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Fractures, Flow & Seismicity. The Art of Coupling. Tanneke Ouboter, Brecht Wassing, Peter A. Fokker. The Art of. Coupling. Context. Enhanced Geothermal Energy operations Produce heat from hot impermeable rock Well doublet Stimulation by activating natural fracture network

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The Art of Coupling


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slide1

Fractures, Flow & Seismicity

The Art of Coupling

Tanneke Ouboter, Brecht Wassing, Peter A. Fokker

slide2

The Art of

Coupling

context
Context
  • Enhanced Geothermal Energy operations
    • Produce heat from hot impermeable rock
    • Well doublet
    • Stimulation by activating natural fracture network
    • Shear failure of existing fractures
    • Seismicity
  • Direct interest for modeling gas shales
coupled system the physical interaction
Coupled system – the physical interaction
  • Geomechanics
    • Stress, strain, fracture shearing and opening
    • Poro-elastic stresses
    • Thermo-elastic stresses
  • Temperature
    • Heat diffusion
    • Convection by flow
  • Flow
    • Darcy flow; Volume balance
    • Fracture permeability
    • Fluid viscosity
slide5

FLUID FLOW

GEO-MECHANICS

HEAT TRANSPORT

Darcy’s lawMass balance

Stress, StrainForce equilibrium

Fourier’s lawHeat conservation

Porosity,permeability

Density, viscosity

Advective heat transport

Poro-elasticity

Thermal expansion

Frictional heating

approach
Approach
  • Full coupling
  • Effective continuous properties
  • FLAC-3D, using “softening ubiquitous joints”
  • Implementation of constitutive behavior of fractures
  • For now: Geomechanics & Flow
coupled behavior
Coupled behavior

Geomechanics => Flow properties

  • Fracture permeability due to opening (Poiseuille flow)
    • kf = w2 / 12

Effective

    • k|| = c w3 / 12 L;
    • k┴ = 0
  • Fracture orientation gives non-diagonal permeability tensor
  • Porosity changes inducing pressure change (still to be done)
coupled behavior1
Coupled behavior
  • Mechanical deformation
    • Normal deformation due to fracture pressurization
    • Reversible fracture opening
    • Shear deformation due to Mohr-Coulomb failure
    • Permanent fracture opening
slide10
The Art of Coupling

Tensile failure – elastic

example pressure field homogeneously fractured medium
Example pressure field: Homogeneously fractured medium

W1 > W2

Low pore pressure

Reactivated

fractures & deformation

High pore pressure

The Art of Coupling

Permeability tensor

W1

W2

orientation fracture system

Well

fracture zones
Fracture zones

Stress field

Fracture reactivation

Flow preferences

Well

The Art of Coupling

Dynamic modeling:

  • Output:
  • Total area of slipping fracs
  • Displacement
  • Stress changes
  • Time and place
results
Results

The Art of Coupling

slide16
The Art of Coupling

Fluid flow through fracture zone

Time

preliminary 3d results
Preliminary 3D results
  • pore pressure profile in the fracture and matrix.
  • shear strain in the reactivated fracture zone.
  • enhanced permeability in the reactivated fracture zone.
slide19

5 min

shear strain (-)

pore pressure (Pa)

permeability (m2/(Pa/sec))

23 min

Constant joint friction angle – no shear softening

slide20

t= 24 min

shear strain (-)

pore pressure (Pa)

permeability (m2/(Pa/sec))

t= 25 min

Constant joint friction angle – no shear softening

further 3d results
Further 3D results
  • Softening of joints
  • Random friction angle in fault zone
  • Model to represent Soulz GPK-3 injection well with one fault zone
  • Reactivation represented
  • Calculation of stress drop for seismic moment
  • More calibration required
slide22

joint friction angle

(°)

injection well

35

friction angle

25

shear strain

0

1e-4

slide23

t=10200 sec ≈ 3 hours

t=20400sec ≈ 6 hours

random joint friction angle –shear softening

slide24

t=26400 sec ≈ 7.5 hours

random joint friction angle –shear softening

slide25

fracture zone

t=7.5 hours

pore pressure (Pa)

Injection well

slide27

t=7.5 hours

fracture zone

permeability (m2/(Pa/sec))

Injection well

slide28

t=7.5 hours

fracture zone

fractures

no reactivation

reactivation

reactivation

injection well

in fracture zone1

Injection well intersecting fracture zone

no reactivation

reactivation

reactivation

distance to seismicity front

z

X // fracture

In Fracture Zone
stress drop
Stress drop
  • Calculation of stress drop for seismic moment
    • t0: initial stress
    • td: residual stress
    • Dss: static stress drop(Cappa, 2011).
conclusions
Conclusions
  • Coupled model implemented in FLAC
    • Continuum description
    • Tensile fracture opening
    • Fracture reactivation
    • Fracture network permeability
    • To be done: Porosity
  • Progressive reactivation demonstrated
challenges
Challenges
  • Calibration
    • Injectivity
    • Seismicity
    • Model applicability
  • Temperature
    • Heat diffusion and convection
    • Dual-temperature system?