Fault Weakening During Short Seismic Slip Pulse Experiments: The Role of Pressurized Water and Implications for Induced Earthquakes in the Groningen Gas Field

High-velocity friction experiments on simulated fault gouges sheared at high normal stress and to low displacement are particularly relevant to induced seismicity, which is becoming an important topic in fault mechanics. Using a new, improved set-up, which allows simulation of fault stress and fluid pressure ((Formula presented.)) conditions approaching in-situ reservoir values, we performed ring-shear experiments on simulated fault gouges prepared from the source-, reservoir-, and caprock-formations of the Groningen gas field. Pre-sheared gouges were subjected to a rotational slip pulse reaching ∼1.0 m/s peak velocity and 13–16 cm total displacement at effective normal stresses ((Formula presented.)) of 5–31 MPa and (Formula presented.) up to 5 MPa, using water or dry nitrogen as pore fluid. All water-saturated gouges show strong dynamic weakening within a few cm of slip, with the lowest dynamic friction (0.2–0.4) measured at the highest (Formula presented.). By contrast, the weakening was subtle in experiments using nitrogen. Our analyses focus on the high- (Formula presented.) experiments, which are more realistic and show a distinct dependence of constitutive parameters (e.g., slip-weakening rate) on (Formula presented.), in the form of empirical linear, power-law or exponential relations. The results provide much-needed constraints for numerical modeling of induced rupture propagation in the Groningen field. Based on temperature- and (Formula presented.) -measurements made in near-direct contact with the active shear band, and using “post-mortem” microstructures, we exclude previously-proposed dynamic weakening mechanisms (e.g., flash heating or thermal pressurization) and suggest that water pressurization at heated asperity or grain contacts explains the weakening seen in our high- (Formula presented.) experiments

Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates

Previous studies show that organic-rich fault patches may play an important role in promoting unstable fault slip. However, the frictional properties of rock materials with nearly 100 % organic content, e.g., coal, and the controlling microscale mechanisms remain unclear. Here, we report seven velocity stepping (VS) experiments and one slide-hold-slide (SHS) friction experiment performed on simulated fault gouges prepared from bituminous coal collected from the upper Silesian Basin of Poland. These experiments were performed at 25-45 MPa effective normal stress and 100?C, employing sliding velocities of 0.1-100 μm s-1 and using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip-weakening behavior at shear displacements beyond ~ 1-2 mm, from a peak friction coefficient approaching ~ 0:5 to (nearly) steadystate values of ~ 0:3, regardless of effective normal stress or whether vacuum-dry or flooded with distilled (DI) water at 15 MPa pore fluid pressure. Analysis of both unsheared and sheared samples by means of microstructural observation, micro-area X-ray diffraction (XRD) and Raman spectroscopy suggests that the marked slip-weakening behavior can be attributed to the development of R-, B-and Y-shear bands, with internal shear-enhanced coal crystallinity development. The SHS experiment performed showed a transient peak healing (restrengthening) effect that increased with the logarithm of hold time at a linearized rate of ~ 0:006. We also determined the rate dependence of steady-state friction for all VS samples using a full rate and state friction approach. This showed a transition from velocity strengthening to velocity weakening at slip velocities > 1 μm s-1 in the coal sample under vacuum-dry conditions but at > 10 μm s-1 in coal samples exposed to DI water at 15 MPa pore pressure. The observed behavior may be controlled by competition between dilatant granular flow and compaction enhanced by the presence of water. Together with our previous work on the frictional properties of coal-shale mixtures, our results imply that the presence of a weak, coal-dominated patch on faults that cut or smear out coal seams may promote unstable, seismogenic slip behavior, though the importance of this in enhancing either induced or natural seismicity depends on local conditions

Temperature and Gas/Brine Content Affect Seismogenic Potential of Simulated Fault Gouges Derived From Groningen Gas Field Caprock

We investigated the rate-and-state frictional properties of simulated anhydrite-carbonate fault gouge derived from the basal Zechstein caprock overlying the seismogenic Groningen gas reservoir in the NE Netherlands. Direct shear experiments were performed at in situ conditions of 50–150 °C and 40-MPa effective normal stress, using sliding velocities of 0.1–10 μm/s. Reservoir pore fluid compositions were simulated using 4.4 Molar NaCl brine, as well as methane, air, and brine/gas mixtures. Brine-saturated samples showed friction coefficients (μ) of 0.60–0.69, with little dependence on temperature, along with velocity strengthening at 50–100 °C, transitioning to velocity weakening at 120 °C and above. By contrast, gas filled, evacuated and partially brine-saturated samples showed μ values of 0.72 ± 0.02 plus strongly velocity-weakening behavior accompanied by stick slip at 100 °C (the only temperature investigated for gas-bearing and dry samples). A microphysical model for gouge friction, assuming competition between dilatant granular flow and thermally activated compaction creep, captures the main trends seen in our brine-saturated samples but offers only a qualitative explanation for our gas-bearing and dry samples. Since the reservoir temperature is ~100 °C, our results imply high potential for seismogenic slip nucleation on faults that cross cut and juxtapose the basal Zechstein anhydrite-carbonate caprock against the Groningen reservoir sandstone, specifically in the gas-filled upper portion of the reservoir system

Frictional properties of simulated shale-coal fault gouges: Implications for induced seismicity in source rocks below Europe's largest gas field

We report 21 frictional sliding experiments performed on simulated fault gouges prepared from shale-coal mixtures. Our aim was to investigate the effects of local coal seam smearing on the frictional properties and induced seismogenic potential of faults cutting the Upper Carboniferous source rocks underlying the Groningen gas reservoir (Netherlands). We used shale/siltstone core recovered from beneath the Groningen reservoir plus Polish bituminous coal of similar age and origin to coals locally present in the Groningen source rocks. We performed friction experiments in velocity stepping, constant velocity, slide-hold-slide (SHS) and slide-unload-slide (SUS) modes, under near in-situ conditions of 100 °C and 40 MPa effective normal stress, employing sliding velocities of 0.1–100 μm/s and a variety of pore fluids. Samples with 0–50 vol% coal showed friction coefficients ~0.45, with minor slip weakening. Samples with ≥50 vol% coal showed marked slip-weakening from peak friction values of ~0.47 to ~0.3, regardless of experimental conditions, presumably reflecting strain localization in weak coal-rich shear bands, possibly accompanied by changes in coal molecular structure. However, re-sliding experiments (SUS) showed that slip-weakening is limited to small initial displacements (2–3 mm), and does not occur during slip reactivation. At (near) steady state, almost all experiments performed at in-situ stress, pore water pressure (15 MPa) and temperature conditions exhibited stable, velocity strengthening behaviour, regardless of coal content. By contrast, under dry and gas-saturated (CH4, Argon) conditions, or using water at 1 atm, 50:50 (vol%) shale-coal mixtures showed velocity-weakening and even stick-slip. Our results imply that faults in the Groningen Carboniferous shale-siltstone sequence are not prone to induced earthquake nucleation at in-situ conditions, even when coal-bearing or coal-enriched by smearing. However, the mechanisms controlling coal friction remain unclear at the sliding velocities studied, and the evolution of coal friction at seismic slip velocities remains unknown

Healing Behavior of Simulated Fault Gouges From the Groningen Gas Field and Implications for Induced Fault Reactivation

We investigated the frictional strength recovery (healing) and subsequent reactivation and slip-weakening behavior of simulated fault gouges derived from key stratigraphic units in the seismogenic Groningen gas field (N. E. Netherlands). Direct-shear, slide-hold-slide (SHS) experiments were performed at in situ conditions of 100 °C, 40 MPa effective normal stress and 10–15 MPa pore fluid pressure (synthetic formation brine). Sheared gouges were allowed to heal for periods up to 100 days before subsequent reshearing. The initial coefficient of (steady) sliding friction μ was highest in the Basal Zechstein caprock (μ = 0.65 ± 0.02) and Slochteren sandstone reservoir (μ = 0.61 ± 0.02) gouges, and the lowest in the Ten Boer claystone at the reservoir top (μ = 0.38 ± 0.01) and in the Carboniferous shale substrate (μ ≈ 0.45). Healing and subsequent reactivation led to a marked increase (∆μ) in (static) friction coefficient of up to ~0.16 in Basal Zechstein and ~0.07 in Slochteren sandstone gouges for the longest hold periods investigated, followed by a sharp strength drop (up to ~25%) and slip-weakening trajectory. By contrast, the Ten Boer and Carboniferous gouges showed virtually no healing or strength drop. Healing rates in the Basal Zechstein and Slochteren sandstone gouges were significantly affected by the stiffness of different machines used, in line with the Ruina slip law, and with a microphysical model for gouge healing. Our results point to marked stratigraphic variation in healed frictional strength and healing rate of faults in the Groningen system, and high seismogenic potential of healed faults cutting the reservoir and Basal Zechstein caprock units, upon reactivation

Seismic Slip-Pulse Experiments Simulate Induced Earthquake Rupture in the Groningen Gas Field

Rock materials show dramatic dynamic weakening in large-displacement (m), high-velocity (∼1 m/s) friction experiments, providing a mechanism for the generation of large, natural earthquakes. However, whether such weakening occurs during induced M3-4 earthquakes (dm displacements) is unknown. We performed rotary-shear experiments on simulated fault gouges prepared from the source-, reservoir- and caprock formations present in the seismogenic Groningen gas field (Netherlands). Water-saturated gouges were subjected to a slip pulse reaching a peak circumferential velocity of 1.2–1.7 m/s and total displacements of 13–20 cm, at 2.5–20 MPa normal stress. The results show 22%–81% dynamic weakening within 5–12 cm of slip, depending on normal stress and gouge composition. At 20 MPa normal stress, dynamic weakening from peak friction coefficients of 0.4–0.9 to 0.19–0.27 was observed, probably through thermal pressurization. We infer that similar effects play a key role during induced seismic slip on faults in the Groningen and other reservoir systems

Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates

Previous studies show that organic-rich fault patches may play an important role in promoting unstable fault slip. However, the frictional properties of rock materials with nearly 100 % organic content, e.g., coal, and the controlling microscale mechanisms remain unclear. Here, we report seven velocity stepping (VS) experiments and one slide-hold-slide (SHS) friction experiment performed on simulated fault gouges prepared from bituminous coal collected from the upper Silesian Basin of Poland. These experiments were performed at 25-45 MPa effective normal stress and 100?C, employing sliding velocities of 0.1-100 μm s-1 and using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip-weakening behavior at shear displacements beyond ~ 1-2 mm, from a peak friction coefficient approaching ~ 0:5 to (nearly) steadystate values of ~ 0:3, regardless of effective normal stress or whether vacuum-dry or flooded with distilled (DI) water at 15 MPa pore fluid pressure. Analysis of both unsheared and sheared samples by means of microstructural observation, micro-area X-ray diffraction (XRD) and Raman spectroscopy suggests that the marked slip-weakening behavior can be attributed to the development of R-, B-and Y-shear bands, with internal shear-enhanced coal crystallinity development. The SHS experiment performed showed a transient peak healing (restrengthening) effect that increased with the logarithm of hold time at a linearized rate of ~ 0:006. We also determined the rate dependence of steady-state friction for all VS samples using a full rate and state friction approach. This showed a transition from velocity strengthening to velocity weakening at slip velocities > 1 μm s-1 in the coal sample under vacuum-dry conditions but at > 10 μm s-1 in coal samples exposed to DI water at 15 MPa pore pressure. The observed behavior may be controlled by competition between dilatant granular flow and compaction enhanced by the presence of water. Together with our previous work on the frictional properties of coal-shale mixtures, our results imply that the presence of a weak, coal-dominated patch on faults that cut or smear out coal seams may promote unstable, seismogenic slip behavior, though the importance of this in enhancing either induced or natural seismicity depends on local conditions

Temperature and Gas/Brine Content Affect Seismogenic Potential of Simulated Fault Gouges Derived From Groningen Gas Field Caprock

We investigated the rate-and-state frictional properties of simulated anhydrite-carbonate fault gouge derived from the basal Zechstein caprock overlying the seismogenic Groningen gas reservoir in the NE Netherlands. Direct shear experiments were performed at in situ conditions of 50–150 °C and 40-MPa effective normal stress, using sliding velocities of 0.1–10 μm/s. Reservoir pore fluid compositions were simulated using 4.4 Molar NaCl brine, as well as methane, air, and brine/gas mixtures. Brine-saturated samples showed friction coefficients (μ) of 0.60–0.69, with little dependence on temperature, along with velocity strengthening at 50–100 °C, transitioning to velocity weakening at 120 °C and above. By contrast, gas filled, evacuated and partially brine-saturated samples showed μ values of 0.72 ± 0.02 plus strongly velocity-weakening behavior accompanied by stick slip at 100 °C (the only temperature investigated for gas-bearing and dry samples). A microphysical model for gouge friction, assuming competition between dilatant granular flow and thermally activated compaction creep, captures the main trends seen in our brine-saturated samples but offers only a qualitative explanation for our gas-bearing and dry samples. Since the reservoir temperature is ~100 °C, our results imply high potential for seismogenic slip nucleation on faults that cross cut and juxtapose the basal Zechstein anhydrite-carbonate caprock against the Groningen reservoir sandstone, specifically in the gas-filled upper portion of the reservoir system

Frictional properties of simulated shale-coal fault gouges: Implications for induced seismicity in source rocks below Europe's largest gas field

We report 21 frictional sliding experiments performed on simulated fault gouges prepared from shale-coal mixtures. Our aim was to investigate the effects of local coal seam smearing on the frictional properties and induced seismogenic potential of faults cutting the Upper Carboniferous source rocks underlying the Groningen gas reservoir (Netherlands). We used shale/siltstone core recovered from beneath the Groningen reservoir plus Polish bituminous coal of similar age and origin to coals locally present in the Groningen source rocks. We performed friction experiments in velocity stepping, constant velocity, slide-hold-slide (SHS) and slide-unload-slide (SUS) modes, under near in-situ conditions of 100 °C and 40 MPa effective normal stress, employing sliding velocities of 0.1–100 μm/s and a variety of pore fluids. Samples with 0–50 vol% coal showed friction coefficients ~0.45, with minor slip weakening. Samples with ≥50 vol% coal showed marked slip-weakening from peak friction values of ~0.47 to ~0.3, regardless of experimental conditions, presumably reflecting strain localization in weak coal-rich shear bands, possibly accompanied by changes in coal molecular structure. However, re-sliding experiments (SUS) showed that slip-weakening is limited to small initial displacements (2–3 mm), and does not occur during slip reactivation. At (near) steady state, almost all experiments performed at in-situ stress, pore water pressure (15 MPa) and temperature conditions exhibited stable, velocity strengthening behaviour, regardless of coal content. By contrast, under dry and gas-saturated (CH4, Argon) conditions, or using water at 1 atm, 50:50 (vol%) shale-coal mixtures showed velocity-weakening and even stick-slip. Our results imply that faults in the Groningen Carboniferous shale-siltstone sequence are not prone to induced earthquake nucleation at in-situ conditions, even when coal-bearing or coal-enriched by smearing. However, the mechanisms controlling coal friction remain unclear at the sliding velocities studied, and the evolution of coal friction at seismic slip velocities remains unknown

Seismic Slip-Pulse Experiments Simulate Induced Earthquake Rupture in the Groningen Gas Field

Rock materials show dramatic dynamic weakening in large-displacement (m), high-velocity (∼1 m/s) friction experiments, providing a mechanism for the generation of large, natural earthquakes. However, whether such weakening occurs during induced M3-4 earthquakes (dm displacements) is unknown. We performed rotary-shear experiments on simulated fault gouges prepared from the source-, reservoir- and caprock formations present in the seismogenic Groningen gas field (Netherlands). Water-saturated gouges were subjected to a slip pulse reaching a peak circumferential velocity of 1.2–1.7 m/s and total displacements of 13–20 cm, at 2.5–20 MPa normal stress. The results show 22%–81% dynamic weakening within 5–12 cm of slip, depending on normal stress and gouge composition. At 20 MPa normal stress, dynamic weakening from peak friction coefficients of 0.4–0.9 to 0.19–0.27 was observed, probably through thermal pressurization. We infer that similar effects play a key role during induced seismic slip on faults in the Groningen and other reservoir systems.Applied Geophysics and Petrophysic