Fracturing of porous rock induced by fluid injection

Abstract We monitored acoustic emission (AE) activity and brittle failure initiated by water injection into initially dry critically stressed cylindrical specimens of Flechtingen sandstone of 50 mm diameter and 105–125 mm length. Samples were first loaded in axial direction at 40–50 MPa confining pressure at dry conditions close to peak stress. Subsequently distilled water was injected either at the bottom of specimen or via a central borehole at pore pressures of 5–30 MPa. Water injection into stressed porous sandstone induced a cloud of AE events located close to the migrating water front. Water injection was monitored by periodic ultrasonic velocity measurements across the sample. Propagation of the induced cloud of AE was faster in the direction parallel to bedding than normal to it, indicating permeability anisotropy. Water injection was associated with significant AE activity demonstrating increased contribution of tensile source type. Brittle failure was accompanied by increased contribution of shear and pore collapse source types. At a critical pore pressure, a brittle fault nucleated from a cloud of induced AE events in all samples. Microstructural analysis of fractured samples shows excellent agreement between location of AE hypocenters and macroscopic faults.

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