Monday, 12 August, 2024 / Edition 20

Last week, we looked at numerical simulation of fault reactivation and induced seismicity by hydraulic injection. In this edition of Core Elements, we follow the same topic via findings in recently published experimental works.

Rasoul Sorkhabi

Editor, Core Elements

Roughness Matters

Induced earthquake in Pohang, South Korea in 2017/ Wikipedia Commons

Lei Wang and researchers in Germany recently published two articles that investigate how fracture roughness impacts injection-induced seismicity.

Important technical terms: These terms are helpful to know when reviewing these studies.

• A surface may be smooth or rough. Faults are usually rough. That roughness is called fracture asperity.

• Focal mechanism: Deformation and orientation of slip in the seismic source

• Moment tensor: A mathematical representation of deformation at the source based on force couples

• Double-couple component: The idealized earthquake mechanism for shear faulting in a homogenous, isotropic, elastic rock

• Non-double-couple component: An earthquake mechanism that deviates from idealized models because of rock anisotropy and fault curvature

• The Gutenberg–Richter law expresses the relationship between magnitude and frequency of earthquakes in any given region and time period.

Experiment design: Researchers cored plugs from Bentheim sandstone for a triaxial deformation experiment.

You can learn more about what triaxial testing entails by watching this video, but here is a basic outline of the process used in this experiment:

1. One sample with a smooth surface was saw-cut; two other samples had fractured rough surfaces.

2. Fluid was injected into the fault surface at a constant confining pressure of 35 MPa and fluid pressure of 5 MPa.

3. Acoustic emission events associated with micro-fracturing were recorded using high-resolution piezoelectric transducers attached to the sample surfaces.

4. The researchers also conducted a numeral simulation with input from the experimental results.

Results for smooth fault surfaces:

• Injection-induced acoustic emissions were uniformly distributed.

• They showed more double-couple components and less variability of focal mechanism for acoustic emission events.

• Differential stresses were high but localized to a narrow band adjacent to the fault plane.

Results for rough fault surfaces:

• These faults showed significant non-double-couple components of acoustic emission sources and a high degree of heterogeneity for focal mechanism.

• The clustered acoustic emission events occurred around highly stressed asperities, where induced local slip rates are higher and Gutenberg-Richter values for magnitude-frequency relation are lower.

• There was persistent and large-scale stress heterogeneity and off-fault damage, probably due to secondary fracturing.

• Fracture roughness slowed injection-induced slips.

Why it matters: This study suggests that real-time monitoring of induced seismicity during fluid injection may help localize seismic activity, improve forecasting for runaway seismic events, and better guide injection operations in reservoir rocks with segmented and rough faults.

Go deeper: Read the full study in EPSL and PNAS.

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The Impacts of Stress vs. Fluid Pressure

Photo source: USGS Survey

Zhang and colleagues conducted a study to understand the impacts of stress-driven vs. fluid pressure-driven friction on induced seismicity.

Experiment design:

• The researchers used cylindrical samples from granite outcropped in the Gonghe basin in the Qinghai province of China.

• Nine plugs, 50 by 100 mm, were diamond sawcut to create faults with dip angles of 45 degrees.

• Researchers used sandpaper of different particle sizes to roughen the fault surfaces.

• Triaxial stresses with concurrent fluid flow were used to create slip on the fault surfaces.

• Confining pressures and fluid pressures ranged from 0 to 60 MPa.

• Fault permeability was measured before and after slip.

Results for stress-driven conditions:

• Fault slip occurred when stress exceeded the peak static friction and shear displacement began to increase.

• As axial stress increased, fault reactivation occurred in stick-slip phases with a drop in stress and a jump in shear displacement.

• The friction coefficient gradually increased with slip events, because the rocks on the fault plane underwent strain-hardening during each slip.

Results for fluid pressure-driven conditions:

• Fault slip occurred only after the pore fluid pressure within the fault caused an overpressure.

• Uneven distribution of pore fluid pressure was marked by high-pressure areas near the fluid injection site, causing sliding within the fault.

• Other areas on the fault plane remained locked until more fluid was injected and shear stress exceeded friction.

• As permeability decreased, the magnitude of overpressure increased.

The bottom line: Fault roughness increases the coefficient of friction and permeability.

Go deeper: Read the full article in the IJRMMS.

Injection Rate's Effect on Induced Seismicity

Klee048/Shutterstock.com

Researchers Liu and Si have conducted triaxial deformation experiments on granite samples and analyzed slip characteristics by varying the fluid pressurization rate, confining pressure, and stress state of faults.

Results:

• Pressurization rate and confining pressure affected the onset of fault reactivation.

• At a low pressurization rate of 0.5 MPa per minute, the injection pressure required for fault reactivations agreed with theoretical estimation: The fault slip was slow and prolonged.

• At higher pressurization rates, the injection pressure required for induced seismicity increased and the fault slip was rapid.

• An increase in confining pressure also elevated the injection pressure required for fault reactivation.

• The faults displayed a stick-slip behavior (repeats of locking and moving) at high-pressure conditions.

• Different initial stress states had little influence on the slip mode after the onset of fault reactivation.

Why it matters: Studies like this help us understand and control induced seismicity in enhanced geothermal systems.

Go deeper: Read the full study in Applied Sciences.

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✉️ To get in touch with Rasoul, send an email to editorial@aapg.org.