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The Art of Coupling

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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  1. Fractures, Flow & Seismicity The Art of Coupling Tanneke Ouboter, Brecht Wassing, Peter A. Fokker

  2. The Art of Coupling

  3. 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

  4. 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

  5. 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

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

  7. 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)

  8. Coupled behavior • Mechanical deformation • Normal deformation due to fracture pressurization • Reversible fracture opening • Shear deformation due to Mohr-Coulomb failure • Permanent fracture opening

  9. The Art of Coupling Tensile failure – elastic

  10. The Art of Coupling Tensile failure – elastic

  11. The Art of Coupling Shear failure shift along fracture plane

  12. The Art of Coupling Shear failure

  13. 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

  14. 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

  15. Results The Art of Coupling

  16. The Art of Coupling Fluid flow through fracture zone Time

  17. 3D model setup

  18. 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.

  19. 5 min shear strain (-) pore pressure (Pa) permeability (m2/(Pa/sec)) 23 min Constant joint friction angle – no shear softening

  20. t= 24 min shear strain (-) pore pressure (Pa) permeability (m2/(Pa/sec)) t= 25 min Constant joint friction angle – no shear softening

  21. 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

  22. joint friction angle (°) injection well 35 friction angle 25 shear strain 0 1e-4

  23. t=10200 sec ≈ 3 hours t=20400sec ≈ 6 hours random joint friction angle –shear softening

  24. t=26400 sec ≈ 7.5 hours random joint friction angle –shear softening

  25. fracture zone t=7.5 hours pore pressure (Pa) Injection well

  26. Injection well intersecting fracture zone pore pressure (Pa) z X // fracture In Fracture Zone

  27. t=7.5 hours fracture zone permeability (m2/(Pa/sec)) Injection well

  28. t=7.5 hours fracture zone fractures no reactivation reactivation reactivation injection well

  29. Injection well intersecting fracture zone no reactivation reactivation reactivation distance to seismicity front z X // fracture In Fracture Zone

  30. Stress drop • Calculation of stress drop for seismic moment • t0: initial stress • td: residual stress • Dss: static stress drop(Cappa, 2011).

  31. Conclusions • Coupled model implemented in FLAC • Continuum description • Tensile fracture opening • Fracture reactivation • Fracture network permeability • To be done: Porosity • Progressive reactivation demonstrated

  32. Challenges • Calibration • Injectivity • Seismicity • Model applicability • Temperature • Heat diffusion and convection • Dual-temperature system?

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