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Seismic and Aseismic Slip During Hydraulic Fracturing
02-01-12Stephen Perry
Sources
• Seid Bourouis and Pascal Bernard. Evidence for coupled seismic and aseismic fault slip during water injection in the geothermal site of Soultz (France), and implications for seismogenic transients. (2007).
• Cornet et al. Seismic and Aseismic Slips Induced by Large-Scale Fluid Injections. (1997).
• Calò et al. Large-scale aseismic motion identified through 4-D P-wave tomography. (2011).
Road Map
• Basics of Hydraulic Fracturing:– Explain how the process works– Methods to determine in-situ stresses and tensile
strength• Soultz Experiment– Findings of induced seismicity– Does the overall slip make sense for the observed
seismicity?– Seismic vs. Aseismic Slip and its effect on repeating
earthquakes
Goals of Hydraulic Fracturing• Characterize hydraulic behavior of rock• Determine in-situ stress field
Fundamentals of Structural Geology 2005.
Pressure and Flow Rate vs. TimePr
essu
re ->
Flow
Rat
e ->
Fundamentals of Structural Geology2005.
Calculating Tensile Strength• Stress State: σ = P + SH - 3Sh
– Fluid Pressure Required for Tension: P > 3Sh - SH
– Pressure Required to Cause Fracture:P = Pc = 3Sh - SH + T
– Pressure to Reopen Fracture:P = Pr = 3Sh - SH
– Tensile Strength:T = Pc - Pr
+SH (tension)
-3SH (compression)
Uniaxial Compression by a stress SH
Realistic Stress Field
+SH - 3Sh
-3SH + Sh
Finding the Principal Stresses
– Vertical Principal Stress: Sv = ρgD
– Smaller Horizontal: Sh = Ps
– Larger Horizontal: SH = 3Ps - Pc + T = 3Ps - Pr
1993 Soultz Water Injection Experiment
Bourouis and Bernard 2007.
Fluid Injection Rates and Surface Pressure
Cornet et al. 1997.
Loss of Water at Depth
Cornet et al. 1997.
Recorded Induced Seismicity
Cornet et al. 1997.
Slip on the Borehole
The slip vector S was determined to be 1.3 cm on this section of the borehole
Cornet et al. 1997.
Can the Earthquake Catalog Explain this Slip?
• Shear Stress Drop: Δσ = 7π/16*G*D/a9 MPa = 7π/16*(2*104 MPa)*(4 cm)/ (a)a ≈ 122 m
• Moment Magnitude: M0 = G*S*D = 16/7*Δσ*a3
M0 = 16/7 * (90*106 dyne/cm2) * (122*102 cm)3
M0 = 3.73 * 1020 dyne-cm
• The magnitude 1.9 earthquake is assumed to have a moment magnitude of 5.4 * 1018 dyne-cm
• No, it cannot explain the slip….there must be aseismic slip
Bourouis Examination of Multiplets
Bourouis and Bernard 2007.
Cumulative Slip and Pressure vs. Injection Rate
Each color represents a different multiplet. We can see their cumulative slip along with the injection rate and pressure at the well head at the time
Time
Bourouis and Bernard 2007.
Time Dependence of Seismicity
The time decay of the seismic rate decays as 1/t0.85 Bourouis and
Bernard 2007.
Indications on Friction Law
• The time evolution of reloading is consistent with reloading being driven by creep and creep being governed by rate-strengthening friction with friction:
• μ α log(v)
Extra Slides
Stresses on the Borehole
Fundamentals of Structural Geology 2005.