International audienceIn order to better understand the interaction between pore-fluid overpressure and failure patterns in rocks we consider a porous elasto-plastic medium in which a laterally localized overpressure line source is imposed at depth below the free surface. We solve numerically the fluid filtration equation coupled to the gravitational force balance and poro-elasto-plastic rheology equations. Systematic numerical simulations, varying initial stress, intrinsic material properties and geometry, show the existence of five distinct failure patterns caused by either shear banding or tensile fracturing. The value of the critical pore-fluid overpressure at the onset of failure is derived from an analytical solution that is in excellent agreement with numerical simulations. Finally, we construct a phase-diagram that predicts the domains of the different failure patterns and at the onset of failure

Borehole fluid injection is commonly used for geological sequestration of carbon dioxide, underground storage of natural gas, waste injections, and during stimulations and development of geothermal and hydrocarbon reservoirs. Typically, the injection process induces significant seismicity, with some earthquakes as large as magnitude four. Induced seismicity has also been observed around producing hydrocarbon boreholes. Recently, it has been argued that some induced seismicity data can be explained by a highly nonlinear fluid diffusion mechanism or by the propagation of fluid pressure pulses. The nature of the nonlinearity and the mechanisms by which a pressure pulse can trigger seismicity are still uncertain. In this paper I show that the same spatiotemporal variation of seismicity can be explained and predicted by linear diffusion coupled to deformation of a linear poroelastic medium. By calculating the propagation of Coulomb Yielding Stress (CYS) perturbation with time, it is demonstrated that seismicity can be triggered by this perturbation. The change of CYS along the diffusion front is caused by seepage forces, which are body forces generated by fluid pressure gradients, and can explain induced seismicity during borehole fluid injection and extraction. Using published experimental data, I demonstrate how the spatiotemporal distribution of fluid‐induced seismic events can be used for reservoir modeling and characterization.

A B S T R A C TA key task of exploration geophysics is to find relationships between seismic attributes (velocities and attenuation) and fluid properties (saturation and pore pressure). Experimental data suggest that at least three different factors affect these relationships, which are not well explained by classical Gassmann, Biot, squirt-flow, mesoscopicflow and gas dissolution/exsolution models. Some of these additional factors include (i) effect of wettability and surface tension between immiscible fluids, (ii) saturation history effects (drainage versus imbibition) and (iii) effects of wave amplitude and effective stress. We apply a new rock physics model to explain the role of all these additional factors on seismic properties of a partially saturated rock. The model is based on a well-known effect in surface chemistry: hysteresis of liquid bridges. This effect is taking place in cracks, which are partially saturated with two immiscible fluids. Using our model, we investigated (i) physical factors affecting empirical Brie correlation for effective bulk modulus of fluid, (ii) the role of liquids on seismic attenuation in the low frequency (static) limit, (iii) water-weakening effects and (iv) saturation history effects. Our model is applicable in the low frequency limit (seismic frequencies) when capillary forces dominate over viscous forces during wave-induced two-phase fluid flow. The model is relevant for the seismic characterization of immiscible fluids with high contrast in compressibilities, that is, for shallow gas exploration and CO 2 monitoring.

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