Fracture Dynamics and Its Induced Seismicity

Fractures strongly affect the hydraulic properties of rocks as they often serve as preferential flow paths for fluids. Therefore, fracture characterization is crucial to determine the scale of the fluid transport in geological settings (e.g., the volume of gases that reaches the surface from magma), given their strong contribution to fluid transports in geological settings (e.g., impermeable bedrock aquifers, volcanoes, and petroleum reservoirs). To understand the fracture dynamics and its induced seismicity, Dr. Askari's team developed several experimental apparatuses in the Physical Modeling Laboratory (PML). Specifically, the team studied a specific seismic mode, called the Krauklis wave, that is generated within fluid-filled fractures. Since the Krauklis wave is initiated from fractures, it can be used to estimate their sizes and the fluids involved.

In Cao et al. 1, we studied the fundamental parameters affecting dispersion, dissipation, and resonance properties of the Krauklis waves, such as fluid viscosity, fracture geometry, fracture compliance, and stiffness ratio, to enhance the interpretation of the Krauklis wave for fracture characterization. We developed an experimental apparatus to study the Krauklis wave within a trilayer model consisting of a pair of aluminum plates and a mediating viscous fluid layer. We utilized a piezoelectric source and miniature pressure transducers in our measurements. Through comprehensive experiments, we noted that the phase velocity, resonant frequencies, and quality factors of the Krauklis wave (1) increase with the expansion of the fracture aperture and (2) decrease with the increase of the fluid viscosity. Additionally, (3) phase velocity, resonant frequencies, and attenuation decrease with the increase of mechanical compliance. In addition, we found out that fracture geometry can significantly affect the Krauklis wave as in two modeled wedge-shaped and rough fractures, the phase velocity of the Krauklis wave decreased.

The Sompi analysis of a Krauklis wave record the fracture apertures of (a) 1.5 mm, (b) 2.5 mm, (c) 4.5 mm, and (d) 8.5 mm (from Cao et al. 1).

Visualization of the Krauklis wave propagation with a trilayer model using the dynamic photoelasticity experimental apparatus (from Cao et al. 2).

In Cao et al. 2, we developed an optical apparatus based on the dynamic photoelasticity technique to visualize the propagation of the Krauklis wave within fluid-field fractures. Using this apparatus, we visualized and analyzed the propagation of the Krauklis wave within an analog fluid-filled fracture. We demonstrated that the dynamic photoelasticity technique can quantitatively describe seismic wave propagation with a quality similar to experiments using conventional transducers (receivers) while additionally visualizing the seismic stress field. We also determine the capability of the method to analyze seismic data in the case of complex geometry by modeling a saw-tooth fracture. We showed that the fracture’s geometry could strongly affect the characteristics of the Krauklis wave as we noted a higher Krauklis wave velocity for the saw-tooth case, as well as greater perturbation of the stress field.

In Cao et al. 3, we focused on the dynamics of fluid-filled fractures using the dynamic photoelasticity technique. We studied two phenomena stemming from the solid-fluid interaction that can significantly fracture dynamics. The first phenomenon is cavitation (also called bubble nucleation in literature), which is initiated when the pressure fluid within a fracture suddenly drops below its vapor pressure, forming the nucleation of vapor bubbles. Cavitation has a significant effect on fracture dynamics. We showed that fracture geometry can contribute to cavitation when the fluid viscosity is low. Another phenomenon pertinent to fracture dynamics is fracture opening due to an increase in fluid pressure. We demonstrated that bubbles accumulated at the tip of a fracture can enhance fracture opening.

In an internationally collaborative project, we studied the deformation of fractures and vugs in carbonate rocks due to changes in the effective stress by conducting a series of loading-unloading experiments (up to 20 MPa effective stress) using a high-resolution 3D X-ray computed tomography technique. We showed that after loading, the porosity of carbonate rocks decreased exponentially, followed by an increase during unloading; however, it did not recover to its initial value.

Currently, Dr. Askari's team is working on two NSF-funded projects pertinent to fracture dynamics and its induced seismicity. In one research3, we have developed a large-scale apparatus in a laboratory-controlled condition to simulate long period (LP) events that are observed in magmatic settings. Our results will improve the interpretation of LP events to enhance estimating the scale of magma transports in volcanoes. In another research, we are developing a novel remote sensing technique based on the enhanced Moiré technique to record seismological signals at locations close to volcanic craters. Those seismological signals can provide crucial information about gas and magma through the shallow crust.

(a-g) Image sequence of the bubbles expanding and collapsing for a saw-tooth crack model visualized by the photoelasticity apparatus. (h) The tracked pixel brightness intensity at the saw-tooth location indicated by a white arrow in (a–g). From Cao et al. 3.

3-D visualizations of (a) digital rock and corresponding (b) pore network model. In (a), the blue and blue areas correspond to the pore space and the matrix respectively. In (b), blue spheres are the pores and white tubes are the throats (from Yang et al.).