The role of lithological layering and pore pressure on fluid‐induced microseismicity

Abstract

The success of hydraulic fracturing treatments is often judged by the shape and size of the resulting microseismic cloud. However, it is challenging to predict the anticipated microseismic cloud prior to treatment. We use geomechanical modeling to predict the distribution of the microseismicity prior to the hydraulic fracture treatment. We analyze the likelihood of tensile and shear failure due to 1-D variations in local stresses and rock strengths, induced by layering and pore pressure, for two field cases. The deviation in the local stresses from the regional stress field is induced by vertical variations in stiffness. This promotes failure of the stronger layers instead of the weaker ones since the stronger layers can become essentially load bearing. The simulations and field studies show that (1) microseismic events tend to locate preferentially where layers reach tensile failure due to fluid injection, and the number of events tends to decrease in layers that do not reach tensile failure; (2) shear initiation can occur in different layers from those failing in tension, thereby creating additional fluid migration paths; and (3) reactivation of preexisting fractures may occur due to fluid migration, even if their orientations are unfavorable. Numerical modeling is a significant aid in understanding the interplay of regional and local stresses and associated in situ failure due to variations in rock strength, pore pressure, and stiffnesses. In a wider perspective, it gives fundamental insights into the understanding of earthquakes and fault localization, the mechanisms of fracture development, and the role of fractures on fluid circulation and on the in situ stress field.

Human‐induced seismicity and large‐scale hydrocarbon production in the USA and Canada

“…Unfortunately, it is typically measured only at a few depths. Geochemistry, Geophysics, Geosystems 10.1002/2017GC006915 Furthermore, numerical simulations show that the magnitude of both horizontal stresses can vary significantly with depth [Roche and Van der Baan, 2015], because material and stress heterogeneity are likely to be coupled. All three principal stresses are required to find the likelihood of shear reactivation of a pre-existing fault of a given orientation, for instance, using slip tendency analysis [Morris et al, 1996].…”

“…The pore pressure regime also plays an important role [Sibson, 1985[Sibson, , 1990. Geochemistry, Geophysics, Geosystems 10.1002/2017GC006915 Furthermore, numerical simulations show that the magnitude of both horizontal stresses can vary significantly with depth [Roche and Van der Baan, 2015], because material and stress heterogeneity are likely to be coupled. Understanding the regional and local variability in induced seismicity thus requires an improved knowledge of the orientation and magnitude of all components of the in situ stress field.…”

The role of aseismic slip in hydraulic fracturing–induced seismicity

“…This deviates from a simple model of a critically stressed crust ( 54 ); however, we remark that (i) the faults appear to terminate at the top of the Late Devonian Wabamun Formation (Fig. 2) and therefore represent relic structures that formed at a time when the stress field was likely different from the present day and (ii) sedimentary basins exhibit geomechanical layering with variable stress between stronger and weaker layers, such that if the crust is critically stressed at some depths, it cannot be critically stressed throughout the sedimentary section ( 55 ). The low estimated value of the fault stress ratio in the Duvernay is consistent with the shear stress drop there because of the long-term slip along the creeping sections of the fault through overpressured shales or sandstones.…”

Dilatancy behaviour and permeability evolution of sandstone subjected to initial confining pressures and unloading rates

“…Similarly, plastic behaviour of rocks was not uncommon especially for shales and coal-rich layers but less so for limestone and sandstone [ 10 , 11 ]. Roche & van der Baan concluded that the anisotropic Young's moduli tended to reverse the effect of plasticity strain, thus decreasing the likelihood of failure in the shales and coals [ 12 ]. These studies show that sandstone has obvious dilatancy behaviour.…”