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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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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 • Shear failure of existing fractures • Seismicity • Direct interest for modeling gas shales
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
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 • Full coupling • Effective continuous properties • FLAC-3D, using “softening ubiquitous joints” • Implementation of constitutive behavior of fractures • For now: Geomechanics & Flow
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 behavior • Mechanical deformation • Normal deformation due to fracture pressurization • Reversible fracture opening • Shear deformation due to Mohr-Coulomb failure • Permanent fracture opening
The Art of Coupling Tensile failure – elastic
The Art of Coupling Tensile failure – elastic
The Art of Coupling Shear failure shift along fracture plane
The Art of Coupling Shear failure
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 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 The Art of Coupling
The Art of Coupling Fluid flow through fracture zone Time
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.
5 min shear strain (-) pore pressure (Pa) permeability (m2/(Pa/sec)) 23 min Constant joint friction angle – no shear softening
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 • 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
joint friction angle (°) injection well 35 friction angle 25 shear strain 0 1e-4
t=10200 sec ≈ 3 hours t=20400sec ≈ 6 hours random joint friction angle –shear softening
t=26400 sec ≈ 7.5 hours random joint friction angle –shear softening
fracture zone t=7.5 hours pore pressure (Pa) Injection well
Injection well intersecting fracture zone pore pressure (Pa) z X // fracture In Fracture Zone
t=7.5 hours fracture zone permeability (m2/(Pa/sec)) Injection well
t=7.5 hours fracture zone fractures no reactivation reactivation reactivation injection well
Injection well intersecting fracture zone no reactivation reactivation reactivation distance to seismicity front z X // fracture In Fracture Zone
Stress drop • Calculation of stress drop for seismic moment • t0: initial stress • td: residual stress • Dss: static stress drop(Cappa, 2011).
Conclusions • Coupled model implemented in FLAC • Continuum description • Tensile fracture opening • Fracture reactivation • Fracture network permeability • To be done: Porosity • Progressive reactivation demonstrated
Challenges • Calibration • Injectivity • Seismicity • Model applicability • Temperature • Heat diffusion and convection • Dual-temperature system?