PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering

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

SGP-TR-198

3-D GEOMECHANICAL MODELING OF THE STRESS FIELD IN THE NORTH GERMAN

BASIN: CASE STUDY GENESYS-BOREHOLE GT1 IN HANOVER GROß-BUCHHOLZ

Ernesto Meneses Rioseco1, Jörn Löhken

2, Rüdiger Schellschmidt

1, Torsten Tischner

3

1Leibniz Institute for Applied Geophysics

Stilleweg 2

Hannover, Niedersachsen, 30655, Germany

e-mail: Ernesto.MenesesRioseco@liag-hannover.de, Ruediger.Schellschmidt@liag-hannover.de

2DYNAenergetics GmbH & Co. KG

Kaiserstr. 3

Troisdorf, Nordrhein-Westfalen, 53840, Germany

e-mail: Joern.loehken@dynaenergetics.com

3Federal Institute for Geosciences and Natural Resources

Stilleweg 2

Hannover, Niedersachsen, 30655, Germany

e-mail: Torsten.Tischner@bgr.de

ABSTRACT

Understanding the behavior of geothermal reservoirs

has increasingly gained in relevance to academic,

public and environmental institutions in the last two

decades. Among numerous projects operating

worldwide, the project ―Hydro-mechanical response

of geothermal reservoirs in the stress field generated

by complex geological structures‖ is a subproject of

the interdisciplinary research association

―Geothermal energy and high-performance drilling

techniques‖ (Geothermie und

Hochleistungsbohrtechnik ―gebo‖) in Lower Saxony,

Germany. The aim of this subproject is to improve

the understanding of the hydro-mechanical behavior

of geothermal reservoirs during and post- drilling and

-stimulation. We focus in the present study on the

present-day stress field characterizing the reservoir,

caused by the complex geologic structures

surrounding it. This will ultimately provide the

loading conditions necessary for the subsequent

modeling of the hydro-mechanical response of such

geothermal reservoir.

Using COMSOL Multiphysics as finite element code,

3-D present-day geomechanical modeling of the

stress field in a complex lithologically stratified

region containing salt diapirs typical of the North

German Basin have been carried out. In particular,

the region surrounding the drilling demonstration site

GeneSys-Borehole GT1 in Hanover Groß-Buchholz

has been chosen as the study area.

Geological/geophysical constraints constitute the real

structure of sediments provided by the geotectonic

atlas of NW Germany as well as measurements on

density and seismic velocities performed within the

framework of GeneSys-Borehole GT1.

Modeling results of the stress field are in good

agreement with measurements of the stress field

obtained from borehole breakouts and tensile

fracture. In particular, the observed depth-dependent

changes in the direction of the maximum horizontal

stress component are reproduced remarkably well in

the deeper section of the modeling domain. The

obtained magnitude of the horizontal components of

the stress seems to be lower than the values gained in

an initial mini-frac test performed in the area under

investigation. This suggests that additional far-field

stresses may play a significant role in the study

region. Moreover, model results show that depending

on the rheological properties assumed for salt the

resulting stress field in the targeted domain shows

different patterns. Especially when allowing for far-

field stresses, the impact of the different adopted

rheological models for salt on the resulting stress

field is even more striking.

INTRODUCTION

With the world demographic explosion and the rapid

economic growth of developing countries such as

China, India and Brazil, the energy demand has

dramatically increased in the last years. Rocketing

prices of conventional energy, specifically oil, gas

and coal together with the worrying man-made

climate change has resulted in an interest growth for

sustainable, non-conventional energy sources.

Environmentally friendly, renewable energy has

gained momentum worldwide. One promising is

geothermal energy, from which in contrast to solar

and wind energy, replenishable heat and electricity

are produced regardless of diurnal and seasonal

variations in weather conditions.

In particular, three prominent regions in Germany

have been recognized as appropriate for the

production of hydro-geothermal energy: the South

German Molasse Basin, the Upper Rhine Graben, and

the North German Basin (e.g. Agemar et al., 2012;

Pester et al., 2010; Schellschmidt et al., 2010,

http://www.geotis.de). This latter, unlike the two

other preceding examples that are distinguished by

highly conductive units deep underground, is

characterized by exceedingly low-permeability

sedimentary rocks. Geothermal resource exploitation

under this condition requires the generation of

relatively large artificial fractures to enhance the heat

exchange and hence the performance of the

geothermal reservoir.

Among diverse projects being in progress across the

world, the project ―Hydro-mechanical response of

geothermal reservoirs in the stress field generated by

complex geological structures‖ is a subproject of the

interdisciplinary research association ―Geothermal

energy and high-performance drilling techniques‖

(Geothermie und Hochleistungsbohrtechnik ―gebo‖)

in Lower Saxony, Germany (see for instance

Reinicke et al., 2010). The main goal of this

subproject is to enhance the understanding of the

hydro-mechanical behavior of a geothermal reservoir

during and post- drilling and -stimulation. The focus

of the present study, however, lies on the present-day

stress field characterizing the reservoir, caused by the

complex geologic structures surrounding it.

Knowledge and understanding on the current state of

upper-crustal stresses is crucial when addressing

geothermal as well as hydrocarbon energy extraction,

shale gas derivation, geological nuclear waste

repository, the storage of carbon dioxide, the

construction of stable tunnels, underground caverns

and shafts in the mining industry for the final

excavation and exhaustion of natural resources (Zang

& Stephansson, 2010).

The present-day stress field in the North German

Basin is particularly complicated by the numerous

salt diapirs and salt layers present in this area (e.g.

Hecht et al., 2003; Kaiser et al., 2005; Marotta et al.,

2002; Maystrenko et al., 2012; Röckel & Lempp,

2003; Scheck et al., 2003a, b; Scheck-Wenderoth &

Lamarche, 2005, and references therein).

Figure 1: Map showing the North German Basin

with the diverse salt structures inherent of

this area. This map has been drawn with

the help of the geotectonic atlas of NW

Germany (Baldschuhn et al., 2001).

Yellow circle denotes the demonstration

site GeneSys-Borehole GT1 in Hanover

Groß-Buchholz. Salt domains are

depicted in light blue. Note the two

relatively large salt diapirs Lehrte and

Benthe in the neighboring area of the

demonstration site.

Especially the region surrounding the drilling

demonstration site GeneSys-Borehole GT1 in

Hanover Groß-Buchholz has been chosen as the

study area (Fig. 1) (Jung et al., 2005; Orzol et al.,

2005; Tischner et al., 2010). Substantial mechanical

and hydraulic data collected in this demonstration site

have provided valuable geological and geophysical

constraints for the modeling. In particular, the project

GeneSys includes the application of water-frac

techniques to sedimentary rocks. A more detailed

description of target, goals as well as milestones

reports of this demonstration site can be found in

www.genesys-hannover.de. Ultimately, the

measurements made in GeneSys-Borehole GT1 will

also serve to validate the model results.

Roughly speaking, hydraulic fracturing means

pumping fluids (predominantly water-rich) into a

borehole until the fluid pressure at depth reaches a

threshold value, resulting in rock failure. Hydraulic

fractures, in turn, are fundamentally extensional

fractures and hence their predominant pathways are

determined by the orientation of the principal

horizontal stress components. Well planning

includes, thus, a previous thorough analysis and

modeling of the current state of the stress field in

combination with a proper choice of the drilling path.

In general, information on the operating stresses is of

special interest when it comes to understanding the

tectonics of a region. Specifically in-situ stresses

have a major influence on the short- and long-term

spatiotemporal evolution of hydraulically stimulated

geothermal reservoirs. Therefore, knowledge on the

local stress field in the target domain is of paramount

importance for every study involving the complex

hydro-mechanical response of geothermal reservoirs.

Besides, any fluid-induced deformation analysis of

the uppermost crust or seismicity study (Jahr et al.,

2006; Moeck et al., 2009; Moeck & Backers, 2011) is

based on the stress field. Altogether, studying the

present-day stress field in the area under investigation

is a prime ingredient for an in-depth and complete

comprehension of the prominent mechanisms

involved in a geothermal reservoir.

There are essentially two ways of estimating the

stress field in sedimentary rocks: numerical modeling

and in situ stress measurements, the latter however

limited to accessible depths (e.g. Bell, 1996, 2003;

Zoback, 2008, and references therein). Both ways

complement each other to give a more robust picture

of the local/regional stress field in a region under

investigation. In particular, the presence of complex

geological structures such as active faults, grabens,

and salt diapirs makes the regional stress field picture

considerably heterogeneous. As mentioned before,

the Northern German Basin is characterized by the

occurrence of intense salt diapirism (e.g. Maystrenko

et al., 2012, and references therein). Hence, any

regional model concerning the assessment of the

stress field in this region should include the effect of

salt diapirs on the local stress field. Although

remarkable works (e.g. Moeck et al., 2008, and

references therein) have been achieved to assess the

recent stress field in the Northeast German Basin

region, to the best of our knowledge there is no 3D

geomechanical modeling containing the effect of

neighboring salt diapirs on the present-day stress

field in the Northwest German Basin region. Hence,

the focus of this work is directed to the numerical

simulation of the stress field in the area of interest.

What do we know about the stress field in the

North German Basin?

The study area is part of the Lower Saxony Basin,

which is part of the North West German Basin

(NWGB), which in turn forms part of a major system

of basins regarded as the North German Basin

(NGB). The NGB is a typical passive-rifting

sedimentary basin that originated in Stephanian to

Early Rotliegend times (e.g. Kockel, 2002, and

references therein). A more than 10 km thick

Permian-to-Cenozoic sedimentary veneer

characterizes locally this basin, that is considered to

be primarily the result of thermal subsidence

(cooling) after a previous Permian mantle plume

pulse (e.g. Kockel, 2002; Ziegler, 1990). This basin is

distinguished by a composite of several deep rift

basins that have been tectonically active

intermittently during the Mesozoic. Its tectonic

evolution has been discussed at length in numerous

works (e.g. Baldschuhn et al., 1991; Betz et al., 1987;

Kockel, 2002; Kossow & Krawczyk, 2002; Ziegler,

1990, and references therein). The evolution of the

North East German Basin is especially marked by

intensive salt tectonics, and most specifically

diapirism. These salt movements have led to the

formation of a number of salt structures including salt

ridges, pillows, and diapirs (see for instance Fig. 1,

2). Zechstein salt is considered to play a prominent

role because of its mechanically decoupling character

(e.g. Röckel & Lempp, 2003). Rheological properties

of Zechstein evaporites, and most specifically its

inherent isotropic state of stress, resulted in different

stress patterns for the supra-salinar (Triassic and

younger depositions), the salinar (Zechstein), and the

subsalinar (lower-Permian and older depositions,

including crystalline) successions. While the

maximum horizontal stress component in the

saprasalinar layers reveals a rather scattering pattern

(so-called fan-like pattern), the stress components in

the salinar layer exhibit an isotropic state without any

horizontal or vertical preferential orientation. In the

subsalinar successions the maximum horizontal

component displays a robust N-S orientation in the

middle of the NGB, with small departures of this

orientation in the western and eastern part of the

basin (e.g. Röckel & Lempp, 2003). As for the

magnitude of the principal stress components, it is

broadly acknowledged that, essentially, gravitational

loading is what drives the state of stress in passive

sedimentary basins. In this case, the vertical stress is

determined by the overburden weight, and the

maximum horizontal stress is expressed as a fraction

of the vertical stress component. It is noteworthy that

this is the present-day general stress state in the

NGB, however, this stress state may be locally

altered by the presence of neighboring salt diapirs

and other significant salt-bearing sediment sequence

(e.g. Heidbach et al., 2007, and references therein).

Vast detailed information about the present-day stress

state can be found in several key reports of the World

Stress Map (e.g. Heidbach et al., 2009).

MODEL SETUP

The geological setting for the modeling domain,

which comprises the region of Groß-Buchholz

Hanover, was constructed taking into account data

from the geotectonic atlas of Northwest Germany

(Baldschuhn et al., 2001). Fig. 2 shows lithological

cross-sections of the study area. Fig. 3 illustrates as

an example the base of Lower Bunter. The location

of the borehole is depicted with a yellow circle. As

can be seen from Fig. 3, there exist some areas where

data is missing. For instance, west of the borehole is

one of these zones. Also in the municipal area of

Hanover such domains are present in deeper layers

such as Early Triassic. In the geotectonic atlas of

Northwest Germany such areas were not assigned

any data due to the combination of two facts: (I)

complex geology and (II) amount of available data,

which make an interpolation procedure not

geologically reasonable for these zones.

Figure 2: Different seismic profiles crossing the

study area. The cross-sections displayed

correspond to the seismic profiles 1 and 2

respectively highlighted in the upper

picture. Map and cross-sections were

obtained from the geotectonic atlas of NW

Germany (Baldschuhn et al., 2001).

Figure 3: Basis of Lower Bunter in the region of

Hanover. This map originates from the

geotectonic atlas of NW Germany

(Baldschuhn et al., 2001). Data is missing

where isolines are not present. Yellow

circle shows the position of Groß-

Buchholz Hannover.

For the modeling, the depths characterizing those

domains were calculated using a cubic interpolation

function. This is based on the fact that also the

geological maps used for the construction of the

geological setting in the study area originated

essentially as a result of the interpolation of point and

line data on areas, where rather less mathematical

interpolation procedures than geological assumptions

were considered. Hence, the data provided by the

geotectonic atlas of Northwest Germany reflect rather

an estimate picture where only locally accurate data

is available. The geological model is certainly the

best available model but not necessarily everywhere

correct. That is why the geological stratification of

the area under investigation was enhanced and

complemented with more recent and accurate data

provided by the borehole demonstration site GeneSys

in Groß-Buchholz Hanover.

Fig. 4 illustrates all geological horizons considered.

In particular, the outcropping of the upper layer on

the surface is clearly visible in Fig. 4. Another

striking feature of Fig. 4 relates to lower layers going

in part through upper layers in the neighborhood of

salt diapirs, as expected from the process of salt

diapirism. An additional aspect seen in Fig. 4,

possibly driven by salt tectonics as well, represents

the from-south-to-north-going depression of the

geological horizons between the two salt diapirs,

which intensifies in the northeast direction.

The lateral dimensions of the salt diapirs are given

for each layer in the geotectonic atlas. In some cases,

however, these cannot be sharply distinguished from

fault zones or by salt diapirism disturbed horizons,

see Fig. 3 (southwest of Hanover). For the modeling,

the diapirs contour in three layers at different depths

were taken from the geotectonic atlas and then

linearly interpolated between them. To ease the

finite-element grid generation of such complex

geometrically stratified domain, the geometry of the

salt diapirs was extended to the bottom of the model.

It is worth noting that for the calculations performed

in the numerical modeling these extensions of the salt

diapir were ignored.

Figure 4: Geological structure of the area

surrounding the demonstration site Groß-

Buchholz Hanover. Evident in the picture

is salt diapirs that pinch out of the

neighboring sedimentary layers and the

resulting arching and buckling of the

sedimentary layers. The notation used

follows: t=Tertiary/Tertiär, kro=Upper

Cretaceous/Oberkreide, kru=Marine

Lower Cretaceous/Marine Unterkreide,

jo=Upper Jurassic/Malm, jm=Middle

Jurassic/Dogger, ju=Lower Jurassic/Lias,

k=Upper Triassic/Keuper, so=Upper

Bunter/Röt, su=Lower Bunter/Unterer

Buntsandstein, z=Upper

Permian/Zechstein. Each plane

corresponds to the base of the layer.

Because the rheological model assumed at this stage

of the work is elastic or poroelastic, only elastic

parameters of all considered lithological layers are

required. The elastic parameters were calculated from

real seismic velocity and density measurements

performed in the borehole test well Groß-Buchholz

GT1. The Young’s modulus (E) and the Poisson’s

ratio (ν) for each lithological layer are derived from

the measured compression wave velocity (Vp), the

shear wave velocity (Vs), and the density (ρ) as

follows:

[

] ( ⁄ )

( ⁄ )

Fig. 5 displays the parameters for the different

compositional layers, which were calculated based on

density (TLD)- and seismic (DSI)- measurements. In

some sections no density measurements were

recorded what led to data incompleteness. This data

was then complemented with some other density

measurements carried out in other borehole in North

Germany with comparable lithological stratification.

Note the red dashed lines depicted in Fig. 5, which

exhibit the different layer boundaries that result from

the analysis of well cuttings as well as the

geotectonic atlas.

Figure 5: Density, Young’s modulus, and Poisson’s

ratio obtained from borehole

measurements conducted at the drilling

place Groß-Buchholz. Raw data is shown

with blue color while smoothed data is

displayed with red color. Red dash lines

correspond to depths from borehole

measurements. Depths stemming from the

geotectonic atlas of NW Germany are

depicted with black dashed lines.

Table 1: Material and rheological parameters for

each lithological layer, which are used for

the 3-D geomechanical modeling.

Compositional

layers

Depth

(m)

Density

(kg/m3)

Young’s

modulus

(GPa)

Poiss

on’s

ratio

(-)

Younger

covering layers

to Lower

Cretaceous

To

1187

2450 25 0.25

Lower Jurassic 2380 2666 25 0.31

Upper Triassic 2850 2724 49 0.26

Middle Triassic 3160 2755 65 0.29

Upper Bunter 3331 2670 52 0.27

Lower Bunter 3982 2710 59 0.22

Upper Permian 4032 2171 35 0.28

Lower Permian 5000 2650 45 0.24

Middle Triassic

halite

2948-

3038

2202 38 0.26

Upper Bunter

halite

3331-

3430

2071 34 0.25

Significant inconsistencies are clearly detectable for

the depth of the marine Lower Cretaceous, Lower

Jurassic, Upper Triassic, and Upper Bunter. In the

case of the marine lower Cretaceous the geotectonic

atlas essentially reveals the reflection on the upper

boundary of the Wealden whereas in the case of

Upper Bunter it displays the reflection of the Upper

Bunter halite. In other words, these discrepancies

result from the fact that these horizons are separated

by compositional layers characterized by different

physical properties. This can be better seen for the

density and the Young’s modulus in Fig. 5. The

layers depths of Lower Jurassic and Upper Triassic

were displaced either 92 m upwards or 89 m

downwards according to the mismatch between the

values suggested by the geotectonic atlas and the

actual registered values in borehole.

Summing up, the analysis of the borehole data

collected in GeneSys GT1 (Groß-Buchholz,

Hanover) leads to the differentiation of the following

layers: a cover layer until marine Lower Cretaceous,

Lower Jurassic, Upper Triassic, Upper Bunter,

Middle Triassic, and Lower Bunter as well as a halite

layer within Upper Bunter and Lower Bunter,

respectively. The lithological layering obtained in

GeneSys GT1 is displayed in Fig. 6. The material and

rheological parameters, that were averaged and

considered representative for each compositional

layer, are listed in table Tab. 1. The values for these

parameters for the Upper Permian and Lower

Permian were taken from Zimmer, 2001 and

Trautwein, 2005, respectively.

Figure 6: Stratigraphic layering from drilling core

analyses carried out at the drilling site

Groß-Buchholz Hanover (Hesshaus et.

al.,2010).

The assumed 3-D geological model for the numerical

modeling is illustrated in Fig. 7. A model block of

dimensions 31,5 km x 30 km x 5 km is composed of

eleven different lithological layers. Also the salt

diapirs Benthe und Lehrte are included in the model.

It is worth mentioning that these two salt diapirs are

incorporated in the model with their real complex

geometry. The borehole Groß-Buchholz is situated in

the picture at the intersection point of the middle

slices (x=16,5 km and y=17,5 km).

Figure 7: Geological model that is used for the

modeling setup with COMSOL-

Multiphysics. The modeling domain

dimensions are 31,5 km x 30 km x 5 km.

Yellow circle and dashed line depict the

drilling site GeneSys GT1 at Groß-

Buchholz Hanover. Note that salt material

has been displayed with white color. The

Poisson’s ratio assigned to salt material

in this case is 0.28.

For the ultimate numerical computation the isolines

of every compositional layer in the geotectonic atlas

were assigned a 1500 m x 1500 m grid and

subsequently read by Comsol Multiphysics

(software), which is an all-purpose, robust finite-

element numerical tool particularly suitable for the

solution of geomechanical problems involving

complex geometries. This already defines the

resolution of the mesh. Obviously a finer

discretization would better resolve small-scale

geometrical features, however at the expenses of a

more computationally intensive modeling. Further

refinements of the mesh were not successful since in

particular in the immediate vicinity of the salt diapirs

complex small-scale structures form, which sharp

angles and cross-sections at the interface with the salt

diapirs make difficult a proper generation of a finite-

element mesh.

As mentioned earlier, it may well be that for some

lithological layers data from the geotectonic atlas is

either missing or overlaps with overriding or

underlying layers. The reason for this issue is that

while compiling data for the mapping several

problems were encountered. On one hand data was

missing in some regions and on the other hand the

gridding and interpolation of data did consider the

lateral expansion of the layers but not their vertical

interlocking. In such cases, progressively going from

top to down, a minimum thickness for each layer is

assumed, that is consistent with the layer thickness of

the respective geological formations suggested by the

borehole analyses.

THE INFLUENCE OF THE RHEOLOGICAL

PROPERTIES OF SALT ON THE LOCAL

STRESS FIELD

Among common sediments, halite or salt-dominated

sediments have special thermal, mechanical and

rheological properties and behavior (Fossen, 2010).

Particularly the rheological behavior of salt has been

enigmatic to geoscientists for the last twenty years

(e.g. Hunsche & Schulze, 1994; Silberschmidt &

Silberschmidt, 2000, and references therein). A great

deal of studies addressing the impact of the presence

of different salt structures on the stress field in

adjacent sediments has been conducted recently (e.g.

Fredrich et al., 2003, 2007; Luo et al., 2012;

Nikolinakou et al., 2012; van der Zee et al., 2011, and

references therein). One of the major issues discussed

in the published literature relates to the constitutive

behavior of salt, namely its inability to accommodate

deviatoric stresses. This implies a reorientation of the

stress field at the interface between salt and the

adjacent non-salt material. This is in turn due to the

isotropic state of stress generally established within

salt bodies. To comply with this requirement, the

stress field in the vicinity of the interface between

salt conglomerates and surrounding sediments may

thus be considerably complicated and altered from its

far-field configuration. Well known is the creep

rheological behavior of salt observed in several

laboratory experiments and modelled by a number of

authors (e.g. Fredrich et al., 2003, 2007; van der Zee

et al., 2011). As a starting point in this work,

however, a stationary approach has been adopted,

capturing the main rheological properties of salt-

bearing sediments.

RESULTS

Stress field

As previously mentioned in the introductory part of

this work, the stress field in the Northwest German

Basin was investigated in several studies (e.g.

Fleckenstein et al., 2003; Kockel, 2002; Röckel &

Lempp, 2003). As a result of these works, one major

conclusion was drawn about the state of the stress

field in the North German Basin area: above the

Upper Permian layer the stress field is heterogeneous

and dominated by local geological units whereas

beneath the Upper Permian layer the stress field

exhibits a uniformly favored North-South fan-shaped

orientation. The reason of this decoupling in the

stress field seems to lies in the presence of a halite

layer within the Upper Permian layer. The

rheological properties of this halite layer result in a

mechanical detachment between the subsaliniferous

bed and suprasalt sediments, interrupting the pattern

of the stress field in the region. In suprasalt sediments

the stress field is controlled rather by local/regional

geological formations such as salt diapirs, faults, and

topological features. Some examples of these effects

are discussed at length in Fleckenstein, 2003.

Several logs measurements starting from a depth of

1000 m were carried out in the demonstration site

GeneSys GT1 borehole in Hanover Groß-Buchholz.

The realization of a six-arm caliper tool as well as

acoustic and electric display methods (UBI, FMI)

enabled the recording and further processing of

borehole breakouts and tensile fractures that took

place at the borehole wall. With the help of this data

it was possible to generate a vertical profile of the

stress field for the region of Hanover (Tingay et al.,

2008). Fig. 8 shows the results of the analyses (T.

Röckel, 2010, personal communication) for the

minimum principal stress component. The vertical

stress component was assumed as the largest one

according to studies conducted by Röckel & Lempp,

2003 supporting this stress regime.

In the depth interval between 1000 m and 2600 m the

direction of the minimum horizontal stress

component changes from 80° in the north direction to

170°, indicating a reorientation from East-West to

North-South. The direction of the minimum

horizontal stress component swings again at a depth

of 3500 m to 120°. Altogether, the orientation of the

minimum horizontal stress component shows a

―swirl-pattern‖ over depth.

The depth-dependent changes in the direction of the

stress field substantially complicate the planning and

design of the deflection of the drilling path, which is

a requirement for cost-effective, efficient geothermal

energy production using deep-circulation or multi-

fracture schemes. In such cases the deviation of the

drilling path should be conducted in the direction of

the minimum horizontal stress component so that the

drilling trajectory does not coincide with the plane of

the fracture propagation, which is in the direction of

the maximum horizontal stress component. In the

unfavorable case a hydraulically induced fracture

would extend over a large borehole segment,

provided there is no stress-related barrier inhibiting

the further propagation of the fracture. In such

situations hydraulic stimulations in any position of

the borehole would only enhance the first induced

fracture but not trigger any other fracture.

Figure 8: Left: Modeling results for the magnitude

of the principal stress components as well

as calculated according to equation 2.

Right: Results for the direction of the

minimum horizontal stress component

obtained from breakout analyses and

tensile fractures conducted at the drilling

site Groß-Buchholz GT1, as well as

modeling results of this horizontal stress

component.

Numerical simulations of the stress field were

performed with the finite-element program COMSOL

Multiphysics. The modeling domain as well as the

lithologically stratified sequence of compositional

layers are displayed in Fig. 4, 6, and 7. This block is

subjected to gravitational load and its elastic and/or

poroelastic response is investigated in terms of the

resulting stress field. The lower boundary of the

model is fixed and the upper model boundary can

experience vertical displacement (subsidence). At the

lateral boundaries of the model no compression or

extension is allowed, only motion is permitted in the

direction perpendicular to the normal of the lateral

surfaces.

The combination of several assumptions such as (I)

absence of far-field stresses, (II) negligible pore

pressure, and (III) some special selection of boundary

conditions explained above make it possible to

express the minimum horizontal stress component

( ) in terms of the vertical stress component ( ) for

a horizontally layered sequence of geological

formations:

As mentioned before, when considering linear

elasticity according to Hook’s law as the prevailing

rheological response of such a domain for the time-

scale and spatial-scale considered, the tectonic

stresses generated depend in general on the Young’s

modulus and Poisson’s ratio of the respective

lithological layer (e.g. Karato, 2008; Ranalli, 1995;

Turcotte & Schubert, 2002, and references therein}.

Since at the first stage of simulations the interest lies

on the orientation of the stress field, the pore pressure

is ignored. Note that the pore pressure only

influences the magnitude of the principal stress

components but not their respective directions.

Hence, instead of a poroelastic model only the elastic

behavior of the model at hand is addressed as a

starting point. Further analyses aimed at comparing

model values with values obtained from a mini-frac

test of the minimum principal stress component do

include a hydrostatic state of pore pressure.

Model results are displayed in Fig. 8. As can be seen

in Fig. 8 (left), there is almost full overlap of the

minimum and intermediate principal stress

components. The difference is limited to 2-5%.

Additionally, the minimum principal stress

component is calculated according to equation 2,

showing almost exact agreement with the values

obtained from the modeling. This indicates that no

considerable perturbation of the magnitude of

principal horizontal stresses is caused by the

geological structures embedded in the modeling

domain. However, when it comes to the direction of

the minimum principal stress component it is clear

from Fig. 8 (right) that the complex topology of the

compositional layers does substantially influence its

orientation.

Together with the model direction of the minimum

principal stress component Fig. 8 (right) shows the

data obtained from the measurements conducted at

the borehole GeneSys GT1 Groß-Buchholz. Except

in the vertical section between 1200 m and 2400 m

the model direction of the minimum principal stress

shows good agreement with the in situ

measurements. In particular, modeling results seem

to reproduce remarkably well the significant change

in the direction of the minimum principal stress from

180° in upper Triassic to 120° in Lower Triassic.

Similarly, model results are consistent with the

observed rotation of the direction of the minimum

principal stress within Triassic.

Several steps were taken in an attempt to reconcile

the discrepancies between model results and in situ

measurements of the direction of the minimum

principal stress component for the vertical section

comprising the layers overriding Middle and Lower

Jurassic. For instance, this vertical section was

refined including additional compositional layers

such as Wealden. In addition to that the density and

the elastic parameters of layers encompassed within

this vertical section were varied over a relatively

broad range of acceptable values. None of these

attempts were successful in capturing the measured

direction of the minimum principal stress in the upper

vertical section, so that such possibilities can be

excluded.

Alternatively, different rheological properties of

halite-dominated structures together with the effect of

far-field stresses can be held to account for such

direction of the minimum principal stress in the

above mentioned upper part of the modeling domain.

This is the focus of analyses described in the next

section.

The impact of different salt rheological properties

on the local stress field

The modeling of the stress field conducted in the

previous section used the physical and rheological

parameters of the halite-dominated layers in Middle

Triassic, Upper Bunter, and Upper Permian as well as

the salt diapirs embedded in the modeling domain

from the analyses of the borehole measurements. In

this case, in contrast to the remaining compositional

layers, all salt structures are distinguished by a

significantly lower density, a comparatively smaller

Young’s modulus, and a relatively marginal

Poisson’s ratio (see Fig. 5).

Because of the ability of salt to flow its rheological

behavior should not only be described by linear

elasticity. Salt deformation is controlled also by creep

mechanisms and these may significantly influence

the stress field in adjacent sediments to salt diapirs

and salt layers. In general, the most striking effect of

salt units embedded in sediments on the stress state

results from the fact that salt cannot withstand any

differential stress and as a consequence creeps when

any deviatoric stress is exerted (e.g. Fredrich et al.,

2003, 2007; van der Zee et al., 2011, and references

therein).

As a proxy for the different rheological behaviors of

salt, two conventional approaches have been adopted,

following Fleckenstein et al., 2003 and Thiercelin &

Roegiers, 2000. While Fleckenstein et al., 2003

reduce the Young’s modules of salt making it

weaker, Thiercelin & Roegiers, 2000 simulate the

expected lithostatic stress state of salt for all the

principal stress components by assuming a Poisson’s

ration value of 0.5 and letting unchanged the Young’s

modulus.

The elastic parameters of the three major salt

structures (Upper Permian salt layer, as well as salt

diapirs Benthe and Lehrte) were systematically

varied one after the other for each salt unit separately,

according to the ―weak‖ and ―lithostatic‖ approaches

mentioned before. The aim here is to possibly

differentiate the effect that each salt structure and its

rheological deformation mode may have on the local

stress field. Model results are shown in Fig. 9, and

10. For both considered rheological approaches,

model results show that the direction of the minimum

principal stress component is greater influenced by

the salt diapir Lehrte than by the salt diapir Benthe.

When the latter is characterized by a ―weak‖ or

―lithostatic‖ rheological state, at a depth of around

3400 m a uniform northeast-southwest orientation of

the minimum principal stress is established, which

rotates then again 90° in Lower Triassic.

Figure 9: Rheologically “weak” salt. Left:

Modeling results of the magnitude of the

principal stress components. Right:

Results concerning the direction of the

minimum horizontal stress component

obtained from breakout analyses and

tensile fractures carried out at the drilling

site Groß-Buchholz GT1, as well as the

direction of this stress component

resulting from modeling. The notation

used (see legend upper right) show which

modeling domain is assigned a

rheologically “weak” salt. B: Benthe salt

dome only, L: Lehrte salt dome only, Z:

Zechstein layer only, BL: Benthe and

Lehrte salt domes, BLZ: Zechstein layer

together with Lehrte and Behnte salt

domes. Model 1 relates to the unchanged

initial model where no variation of the

rheological properties of salt is

considered.

Figure 10: “Lithostatic” Salt. Left: Modeling results

of the magnitude of the principal stress

components. Right: Results concerning

the direction of the minimum horizontal

stress component obtained from breakout

analyses and tensile fractures carried out

at the drilling site Groß-Buchholz GT1, as

well as the direction of this stress

component resulting from modeling. The

notation used (see legend upper right)

shows which modeling domain is assigned

a rheologically so-called “lithostatic”

salt. B: Benthe salt dome only, L: Lehrte

salt dome only, Z: Zechstein layer only,

BL: Benthe and Lehrte salt domes, BLZ:

Zechstein layer together with Lehrte and

Behnte salt domes. Model 1 relates to the

unchanged initial model where no

variation of the rheological properties of

salt is considered.

Similar effects are obtained when varying the

rheological parameters of the salt layer contained in

the Upper Permian layer. However, in this case some

noticeable differences in the direction of the

minimum principal stress component are present. The

above described rotation of the minimum principal

stress by 90° occurs only when this salt layer is

assumed in ―lithostatic‖ state. When the salt layer is

assigned rheologically ―weak‖ properties no such

rotation of the minimum principal stress takes place

and the direction of the stress field remains almost

constant throughout the entire considered depth.

The effect of far-field stress in combination with

salt rheology on the local stress field

End of June 2010 a first mini-frac test was performed

at the GeneSys-borehole GT1 Groß-Buchholz, where

depending on the assumed pore pressure and Biot-

Willis coefficient (Biot, 1956) a 50-bar-higher value

of the magnitude of the horizontal principal stresses

than the value obtained from the modeling was

measured. As shown in the previous section, such

difference between measured and model magnitude

of the horizontal principal stresses can be explained

assuming for instance ―weak‖ rheological properties

of salt, however at the expenses of the loss in

information of the direction of the stress field in the

upper section of the model. That is why other effects

on the local stress picture such as those caused by

far-field stresses need to be studied.

Based on regional tectonics as well as recent studies

on the regional stress field, a compressional strain by

about 0.01% at 40° in the north direction is applied

resulting in a superpositional stress to the local stress

field. Model results are illustrated in Fig. 11. Using

Hook’s law for the respective compositional layer

this compressional strain translates into additional

stresses which depending on the rheological

properties of salt assumed are accommodated in the

entire modeling domain.

Figure 11: Left: Results involving the direction of the

minimum horizontal stress component

obtained from breakout analyses and

tensile fractures performed at the drilling

site Groß-Buchholz GT1, as well as the

direction of this stress component

resulting from modeling including the

effect of the rheologically “weak” salt.

Additionally, the direction of the minimum

horizontal stress component is shown for

the case where an external compression to

the modeling domain is applied (green

dashed line). Right: Analogous to the left

graph, the corresponding curves and

results are displayed, however in this case

a rheologically “strong” salt has been

adopted with a Poisson’s ratio of 0.27.

Analogous to the considerations made in the previous

section, two scenarios were selected where the

present salt structures were given ―weak‖ rheological

properties and a ―lithostatic‖ stress state. Fig. 9 (left)

shows model results corresponding to the case where

all salt layers considered in the model are assigned a

smaller value of Young’s modulus, what we call

―weak‖ salt. In this specific case model results show

that with the addition of carefully chosen far-field

stresses (e.g. external deformation by compression),

the direction of the minimum principal stress

obtained from the modeling agrees reasonably well

with that measured at the GeneSys-borehole GT1

Groß-Buchholz Hanover. Besides, also in this

scenario the increase in the magnitude of the model

minimum horizontal stress component is equal to the

required 50 bar. Fig. 11 (right) displays model results

for the scenario where salt structures are given a

lower value of Poisson’s ratio to mimic rather

stronger rheological properties of salt, what we call

―strong‖ salt. In this case the entire modeling block

behaves rigidly as a non-deformable unit and the

stress field is homogenously oriented. The external

deformation exerted on the modeling block

dominates the stress field within the block, adopting a

from-the-surface-to-the-bottom-of-the-model uniform

orientation in the direction of approximately 130°.

All otherwise observed rotations of the stress field

over depth are no longer reproducible since they are

suppressed by far-field stresses.

To sum up, model results confirm the observation

made in North German Basin domain of locally

perturbed stress field depending on the salt structures

present nearby. In particular, the orientation of the

stress field in a salt-dominated geological province

such as the North German Basin is highly sensitive to

the rheological properties and rheological mode of

deformation assumed for salt. Based on modeling

results and in situ measurements of the stress field it

can be conclusively said that the combination of the

special rheological properties of salt together with

tectonic far-field stresses and the interplay between

them play a key role in understanding the state of the

stress field in the study area.

CONCLUDING REMARKS

The North German Basin is distinguished by the

abundance of salt domes and salt structures in

general. This makes the present-day regional stress

field significantly heterogeneous. The region

surrounding the drilling demonstration site GeneSys

GT1 is directly neighbored by two relatively large

salt diapirs, i.e. Lehrte and Benthe, and as such

constitutes a representative part of the North German

Basin. Due to the wealth of data collected in this

area, this place has been chosen as the focus of our

numerical simulations. We concentrate in this work

on the modeling of the present-day stress field and

possible controlling factors driving it. To do so,

different scenarios have been modeled varying the

parameter space. The integrated data provided by the

GeneSys project together with additional data gain

from laboratory measurements of samples from the

study area served to constrain the model and score

model results. Although adopting a fairly simplified

rheological behavior of salt-dominated sediments, the

model stress field in the region around the

demonstration site GeneSys GT1 in Groß-Buchholz

Hanover seems to agree favorably well with

measurements conducted at this place. Model results

corroborate the previous broadly accepted view of

local departures of the general pattern of the stress

field characterizing common passive sedimentary

basins. In addition, the orientation of the stress field

in the vicinity of the salt domes and salt-bearing

layers turns out to be particularly sensitive to the

rheological properties as well as the rheological kind

of deformation adopted for salt. Modeling results

demonstrate that the superposition of specific

rheological properties of salt along with tectonic far-

field stresses and the interaction between them seems

to be a key ingredient in understanding the present-

day state of stress in area under investigation.

Nevertheless, it is worth mentioning that these

modeling constitute first steps towards a more

realistic and comprehensive modeling of the

rheological behavior of salt. Future efforts are

planned to strengthen the reliability of the model

results.

ACKNOWLEDGEMENT

This work has been conducted within the framework

of the project gebo ―Geothermie und

Hochleistungsbohrtechnik‖ (Geothermal energy and

high-performance drilling techniques). We

acknowledge the financial support provided by the

Ministerium für Wissenschaft und Kultur (MWK)

―Ministry of science and culture‖ in Lower Saxony as

well as Baker Hughes. We wish to extend our

appreciation to Franz Binot for his instrumental

contributions in our generating a proper lithologically

stratified uppermost crust of the modeling domain.

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