Initiation of Rock Fracture by Fluids of Different Viscosities

—The acoustic emission (AE) patterns observed in laboratory conditions at the initiation of fracture in rock samples by fluids of different viscosities are revealed. A series of experiments with sandstones and granites of order-of-magnitude different porosities and fluids of viscosities ranging within two orders of magnitude have been conducted. The effects of fluid injection into dry samples and pore pressure increase in saturated samples are examined. Pore pressure was varied both in abrupt steps and in smooth increase-and-decrease cycles. In the case of fluid injection into dry samples, the time delay of AE activation corresponds to the fluid front propagation time calculated in the piston-type model of air displacement by a fluid in a pore space. In the case of fracture initiation by steps in pore fluid pressure in saturated samples, the time delay of the response is substantially longer than predicted by the linear piezo conductivity model with constant hydraulic diffusivity and significantly shorter than the time of fluid front propagation in a dry sample. The experiments with smooth variations in the pore pressure in saturated samples revealed characteristic changes in AE activity: the minimum b -values (slope of magnitude–frequency relationship) fall in the intervals of the maxima in acoustic responses, and the maximum b -values occur during the rise and fall phases of pore pressure. The results of the experiments on the rock samples of different porosities with fluids of different viscosities can be useful in the analysis of field data both in the regions of anthropogenic seismicity associated with reservoir operation and fluid injection in wells and in the interpretation of the seismicity patterns due to tectonic and volcanic activity in the subduction zones.