Monday, 5 August, 2024 / Edition 19

While looking back at past editions of Core Elements, I noticed I had skipped over one of my own core interests: structural geology and rock deformation. For the next two editions, we will turn our attention toward induced seismicity and fault reactivation by hydraulic injection. This phenomenon occurs in oil and gas shale fields, enhanced geothermal systems, carbon dioxide sequestration reservoirs, and wastewater disposal sites.

But before we dive in, a note about the physical basis of all these studies: Massive fluid injection into subsurface rocks alters pore fluid pressure and stress conditions, and eventually reactivates pre-existing, critically-stressed faults—those prone to slip and rupture. But the devil is in the details, and details are less known.

Rasoul Sorkhabi

Editor, Core Elements

Induced Seismicity Via Fluid Injection into a Fault Surface

Courtesy of the Alberta Geological Survey

An article in Computers and Geotechnics by Yan and colleagues reports on a numerical simulation of fluid injection directly into a fault surface.

Important context: To understand this study and its findings, we need to first remember that rock deformation and fracturing are mathematically described by Hooke’s law of stress-strain relationship and the Mohr-Coulomb failure criteria (failure envelope).

Three principle stress axes:

• For a three-dimensional rock body, three stress axes at a given time are applied onto a fracture surface: Vertical, minimum horizontal, and maximum horizontal.

• In normal faults, the maximum stress is vertical; in reverse faults the minimum stress is vertical; and in strike-slip faults the intermediate stress is vertical.

The input parameters by Yan and colleagues included:

• Rock parameters: Young’s modulus, Poisson’s ratio, density, porosity, permeability, and Biot coefficient

• Fault parameters: stiffness (normal and shear), friction coefficient (static and dynamic), fault aperture (initial and maximum), critical slip distance, and dilation angle

• Fluid parameters: density, viscosity, and compressibility

Field analog: An induced earthquake of 5.5 magnitude near an enhanced geothermal system site in Pohang, South Korea, on 15 November 2017, was the basis for this numerical simulation.

The study found:

1. Seismic slip is determined by a sudden increase in shear stress at the boundaries of aseismic slip zone.
2. The maximum seismic event occurs near the injection site, and seismic events move along the pressure propagation front.
3. For strike-slip faults, aseismic slip along fault strike is greater than along fault dip. For normal and reverse faults, aseismic slip along fault dip is greater.
4. Induced fault slip area and displacement for strike-slip faults are greater than those for reverse faults—hence, more earthquakes.
5. Although the slip tendency for normal faults is smallest, the seismic event magnitude is the largest at the shut-in stage. This is possibly because of tensile failure and higher permeability induced at normal faults during the injection phase.

BUT points 4 and 5 found are not supported by the distribution size of natural earthquakes.

Go deeper: Read the full article here.

Sponsored

The world’s most widely trusted geophysical interpretation solution

Integrate petrophysical, geological, and geophysical data to create regional reservoir models and maps, all in a user-friendly interface.

The Role of Fracture Stiffness in Induced Seismic Events 

A 2024 study by Sun and colleagues models the fault and rock matrix as poroelastic media.

Poroelastic theory:

• Under this theory, the rock is a porous linear elastic material coupled with fluid diffusion. The overall response of the rock matrix to fluid injection is responsible for reactivation of pre-existing faults.

• The theory was developed by Maurice A. Biot in 1941 and published in the Journal of Applied Physics.

Model: The authors used a three-dimensional, finite element, coupled fluid-flow deformation model, COMSOL Multiphysics, to estimate the likelihood of induced earthquakes on pre-existing faults during a multistage fracking operation.

Field analog: The model was based on information from the Silurian-age Longmaxi Formation—a major shale gas play in the Sichuan Basin. The ten-stages of fracking a 3790 m-deep horizontal well were monitored for microseismicity with eight downhole geophones mounted in a nearby vertical well.

Critical stiffness: To estimate critical stiffness, the authors incorporated the magnitude and change rate of effective stress (normal stress minus pore fluid pressure). Both contribute to rock instability.

• Stiffness is the resistance of material to elastic deformation.

• It is expressed by Young’s modulus or the ratio of stress to strain.

• In hydraulic fracturing, a rock with high stiffness (high Young’s or elasticity modulus) results in more narrow fractures.

Findings:

• There was a positive correlation between the magnitude of effective stress and critical stiffness. There was a negative correlation between the change rate of effective stress and critical stiffness.

• Instability is prominent when the magnitude of the effective stress increases (constant injection at each fracturing stage) and the change rate of effective stress decreases (the injection process is abruptly stopped).

• A closer distance of the fault to the injection site reduces the Coulomb failure stress and enhances the critical stiffness.

• The study also considered three different positions along the height of fault. The middle position, where fault is within the reservoir, is most unstable, while the fault position above the reservoir layer is more stable than that below the reservoir.

The bottom line: During fracking, fault location and fluid injection rate should be considered to reduce potential induced seismicity.

Go deeper: Read the full article in Petroleum Science.

👍 If you enjoyed this edition of Core Elements, consider supporting AAPG's brand of newsletters by forwarding to a friend or colleague and signing up for our other newsletters here.

➡️ Was this newsletter forwarded to you? Subscribe to Core Elements here.

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