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.