Abstract Longmaxi formation shales are the major target reservoir for shale gas extraction in Sichuan Basin, southwest China. Swarms of earthquakes accompanying hydraulic fracturing are observed at depths typifying the Longmaxi formation. Mineral composition varies broadly through the stratigraphic section due to different depositional environments. The section is generally tectosilicate‐poor and phyllosilicate‐rich with a minor portion the converse. We measure the frictional and stability properties of shale gouges taken from the full stratigraphic section at conditions typifying the reservoir depth. Velocity‐stepping experiments were performed on representative shale gouges at a confining pressure of 60 MPa, pore fluid pressure of 30 MPa, and temperature of 150°C. Results show the gouges are generally frictionally strong with friction coefficients ranging between 0.50 and 0.75. Two phyllosilicate + TOC (total organic carbon)‐poor gouges exhibited higher frictional strength and velocity weakening, capable of potentially unstable fault slip, while only velocity strengthening was observed for the remaining phyllosilicate + TOC‐rich gouges. These results confirm that the frictional and stability properties are mainly controlled by phyllosilicate + TOC content. Elevating the temperature further weakens the gouges and drives it toward velocity weakening. The presence of observed seismicity in a majority of velocity‐strengthening materials suggests the importance of the velocity‐weakening materials. We suggest a model where seismicity is triggered when pore fluid pressures drive aseismic slip and triggers seismic slip on adjacent faults in the same formation and distant faults in the formations above/below. The effect of pore pressure transients within low‐permeability shale gouges is incorporated. Our results highlight the importance of understanding mechanisms of induced earthquakes and characterizing fault properties prior to hydraulic fracturing.
Abstract Microearthquakes accompanying shale gas recovery highlight the importance of exploring the frictional and stability properties of shale gouges. Aiming to reveal the influencing factors on fault stability, this paper explores the impact of mineral compositions, effective stress and temperature on the frictional stability of Longmaxi shale gouges in deep reservoirs located in the Luzhou area, southeastern Sichuan Basin. Eleven shear experiments were conducted to define the frictional strength and stability of five shale gouges. The specific experimental conditions were as follows: temperatures: 90–270°C; a confining stress: 95 MPa; and pore fluid pressures: 25–55 MPa. The results show that all five shale gouges generally display high frictional strength with friction coefficients ranging from 0.60 to 0.70 at the aforementioned experiment condition of pressures, and temperatures. Frictional stability is significantly affected by temperature and mineral compositions, but is insensitive to variation in pore fluid pressures. Fault instability is enhanced at higher temperatures (especially at >200°C) and with higher tectosilicate/carbonate contents. The results demonstrate that the combined effect of mineral composition and temperature is particularly important for induced seismicity during hydraulic fracturing in deep shale reservoirs.
Basalt is a major component of the earth and moon crust. Mineral composition and temperature influence frictional instability and thus the potential for seismicity on basaltic faults. We performed velocity-stepping shear experiments on basalt gouges at a confining pressure of 100 MPa, temperatures in the range of 100–400 °C and with varied obsidian mass fractions of 0–100% under wet/dry conditions to investigate the frictional strength and stability of basaltic faults. We observe a transition from velocity-neutral to velocity-weakening behaviors with increasing obsidian content. The frictional stability response of the mixed obsidian/basalt gouges is characterized by a transition from velocity-strengthening to velocity-weakening at 200 °C and another transition to velocity-strengthening at temperatures >300 °C. Conversely, frictional strengths of the obsidian-bearing gouges are insensitive to temperature and wet/dry conditions. These results suggest that obsidian content dominates the potential seismic response of basaltic faults with the effect of temperature controlling the range of seismogenic depths. Thus, shallow moonquakes tend to occur in the lower lunar crust due to the corresponding anticipated higher glass content and a projected temperature range conducive to velocity-weakening behavior. These observations contribute to a better understanding of the nucleation mechanism of shallow seismicity in basaltic faults.
Abstract Induced seismicity triggered during hydraulic fracturing for shale gas exploitation in the Sichuan Basin has aroused wide public concern with these earthquakes closely correlated with the reactivation of faults within the reservoir. The target shale reservoirs in the Sichuan Bain are currently located in the Longmaxi formation. To explore instability on these faults, we conducted shear experiments on simulated fault gouges under hydrothermal conditions to assess frictional stability and also evaluated the stress perturbations resulting from multistage hydraulic fracturing. Experimental results show that the frictional instability on these faults is primarily controlled by both mineral composition and the applied temperatures. With a decrease in the contents of phyllosilicate minerals or an increase in applied temperatures, the shale faults exhibit a transition from velocity-strengthening to velocity-weakening behaviour, indicating the potential for unstable fault slip. The stress perturbations resulting from multistage hydraulic fracturing were calculated from a dislocation model of the fluid-driven fracture. These results show that the stress changes resulting from multistage hydraulic fracturing are sufficient to reactivate adjacent unstable faults in the shale reservoir and trigger the seismicity. Our experimental and modelling results highlight the importance of shale composition, temperature, and stress perturbations on shale fault stability. These results have significant implications for understanding the shale fault instability and the potential triggering of induced seismicity during hydraulic fracturing in the Sichuan Basin.
Abstract The presence of metamorphic epidote on faults has been implicated in the transition from stable to unstable slip and the nucleation of earthquakes. We present structured laboratory observations of mixed epidote and simulated Pohang granodiorite (analogous to the EGS‐enhanced geothermal system site) gouges to evaluate the impact of heterogeneity and contiguity of epidote‐patch structure on frictional instability. Experiments are at a confining pressure of 110 MPa, pore fluid pressures of 42–63 MPa, temperatures 100–250°C and epidote percentages of 0–100 vol.%. The simulated Pohang granodiorite gouge is frictionally strong (friction coefficient ∼0.71) but transits from velocity‐strengthening to velocity‐weakening at temperatures >150°C. This velocity‐weakening effect is amplified in approximate proportion to increasing epidote content. Modes of epidote precipitation likely control the size and contiguity of the epidote‐only patches and this in turn changes the response of 50:50 epidote‐granodiorite mixed gouges for different geometric configurations. However, 50:50 epidote‐granodiorite mixtures that are variously homogeneously mixed, encapsulated and checkerboarded in their structures are insensitive to their geometries – all reflect the high frictional strength and strong velocity‐weakening response of 100:0 pure epidote. This suggests that the epidote present as thin coatings on fractures/faults can enhance velocity‐weakening behavior, independent of individual patch size and can thereby support the potential seismic reactivation of faults. Considering the frictional and stability properties of epidote at conditions typical of shallow depths, the presence of low‐grade metamorphism exerts a potentially important control on fault stability in granitoids with relevance as a marker mineral for susceptibility to injection‐induced seismicity.
Abstract Fluid injection into enhanced geothermal system (EGS) reservoirs can reactivate subsurface fractures/faults and trigger earthquakes—requiring that frictional stability and permeability evolution characteristics are adequately evaluated. This behavior potentially becomes more complicated when the impacts of temperature and cycled thermal stresses, and the resulted damage accumulation on both stability and transport characteristics are getting involved. We conducted coupled shear-flow experiments on saw-cut fractures recovered from an analog surface outcrop representative of a reservoir at 2450 m in the Gonghe Basin of northwestern China. The rocks were subjected to variable numbers of repeated heating-quenching (25-180-25 °C) cycles for shear-flow experiments at an effective stress of ~ 3 MPa and with velocity stepped between 10-1-10-1-10 μm/s. The smooth fractures return frictional coefficients in the range ~ 0.69 to 0.72 and are little affected by the thermal cycling. The frictional stability parameter ( a – b ) decreases and the instantaneous permeability increases with an increase in the number of heating-quenching cycles, during which intergranular and intragranular microcracks were generated in fracture surface. The above results indicate that the heating-quenching cycles during hydraulic fracturing of geothermal reservoir could affect both the fracture frictional instability and permeability evolution.
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