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A Methodology for the Hydro-mechanical Characterisation of EGS reservoirs

Content:

1. Principal task in EGS development 2. A methodology: HEX-S code 3. Example: Coso geothermal field, USA 4. Example: Europ. EGS project Soultz, France

Mégel Th., Kohl Th. GEOWATT AG, CH-Zürich ENGINE Workshop3: Stimulation of reservoir and induced microseismicity 29-30th June 2006, CH-Zürich Session: Reservoir characterisation during stimulation

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Year Production [MWh]

Principal task in EGS development

C Exp D Production A

Life cycle phases

1. Maximise production 2. Guarantee production Principal Tasks:

1 2 Reservoir parameters of production ?

Production

Q [l/s] = Function of permeability k(x,y,z,t) T [°C] = Function of permeability k(x,y,z,t) permeability k(x,y,z,t) Fractured reservoir

?

Characterising the reservoir = Identifying k(x,y,z,t)

Reservoir impedance Heat exchange surfaces

Conditioning the reservoir = Creating/modifying k(x,y,z,t)

Strong interrelation

• f knowledge

Flow entry Flow exit

Principal task in EGS development

Stimulation Tests/measurements/data analysis

Data for characterising an EGS reservoir

Before drilling After drilling After stimulation

• 1. well

Regional/local geological model Geophysical survey data Cuttings (z) UBI/ARI... logs (z) Temp logs (z,t) Stress field (z) After hydraulic tests

• 1. well

Phase Borehole scale Reservoir scale Flow log (z,t) Pressure (z,t) Pressure (z,t) Microseismic locations (x,y,z,t)

• 2. well
• 3. well

Identifying k(x,y,z,t) Predict k(x,y,z,t) for a stimulation scenario

HEX-S

Hydro-mechan. Physics

A methodology: HEX-S code

• 1. Hydraulic structure model
• 2. Hydromech. calc. of k(x,y,z,t)

Identification of hydraulic structures from existing data sets Identification of hydraulic structures from existing data sets Mapping of the deterministic structures to a FE-grid Mapping of the deterministic structures to a FE-grid Mapping of stochastic structures to the FE-grid = k(x,y,z,t=0) Mapping of stochastic structures to the FE-grid = k(x,y,z,t=0) Pressure development in the reservoir (FE-grid) Pressure development in the reservoir (FE-grid) Adaptation of k(x,y,z,ti) in FE-grid Adaptation of k(x,y,z,ti) in FE-grid Shearing and opening of fractures Shearing and opening of fractures Injection flow (t) foreseen Injection flow (t) foreseen

Principal concept of HEX-S

timestep ti

A methodology: HEX-S code

Example: Stimulation GPK4, development of fracture apertures

A methodology: HEX-S code

Implementation of hydraulic structures

• by compliance
• by shearing
• by jacking : σn,eff <0

( ) ( )

dil S s dil basic eff n

U a K U Φ ⋅ = ∆ = Φ + Φ ⋅ − = ∆ tan ; tan

,

τ σ τ τ

ref n eff n

a a

, ,

9 1 σ σ ⋅ + = Representation: Circular fracture zones subdivided into circular slip patches Mechanics of fracture aperture a:

A methodology: HEX-S code

Deterministic fracture zones Stochastic fracture zones

GPK2 GPK3 GPK4

Representation of hydraulic structures

Example: European EGS project Soultz-sous-Forêts, France

+

• borehole logs (UBI, T, flow,...)
• structures from microseismicity
• others

statistical interpretation of “known” data:

• stat. distribution of structure parameters

data used “known” data:

Example: Coso geothermal field, USA

Representation of hydraulic structures

38 pad

38A-9 38B-9 38C-9 34-9RD2

• 600
• 400
• 200

200

[ ]

• 200

200 400 38C-9 38b-9 38a-9 38-9 34-9RD2

• 3000
• 2500
• 2000
• 1500
• 1000

z [m]

500

x [m]

• 500

y [m]

38C-9 38b-9 38a-9 38-9 34-9RD2

38A-9 38C-9 38B-9 34-9RD2

• P (z)
• Initial permeability: 5e-16 m2 (defines the initial apertures)

Initial hydraulics

• shmin (z)
• SHmax (z)
• Sv (z)
• Azi of SHmax (11°)

Stress field

(linear function with depth)

• E-modulus (6e10 Pa)
• Poissons ratio (0.25)

Rock parameters

• Dip, Azi of dip, depth
• Shear friction angle (31°)
• Shear dilation angle (3°)
• 90% reference closure stress (30e6 Pa)
• Fracture zone density [2e-3 m-1]
• Fracture zone radii (500 m)
• Slip patches radii (40 m)

Network of hydraulically active fracture zones

Representation of hydraulic structures

Example: Coso geothermal field, USA

1. Extracting dip + azidip from FMS- logs for the depth range of each identified FZ under task 1 2. Determination of the set of

• rientations with the highest
• ccurrences
• 2. Identification of the orientation

1. Extracting the depth range of zones of lost circulation 2. Extracting depth range of signals (significant deviations in gradient) in temperature logs 3. Others ?

• 1. Identification of the hydraulic activity

Method Task

Representation of hydraulic structures

Example: Coso geothermal field, USA

Representation of hydraulic structures

• 1. Identification of the hydraulic activity, Pad 38A

Lost circulation zones Temperature gradient

Example: Coso geothermal field, USA

Representation of hydraulic structures

• 2. Identification of the orientations

Example: Coso geothermal field, USA

Representation of hydraulic structures

Orientations + depths of deterministic FZ for HEX-S model

34-9RD2 38A/B/C-9

Deterministic hydraulic structures

Example: Coso geothermal field, USA

Stochastic hydraulic structures

Representation of hydraulic structures

Orientation of all fractures with an assumed hydraulic relevance, with the sign of the corresponding BH Orientations + frequency of all hydraulic active fractures from pad 38

Example: Coso geothermal field, USA

Stochastic hydraulic structures

Representation of hydraulic structures

red: deterministic structures blue: stochastic structures

Example: Coso geothermal field, USA

FE-grid for transient hydraulic calc.

Transient hydro-mechanical calculation

• 3000
• 2000
• 1000

x3

• 4000
• 2000

2000 4000

x1

• 4000
• 2000

2000 4000

x2

X Y

Numerical FE Grid Element size 25m ~340‘000 elements

X Y Z

34-9 RD2 38C-9 38B-9 38A-9

Each element has a permeability corresponding to the apertures of the intersecting fractures

k(x,y,z,ti)

Example: Coso geothermal field, USA

Example of pressure field

• Strongly anisotropic pressure

distribution

• Radial field only around injection

point

• Pressure envelope oriented along

fracture orientation

• Pressure wave propagation in

areas with highest degree of fracturing After 15 hours of injection into 34-9 RD2

Time [s] Pressure [Pa]

20000 40000 60000 80000 0.0E+00 2.0E+06 4.0E+06 6.0E+06 8.0E+06 1.0E+07 1.2E+07

Example: 5 stochastic realizations

34-9 RD2

Example: Coso geothermal field, USA

Transient hydro-mechanical calculation

injection into 34-9 RD2

• Calc. fracture shearing

34-9 RD2 Injection into 34-9 RD2 Colour code = shear displacement (red: big displ.)

Example: Coso geothermal field, USA

Transient hydro-mechanical calculation

k(x,y,z)

Example: Europ. EGS project Soultz, France

Transient hydro-mechanical calculation

• Numerical FE Grid
• Element size 25m
• ~400‘000 elements
• 6000
• 5000
• 4000
• 3000

z

• 4000
• 2000

2000 4000

x

• 6000
• 4000
• 2000

2000 4000

y

X Y Z X Y Z

GPK2 GPK4 GPK3

GPK2 GPK3 GPK4

• deterministically defined
• stochastically defined

structures

mapping ap k(x,y,z,t=0)

Example: Europ. EGS project Soultz, France

Transient hydro-mechanical calculation GPK3, 2003

At GPK3

time [s] d Pdh [Pa] Flowrate [l/s]

100000 200000 300000 400000 500000 2E+06 4E+06 6E+06 8E+06 1E+07 1.2E+07 1.4E+07 1.6E+07 1.8E+07 2E+07 10 20 30 40 50 60 70 80 90 100 Measured (depth corr.) Model Flow

IIdata : 1 IImodl : 1 IIdata : 0.78 IImodl : 0.88 IIdata : 0.70 IImodl : 0.84 IIdata : 0.60 IImodl : 0.80 IIdata : 0.80 IImodl : 0.94 IIdata : 1.04 IImodl : 1.20 IIdata : 1.01 IImodl : 1.25

Y X Z

1.70E+07 1.60E+07 1.50E+07 1.40E+07 1.30E+07 1.20E+07 1.10E+07 1.00E+07 9.00E+06 8.00E+06 7.00E+06 6.00E+06 5.00E+06 4.00E+06 3.00E+06 2.00E+06 1.00E+06

Around GPK3

• Fit of downhole pressure history
• Highly non-linear processes
• Permeability variation due to shearing
• Flow aligned along seismic structures
• Seismicity connected to zones of high pressure

Example: Europ. EGS project Soultz, France

Transient hydro-mechanical calculation GPK3, 2003

• Microseis. loc. GPK3 stimulation

Fracture shearing in HEX-S model

Similar spatial extension

after 1 day injection

plan view plan view

Example: Europ. EGS project Soultz, France

Transient hydro-mechanical calculation GPK4, 2004

time [s] ∆Pdh [MPa] Qin Omega

100000 200000 300000 5 10 15 20 10 20 30 40 50 60 70 80 90 100

Ref.pressure 45.0 MPa

Pressure measured Pressure from HEX-S model

time [s]

100000 200000 5E+06 1E+07 1.5E+07 2E+07 2.5E+07

IImodl : 6.9-7.4 IImodl : 9.7-10.7

short transients short transients general characteristics at t = 15’500 s at t = 16’600 s Maximum Pdh + 1.5 MPa + 1.05 MPa increasing to 45 l/s 21 MPa 15.5 MPa ∆Pdh at 30 l/s HEX-S prediction Measurements

Example: Europ. EGS project Soultz, France

Transient hydro-mechanical calculation GPK4

• Microseis. loc. GPK4 stimulation

Fracture shearing in HEX-S model (predicted before stimulation test)

• Similar spatial extension
• Similar path of development

1 day of injection

view from west view from west

Conclusions

• Production must become sufficiently predictable for conditions at a given site
• Production parameter Q [l/s] depends on reservoir impedance = fkt { k(x,y,z,t) }
• Production parameter T [°C] depends on heat exch.surfaces = fkt { k(x,y,z,t) }
• Task 1:

Making production [MW] predictable = sufficient characterisation of k(x,y,z,t) of a reservoir

• Task 2:

Improve/enhance production [MW] = predict k(x,y,z,t) due to enhancement activities

• Methods/models/codes as HEX-S are needed to fulfil these tasks.