Earthquake physics (2)

The current models predicting fault geometry and growth are limited to theoretical considerations mostly from fracture mechanics theories (Rice, 1968; Palmer and Rice, 1973, Rudnicki, 1980) and restricted datasets from geological investigations. Consistent with fracture mechanics theories, in structural geology, a fault cutting through heterogeneous layers is described with elliptical tip-lines and maximum displacement in its center (Barnett et al., 1987). Although, fracture mechanics can explain fault propagation at the fault tip through a process zone, it cannot properly predict the lateral fault growth into cross-fault damage zone and eventual localization of strain into the fault core (Figure 1a-b). In addition, the theories fail to consider the effect of fault segmentation and asperities, on displacement partitioning. We report that new advances in numerical modelling of faults and direct fault imaging and property extraction from seismic data utilizing Deep Learning reveals a realistic 3D structure of fault and its mechanical behavior, which in fact shows discrepancy to the previous fault mechanical models. Statistically, faults are normally considered as fractal phenomena, which is consistent with most of the fracture mechanics theories. This means that the fault should repeat the same mechanical behavior at different scales. However, new fault studies at different scales using different methods, suggest that faults can change their behavior at different scales and there is no scale-invariant relation between the fault attributes (Kolyukhin and Torabi, 2012, 2013; and Torabi et al., 2023). 

Figure 1. a) An illustration of a fault plane and its surrounding fault core and damage zones from structural point of view. b) Fault concept from fracture mechanics view. c) A 3D structure of a fault with displacement distributions from seismic data. Modified after Torabi et al., 2023.

Faults can either serve as permeable pathways or seals for fluids at a variety of scales. Hydraulic conductivity and leakage rates from faults are one of the unresolved key issues within fault seal analysis, especially for CO2 Capture and Storage (CCS) where reservoir integrity is critical, but also in geothermal operations and radioactive waste disposal. Investigation of factors that may alter the hydraulic conductivity in faults are therefore of great importance when evaluating fluid flow. Fault cores are often characterized as fault breccia or fault gouge, based on fault core fragment content. Fault gouge consist of fine-grained materials such as very-fine sand, silt and clay. The presence of phyllosilicates in fault gouge may hinder fluid flow and can therefore have a large negative effect on hydraulic conductivity. The Shale Gouge Ratio algorithm provide a widely used and well-established methodology to estimate the proportion of shaly material in a mixed clastic sequence fault zone (Freeman et al. 1998). Although this algorithm can provide a general impression on fault properties (e.g. sealing or leaking), it has certain limitations when estimating hydraulic conductivity in fault cores. By performing laboratory experiments on pre-fractured sandstones, with a varying ratio of clay and sand, we aim to better understand the effect of gouge ratio on hydraulic conductivity in faulted/fractured rocks. Several laboratory experiments were conducted to investigate the relationship between fault gouge ratio and hydraulic conductivity on pre-fractured Berea sandstone samples, which were tested under water (saline and freshwater) and oil saturated conditions. A new setup was designed to conduct shear movement in a triaxial cell, allowing for simulations of in-situ reservoir fault stress and pore pressure. Different gouge ratios (e.g. varying amount of clay and fine sand) were mixed in the laboratory and applied to machined fracture planes on cylindrical samples. Testing consisted of applying isotropic confinement stress and subsequent shear displacement. Hydraulic conductivity was estimated based on pore pressure and pore fluid flow measurements, at different stress stages during tests. Data obtained from a detailed experimental study show that fault gouge ratio play an important role on the hydraulic conductivity of fracture planes. The dataset indicates that fault gouge mineralogy, grain size and sorting, as well as pore fluid are important factors that control fluid flow in fractured sandstone units.

In recent decades, it has become increasingly evident that various human activities, including water waste injection, hydraulic fracturing, and geothermal energy production, can contribute to seismic events. To effectively mitigate the risks associated with induced seismicity, it is crucial to comprehend how fluid injection-related factors influence seismic response and evolution. Therefore, gaining an understanding of the effects of these parameters is vital in developing strategies to manage induced seismicity. Experimental and numerical studies indicate that varying injection patterns and rates can mitigate seismicity. However, the mechanism involved in seismicity mitigation of faulted porous medium remains unclear. In the current study, we perform injection-driven fault reactivation experiments on the faulted (saw-cut) Red Felser sandstone to provide new insight into the effect of fluid pressure cycling and rate on fault slip behavior and seismicity evolution. Three different injection rates were applied: low, medium, and high rates of 2 MPa/min, 1 MPa/min, and 0.2 MPa/min, respectively. Three types of injection patterns were also used, including cyclic recursive, monotonic, and stepwise injections. Our results show that the rate at which pore pressure increases plays a more significant role in determining the likelihood of triggering earthquakes than the magnitude of the pressure itself. For seismicity mitigation, a low injection rate in terms of b-value, slip velocity, number of events, and total AE energy is desirable. In addition, results from samples subjected to different injection patterns show that the cyclic recursive pattern can induce fault dilation and comparison that changes the critical stiffness of the fault, and by increasing the number of injection cycles available hydraulic energy budget, maximum slip velocity, and shear stress drop increases. Besides compared to the stepwise and monotonic injection patterns, the cyclic recursive injection scenario showed higher peak slip velocity. A proper injection strategy needs to consider various factors, such as fault drainage, critical shear stress, injection rate, and injection pattern (frequency and amplitude). Our results demonstrate that selecting proper stress/pressure amplitude, waiting time, and pressurization rate for injection design strategy can help to reduce seismicity risk.

The risk of induced seismicity is often managed using methods based on the empirical scaling relationship between fluid injection volumes and cumulative seismic moment. Mitigation options involve reducing the rate of fluids injection when induced earthquake magnitudes reach a certain threshold. Field observations show that the magnitudes of some of the largest induced earthquakes exceed the values typically used to forecast the maximum magnitude of events. These larger events are hosted by pre-existing faults that are often considered to be hydraulically connected to the injected fluid source. To address the apparent limitations of these forecasted magnitudes, we investigate the correlation between spatial and temporal evolution of fracturing/faulting and measured physical properties of reservoir rocks and seismic parameters of laboratory earthquakes.
Intact core plugs from the Horn River Basin shale play (BC,Canada) were characterized in terms of their density, elastic moduli, ultrasonic waves velocity and seismic anisotropy, then loaded to failure at reservoir conditions (~2km depth) in a triaxial apparatus. Acoustic emissions were measured to monitor the evolution of seismic parameters during the progressive load-induced degradation leading to failure. To investigate fluid induced reactivation of a preexisting fault within the reservoir underburden, we injected pore fluids via a borehole in a composite sample consisting of a granite saw-cut and shale core. 
During the prefailure stage, after yielding we observed a 3% decrease in P-wave velocity, an increase in acoustic emission rate to 0.35s-1 and a max relative magnitude of -4.35. During failure and sudden stress drop stage, emission rate increased to 24.7s-1, P-wave velocity dropped by 6% and relative max magnitude increased to -2.84. Acoustic events location shows that the evolution of seismic parameters at the transition between pre to co-failure corresponds to the development of a larger, throughgoing fault. During the composite sample test, injection of pore fluid at 40MPa reactivated the preexisting fault within the underburden. Then, at constant/decreasing pore pressure, location of acoustic events shows progressive fault reactivation within the underburden and fault growth within the reservoir. The development of throughgoing fault was associated with a decrease in P-wave velocity, and an increase in seismicity rate and magnitude, as the throughgoing propagated from the underburden into reservoir. 
Our results show that a proactive risks mitigation strategy can be based on real time observation and monitoring of diagnostic systematic changes in physical properties and seismic parameters of the reservoir-underburden system.