Abstract 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.

中文翻译：

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模拟页岩-煤断层泥的摩擦特性：对欧洲最大气田下方烃源岩诱发地震活动的影响

摘要 我们报告了在由页岩-煤混合物制备的模拟断层泥上进行的 21 次摩擦滑动实验。我们的目的是研究局部煤层涂抹对摩擦特性和诱发地震潜力的影响，该断层切割格罗宁根气藏（荷兰）下的上石炭统烃源岩。我们使用了从格罗宁根储层下方回收的页岩/粉砂岩岩心，以及与格罗宁根烃源岩中当地存在的煤具有相似年龄和来源的波兰烟煤。我们在 100 °C 和 40 MPa 有效法向应力的近原位条件下，使用滑动速度进行了速度步进、等速、滑动保持滑动 (SHS) 和滑动卸载滑动 (SUS) 模式的摩擦实验0.1–100 μm/s 和各种孔隙流体。含 0-50 vol% 煤的样品显示摩擦系数 ~0.45，滑动减弱。无论实验条件如何，煤含量≥50 vol% 的样品都显示出从~0.47 到~0.3 的峰值摩擦值的显着滑移减弱，这可能反映了弱富煤剪切带中的应变局部化，可能伴随着煤分子结构的变化。然而，重新滑动实验 (SUS) 表明，滑动减弱仅限于小的初始位移（2-3 毫米），并且不会在滑动重新激活期间发生。在（接近）稳定状态下，几乎所有在地应力、孔隙水压力 (15 MPa) 和温度条件下进行的实验都表现出稳定的速度强化行为，而与煤含量无关。相比之下，在干燥和气体饱和（CH4、氩气）条件下，或使用 1 个大气压、50 的水：50 (vol%) 页岩-煤混合物表现出速度减弱甚至粘滑。我们的结果表明，格罗宁根石炭纪页岩-粉砂岩层序中的断层在原位条件下不易诱发地震成核，即使在含煤或通过涂抹富集煤时也是如此。然而，在所研究的滑动速度下控制煤摩擦的机制仍不清楚，在地震滑动速度下煤摩擦的演变仍然未知。