Using data from nodal geophones and broadband seismometers, this study investigates the seismicity near Red Deer, Alberta, a region with increasing cases of hydraulic fracturing (HF)-induced earthquakes. A cluster of 417 events was detected, and their spatial distribution and focal mechanisms reveal a NE trending rupture area with two strike-slip fault planes. Reactivation of preexisting faults by pore pressure diffusion is likely responsible for the occurrence of the earthquake sequence following the M L 4.18 mainshock. The temporal sequence of reactivated fault orientations suggests apparent changes in the local stress field following the mainshock, which is also responsible for a remotely triggered cluster observed 1 month after the mainshock. This secondary triggering process enhances our understanding of the trailing effect of HF-induced seismicity. Plain Language Summary Since 2018, the Red Deer region in Alberta, Canada, has experienced an increasing number of earthquakes, most of which are associated with nearby hydraulic fracturing operations. In this study, we analyze data from a dense array of seismic sensors and regional seismometers to detect and locate events surrounding a hydraulic fracturing site near Red Deer from 4 March to 10 April 2019. The spatial distribution of the earthquakes defines a complex fault system that was activated at two different times. The results in this study signify stress changes in the shallow crust in connection with the 4.18 magnitude earthquake on 4 March 2019. Modifications to the regional stress regime are relatively long-lived, as suggested by the continued occurrences of smaller earthquakes 1 month after the mainshock.

This study analyzes earthquake recordings from four near‐source (<10 km) stations near Fox Creek, Alberta, a region known for hydraulic fracturing‐induced seismicity. We examine the spatiotemporal variations of focal mechanisms and seismic anisotropy in the sedimentary strata. The focal mechanisms of surrounding earthquake swarms are generally consistent with the strike‐slip mechanism of the ML 4.6 earthquake, favoring a flower type of fault structure. The NE‐SW‐orientated fast splitting direction, determined from the shear wave splitting measurements, reflects the combined effects of (1) N‐S faults and (2) NE‐SW time‐dependent hydraulically stimulated fractures. The latter effect dominates the apparent anisotropy during the days leading to the mainshock, while its contributions are reduced by 60–70% after the mainshock. Loss of fluid into the fault damage zone, which causes the closure of fractures, is responsible for the observed spatiotemporal variation of seismic anisotropy near the hydraulic fracturing well.

Although hydraulic fracturing-induced earthquakes have been widely reported in Alberta, Canada, only one seismic cluster (the Cordel Field) has thus far been linked to wastewater disposal (WD). In this study, we report a statistically significant spatiotemporal correlation between recent earthquakes and nearby WD wells near Musreau Lake—the second disposal-induced earthquake swarm in Alberta. This newly occurred swarm contains five events with local magnitudes ML&gt;3 from January 2018 to March 2020, forming into three tightly spaced clusters. The refined locations and focal mechanisms suggest a ∼10 km long northwest–southeast-trending rupture along the northern Rocky Mountains that developed over time, during which both poroelastic effects and static stress transfer played key roles. Through a statistical analysis of all reported induced earthquake clusters in the western Canada sedimentary basin (WCSB), we propose a linear predictive relationship (i.e., the “Interpolated Strike Orientation” model) between fault rupture direction and fault distance to the Rocky Mountains. This observation-based model, which is supported by both the focal mechanisms of the natural earthquakes and the nearby northwest-striking geological faults, is a new and useful reference for future assessments of seismic hazard in the WCSB.

Carbonate rocks have a complex pore structure, show strong heterogeneity, and have a wide range of velocities that lead to more complicated velocity-porosity relationships compared with sandstones. We designed and prepared 72 carbonate synthetic cores with known pore structures according to the control variate principle. We measured the P- and S-wave velocities of these cores by an ultrasonic pulse transmission method, analyzed the effects of the pore aspect ratio (AR) and pore size [Formula: see text] on velocities, and compared the experimental results with predictions of effective medium theories (EMTs). The matrix of our synthetic cores was consolidated mixture of carbonate cuttings and epoxy. We randomly imbedded predesigned penny-shaped silicone disks or expandable polystyrene balls into the matrix during the core preparation process to simulate secondary pores. The experimental results indicated that Han’s empirical linear velocity-porosity relation was a good prediction for cores with only interparticle pores. Secondary pores played an important role in the velocity variation of carbonates. Cores with a larger AR had faster velocities. Different ARs could lead to velocity variations as high as [Formula: see text] at a given porosity. When the wavelengths [Formula: see text] were larger than the pore size, cores with larger secondary pores found higher velocities under the same pore shape, pore fluid, and porosity condition. Different pore sizes could contribute to nearly 15% velocity variation at a given porosity. The comparison between our measurements and EMT predictions indicated that for carbonate rocks with a complicated pore structure, the self-consistent model gave more reliable predictions when the secondary pore size was relatively small ([Formula: see text]) and Kuster and Toksoz formulations as well as the differential effective medium model gave more satisfactory results when the secondary pore size was relatively large ([Formula: see text], or even smaller).

Underground rocks usually have complex pore system with a variety of pore types and a wide range of pore size. The effects of pore structure on elastic wave attenuation cannot be neglected. We investigated the pore structure effects on P-wave scattering attenuation in dry rocks by pore-scale modeling based on the wave theory and the similarity principle. Our modeling results indicate that pore size, pore shape (such as aspect ratio), and pore density are important factors influencing P-wave scattering attenuation in porous rocks, and can explain the variation of scattering attenuation at the same porosity. From the perspective of scattering attenuation, porous rocks can safely suit to the long wavelength assumption when the ratio of wavelength to pore size is larger than 15. Under the long wavelength condition, the scattering attenuation coefficient increases as a power function as the pore density increases, and it increases exponentially with the increase in aspect ratio. For a certain porosity, rocks with smaller aspect ratio and/or larger pore size have stronger scattering attenuation. When the pore aspect ratio is larger than 0.5, the variation of scattering attenuation at the same porosity is dominantly caused by pore size and almost independent of the pore aspect ratio. These results lay a foundation for pore structure inversion from elastic wave responses in porous rocks.

Fractures greatly increase the difficulty of oil and gas exploration and development in reservoirs consisting of interlayered carbonates and shales and increase the uncertainty of highly efficient development. The presence of fractures or layered media is also widely known to affect the elastic properties of rocks. The combined effects of fractures and layered media are still unknown. We have investigated the effects of fracture structure on wave propagation in interlayered carbonate and shale rocks using physical models based on wave theory and the similarity principle. We have designed and built two sets of layered physical models with randomly embedded predesigned vertically aligned fractures according to the control variate principle. We have measured the P- and S-wave velocities and attenuation and analyzed the effects of fracture porosity and aspect ratio (AR) on velocity, attenuation, and power spectral dimension of the P- and S-waves. The experimental results indicated that under conditions of low porosity ([Formula: see text]), Han’s empirical velocity-porosity relations and Wang’s attenuation-porosity relation combined with Wyllie’s time-average model are a good prediction for layered physical models with randomly embedded fractures. When the porosity is constant, the effect of different ARs on elastic wave properties can be described by a power law function. We have calculated the power spectrum fractal dimension [Formula: see text] of the transmitted signal in the frequency domain, which can supplement the S-wave splitting method for estimating the degree of anisotropy. The simple power law relation between the power spectrum fractal dimension of the P-waveform and fracture density suggests the possible use of P-waves for discriminating fracture density. The high precision and low error of this processing method give new ideas for rock anisotropy evaluation and fracture density prediction when only P-wave data are available.

Existing methods of well-logging interpretation often contain errors in the exploration and evaluation of carbonate reservoirs due to the complex pore structures. The differences in frequency ranges and measurement methods deviated between the acoustic well logs and indoor ultrasonic tests cause inconsistent results. Based on the elastic wave equation and the principle of the control variable method, a 2D axisymmetric borehole model with complex pore structures was developed, and the numerical simulation method for acoustic log was constructed. The modeling results show that the power function can well describe the effects of pore structure on the acoustic waves, while the velocity of the Stoneley wave is not sensitive to the pore structure. Crack-like pores with pore aspect ratio (AR) less than 0.1 significantly affect the velocities of P- and S-waves, whereas “spherical” pores have fewer effects. The models with larger pore sizes have high velocities of P- and S-waves. The velocities calculated by the equivalent medium theory are always higher than the numerical simulation results. The velocity deviation caused by the difference in frequency is much smaller than the pore structure. A fractal approach to quantify the effects of pore structures is applied in the acoustic logging data. The fractal dimension increases with the pore AR or size when the porosity is constant, which can be described by a simple power function. This gives us new ideas and methods for pore structure evaluation in the lower frequency range than the conventional petrophysical model.

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