Abstract—We present the results of the laboratory studies on fluid initiated fracture in the samples of porous-fractured rocks that have been initially saturated with a pressure-injected fluid and then tested under increasing fluid pressure in saturated rocks. The tests were conducted at the Geophysical observatory “Borok” of Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences. The laboratory is equipped with electrohydraulic press INOVA-1000. The experiments were conducted on the rock samples with substantially different porosity. The tested samples were made of Buffalo sandstones, granites from the well drilled in a seismically active region and of granites from the well in the Voronezh crystalline massif. The permeability of granite samples was varied by their controlled artificial cracking by successive heating and cooling. The experimental procedure was set up in the following way. A preliminarily dried sample was initially subjected to uniaxial loading in uniform compression (confining pressure). Loading was performed at a constant strain rate until the moment when the growth rate of acoustic emission (AE) activity began to accelerate which indicated that the stress level approaches ultimate strength. Since that, the loading rate was decreased by an order of magnitude, and water was infused into a sample from its top face. The bottom end of a sample was tightly sealed and impermeable to water. After this, the pore pressure in the sample that had got saturated with water to that moment was raised in steps whose amplitudes were varied. The obtained results of the laboratory studies show that the character and intensity of fluid initiation of fracture markedly differ under primary fluid injection into the porous-fractured samples and under the subsequent increases of the pore pressure in the saturated samples. The time delay of acoustic response relative to fluid initiation and the amplitude of the response proved to be larger in the case of water injection into dry samples than in the case of raising the pore pressure in saturated samples. The AE response to the decrease in the pore pressure was also detected in the experiments. The theoretical analysis of fluid propagation in a pore space of an air-filled sample in the model with piston-type air displacement has shown that in the case of water injection into a dry sample, the fluid pressure front propagates more slowly than in the saturated sample.

中文翻译：

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干水饱和岩石中裂缝的流体起裂

摘要——我们介绍了多孔裂缝岩石样品中流体引发裂缝的实验室研究结果，这些岩石样品最初用压力注入流体饱和，然后在饱和岩石中增加流体压力下进行测试。测试是在俄罗斯科学院施密特地球物理研究所的地球物理天文台“Borok”进行的。实验室配备电液压力机INOVA-1000。实验是在孔隙率显着不同的岩石样品上进行的。测试样品由布法罗砂岩、地震活跃区钻井中的花岗岩和沃罗涅日结晶地块中井中的花岗岩制成。花岗岩样品的渗透性是通过连续加热和冷却控制人工开裂而改变的。实验程序按以下方式设置。初步干燥的样品首先在均匀压缩（围压）下承受单轴载荷。加载以恒定应变率进行，直到声发射 (AE) 活动的增长率开始加速，这表明应力水平接近极限强度。从那以后，加载速率降低了一个数量级，并且水从其顶面注入到样品中。样品的底端被紧密密封并且不透水。此后，到那时为止已经被水饱和的样品中的孔隙压力以幅度变化的阶跃升高。室内研究的结果表明，在初始流体注入多孔裂缝样品和随后饱和样品中孔隙压力增加的情况下，裂缝的流体起裂特征和强度显着不同。声学响应相对于流体起始的时间延迟和响应幅度证明在将水注入干燥样品的情况下比在提高饱和样品中的孔隙压力的情况下更大。在实验中还检测到对孔隙压力降低的 AE 响应。活塞式空气置换模型中充气样品孔隙空间中流体传播的理论分析表明，在将水注入干燥样品的情况下，