February 13-15, 2006

Hydromechanical modeling of fractured crystalline reservoirs

hydraulically stimulatedS. Gentier*, X. Rachez**, A. Blaisonneau*,

*BRGM** Itasca Consultants->BRGM

BRGM/Geo-Energy unit

February 13-15, 2006

Engine

> 2

In situ hydraulic stimulation tests at Soultz-sous-Forêts> Irreversible increase of the

permeability around the wells but not in the same proportions for the all the wells

0

1

2

3

4

5

6

7

8

9

1 0

1 1

1 2

1 3

Pd

h-P

o (

MP

a)

0 5 1 0 15 20 2 5 30 35 40 4 5 50 55 6 0 65 70 7 5 80

Qin (l/s )

Tran-Viet/BGR 10/96

Legende

G PK1 S t im ul at i on 19 9 3 (Est i m at ion )

G PK2 S t im u lat i on 95 JU N1 6

GP K2 S t im ula t io n 9 6 SEP1 8

G PK2 T ests in je ct i on 95 JU L0 1

G PK2 T est s i nj ec t io n 9 6SE P2 9

G PK1 Te st s i nj ec t io n (9 4 Ju ly )

G PK2 T es t in j ect i on ap rè s rep a ra ti on 9 5 AU G 1 5

G PK1 Inj ect i on 96 O C T 13

G PK2 Pro du ct io n 96 O C T1 3

(1) Stim. GPK1 -1993

(2) Stim. GPK2 -1995

(1)

(2)

(3)

(4)(4) Injec. GPK1 -1994

(3) Injec. GPK2 -1996

> Micro-seismic events associated to the hydraulic stimulation tests

Stimulation curves (GPK1/GPK2)

Micro-seismic events (GPK2/GPK3)

Gérard et al., 1997

Gérard et al., 2004

February 13-15, 2006

Engine

> 3

Objectives of our modeling work and of the talk...

> Objective of our work at BRGM is:• to understand which physical mechanisms are

involved in the hydraulic stimulation of the well in crystalline rocks

• to extract the main parameters playing a role in the hydraulic stimulation

• to establish the link with the micro-seismic activity observed during the hydraulic stimulation tests

> Objective of my talk is much less ambitious :• to give you an idea of the first results obtained up to

now by means of some examples extracted from the various hydraulic stimulation tests performed at Soultz-sous-Forêts

February 13-15, 2006

Engine

> 4

• Thermal effect is neglected in a first step for two reasons :

– we consider very short duration test

– we are interested in what it could happen at some distance of the well (the Thermo-Hydro-Mechanical behavior of the near well is in progress with another and more appropriated numerical tool)

Hydro-mechanical modeling approach> Conceptual model :

• The rock mass is considered as a blocks assembly which are separated by discontinuities

• Blocks are deformable and impermeable

400m

400m

1000m

1

2

3

5

6

7

F

• Flow takes place in the fractures exclusively

> Numerical tool : 3DEC code

integrating a real HM coupling based on :• Distinct Element method for the

mechanical part

• Finite difference schema for the hydraulic part of the model in the discontinuities

> Aim : to simulate the interaction between mechanical process (deformations, stresses,…) and hydraulic process (pressures, apertures,…)

February 13-15, 2006

Engine

> 5

What kind of data do have we to construct the model ?> hydraulic stimulation tests :

solicitation in the well

> Stress regime (?): mechanical boundary conditions

• Klee and Rummel (1993)

• Cornet et al. (to be published)

> Fracture network mobilized during the hydraulic stimulation :

• identification of this network from :

– flow logs

– temperature logs

– geological analysis (cutting analysis)

– bore-hole imagery

sH

sh

v

North

East

Pi = r g z

y = 0

z

x

x = z = 0

x=z=0

Injection under P = Pi + P

well

February 13-15, 2006

Engine

> 6

What it could happen during the hydraulic stimulation of a well (if we exclude thermal effect...)

h

H

V

In continuous homogeneous andisotropic medium

H

V

h

But in general, the granite is already fractured

February 13-15, 2006

Engine

> 7

More in details...

Un

Us

V

H

H

h

Evolution of the hydraulic aperture is linked to the normal displacement (Un) and the tangential displacement (Us)

closure of the fracture

UnUs

initial state

opening : reduction of the normal component

release of the shearing

Increase of the aperture Well

To

T1

T2

Tf

February 13-15, 2006

Engine

> 8

Four examples...

To illustrate our Hydro-Mechanical modeling approach, we are going to consider the influence of the following parameters :

> number of fractures involved in the stimulated network (GPK1)

> orientation and dip for a given fracture network (GPK2)

> heterogeneity of the hydro-mechanical properties of fractures (GPK3)

> stress regime (GPK4)

February 13-15, 2006

Engine

> 9

Influence of the number of fractures (GPK1)

Hydraulic apertures in the fracture zones

1

2

3

5

6

7

2

3 45 1

4

3

#1 : the most permeablein situ

1

2

3

5

6

7

F

0 10 20 30 40Flowrate [l.s-1]

2

4

6

8

10

Ove

rpre

ssu

re a

pp

lied

in w

ell [

MP

a]

GPK1 - Model with 7 fracturesreal injection testtotal flowrate at wellflowrate in fracture #1flowrate in fracture #2flowrate in fracture #3flowrate in fracture #4flowrate in fracture #5flowrate in fracture #6flowrate in fracture #7

Model with 7 fractures

#1?

0 10 20 30 40Flowrate [l.s-1]

2

4

6

8

10

Ove

rpre

ssu

re a

pp

lied

in t

he

wel

l [M

Pa]

GPK1 - Model with 8 fracturesreal injection testtotal flowrate at wellflowrate in fracture #1flowrate in fracture #2flowrate in fracture #3flowrate in fracture #4flowrate in fracture #5flowrate in fracture #6flowrate in fracture #7flowrate in fracture #8

Model with 8 fractures

#1#8

Extra fracture (depth 2884 m, dip 80°, dip-dir 230°) connecting two fractures in the upper part of the open hole

No significant change in the global behavior but significant change in the fracture #1 : better fitting with the in situ flow log data

# 4, 5, 6

February 13-15, 2006

Engine

> 10

Model with 7 fractures

Maximum Aperture = amax = 0.25mmConnection with other fractures

GPK1

Model with 8 fractures

Maximum Aperture # 0.20mmFew meters from well

GPK1

View in plane of Fracture #1 - Overpressure P=10.0 MPa

Influence of the number of fractures (GPK1)

Extra fracture

February 13-15, 2006

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> 11

Influence of the geometry (GPK2) Tangential displacements P = 14 MPa

Shearing propagates from the top to the bottom of the open hole

Regular network

Us max 5 cm

Statistical network

Shearing is concentrated in the upper part of the open hole

Us max 6 cm

Us max 2.5 cm

Shearing is concentrated in the lower part of the open hole

N 250° -> N 290°

February 13-15, 2006

Engine

> 12

75% of fluid flow

Heterogeneity of the hydro-mechanical properties (GPK3)

4905m

4930m

4960m

5015m

4980m

4750m

4860m

4% of fluid flow

Dezayes et al. (2004)

February 13-15, 2006

Engine

> 13

0

10

20

30

40

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60

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80

90

100

-5100-5000-4900-4800-4700-4600-4500-4400

Depth (m)

% F

low

rate

(25

l/s

)

in situ

Hyp. 1

Hyp. 3

P = 10.5 MPa

Influence of the heterogeneity in the hydro-mechanical properties (GPK3)

Overpressure (MPa)

Flow rate (l/s)

Well (model)

F0

F1

F2

F3

F4

F6

F5

F7

Well (in situ)

February 13-15, 2006

Engine

> 14

Heterogeneity of the hydro-mechanical properties (GPK3)

Shear displacements2D/cross section (EW)

Us max 1 cm

P = 15 MPa

Slip : points of ruptureMicro-seismicity ?

Existence of a very permeable fracture

limited extension of shear displacements for this range of overpressures

Increase of the permeability remains moderated

W E

February 13-15, 2006

Engine

> 15

Stress regime ?

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 50 100 150

Stress (MPa)

Dep

th (

m)

Sh(1)

SH(1)

SV(1)

Phyd

SV(2)

SH(2)

Sh(2)

Shmin(2)

Shmax(2)

hPhyd

H

V

?

1. Klee and Rummel (1993)

H : N170°2. Cornet et al. (2006?)

H : N 175°

Strike slip regime

Normal fault stress regime

February 13-15, 2006

Engine

> 16

P = 18,3 MPaInfluence of the stress regime (GPK4)

Normal fault stress regime

Strike slip regime

Us max 6 cm

Us max 12 cm

x 2

Tangential displacements more concentrated in some fractures

Tangential displacements more spread

February 13-15, 2006

Engine

> 17

Conclusions>Increase of the permeability could be explained by :

• shear mechanisms which are developed only in some fracture zones depending of :

– geometry and connectivity of the fracture network / stress field

– heterogeneity in the hydro-mechanical properties of the fracture in the network

This modeling approach can help to understand better a geothermal site but it must be based on a good geological and structural knowledge of the site

>Difficulties in relationship with the site :• definition of the in situ stress regime

• definition of the fracture network. The model is very sensitive and requires good structural data

• how this main stimulated fracture network is connected to the global fracture network constituting the real volume of the exchanger?

>Difficulties in relationship with the model :• which law of behavior to consider for the main fracture zone and how to

define the associated hydro-mechanical parameters ?