Subsurface feasibility study on the feasibility of using the Eavor Loop™ technology to extract
geothermal heat as a source for the Amer heat network in the Tilburg area. This study focused on thegeological and geomechanical interpretation and modelling of the subsurface in an economic context
and identified a suitable rock formation for placing the full loop system. A surface screening identified
several suitable surface locations from where the base case well design of 12 horizontal multi-laterals
could be drilled and operated Unfortunately, the preferred city surface location was unavailable,
resulting in the selection of a surface location that posed limitations to the optimal well design at
depth. The horizontal length of the laterals therefore had to be shortened from 3,000m to 2,100 m
length as the horizontal legs now had to be drilled updip into the reservoir instead of slightly downdip.
For the most optimal scenario, the heat output was calculated using the Eavor heat flow model, andthe economic feasibility was based on an estimated CAPEX of € 54.5 MM to construct the Tilburg
Eavor-Loop™, yielding an annual heat production of 74,000 MWh.
A Drilling optimisation desktop study on the design of a more automated and dedicated Eavor Loop
rig for more cost-effective drilling and completion that is able to drill 24/7 in urban environments.
Given that drilling efficiency (and therefore speed) is the key factor affecting drilling costs forgeothermal wells, related parameters such as drill bit design, mud system, power, and rig control
precision have been analyzed. A new design of a dedicated rig constructed that involves automatedpipe handling and reduction of manual handling of bottom hole assembly components that reduces
the time required for mechanical steps when compared to conventional drilling. Furthermore, the new
rig design provides more accurate control of drilling parameters, reducing the need to pull out the drill
string to replace the drill bit, resulting in significant time reductions. To reduce the required footprint
for drilling in an urban environment, a dual activity drilling system (dual-derrick) was compared withdrilling by 2 conventional rigs drilling simultaneously. The study concluded that the dual-derrick option
is preferred saving costs by 16% and reducing drilling time by 43%.
An assessment of environmental impact, safety and CO2 footprint of Eavor Loop based on numericalsimulations over its lifetime. The study assessed the possible occurrence of undesirable effects (i.e.,2

TKI reference 1921406 – Publicleakage, well bore collapse, induced seismicity) and compared the carbon footprint of the Eavor Looptechnology with conventional geothermal applications. As part of the evaluation, thermal
performance and Eavor Loop construction / operational characteristics were analyzed through a Life
Cycle Assessment (LCA) and quantitative assessment of the CO2 per GJ produced. This showed that
Eavor Loop emissions are 20% improved compared to conventional systems, saving over 2kg CO2eq/GJ. The thermal response of the Eavor Loop for 30 years at constant flow rate conditions wascalculated using a dedicated numerical thermal model. It shows that the Eavor Loop has a long lifetime
marked by a very moderate linear decline of the production temperature and power over a lifetimeof 10s of years, with minor thermal interference of laterals if placed with a spacing of ca 70 m. Basedon the thermal simulations, stress changes were analyzed with a semi-analytical approach, assessing
underlying sensitivities. Results indicated that potential leakage due to issues with borehole-stability
and reactivation of pre-existing faults is not likely to be risks for safe operation of the Eavor Loop overits full life-time.
A system integration study of the Eavor Loop in the district heating network of Tilburg (Amer)
involving the evaluation of two main components: 1) physical connection to the existing grid with thetechnical capacity to handle the thermal energy generated by the Eavor Loop, and 2) simulation ofheat demand and temperature profiles to determine the feed-in capacity of the loop ensuring thequality of heat delivered to customers. The physical connection between the Eavor Loop and the heat
grid was defined as a connection point in the existing grid and a new connection pipeline. The technicaldesign has been modeled with Sishyd and a concept budget has been made, with increasing accuracy
in later project phases. Two simulations have been completed to understand the impact of the Eavor
Loop on heat grid temperatures: one by EnNatuurlijk using historical data, and one by TNO using the
Design Toolkit software. The simulations by TNO show that a temperature drop of the network mainly
occurs in high heat-demand periods and that the heat grid part “Kraaiven” is most affected if nomeasures are taken. Note that the simulations were based on a theoretical model at feasibility stage
and more detailed modeling and optimisation is required to increase simulation model confidence.
Evaluation of the wider implementation of the Eavor Loop technology in the Netherlands. The study
determined the subsurface potential for the Eavor Loop in the Netherlands by comparing subsurface
conditions to Eavor's placement criteria. These criteria were divided into three subsurface categories:
those specific to the formation, those specific to the location, and those specific to the local situation(e.g. characteristics that vary locally and cannot be studied using a national scale data base). The study
concluded that the most favorable conditions for Eavor Loop placement occur in the West Netherlands
Basin, Ruhr Valley Graben, Lauwerszee Trough, and the Lower Saxony Basin, where older units like theCarboniferous Limestone and Devonian rocks are of interest but for which limited information isavailable. These units are usually buried to depths exceeding 4500m.
Overall, the ELFO project has demonstrated that the Eavor-Loop™ is a more attractive geothermal
production technology than conventional systems from a societal and environmental perspective. It
is a closed loop, highly controllable system that does not depend on a permeable reservoir and has noenvironmental interaction with the subsurface or CO2 emissions while in production. To achieve
deployment of the technology at a large scale in a cost-effective manner, dedicated rigs, like thoseevaluated in this project, are required to drill these types of wells in an urban environment. Only withfurther innovations to improve drilling speed and cost-reductions, like those identified from thisproject, the technology is economically viable for large scale deployment.

3

TKI reference 1921406 – Public

Contents
1.

Nederlandse samenvatting ................................................................................................... 8

2.

General .............................................................................................................................. 11
2.1. Introduction .......................................................................................................................... 11
2.1.1.
The Eavor-Loop ............................................................................................................. 11

3.

4.

5.

6.

2.2.

Objectives.............................................................................................................................. 14

2.3.

Project partners & roles ........................................................................................................ 14

2.4.

Duration ................................................................................................................................ 15

2.5.

Overview technical scope ..................................................................................................... 15

Results and discussion ........................................................................................................ 17
3.1.

Summary ............................................................................................................................... 17

3.2.

WP1: Subsurface feasibility study of the Tilburg area .......................................................... 17

3.3.

WP2: Optimised drilling rig design & drilling practices......................................................... 19

3.4.

WP3: Environmental impact and safety ............................................................................... 20

3.5.

WP4: System integration of Eavor Loop into district heating............................................... 23

3.6.

WP5: Outlook for wider implementation in the Netherlands .............................................. 24

3.7.

KPI table ................................................................................................................................ 27

Conclusions and recommendations ..................................................................................... 29
4.1.

Conclusions ........................................................................................................................... 29

4.2.

Recommendations ................................................................................................................ 29

Project execution................................................................................................................ 31
5.1.

Technical & organisational issues ......................................................................................... 31

5.2.

Changes in project plan......................................................................................................... 31

5.3.

Knowledge transfer ............................................................................................................... 31

Contribution to research area ............................................................................................. 32
6.1.

TKI Urban Energy MMIP’s ..................................................................................................... 32

6.2. Dutch economy ..................................................................................................................... 32
6.2.1.
Eavor Loop Market ........................................................................................................ 32
6.2.2.
Financial ........................................................................................................................ 33
6.2.3.
Business case end-user ................................................................................................. 33
6.3. Strengthening Dutch knowledge position ............................................................................ 33
6.3.1.
Subsurface feasibility .................................................................................................... 33
6.3.2.
Improved understanding of heat transfer, borehole stability and life-cycle footprint ofclosed loop geothermal systems .................................................................................................. 33
6.3.3.
Improved drilling processes .......................................................................................... 34

4

TKI reference 1921406 – Public

6.3.4.
6.3.5.

ELFO use case for Design Toolkit; a heat system integration tool ................................ 34
Improved understanding of applicability elsewhere in the Netherlands ..................... 34

7.

Spin off............................................................................................................................... 35

8.

References ......................................................................................................................... 36

TablesTable 1: Parameters used in the Tilburg Case study ............................................................................. 22

FiguresFigure 1: Schematics of different Eavor-Loop™ (EL) designs with indicative depth & length ranges andmulti-lateral design. The Daisy-Chain configuration was demonstrated in a Canadian Pilot (2019). The
James Joyce configuration is the proposed design for the ELFO project. The Eavor-Deep design iscurrently being drilled in the USA. ........................................................................................................ 12
Figure 2: CFD example showing lateral interaction after 10 years of operation, based on 75m and 65m
vertical and lateral spacing, respectively .............................................................................................. 13
Figure 3: the 5 work packages (WP's) and their leads .......................................................................... 17
Figure 4: Stylised schematic of the Eavor-Loop™ planned near Tilburg. For perspective purposes, thesize of the Eifel Tower is shown on the right; the Eavor Loop system is planned at a depth of 10x theheight of the Eifel Tower....................................................................................................................... 19
Figure 5: InnoRig XL90D dual derrick: two rigs with one combined floor and pipe handling system .. 20
Figure 6: Grid connection options ........................................................................................................ 24
Figure 7: prospective areas (in blue) or so-called sweet spots where all criteria deem favourable for EL
development. Left figure shows strict criteria, while right figure shows more relaxed (or loosened)
criteria, resulting in larger prospective areas. ...................................................................................... 26
Figure 8: Subcrop of litho-stratigraphic units at the 90 °C (left) and 110 °C (right) isotherms ............ 26
Figure 9: prospective areas when combining all selection criteria. Left: base-case criteria, right:
loosened criteria. .................................................................................................................................. 27

Glossary of key terms
AquiferA body or layer of permeable rock which can contain or transmit water, forexample a sandstone layer.

Borehole collapse

A shear-type wellbore failure that occurs at low wellbore pressures whentangential stress becomes large, ultimately resulting in failure. Rockfragments fall off the wellbore wall, often leaving an elliptic borehole shapedue to the stress concentration effects described above.

Break out or fracturing Borehole breakouts / fractures are stress-induced enlargements of the
wellbore cross-section. When a wellbore is drilled, the material removed fromthe subsurface is no longer supporting the surrounding rock. As a result, thestresses become concentrated in the borehole wall. Break outs occurs whenthese stresses exceed the required to cause compressive failure of theborehole wall.

5

TKI reference 1921406 – Public

CasingA lining that is installed in a well once it is drilled that surrounds the wellentirely. Casing is typically a hollow steel pipe that lines the inside of
the wellbore to support the well, as the raw sides of the well would collapse
in without support.

DoubletA pair of two wells drilled next to each other into the same aquifer, andenabling hot groundwater to be pumped up through one well, while thecooled-down water is injected back into the same aquifer through the other
well.

(Drilling) Mud system

Also called drilling fluid, aids in the process of drilling a borehole into thesubsurface. It is used to lubricate the drill bit and transport drill cuttings to
the surface.

Dual-derrick rig

Large, load-bearing structure from which the hoisting system and therefore
the drill string is suspended. The derrick provides the height necessary for the
hoisting system to raise and lower the pipe. A dual-derrick rig contains 2 ofderrick structures as part of a single rig, which allows drilling of 2 wells
simultaneously.

Eavor Loop (EL)A new type of deep geothermal system (at ca 3-4 km depth range) which
eliminates or mitigates many of the issues with traditional deep geothermal
systems.

Fractures

A crack or surface of breakage within the subsurface. Wellbore tensile (oropen mode) fractures occur when the minimum principal stress on the
wellbore wall goes below the limit for tensile stress.

Geomechanics

The study of how soils and rocks deform in response to changes of stress,
pressure, temperature, and other parameters. This science is central tounderstanding how drill bits remove rock, characterizing borehole stability,
predicting the stability of perforation tunnels, and designing and monitoring
stimulation programs.

Induced seismicity

Non-tectonic (i.e., non-natural) earthquakes that result from human activities
that alter the stresses and strains in the subsurface

LCA

Life-Cycle Assessment. A systematic methodology used to calculate theenvironmental impacts, by quantifying and comparing the effects of a
product, system, service or geographical entity, in the case of the ELFO project
the assessment of the Eavor Loop as part of the Tilburg/Amer Heat network.

LCM

Lost Circulations Material refers to additives introduced to the drilling
fluid when there are signs of inadvertent returns or the loss of drilling fluid
into the formation. Lost circulation materials are intentionally introduced into
the mud system to stop the flow of drilling fluid into a thief zone or a weak
formation.

Multi-lateral well

A horizontal well constructed with more than one productive lateralbranching off a main wellbore.

SCU

Shear capacity Utilisation. SCU is used to quantify the risk of shear failure (forexample on an existing fault plane, e.g. fault criticality). SCU is between 0 (norisk) and 1 (rock is at shear failure).

Target Formation

Sedimentary layers suitable for heat extraction by Eavor loop.

6TKI reference 1921406 – Public

Temperature Gradient The rate of temperature change with respect to increasing depth in Earth's
interior. As a general rule, the crust temperature rises with depth.
(Thermal) conductivity A property that describes how well a material can transfer heat. A material
with high thermal conductivity allows heat to pass through it easily, while amaterial with low thermal conductivity resists heat flow.

7

TKI reference 1921406 – Public

1. Nederlandse samenvatting
De energieopwekking en -consumptie in Nederland verandert langzaam maar gestaag van fossiele
brandstoffen naar hernieuwbare energiebronnen, veelal door groei van wind- en zonne-energie. Omde energiezekerheid en betaalbaarheid van hernieuwbare energie te verbeteren, zouden ook andere
bronnen, zoals geothermische energie, een groter deel moeten gaan uitmaken van de energiebalans
in de nabije toekomst. Om een grotere bijdrage te kunnen leveren aan de totale energievoorziening
zullen vooral diepe putten, die een hoog vermogen kunnen leveren, nodig zijn.
Om nieuwe en innovatieve ideeën rondom geothermie (sneller) van de grond te krijgen, heeft het
ELFO-project de technische en economische haalbaarheid van zo’n diep, innovatief geothermischsysteem beoordeeld: de Eavor Loop, een put bestaande uit 12 multi-laterals. Het consortium van TNO(onafhankelijk onderzoeksinstituut), Eavor (geothermische energiebedrijf), Huisman Geo BV(boorapparatuur & onderzoek) en EnNatuurlijk (warmtebedrijf) heeft de toepasbaarheid van dezetechnologie als primaire warmtebron voor stadsverwarmingsnetwerken geëvalueerd voor de regio
Tilburg alsmede voor een bredere toepassing elders in Nederland. De uitkomsten van het projectdragen bij aan de doelstelling van de TKI Urban Energy regeling voor Duurzame warmtenetten en
Geothermie (MMIP 4) door aan te tonen dat het haalbaar is om een Eavor Loop-systeem in de regio
Tilburg te ontwikkelen als een hernieuwbare warmtebron voor het bestaande Amerverwarmingsnetwerk. Binnen de huidige markt is de technologie alleen economisch haalbaar met een
CAPEX-subsidie en gegarandeerde SDE++-prijzen. De studie voorziet dat er een mogelijkheid is om deboorkosten verder te verlagen met 20%, waardoor het commercieel wordt zonder CAPEX-subsidie.
Om tot bovenstaande bevindingen te komen, heeft het ELFO-project de volgende aspectengeëvalueerd:
Een ondergrondse haalbaarheidsstudie heeft het gebruik van de Eavor Loop™-technologie om
geothermische warmte te extraheren als bron voor het Amer-warmtenetwerk in de regio Tilburg
beoordeeld door middel van geologische en geomechanische interpretatie en modellering van deondergrond in een economisch context. De ondergrondse haalbaarheidsstudie heeft een geschikte
gesteenteformatie geïdentificeerd voor het plaatsen van het volledige loopsysteem dat voldoende
vermogen kan leveren. Het basisontwerp van het systeem bestaat uit een verticale put tot ongeveer
3,5-4 km diepte met daaraan 12 horizontale multi-laterals van 3,000m lengte elk. Dit ontwerp kon
helaas niet gerealiseerd worden omdat de voorkeurslocatie in de gebouwde omgeving nietbeschikbaar bleek voor het plaatsen van de put en de operationele activiteiten. Om toch dezelfde
gesteenteformatie aan te kunnen boren vanaf een andere locatie in de gebouwede omgeving, moestde lengte van de multi-laterals worden verkort naar 2,100m doordat de laterals nu enigszins updip(omhoog) geboord moeten worden in plaats van horizontaal/iets naar beneden. Desalniettemin levert
dit aangepaste scenario een jaarlijkse warmteproductie van 74.000 MWh. De geschatte CAPEX om deTilburg Eavor-Loop™ te bouwen bedraagt 54,5 MM €.
Een studie naar optimalisatie van boortechnieken en nieuwe boorinstallaties heeft geresulteerd inhet ontwerp van een kosteneffectief, semi-geautomatiseerd en toegepaste boorinstallatie voor deEavor Loop dat 24/7 ingezet kan worden in stedelijke omgevingen. Factoren die de boorefficiëntie
beïnvloeden, met name de snelheid van boren, zijn geanalyseerd en geoptimaliseerd door veel
processen te automatiseren. Tevens is er optimalisatie gedaan in het nauwkeurig controleren en
aanpassen van boorparameters, waardoor de noodzaak om de boor vaak te vervangen wordt
verminderd en hierdoor dus tijdwinst te behalen valt. Een belangrijke verbetering ten opzichte van
bestaande (geothermische) boortechnieken, is het gebruik van een dual-derrick boorinstallatie,
waardoor de alle apparatuur op een kleinere plek geïnstalleerd kan worden en daardoor dus
gemakkelijker te plaatsen is in een stedelijke omgeving.

8

TKI reference 1921406 – Public

Een evaluatie van de milieueffecten, veiligheid en CO2-voetafdruk van de Eavor Loop op basis van
numerieke simulaties laat zien dat ongewenste effecten (zoals lekkage, instorting van de boorput,geïnduceerde seismische activiteit) uit te sluiten zijn en bevestigde de lage CO2-voetafdruk van deEavor Loop-technologie in vergelijking met conventionele geothermische toepassingen. Als onderdeel
van de evaluatie werden thermische prestaties en de bouw- en operationele kenmerken van de Eavor
Loop geanalyseerd door middel van een Levenscyclusanalyse (LCA), alsmede een evaluatie van
geproduceerde CO2 per GJ. Hieruit bleek dat de Eavor Loop-emissies 20% lager zijn in vergelijking met
conventionele systemen, waardoor meer dan 2 kg CO2-eq / GJ wordt bespaard. De thermische responsvan de Eavor Loop gedurende 30 jaar bij constante debietomstandigheden is berekend met behulp
van een gespecialiseerd numeriek thermisch model. Op basis van de thermische simulaties werden
spanningen geanalyseerd met een semi-analytische benadering, waarbij de onderliggende
gevoeligheden werden beoordeeld. Dit gaf aan dat de stabiliteit van de boorput (en mogelijke lekkage)en het potentieel voor reactivering van bestaande breuken geen risico vormen voor een veilige
werking van de Eavor Loop over de volledige levensduur.
De simulaties laten zien dat de Eavor Loop een lange levensverwachting heeft en dat er geen uitputting
van de bron plaatsvindt. De warmteoverdracht tussen bron en Eavor Loop systeem vindt plaats doorde thermische gradient van het relatief koude boorgat en de omgevingstemperatuur. De sterkte vande warmteoverdracht is lineair afhankelijk van dat temperatuurverschil, en blijft relatief constant na
het eerste jaar/maanden, met een geringe afname (typisch <10%) over de levensduur van 20-30 jaar.
Ook over langere tijd blijft dat karakter van afvlakkende afkoeling in stand tot ca 100 jaar. Op langere
termijn zal verdere afkoeling kunnen optreden door thermische interferentie van de laterals, maar dit
is niet meer dan een paar graden en verwaarloosbaar klein. De grootste warmteoverdracht vindt
plaats bij de instroom van de laterals waar het temperatuurverschil tussen het de vloeistof in het
boorgat en temperatuur in het reservoir het grootst is.
De systeemintegratiestudie van de Eavor Loop in het stadsverwarmingsnetwerk van Tilburg (hetAmer netwerk) laat zien dat het mogelijk is om de warmtecapaciteit van de Eavor Loop fysiek op het
bestaande netwerk aan te sluiten. Tevens concludeert de studie dat een temperatuurdaling van het
netwerk voornamelijk optreedt in perioden met een hoge warmtevraag en dat het
warmtenetwerkgedeelte "Kraaiven" het meest wordt beïnvloed als er geen maatregelen worden
genomen. Let op dat de resultaten van deze studie met zorg moeten worden behandeld, omdat het
gebaseerd is op een theoretisch model in de haalbaarheidsfase en er meer gedetailleerde modellering
en optimalisatie vereist is.
Om de bredere implementatie van de Eavor Loop-technologie in Nederland te beoordelen, is het
potentieel van de Nederlandse ondergrond geëvalueerd aan de hand van geologischevoorkeurscondities voor het plaatsen van een Eavor Loop system. Zo is er naar verschillende criteria
gekeken die specifiek zijn voor de gesteenteformatie, voor de locatie of specifiek voor de lokale
situatie. De studie vond dat de meest gunstige condities voor Eavor Loop-plaatsing werden gevonden
in het West-Nederlandse Bekken, Ruhr-vallei Graben, Lauwerszee Trog en het Neder-Saksische
Bekken. Oudere gesteentelagen zoals de Carboon-kalksteen en Devoon gesteenten zijn erg
interessant en mogelijk van belang om verdere uitrol van de technologie te verbreden. Helaas is ermaar beperkte informatie over deze lagen beschikbaar, mede door de diepte (>4500m) waarop ze tevinden zijn. Verdere exploratie naar en kenniswerving van deze diepere lagen via boringen,(seismische) data acquisitie, interpretatie en modelleringen zou een completer beeld geven van demogelijkheden voor deze vorm van geothermie.
Ondertussen is het ELFO-project voor Tilburg doorgegroeid naar de FEED-fase. Partners ENN, EBN en
Eavor werken aan de plannen voor deze fase op basis van de uitkomst van de haalbaarheidsstudie
zoals in dit eindrapport omschreven. De resultaten van WP1 en WP 3 zijn gebruikt voor het succesvol

9TKI reference 1921406 – Public

aanvragen van een NWN-subsidie (“Nieuwe Warmte Nu”) en voor de SDE++ aanvraag. Tevens heeftde ELFO-studie aangetoond dat veiligheids- en milieurisico's goed onder controle zijn. Vanwege een
juridisch geschil met de gemeente Tilburg is een nieuwe exploratievergunning nodig (en aangevraagd)om verdere activiteiten uit te voeren.
Contact
Voor meer informatie kan contact worden opgenomen met Maartje Koning, Project Manager TNO
Applied Geoscience (maartje.koning@tno.nl).
De verschillende werkpakketrapporten zijn via de TKI-Urban Energy gratis te downloaden.

10

TKI reference 1921406 – Public

2. General
2.1. Introduction
Currently the main sources for renewable energy in the Netherland are offshore wind and solar
energy. These sources of energy are being developed with great pace. The downside of thetechnologies is that they only provide power when there is wind or sunshine. To be able to provide amore consistent renewable energy supply and shave off the peaks, geothermal energy can become alarger player in the energy mix in the near future. In general, geothermal energy can be described assustainable heat extracted from the subsurface through wells that can be used to heat houses,
buildings or green houses. The most conventional method is to pump up hot water, circulate it througha heat network and return into the rocks once cooled. The majority of current geothermal wells areshallow wells (~ 500 mtr) that are used as temporary storage systems to collect energy in summer
periods and extract heat in colder periods. To be able to make a larger contribution to the overall
energy supply, geothermal wells with high power output will be required at competitive costs levels
with other renewables.
The success of such deeper geothermal systems is strongly dependent on the geological parametersand heat flow at depth, as well as its economic viability. Nevertheless, geothermal energy has been
attracting a lot of attention in the Netherlands as a sustainable energy source. The sector has
experienced strong growth since its first success in 2007, with more and more market players
becoming active and bringing innovations to this relatively young energy sector.
One of these innovations is a novel multi-lateral well design that can extract heat independent of thequality of the source rock (e.g., the aquifer) as the design is based on a closed-loop system. Where
conventional systems require a relatively permeable aquifer to allow for sufficient and constant flowof the hot water through the rocks into the well bore, the Eavor-Loop™ technology can be operational
in much tighter rocks as it is not dependent on water flow for heat exchange, thereby unlocking vast
amounts of geothermal energy from impermeable rocks otherwise not suited for geothermal energy.
However, for commercial development in the Netherlands this novel technique needs to prove its
feasibility and requires assessment on (1) lowering the costs for drilling, (2) environmental safety andsustainability, and (3) optimised performance for heat networks.
Therefore, the ELFO project (Eavor Loop Feasibility for Tilburg and Outlook for application in the
Netherlands) is aiming to evaluate the feasibility of the Eavor-Loop™ multi-lateral well system.

2.1.1. The Eavor-Loop
The Eavor-Loop™ (EL in short) is a new type of deep geothermal system that comprises of a series ofconnected horizontal well sections (e.g. multi-laterals) drilled to a depth range of around 3-4 km, andeliminates or mitigates many of the issues with traditional deep geothermal systems. The technology
has been demonstrated in a pilot in Canada but has not yet been commercially exploited.
The key difference relative to existing geothermal technology is that it is a completely closed-loop: it
is simply a buried-pipe system, akin to a deep radiator or heat exchanger. Where conventional
geothermal systems do rely on fluid flow through the rock layers, it also is associated with pumping
up dissolved salts and other elements, resulting in corrosion, local clogging and other side effects
which potentially reduces the efficiency of the system over the lifetime.
As the water in the Eavor Loop system doesn’t come in contact with the rock layers, corrosion andclogging of the pipelines in the system and heat network are not common which contributes to thelongevity of the systemin contrast to conventional systems.

11

TKI reference 1921406 – Public

The Eavor Loop technology is scalable and independent of the permeability of aquifers orhydrothermal flow capacity. This makes it possible to utilize these systems in areas without suitablereservoir conditions for conventional geothermal energy without the need for and high-risk
exploration and therefore extends the overall geothermal resource base. Its scale-up is based onstandardised wells which means it is less dependent on the availability of specialised resources. There
are several types of Eavor-Loop™ configuration which depend on overall setting, application andsurface locations (Figure 1). The ELFO project will focus on the feasibility of the James Joyce design
only.

Daisy-Chain

James Joyce

Eavor-Deep

Figure 1: Schematics of different Eavor-Loop™ (EL) designs with indicative depth & length ranges andmulti-lateral design. The Daisy-Chain configuration was demonstrated in a Canadian Pilot (2019). The
James Joyce configuration is the proposed design for the ELFO project. The Eavor-Deep design iscurrently being drilled in the USA.
An Eavor-Loop™ has remarkably consistent and predictable output over long timeframes, as
demonstrated in the pilot project in Alberta, which is now fully operational. As heat is extracted fromthe earth, the radius of the temperature affected area around the wellbore expands; beyond thisradius the reservoir is still at virgin temperature. The radius size is a logarithmic function of time andthe lateral wellbores have minimal interference with each other if spaced properly. Therefore, decline
is addressed by calculating the effective radius and specifying lateral spacing requirements for the
project to ensure minimal wellbore interference.
Computational Fluid Dynamics (CFD) analysis has also been performed on analogue Eavor projects tovalidate and predict the thermal interference between laterals. A simplification showing an elementof symmetry with four laterals with 65 m horizontal spacing and 75 m vertical spacing is shown in
Figure 2 below, after 10 years of operation.

12

TKI reference 1921406 – Public

The CFD analysis shows that lateral interaction will not materially impact the project output over the
30-year project life.

Figure 2: CFD example showing lateral interaction after 10 years of operation, based on 75m and 65m
vertical and lateral spacing, respectively
The James Joyce EL design comprises a multi-lateral system, consisting of 12 semi-horizontal laterals.
The horizontal spacing between the laterals is 65 meters. The well is constructed with two drilling rigs
operating simultaneously from the same surface location. First, the 2 vertical well sections are drilledclose together, and intermediate casing is cemented in place for both wells according to standard oil
and gas industry practice. Then, two deviated wellbores are drilled out of each standard vertical cased
hole and intersecting at the toe of each horizontal section. This process is repeated until all 12 multilateral wellbores are intersected. The vertical spacing between this top and bottom lateral of a single
loop is 75 meters. The laterals are completed with a sealant called RockPipeTM that ensures there is
no leakage from the well bores into the surrounding rock formations. After well construction, thedrilling fluid is displaced with a working fluid designed for improved thermodynamic and operational
performance.
In order to commercially introduce this Eavor Loop system into the Netherlands, a number of hurdles
need to be overcome:•••

Demonstrate that the technique can be applied commercially for the delivery of heat forsubsurface and surface condition in the Netherlands
The cost for drilling has to decrease: Currently this is the biggest factor in the economicalevaluation. For the proposed EL design more than 65km of well section has to be drilled. For
reference, a standard geothermal doublet system at similar depth only consists of 6-8km ofwell bore. The Eavor Loop involves long, horizontal open hole well sections, which are notconventional. The implications for environmental safety and sustainability have not beenaddressed so far in the Netherlands.
The performance characteristics of the Eavor Loop for heat delivery and options for
optimisation of design for heat network system integration need to be accurately mapped out

13TKI reference 1921406 – Public

and understood (i.e., the radiator as well as circulation is significantly different to conventional
geothermal systems).

2.2. Objectives
The main objectives of the ELFO project are 1) to assess the feasibility and applicability of the Eavor
Loop (EL) technology as the primary heat source for the Amer heat network in Tilburg and 2) evaluating
the possibilities for wider adaptation in the Netherlands. The following sub-objectives are addressed:
1
2
3

4
5

Assess subsurface suitability in the Tilburg Area for construction of an EL (James Joyce design),
and prognose associated heat delivery as a function of subsurface and (design) engineeringparameters.
Design a dedicated rig set-up for cost effective drilling and completion of the proposed EL,such that the drilling can be carried out at lower costs on a 24/7 basis in the urban
environment.
Assess environmental impact, safety and CO2 footprint of the EL based on numericalsimulations over its lifetime, capable of ruling out undesirable effects (i.e., leakage, well bore
collapse, induced seismicity), and validating low carbon footprint compared to conventional
geothermal applications.
Develop an optimised EL design and optimise operational parameters for integration in Amerheat network, based on the heat demand, temperature levels, and overall business case.
Provide an outlook for EL in the Netherlands, including best practices and a roadmap for use
in similar and different geological settings.

2.3. Project partners & roles
The ELFO consortium has the following partners:
Partner

Type of organisation

Eavor Europe

Geothermal technology
company

EnNatuurlijk

Heating company(“warmtebedrijf / leverancier”)

Huisman Geo

Drilling company

TNO

Research instituteRole
WP1 & 5 lead
Eavor’s focus is on the applicability of the Eavor
Loop in the Tilburg area and the Netherlands.
WP4 lead
Responsible for the system integration aspects(integration of the Eavor loop in the Amer network)
WP2 lead
Responsible for the design of drilling rigs that will be
required for economically viable and large-scaleconstruction of Eavor loops across the Netherlands.
Project coordinator & WP3 lead
TNO will conduct the overall project management,
geology and the environmental risks such as
induced seismicity, leakage and the CO2 footprint of
the system and supports the study for wider
adaptation in the Netherlands

The consortium partners have been carefully selected such that the whole value chain involved indeveloping a geothermal energy project is represented to assess the Eavor Loop concept: Technology
provider and operator (e.g. Eavor), Drilling company and operator (e.g. Huisman), environmental risk
& well bore integrity assessment, fundamental and industrial research (e.g. TNO) and the end user(e.g. EnNatuurlijk – the Amer network).

14

TKI reference 1921406 – Public

2.4. Duration
The initial project duration was one year, from 01.11.21 till 01.11.22, however the project was
extended by 3 months till 31.01.23.

2.5. Overview technical scope
The ELFO project scope is setup in five Work Packages, in accordance with the five objectives listed in
section 2.2. In addition, WP6 deals with project management and overall coordination.
The work scope is designed to evaluate and mature the applicability of the Eavor Loop technology ata site-specific location in the Tilburg area. As such, it integrates complex systems and their interfaces
by focusing on subsurface, drilling, and safety aspects while optimising this system into the existing
Amer heat network.
The activities per Work Package are given below:
WP
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7

Work plan item
Feasibility study of subsurface of Tilburg area for Eavor Loop
Obtain seismic data
Review and interpret seismic data (Bunter depth map, thickness map, fault
locations, offsets and uncertainties)
Determine bottomhole location EL
Review well data logs & core
Determine regional stress field
Model stress changes due to cooling
Subsurface peer review

Responsible
Eavor
Eavor
Eavor
Eavor
Eavor
Eavor
TNO
Eavor/TNO/EBN

1.8
1.9
1.10
1.11

Determine ROP range for drilling of lateral
Basic well design based on offset wells
Modelling of optimum thermal output
Determine economics

Eavor
Eavor
Eavor
EBN

2

Optimised drilling rig design and drilling practices

Huisman

2.1
2.2

Optimise tripping in/out process in cased holes, aiming at 30% time reduction
Investigate the effect of tripping in/out speed in open hole laterals
Analyse cost benefit of deployment of dedicated rigs and crews (simultaneous
drilling; 24/7 operations)

HuismanHuismanEnvironmental impact & safety
Determine regional stress field
Induced seismicity: Semi-analytical modelling of reservoir stress changes due tocooling
Well leakage modelling w/ thermo-mechanical models
Evaluation of COP - towards end of project

TNO
TNO

System integration EAVOR in district heating
Select tie-in point(s) for Eavor loop
Operating hours, including full-load equivalent, per year for the EAVOR loop
system. Given its technical specifications (e.g. temp & flow) and input WP 1 - 3

ENN
ENN

2.3
3
3.1
3.2
3.3
3.4
4
4.1
4.2

Huisman

TNO
TNO
TNO

ENN

15

TKI reference 1921406 – Public
4.3
4.4
4.5
4.6
4.7
4.8
5
5.1
5.2
5.3

Calculate steady state EL output
Review options for optimum use of EL source
Conceptual design of the EAVOR system tie in, in the existing heat gridCalculate output for non-steady state operation
Multiple scenarios for future temp & demand optimalisations- also non-steadystate operation
Stakeholder orientation regarding de surface location and benefits to the future
growth of the heat system

Eavor
TNO
ENN
Eavor

Outlook for NL and dissemination
Evaluation of suitability of EL for current and future district heating potential(incl greenhouse and industry heat demand), in similar and different geologicalsettings in NL
Technological learning curve perspective

Eavor

Webinar showcasing the Tilburg feasibility study and NL outlook

ENN

ENN

TNO
Eavor
Eavor

16

TKI reference 1921406 – Public

3. Results and discussion
3.1. Summary
This chapter describes the technical scope, results, potential bottlenecks and recommendations ofeach Work Package (Figure 3) at a high level, with details where required. The reader is referred tothe dedicated Work Package reports for more detailed information regarding scope, methodology,
assumptions, results and conclusions.

Figure 3: the 5 work packages (WP's) and their leads

3.2. WP1: Subsurface feasibility study of the Tilburg area
The goal of WP1 was to assess the feasibility of the use of Eavor Loop™ technology for the extractionof geothermal heat to be used as a source for the Amer heat network. The work included a thoroughgeological and geomechanical interpretation of the subsurface in the Tilburg area in the Netherlands,
to assess suitability of the geological conditions both from a drilling perspective and from a heatdelivery point of view. In addition, the subsurface feasibility was evaluated in an economic context.
The first criteria which applies to any geothermal development in the Netherlands involves theselection of an area with available 3D seismic and a heat network with sufficient capacity for utilizationof a new geothermal heat source.
The 3D seismic survey in the Tilburg area was interpreted and depth corrected with standard
geophysical interpretation tools. Faults and target formation tops were identified within uncertainty
ranges. The available seismic survey unfortunately did not cover the full area of interest as identified
by the extent of the heat network, which limited the optimisation of the placement of the loop, which
was further exacerbated by the restrictions in suitable locations in this area. Eventually, one location
for a complete loop (of 12 laterals) was selected taking into account the limitations in the area. The
lateral length is limited in this case due to the fact that from the selected location the laterals will haveto be drilled updip in a Southward direction, following the dip of the target formation. Updip drilling
limits the maximum length of the laterals to approximately 2100m due to drilling mechanics. Longer
laterals would make the project more economic, due to restrictions on potential other available
locations, these could not be considered.

17

TKI reference 1921406 – Public

The temperature gradient and the relevant formation properties of the target Triassic Buntersandstone formation in the selected region was investigated based on offset well data. The
temperature gradient was confirmed to be better than expected at 35 °C/km. The conductivity of the
Bunter was measured as well as the unconfined compressive strength. The conductivity was also a bit
better than expected and the formation strength is sufficient for the stability of the open-hole laterals
throughout the productive life of the loop as shown by modelling of the near well bore stresses.
The drilling plan for the vertical wells was constructed based on available information from nearby
offset wells. Combining the knowledge of the wells drilled with the new requirements for EL resulted
in a concept casing design and time and cost estimate. For the drilling of the laterals in the target
Bunter formation the rate of penetration and the bit life were estimated by comparing rock hardness
with the results from the hard rock drilling research at Utah Forge. The estimates were judged to be
manageable.
The heat output was calculated with the Eavor heat flow model. The current inlet temperature of theAmer heat network at 120 °C is quite high in the wintertime (95°C in the summer). This temperature
is not often needed but the old network is not able to withstand quick temperature changes. This highinlet temperature is not good for the efficiency of the loop, as it requires a low circulation rate. This
problem can be overcome by the use of a heat pump to boost the outlet temperature by taking moreheat out of the return flow. The optimum capacity for a 30-year flat profile was derived to be 9.1 MWin summer months and 7.1MW in winter months.
The economic feasibility is based on an estimated CAPEX of 54.5 MM € to construct the Tilburg EavorLoop™, yielding an annual heat production of 74,000 MWh. OPEX is estimated at 162k €/year andelectricity demand of 660 kW to power an on-site heat pump to meet the heat network demand. The
project is eligible for subsidized energy pricing through the SDE++ program.

18

TKI reference 1921406 – Public

Eifel tower(324 m) for
scale

Figure 4: Stylised schematic of the Eavor-Loop™ planned near Tilburg. For perspective purposes, thesize of the Eifel Tower is shown on the right; the Eavor Loop system is planned at a depth of 10x theheight of the Eifel Tower.

3.3. WP2: Optimised drilling rig design & drilling practices
The work conducted as part of WP2 has the aim to design a dedicated and more automated Eavor
Loop rig for cost effective drilling and completion such that the drilling can be carried out on a 24/7
basis in the urban environment. In order to achieve this, various parameters that affect the efficiencyof drilling, have been analysed to improve the most costly activity of delivering the geothermal well;
the drilling activities.
The drilling speed is influenced by factors such as drill bit design, mud system, available power, andthe control precision of the rig. An automated rig can save time by automating the pipe handlingprocess and reducing manual handling of bottom hole assembly components. Additionally, theautomated rig can provide accurate control of drilling parameters, reducing the number of times thedrill string needs to be pulled out to replace the drill bit. Therefore, the use of a dual derrick rig as acost and time-saving method for drilling the Eavor Loop well system was considered and compared tothe use of two conventional drilling rigs.
The dual derrick system has the ability to drill two wells simultaneously with only one floor crew,
resulting in a smaller crew size and cost savings of around 16%. Combining two rigs into one system
also results in a much space requirements for the required drilling site, meaning a. The required space
to accommodate a dual derrick compared to two conventional rigs is approximately 40% less. The

19

TKI reference 1921406 – Public

smaller footprint for the dual derrick set-up is of great significance for the ability to drill EL systems indense urban areas.
In case there is no dual derrick rig available or when there is not enough space for two conventionalrigs, a single conventional rig set-up was also considered. It was found that this set-up would be lessefficient due to a lack of automation, resulting in additional time per well, a larger crew and half thecapacity of the dual derrick system.
The dual derrick system can complete a project in 170 days compared to 300 days for the conventionalrig, resulting in a cost savings of 19% and a time savings of 43%.

Figure 5: InnoRig XL90D dual derrick: two rigs with one combined floor and pipe handling system

3.4. WP3: Environmental impact and safety
Work Package 3 covers the assessment of environmental impact, safety and CO2 footprint of the Eavor
Loop (EL) based on numerical simulations over its lifetime, capable of ruling out undesirable effects(i.e., leakage, well bore collapse, induced seismicity), and validating low carbon footprint compared
to conventional geothermal applications. The methodological approach includes the following steps

20

TKI reference 1921406 – Public

•
•
•
•
•

Input well path trajectory, well completion and EL operational conditions (i.e., flow rate, inlet
temperature) for the Tilburg Case study as well as relevant subsurface data and interpretation,
thermo-mechanical properties have been adopted from the results of WP1 and WP2 of ELFO.
EL loop performance and near well bore thermal response, based on thermal simulation
Stress response as a function of the predicted near well bore thermal response, superposedon the in-situ stress.
Analysis of Borehole stability and potential for fault reactivation.
Subsequently, based on the thermal performance and EL construction and operational
characteristics, LCA analysis (Life Cycle Assessment) and quantitative assessment of the CO2
per GJ produced has been done.

The layout of the well system is based on key well survey points which have been defined by Eavor,
which consist of 12 laterals, each about 70m apart. For the operation of the Eavor Loop it is assumed
that inlet temperatures will be ~60°C and flow rate of 60 kg/s with the assumption that the water
density at 60°C is 1000 kg/s. A dedicated numerical thermal model has been used to calculate thethermal response of the Eavor Loop for 10 years at constant flow rate conditions. The grid sizes inradial direction have been chosen such that the logarithmic value of the cell centers radial coordinates
is linearly increasing from sub cm-size at the well bore to few meter size a radial distance of 70 m,
which is sufficient to fully cover the transient heat flow over the simulated lifetime. The predicted
thermal power is in excess of 7 MWth.
The simulations indicate that the Eavor-Loop has a long lifetime marked by a very moderate linear
decline of the production temperature and power over a lifetime of 10s of years, with minor thermalinterference of laterals if placed with a spacing of ca 70 m (Holmes et al., 2021, Van Wees, 2021)
Based on the thermal simulations, the stress changes were analyzed with a semi-analytical approachand the following was concluded:
Borehole stability:•••

The borehole wall upon cooling of the EL is stable, except for less than 1 cm of the wall at theentry of the laterals, marked by a Shear Capacity Utilization (SCU) close to or slightly exceeding 1
indicative for frictional instability (in horizontal direction) and strong tensile stresses of -14 MPa(in vertical direction).
The likelihood for tensile failure is dependent on the actual tensile strength which can be
estimated from Cohesion values. For the Triassic drilled in Tilburg, cohesion may not be higher
than 3-6 MPa. Therefore, it will create tensile cracks and break-out under the ca 14Mpa tensilestress, which are limited to the first 1 cm of the borehole wall. In the analysis we did not consider
the impact of Eavor’s proprietary Rock-pipe, which will be placed on the walls of the open holelaterals to seal off the formation from the fluids inside the EL. This could change the rockparameters at the wellbore face, and can to some extent affect the cohesion, friction, and tensile
strength.
The thermal effects, responsible for the tensile fracturing will manifest itself very early in the Eavor
Loop lifetime, possibly already during drilling when mud circulation is causing borehole wallcooling. The fracturing can therefore already occur during the drilling process. Stress effects ofthermal cooling do not noticeably increase over time after the first month of operation.
Consequently, the breaching of Rock-Pipe, after the first month of operation is unlikely. The early
use of Lost Circulations Material (LCM) and Rock-Pipe to plug the crack tip can prevent further
propagation of potential fractures. Rock-Pipe can be reapplied throughout the life of the EavorLoop™.

Potential reactivation of faults:

21

TKI reference 1921406 – Public••

Stress effects have been calculated up to 70 meters from the well bore based on the predicted
temperature response at 120 months and have been tested for stability on pre-existing fracturefavorably aligned in the (locally rotated) stress field, adopted a cohesion of 0.
The results show Shear Capacity Utilization (SCU) for pre-existing fractures leading to failure onlyin the very close vicinity of the borehole (<10 m radius). The associated reactivated fault areas
would be relatively small and most likely not result in seismicity which could be felt at surface –unless stress changes are able to trigger theoretically larger events, albeit at much lower
probability.

Sensitivities:•••

The results for potential fault reactivation are very sensitive to the in-situ stress assumptions.
Adopting a higher horizontal stress gradient of 17 MPa/km instead of the base case 15 MPa/km,
results in more stable predictions, both in terms of SCU for fault reactivation, as well as tensilestresses at the bore hole wall, positively affecting borehole stability.
The sensitivity to fault reactivation has been analyzed for a higher estimate of the friction angleof pre-existing faults (in line with the range given in Table 1). This results in reduced SCU values,
preventing shear failure for existing faults. Well bore stability is still prone to high tensile stresses.
Lower injection temperature will result in larger cooling of the well bore, in particular at the bore
hole wall. In order to test the sensitivity to lower injection temperature, the injection temperature
was reduced from to 40°C. Consequently, this resulted in more accentuated bore-hole instabilities
and a potentially larger instable zone for frictional reactivation of pre-existing faults, further awayfrom the borehole.

Subsequently, based on the thermal performance and EL construction and operational
characteristics, a Life Cycle Assessment (LCA) and quantitative assessment of the produced CO2 per
GJ heat produced has been conducted and a comparison with a conventional geothermal doublet
system was made:• the emissions (kg CO2-eq/GJ) of the EL loop are ca. 20% less compared to a conventional
geothermal doublet system, saving over 2 kg CO2-eq/GJ. In more detail, the emission related tosystem construction is slightly higher than a conventional doublet, given that much longer
sections are drilled. However, the operation-related emissions are characterized by reduced
pumping power and absence of formation gas compared to a conventional doublet system,
which over the lifetime of operations offsets the increase related to construction.
Table 1: Parameters used in the Tilburg Case study
parametervaluetarget formation

Triassic

unit

remarks

lateralslateral length (times two)

2180

m

see well survey points (WP3 report)

start depth

3450

m

see well survey points (WP3 report)end depth3125m

see well survey points (WP3 report)

lateral dip-7.8

degrees

see well survey points (WP3 report)

number of laterals

12horizontal spacing laterals

65mmaximum lateral extent is 325 m from the main lateralvertical spacing laterals

75mreturn is deepest located 32.5 m below specified depths, inlet is
located 32.5 m above specified depths

Build Up Rate (deg/30m)

622

TKI reference 1921406 – Publicdiameter wells

8.5

inch

thermal properties/model

typical porosity

7%

Tilburg area Triassic core plug porosities are in the range 6-10%,~7% on average. ‘Good’ reservoir zones have 8-13% (5-26 mD),
‘poor’ zones 7-8% (0.06 – 0.5 mD) (BP report 'Sedimentology andreservoir quality of the Middle and Upper Bunter Formations')
Limberger et al., 2019. The P3 petrophysical properties database(Bär Reinsch & Bott 2020) has average bulk thermal
conductivities for Triassic sandstone (measured in Germany solikely comparable with the Netherlands) between 2.8 (finesandstone) and 3.6 (coarse sandstone). ‘Average’ sandstone has
3.0. The latter is in line with Limberger.
Chris Dalby core data measurements on Triassic sandstones(relatively nearby wells Werkendam, Waalwijk Noord,
Steelhoven) has average 4.2 for horizontal, saturated.

conductivity laterals3

W m-1 K-1

conductivity vertical sections

2.25

W m-1 K-2

rock heat capacity

3

MJ K-1 m-3 Limberger et al., 2017

fluid heat capacity

4.18
Gies etal., 2021

kJ K-1 kg-1

Young's modulus

20

GPa

Poisson's ratio

0.2

linear expansion coefficient

1.50E-05

K-1

lab experiments (range 0.9-2E-5)

cohesion20

MPalab experiments (range 26-41 Mpa)

friction angle

31degreeslab experiments (range 29-40 Degree)

in-situ stress
SH (maximum total horizontal
stress)sh (minimum total horizontal
stress)21

MPa km-1

oriented in direction of the lateral azimuth ~150, stress reportwUP

15

MPa km-1

stress report wUP, perpendicular to lateralsv (vertical total stress)

22

MPa km-1

stress report wUPhydrostatic pressure

10.6

MPa km-1

brine

temperature model

water
See WP3 report Error! Reference source not found.

mechanical parameters
lab experiments (range 21 -25 Gpa)
lab experiments (range 0.05-0.16)

3.5. WP4: System integration of Eavor Loop into district heatingThis Work package focused on the optimisation of the Eavor Loop (EL) design towards district heating
network integration, showcased for the Amer network. The Eavor loop contains many engineering andoperational parameters, which need to be tailored for system integration in heat networks.
The system integration of the EAVOR Loop in the district heating network consists of 2 sub-workpackages:
1. Physical connection to existing grid with the technical capacity of absorbing the EAVOR loopheat
2. Simulation of heat demand profiles and temperatures to determine the capacity feed-in of
the loop without a negative impact on the quality (temperatures) of the heat delivered tothe customers.

23

TKI reference 1921406 – Public

The physical connection between the EL and heat grid consists of a connection point in the existing
grid and a new connection pipeline between the Eavor inlet and the existing grid (Figure 5). The
technical design has been modelled with Sishyd software. The connection point has been identified
by comparing the most logical (geographical) options, which resulted in one most logical connection
point. For the technical design are based on the technical requirements received from the EAVOR
loop. One important output of the design is the diameter of the connection pipeline. With the concept
design a concept budget could be made. The accuracy of the budget will increase in the upcoming
project phases.

Figure 6: Grid connection options
For the simulation of demand and temperatures two approached have been used. EnNatuurlijk hasmodeled the heat demand for the EAVOR loop with historical data (hourly basis) of the heat grid andtaken into account that there always needs to be a minimum capacity from other heat sources toavoid the cooling down of the transport grid to these other sources. The current temperature regimeof the heat grid has mostly been treated as a given. Still, lowering the supply and return temperature
reduction will be an important point of attention and topic for further optimalisation. For a next phaseof the project future scenario’s for optimalisation can be considered in the project valuation.
Besides the historical simulation, another simulation has been done by TNO using the Design Toolkit
software. With this software several scenarios have been explored to understand the impact of the
EAVOR loop in the heat grid temperatures. Overall, it shows that a temperature drop of the network
mainly occurs in high heat-demand periods and that mainly the industrial area “Kraaiven” is subjected
to a potential temperature drop if no measures are taken.

3.6. WP5: Outlook for wider implementation in the Netherlands
A short study was conducted in which the relevant subsurface conditions were checked against theEavor Loop placement criteria provided by Eavor with the aim of determining the subsurface potential,i.e., prospective layers and areas for the entire county of the Netherlands. The general Eavor Loop

24

TKI reference 1921406 – Public

placement criteria were fine-tuned during the feasibility study (WP1) to address commercial
application of the technique in the Netherlands.
The criteria can be divided in three categories:
1) Criteria which are a function of the specific formation:- Rock thermal conductivity: minimum 2.5 W/m.K- Hole stability: consolidated formation, no reactivity with water- Drillability (abrasiveness/UCS)- Porosity/permeability: no restrictions
2) Criteria which are a function of the location at regional scale:- Temperature gradient: minimum 30 °C/km- Formation minimum temperature: minimum 110 °C- Depth maximum: 4500 meter- Thickness minimum: 30 meter3) Criteria that are very specific to the local situation and which cannot be studied using national
scale databases:- Layer dip angle versus drilling orientation- Stress data
- Faulting (parallel and crossing, defining layer continuity and maximum possible lateral
length).
The most restrictive and therefore most important subsurface criterium for EL development turned
out to be the minimum required temperature of 110 °C in combination with sufficient rock strengthand thermal conductivity.
The criteria which are specific to a formation were compared to the lithological column of theNetherlands. This resulted in the identification of the competent sandstones, siltstones andcarbonates from Jurassic age and older as most favourable candidates: the Bunter sandstones,
Rotliegend sandstones, Dinantian carbonates and possibly the Devonian sandstones. Of the latter two
there is insufficient data in the database to include them in an automated search.
For these lithologies it was investigated where they are present at the right depth to reach theminimum temperature but not too deep for them to be out of reach for existing drilling technology.
A set of maps were generated based on data from various sources, see Figure 7 for the final map
where all criteria are favourable.
The application of a lower temperature constraint, which might be possible with lower drilling costand or lower supply temperature requirements for the heat network, increases the extent of thesuitable area for the prospective formations. However, this doesn't open up entirely new areas orformations.

25TKI reference 1921406 – Public

Figure 7: prospective areas (in blue) or so-called sweet spots where all criteria deem favourable for EL
development. Left figure shows strict criteria, while right figure shows more relaxed (or loosened)
criteria, resulting in larger prospective areas.

Figure 8: Subcrop of litho-stratigraphic units at the 90 °C (left) and 110 °C (right) isotherms
Figure 8 shows the lithology at the depth corresponding to the current formation temperature cut-offof 110 °C and for a potential future lower cut off 90 °C. From these maps it is clear that in large parts

26

TKI reference 1921406 – Publicof the Netherlands the most prospective layers i.e., Bunter (pink colour/Trias) and Rotliegend
sandstones (brown-pinkish colour) are too shallow. From the Dinantian and Devonian prospective
layers there is insufficient data to be able to prepare reliable maps.
A formation thickness of less than 30m is another important restriction that has been evaluated using
various data sources. Only the ThermoGIS database has the required granularity to screen for thiscriterium on the basis of the individual members. This criterium turned out not to limit the potential
area more than the minimum temperature.
The conclusion from WP 5 is that the most favourable conditions for EL placement are found in theWest Netherlands Basin and the Ruhr Valley Graben, the Lauwerszee Trough and the Lower Saxony
Basin (see blue area in Figure 9).
Older units like the Carboniferous Limestone and Devonian rocks may be interesting too but
information about the nature of the rocks is limited. Furthermore, the rocks of these units are usuallyburied to depths exceeding 4500m.

Figure 9: prospective areas when combining all selection criteria. Left: base-case criteria, right:
loosened criteria.

3.7.

KPI table

KPI

Omschrijving

TRL at completion, maincategory

Industrial Research: TRL: 6; demonstrating the (technically & economically)feasible drilling & implementation of the EL technology in the Amer heat
network.

TRL at completion,
subcategory

•
•
•
•

Feasibility study of Tilburg subsurface area for EL: can not be
expressed in TRL level as standard available technology was used.
Laboratory testing of conductivity rock properties (fundamental
research): TRL 2. All other lab experiments are proven technology.
Optimise drilling rig design & practices for EL: TRL 8
Validation of environmental impact, safety and CO2 footprint of EL:
TRL 8 to 9

27

TKI reference 1921406 – Public
KPI

Omschrijving•••

Project success

Optimised EL design and operational parameters for integration toAmer heat network: TRL 8 to 9
Outlook for wider implementation of EL in the Netherlands: notsuitable for TRL classification
EL multilateral technology: TRL 8 to 9

1. The project has been completed in accordance with the original scope. All
milestones have been achieved

Follow up

The project can be seen as the precursor of the NieuweWarmteNu projectcurrently being developed.

Number of peer-reviewed
publications

0

Number of expected peerreviewed publications

0

Number of non-peer-reviewed
publications

1 (at Geoconvention, Calgary, 2023), more publications expected

Number of submitted patents & 0
doi’s
Number of granted licenties

NA

Number of prototypes

NA

Number of demonstrators

This project entails the demonstration of new technology in a specific area. The
project has shown the feasibility of the demonstration and planning is ongoing
to conduct the project.

Number of spin-offs/ spin-outs

The results of this project have been used for SDE application as well as for
exploration license renewal.

Number of new or improved
products/technologies/services
Impact-,Within the current market, the technology is only economically viable withCAPEX subsidy and SDE++ guaranteed pricing.

28

TKI reference 1921406 – Public

4. Conclusions and recommendations
4.1. Conclusions
The overall conclusion is that the project has resulted in determining that an Eavor Loop system for
the Tilburg area can be seen as a possible option for a renewable heat source for the existing Amerheat network.
The sub- conclusions are:
•
•

•
•

•
•

•
•

The Eavor-Loop™ has a long lifetime marked by a very moderate linear decline of the
production temperature and power over a lifetime of 10s of years, with minor thermalinterference of laterals if placed with a spacing of ca 70 m.
Suitable subsurface conditions for Eavor-Loop™ development have been identified resulting
in a potential 13 MWth Eavor-Loop™ capacity. However, restrictions in available surface
drilling locations in urban areas resulted in suboptimal loop design and depth. In this case thelength of the laterals is limited to 2100m because they are planned to run slightly updip andhave a therefore result in a reduced Eavor-Loop™ capacity of 9MWth in the summer months.
Extensive modeling of the stress changes as a result of the gradual cooling and operation of
the Eavor-Loop™ is not expected to induce seismicity or to reactivate existing faults. The
modelling also indicated that the open-hole laterals will be stable over time.
The connection with the heat network is complicated by the fact that this old network requires
an inlet temperature of 120°C in the winter months. An optimisation was carried out with theoutcome that a heat-pump will be needed to boost the temperature in the winter. Lowering
the district heating network temperature in the winter is possible and should be pursued. This
will increase the thermal capacity of the Eavor-Loop™.
The possibility to start and stop the Eavor-Loop™ and use all its daily capacity during peak
hours is valuable but was not quantified in this study.
The base cost estimate has been prepared assuming the use of existing drilling rigs andavailable technology. Under these conditions, the application of the EL is considered
economic, but only if CAPEX subsidy and SDE++ subsidy is taken into account. By comparison,
a 20% reduction in drilling cost can be achieved by using a semi-custom dual derrick Huisman
drilling rig. A cost saving of this magnitude would make this project commercial without CAPEX
subsidy.
Eavor-Loop™ development is not possible everywhere in the Netherlands due to restrictions
in minimum required temperature (110 °C) in combination with sufficient rock strength andthermal conductivity.
The presence of the Carboniferous formation over a large portion of the depth range suitable
for EL developments. The Upper Carboniferous is overall quite shale and coal rich, this not a
viable target for the current versions of the Eavor-Loop™ technology. The Lower
Carboniferous Dinantian platform carbonates do hold some potential and could be derisked
throughout the Netherlands with seismic data acquisition and deep exploration drilling
programs

4.2. Recommendations
The following research questions and recommendations for further work are identified at project
closure:29

TKI reference 1921406 – Public•

WP1: extend laboratory work
o General: design and build an experimental set up that can accurately measurethermal conductivity from core samples.
o For Tilburg area: provide thermal conductivity from rock samples, construct thermal
conductivity “logs”.
o For Tilburg area: determine the tensile strength (Brazilian test)o Extend rock property measurements for relevant rock formations present elsewherein the Netherlands
o Determine correlation between rock properties and drilling ROP•

WP2: enhanced automationo Utilise AI to optimise & automate drilling process with a focus on drilling speed andmud parameterisations.

•

WP3: fault modelingo For Tilburg area: model fault rupture scenarios based on stochastic assumptions ofpre-existing fault geometry and properties. This can be done effectively with linear
elastic fracture models. Sense-check the expectation that potential fault rupture isvery limited in space (non-felt magnitudes) and unlikely to occur.
o For Tilburg area: map active vs. non-active faults and their effect on the mechanicalreactivation response.

•

WP4: heat usage optimisationso In depth assessment and optimisation of use of heat pumps, and added value ofseasonal heat storage in combination with EL•

WP5: update & refineo Evaluate the potential of Dinantian formation through data collection, interpretation,and modelling.
o Update the findings based on outcome of follow-up research questions.

30TKI reference 1921406 – Public

5. Project execution
The ELFO project was coordinated by TNO, with each consortium partner leading a dedicated work
package. Monthly Project Management Board meetings ensured for regular exchange of information,
status updates and provided the opportunity to flag issues. Additional technical meetings within andacross work packages allowed for in depth technical discussions and alignment, though more
integration meetings could have resulted in improved communication about use of parameterization,
assumptions and required input data. All participants were committed to collaborate and considering
the project was fully online, engagement of partners was good.
The project was concluded with an in-person close-out meeting, with consortium partners andstakeholders attending from the Netherlands, Germany and Canada.

5.1. Technical & organisational issues
The geographical spread of the participating organisations in the Consortium is spanning across 2
different time zones, with the majority of the Eavor workforce being located in Calgary (UTC -6 or -7hrs). Workshops, Management Board Meetings and Work Package meetings were therefore
scheduled in the CET afternoons, to allow for as much as possible time overlap for communication andcollaboration. This proved to be a very effective approach and the transnational time zone challenge
was easily overcome.
Minor technical challenges were faced during the laboratory experiments in the iM4RockLab of TNO
Applied Geosciences in Utrecht for measuring mechanical properties of the claystone rock samples.
Unfortunately, it was not possible to extract intact cores from the claystone ELFO_3.
Due to the complexity of accurately measuring thermal conductivity, it was decided to use analogue
data as published by Dalby et al 2018.

5.2. Changes in project plan
Several Work Packages suffered from delays in project execution due to COVID-19 related illnesses.
Therefore, additional time was required, and a 3-month project extension was granted by TKI toensure delivery and integration of results between all Work Packages. The project scope has not been
amended.

5.3. Knowledge transfer
The final reports for each Work Package are publicly available, describing the details of the work
conducted, as well as the findings, conclusions and recommendations.
The results from the laboratory experiments on deriving mechanical rock properties conducted by
TNO on core plugs are made available thought the DINO / NLOG portal.
At the moment of writing this report, a webinar is being planned for the Dutch public to raiseawareness about the technique as an alternative to conventional geothermal systems and more
specifically about the Tilburg case study.
Several abstracts have been submitted (or are being constructed) to disseminate the project results
at international conferences for wider visibility.

31

TKI reference 1921406 – Public

6. Contribution to research area
6.1. TKI Urban Energy MMIP’s
The results of the ELFO study have contributed to the goals of TKI Urban Energy for sustainable heat
networks and Geothermal systems (MMIP 4) by demonstrating feasibility of the Eavor Loop (EL)technology as a renewable heat source for the Amer city heating network in Tilburg. The complexities,
uncertainties and risks regarding system integration, safety, environmental impact of the geothermal
system and reduction of CO2 emissions have been identified and evaluated. It can be concluded that
the Eavor Loop system can be constructed safely in the deep subsurface of the Tilburg area, with very
low risk of induced seismicity, no leakage to the environment, and reduced CO2 emissions compared
to conventional geothermal systems. Additionally, the study has demonstrated that the Eavor Loopsystem can be integrated into the relatively old Amer city heat network, locating the optimal tie-in
point. It shows that a temperature drop of the network mainly occurs in high heat-demand periods
and that mainly the industrial area “Kraaiven” is subjected to a potential temperature drop if nomeasures are taken. A heat-pump will be needed to boost the temperature in the winter to avoid
temperature drops. The optimum capacity for a 30-year flat profile was derived to be 9.1 MW insummer months and 7.1MW in winter months, with a yearly heat production of 74.000 MWh.

6.2. Dutch economy
6.2.1. Eavor Loop Market
Implementation of the Eavor-Loop™ as one of the key geothermal techniques in the Netherlands,
faces several commercialization challenges. These include achieving low drilling costs while being
dependent on factors such as reservoir temperature and lithology for sufficient heat transfer, but also
to ensure drillability of the well laterals and safe operation and production throughout the well’s lifetime. The goal is to construct a rig that can be financed out of the project at a lower cost than existing
systems, with Huisman R&D initially being the main financial contributor to its development.
The ELFO study demonstrates that the Eavor-Loop™ is a more attractive geothermal production
technology than conventional systems from a societal and economic perspective. It is a closed loop,
highly controllable system that does not depend on a permeable reservoir and has no environmental
interaction with the subsurface or CO2 emissions. The combined results from WP1 to WP4 show that
it is feasible to implement the Eavor Loop system into the current heat network of Tilburg, considering
geological, technical, and economic parameters and sensitivities as well as their safety andenvironmental aspects.
Additionally, WP5 shows that there is potential for the Eavor Loop to be developed in other regions in
the Netherlands where traditional systems cannot be applied due to the absence of immature, but it
has been successfully demonstrated in a pilot in Canada, and another initiative code named ‘EavorDeep™’ spud September 2022, which is considered the deepest and hottest geothermal well in history.
The ELFO project shows that the Eavor Loop technology has potential for both direct heat applications
and production of electricity at locations lacking sufficient underground permeability, like the Tilburgarea. To achieve this potential on a large scale, dedicated rigs would be required, which are equippedof drilling these types of wells in an urban environment in a cost-effective manner 24/7. Therefore,
WP2 focused on designing a new semi-automated dual-derrick rig and drilling procedures to provide
efficient, safe, and environmentally friendly Eavor-Loop™ drilling. Current estimates show that ~40%
time and 16-20% of costs savings can be achieved during drilling of an Eavor Loop well throughautomation of tripping, dual-derrick system and faster pipe adding. The automation of many

32

TKI reference 1921406 – Public

processes, including the use of a dual-derrick instead of a single rig, also leads to reductions in crew
size and therefore overall costs. Optimisations to the currently developed dedicated rig are identified
that could enable even faster uptake of the technology.

6.2.2. Financial
The ELFO study has succeeded in demonstrating commercial feasibility of drilling the first loop in the
Netherlands, the Tilburg area. In parallel with the ongoing ELFO study, and SDE++ subsidy application
was being conducted to put the ELFO results into practice and proceed to the FEED stage to ultimately
construct the Eavor Loop Technology as part of the Amer heat network in Tilburg.
The study provided a more accurate estimate of the capital cost of 54,5MM € (instead of the assumed
46MM € at project start) with a unlevered IRR of 8% (instead of the assumed 6.1%) over the project’s
life time of 30 years. The technology is expected to become more cost-effective with economy of scale
and innovation, and eventually will not need any subsidy. The drilling of the loops will create several
times more employment than the fossil drilling industry, and the manufacturing of advanced drillingsystems will also create significant growth in employment.

6.2.3. Business case end-user
In the (near) future, end users of heat, like city heating systems and greenhouses, can no longer be
expected to use biomass for new heat applications. Thereby making geothermal heat the only
alternative for mid and high temperature applications. In areas with good permeability in theunderground, conventional geothermal is more economic. However, with Eavor-Loop™ generated
geothermal heat, it should be possible for city heating systems to become completely CO2 neutral at
an affordable cost for customers.

6.3. Strengthening Dutch knowledge position
The technological thresholds that must be overcome for the Netherlands is proving the technology in
the Dutch sedimentary subsurface and to start a learning curve that will lead to a capital cost level atwhich an Eavor Loop system can be developed without the need for subsidy. Therefore, the ELFO
study focused on the several aspects related to these technology thresholds; a suitable subsurface
able to produce sufficient heat while maintaining operational safety, optimised drilling processes,
system network integration and applicability elsewhere in the Netherlands for wider adaptation.

6.3.1. Subsurface feasibility
The subsurface feasibility research as carried out for the ELFO project has contributed to a robust
prediction of the subsurface near the City of Tilburg, indicating favourable conditions for deploymentof an Eavor Loop system in the Lower German Triassic sandstone rich Volpriehausen, Detfurth andHardegsen members. Given the low uncertainty on Heat in Place it is possible to predict energy yieldwith relatively high confidence. An expected energy yield of 9.1 MW in summer months and 7.1 MWin the winter months and an approximate annual production of 74,000 MWh of clean baseload heating
into the Amer heating network

6.3.2. Improved understanding of heat transfer, borehole stability and life-cycle
footprint of closed loop geothermal systems
Dedicated numerical thermal models have been designed and optimised for the purpose of the ELFO
study with parameterisation applicable to the Tilburg subsurface and operational assumptions of the33

TKI reference 1921406 – Public

Eavor Loop design. The approach is the first in its kind for these type of closed loop systems and shows
the practical usage of the simulation and sensitivity results with robust predictions of stable borehole
conditions, no to low risk of fault reactivation and the impact when lowering the injection
temperature.
The Life Cycle Analysis has improved our understanding of the emission characteristics (kg CO2-eq/GJ)of the Eavor-Loop™. The study shows that these are ca 20% improved compared to a conventional
geothermal doublet system, saving over 2 kg CO2-eq/GJ.

6.3.3. Improved drilling processes
The Eavor Loop is an alternative technique to extract heat from low to non-permeable rock comparedto other methods like closed loop systems in a single vertical well or Enhanced Geothermal Systems(EGS). The EGS method is marked by hydraulic fracking but due to the risk of induced seismicity andthe current political and public debate on this topic, its applicability in the Netherlands is doubtful.
The current market onshore drilling rigs are dedicated to the oil and gas market, with modifications
made to adapt to the geothermal application, providing semi-efficient rigs. However, drilling of Eavor
Loop systems is new to the geothermal market and these semi-efficient rigs will be even less efficient
to drill Eavor Loop systems. The study conducted in WP2 by Huisman B.V., a custom-built drilling
equipment specialist, has shown that innovation can lead to optimsed, dedicated drilling rigs andimproved (automated) drilling procedures to improve efficiency, safety and consistency when drilling
Eavor Loop well systems.

6.3.4. ELFO use case for Design Toolkit; a heat system integration tool
The Design Toolkit, as developed during the WarmingUp Project (MOOI call), has been used evaluate
the operational dispatch of the Eavor Loop in the Amer heat grid as well as to demonstrate its
applicability (and limitations) in this Tilburg use case. The study highlighted several complexities andstruggles when using existing data (e.g. data loading of the heat network grid, requirements to simplify
the heat network to limited number of clusters, availability of detailed heat demand profiles), but it
also showed its value by quantifying the flow distribution based on the heat demand of all consumers
and heat production characteristics of the Eavor Loop. It provided valuable insights into the
optimisation of a city heat network, and how to best mitigate for temperature drops or stagnation offlow within the network. The knowledge obtained during this use case can be used to improvefunctionality of the Design Toolkit.

6.3.5. Improved understanding of applicability elsewhere in the Netherlands
Overall, the ELFO study has resulted in improved knowledge about the suitability of the Tilburg
subsurface for deployment of an Eavor loop system from well placement & integrity perspective,
drilling perspective and heat transfer perspective as well as highlighting favourable regions elsewherein the Netherlands for an Eavor Loop placement. While data availability was found scarce (particularly
for the deeper and older Carboniferous Limestone and Devonian rocks), areas worth investigating
more are found in the West Netherlands Basin, the Roer Valley Graben, the Lauwerszee Trough andthe Lower Saxony Basin.

34

TKI reference 1921406 – Public

7. Spin off
The Eavor Tilburg project has progressed to the FEED stage. Partners ENN, EBN and Eavor are working
on the plans for this stage based on the outcome of the feasibility study. The results of WP1 and WP
3 were utilized for the successful application of a NWN subsidy and for the SDE ++ application. Due toa legal dispute with Tilburg a new exploration permit was required, and the results of WP 3 were used
to show that safety and environmental risks are well under control.

35

TKI reference 1921406 – Public

8. References
ELFO-WP1-Subsurface_Feasibility_Report-v2023.03.28; S. Longfield, V. Soustelle, 2023
ELFO-WP2-Eavor_customised_rig_efficiency-v2023.03.28; A. de Mul, 2023
ELFO-WP3-Environmental_Impact_and_Safety_report-v2023.03.28; J.D. v Wees, 2023
ELFO-WP4-System_Integration_Report_v2023.03.28; H. Droog, J van ‘t Westende, 2023
ELFO-WP5-Outlook_for_NL_report-v2023.03.28; H. Veldkamp, 2023
WP1:
Ames, R. & Farfan, P.F. 1996. The environment of deposition of the Triassic Main Buntsandstein
Formation in the P and Q quadrants, offshore the Netherlands. In: Rondeel, H.E., Batjes, D.A.J. &Nieuwenhuijs, W.H. (eds): Geology of gas and oil under the Netherlands. Kluwer (Dordrecht), 167178.
ASTM, 2014. D 7012-14 standard test methods for compressive strength and elastic moduli of intact
rock core specimens under varying states of stress and temperatures.
Bakx, E., Wassing, B., Buijze, L., 2022. Formation, lithology and region-specific minimum horizontal
stress field in the Netherlands: implications for fault stability in geothermal doublets.
Camelbeeck, T., 1994. The 1992 Roermond earthquake, the Netherlands, and its aftershocks. Geol. En
Mijnbouw 73 (2–4), 181–197.
Camelbeeck, T., Van Eck, T., 1994. The Roer Valley Graben earthquake of 13 April 1992 and its
seismotectonic setting. Terra Nova 6 (3), 291–300.
Cermak, V. & Rybach, L. (1982) Thermal Conductivity and Specific Heat of Minerals and rocks. In:
Geophysics – Physical Properties of Rocks (pp. 305-343).
Carcione, J.M. & F. Cavallini, 2002. Poisson’s ratio at high pore pressure. Geophysical Prospecting,50(1), 97-106.
Carcione, J.M., H.B. Helle, J.E. Santos & C.L. Ravazzoli, 2005. A constitutive equation and generalized
Gassmann modulus for multimineral porous media. Geophysics, 70(2), N17-N26.
Cloetingh, S., van Wees, J.D., Ziegler, P.A., Lenkey, L., Beekman, F., Tesauro, M., Worum, G., 2010.
Lithosphere tectonics and thermo-mechanical properties: an integrated modelling approach forEnhanced Geothermal Systems exploration in Europe. Earth-Sci. Rev. 102 (3–4), 159–206.
Dirkzwager, J.B., Wees, J.D.V., Cloetingh, S.A.P.L., Geluk, M.C., Dost, B., Beekman, F., 2000. Geomechanical and rheological modelling of upper crustal faults and their near-surface expression in
the Netherlands. Global Planet. Change 27, 67.
Dost, B., Goutbeek, F., Van Eck, T., Kraaijpoel, D., 2012. Monitoring induced seismicity in the North of
the Netherlands: status report 2010. Report No. WR 2012-13. Royal Netherlands Meteorological
Institute, De Bilt.
Eck, T.v., Goutbeek, F., Haak, H., Dost, B., 2006. Seismic hazard due to small magnitude, shallowsource, induced earthquakes in the Netherlands. Eng. Geol. 87, 105.
Fjaer, E., R.M. Holt, P. Horsrud & A.M. Raaen, 2008. Petroleum related rock mechanics: Elsevier.

36

TKI reference 1921406 – PublicGölke, M., Coblentz, D., 1996. Origins of the European regional stress field. Tectonophysics 266 (1–4),
11–24.
Grünthal, G., Stromeyer, D., 1992. The recent crustal stress field in Central Europe: trajectories andfinite element modeling. J. Geophys. Res. 97 (B8), 11,805.
Heidbach, O., M. Rajabi, X. Cui, K. Fuchs, B. Müller, J. Reinecker, K. Reiter, M. Tingay, F. Wenzel, F. Xie,
M. O. Ziegler, M.-L. Zoback, and M. D. Zoback. 2018, The World Stress Map database release
2016:
Crustalstresspatternacrossscales.
Tectonophysics,
744,484498. http://doi.org/10.1016/j.tecto.2018.07.007
Hackston, A. & E. Rutter, 2016. The Mohr–Coulomb criterion for intact rock strength and friction – are-evaluation and consideration of failure under polyaxial stresses. Solid Earth, 7(2), 493-508.
Heap, M.J., M. Villeneuve, A.R.L. Kushnir, J.I. Farquharson, P. Baud & T. Reuschlé, 2019. Rock massstrength and elastic modulus of the Buntsandstein: An important lithostratigraphic unit for
geothermal exploitation in the Upper Rhine Graben. Geothermics, 77, 236-56.
Hofstee, C., Benedictus, T., ter Heege, J., Huibregtse, J., van der Meer, B., Nelskamp, S., Orlic, B.,
Pluymaekers, M., Tambach, T., 2009. Feasibility of CO2 storage in the Friesland platform. TNO034-UT-2009-01084
Kombrink, H., Doornenbal, J.C., Duin, E.J.T., Den Dulk, M., Van Gessel, S.F., Ten Veen, J.H., Witmans,N., 2012. New Insights into the geological structure of the Netherlands; results of a detailed
mapping project. Neth. J. Geosci. 91 (4), 419–446.
Lu, Y., W. Li, L. Wang, Z. Li, X. Meng, B. Wang & K. Zhang, 2019. Damage Evolution and Failure Behaviorof Sandstone under True Triaxial Compression. Geotechnical Testing Journal, 42(3).
Plenefisch, T., Bonjer, K., 1997. The stress field in the Rhine Graben area inferred from earthquake
focal mechanisms and estimation of frictional parameters. Tectonophysics 275 (1–3), 71–97.
Pijnenburg, R.P.J., B.A. Verberne, S.J.T. Hangx & C.J. Spiers, 2019. Inelastic Deformation of the
Slochteren Sandstone: Stress‐Strain Relations and Implications for Induced Seismicity in the
Groningen Gas Field. Journal of Geophysical Research: Solid Earth, 124(5), 5254-82.
Somerton, W.H., 1992. Chapter IV. Thermal Expansion of Rocks, in Developments in Petroleum Science,ed. W.H. Somerton.Elsevier, 29-38.
Soustelle, V., J. ter Heege, L. Buijze & B. Wassing, In prep. Thermomechanical parameters ofgeothermal analogue reservoir sandstones in the West Netherlands Basin TNO.
Toews, M. and Holmes, M. (2021): Eavor-Lite Performance Update and Extrapolation to Commercial
Projects, Proceedings, Geothermal Rising Conference, California, USA (2021), vol. 45
Van Adrichem Boogaert, H.A. & Kouwe, W.F.P. 1994. Stratigraphic nomenclature of The Netherlands;
revision and update by RGD and NOGEPA, Section E, Triassic. Mededelingen Rijks Geologische
Dienst, 50, 1-28.
Vucelic, L.A.B., K. Duffaut & P. Avseth, Burial induced changes in physical sandstone properties: A casestudy of North Sea and Norwegian Sea sandstone formations, in SEG Technical Program
Expanded Abstracts 2017, 3626-31.
Wang, J.C., 1984. Young's modulus of porous materials. Journal of Materials Science, 19(3), 801-8.
Worum, G., van Wees, J., Bada, G., van Balen, R.T., Cloetingh, S., Pagnier, H., 2004. Slip tendency
analysis as a tool to constrain fault reactivation: a numerical approach applied to threedimensional fault models in the Roer Valley Rift system (Southeast Netherlands). J. Geophys. Res.
109, 02401.

37

TKI reference 1921406 – Public

Zoback, M.D., Townend, J., 2001. Implications of hydrostatic pore pressures and high crustal strength
for the deformation of intraplate lithosphere. Tectonophysics, 336, 19.
WP3:
Aadnoy, B.S., 1988. Modelling of the Stability of Highly Inclined Boreholes in Anisotropic Rockformations. SPE Drilling and Engineering, sep. 1988, 259-268. SPE 16526.
Aadnoy, B.S., Chenevert, M.E., 1987. Stability of Highly Inclined Boreholes. SPE Drilling andEngineering, dec 1987, 364-374. SPE 16052
Beckers, K.F., Johnston, H.E., 2022. Techno-economic performance of Eavor-Loop 2.0.
PROCEEDINGS, 47 th Workshop on Geothermal Reservoir Engineering. Stanford University,
Stanford, California, February 7-9, 2022, SGP-TR-223
Buijze, L., Van den Bogert, P.A.J., Wassing, B.T.T., Orlic, B., Ten Veen. J., 2017. Fault reactivation
mechanisms and dynamic rupture modelling of depletion-induced seismic events in a
Rotliegend gas reservoir.
Dinkelman, D., Dijkstra, H., De Simon, L., Ros, J., Hanegraaf, M., Veldkamp, H., Van Wees, 2021.
Duurzaamheid Geothermie factsheet, https://www.tno.nl/publish/pages/2137/tno-2021duurzaamheid.pdf
Droniou, J., 2013. Finite volume schemes for diffusion equations: introduction to and review ofmodern methods, <hal-00813613v1>
Gies, C., Struijk, M., Békési, E., Veldkamp, H., van Wees, J.D., 2021. An effective method for paleotemperature correction of 3D thermal models: A demonstration based on high resolution
datasets in the Netherlands, Global and Planetary Change,
https://doi.org/10.1016/j.gloplacha.2021.103445
Holmes et al., 2021; Multilateral-closed-loop-geothermal-systems-as-a-ZELFR.pdf (eavor.com)
Nguyen, D., Miska, S., Yu, M., 2009. Modelling Thermal Effects on Wellbore Stability. SPE paper
133428
Tang, L., and Luo, P., 1998. The Effect of the Thermal Stress on Wellbore Stability. SPE paper 39505
Van Wees, 2021. Audit Report Eavor Loop. TNO - Eavor-Loop™ Audit Report - Eavor
WP5:
Békési, E., Struijk, M., Bonté, D., Veldkamp, H., Limberger, J., Fokker, P. A., . . . Van Wees, J. D.
(2020). An updated geothermal model of the Dutch subsurface based on inversion oftemperature data. Geothermics, 88 doi:https://doi.org/10.1016/j.geothermics.2020.101880
Bonté, D., Van Wees, J. D., & Verweij, H. (2012). Subsurface temperature of the onshore
Netherlands: new temperature dataset and modelling. Netherlands Journal of Geosciences,
91(4), 491-515.
Dalby, C. J. (2018). Characterisation of the thermal conductivity in the Netherlands. Master
ThesisUtrecht University Utrecht.
Fuchs, S., Balling, N., & Förster, A. (2015). Calculation of thermal conductivity, thermal diffusivity andspecific heat capacity of sedimentary rocks using petrophysical well logs. Geophysical JournalInternational, 203(3), 1977-2000.
Gies, C., Struijk, M., Békési, E., Veldkamp, J. G., & Wees, J. D. v. (2021). An effective method for
paleo-temperature correction of 3D thermal models: A demonstration based on high resolution

38

TKI reference 1921406 – Public

datasets in the Netherlands. Global and Planetary Change, 199, 1-15.
doi:https://doi.org/10.1016/j.gloplacha.2021.103445
Hantschel, T., & Kauerauf, A. I. (2009). Fundamentals of basin and petroleum systems modeling
Springer. doi:10.1007/978-3-540-72318-9
Kombrink, H., Doornenbal, J., Duin, E., Den Dulk, M., ten Veen, J., & Witmans, N. (2012). New
insights into the geological structure of the Netherlands; results of a detailed mapping project.
Netherlands Journal of Geosciences, 91(04), 419-446.
Mozafari, M., Gutteridge, P., Riva, A., Geel, K., Garland, J., & Dewit, J. (2019). Facies analysis anddiagenetic evolution of the Dinantian carbonates in the Dutch subsurface. SCAN Utrecht.
(Report for SCAN program, commissioned by the Ministry of Economic Affairs & Climate)
Robertson, E. C. (1998). Thermal properties of rocks. report 88-441,
Veldkamp, J. G., & Hegen, D. (2020). Temperature modelling of the Dutch subsurface at the depth of
the Dinantian (downloadable from https://nlog.nl/en/scan). SCAN Utrecht.

39