Summary

There has been considerable discussion regarding the possibility and likelihood that vertical nodal plane dip-slip events commonly observed in microseismic data are actually horizontal bedding-plane slip events. The vertical nodal plane is roughly parallel to the maximum stress, with no simple explanation for the shear stress necessary to cause it to slip. The horizontal plane is more easily explained by the shear stress concentration at the tip of a vertically propagating hydraulic fracture. Supporting this interpretation is the fact that the range of azimuthal directions from vertical nodal planes is often more complex than commonly occurs with other nodal plane orientations. The implication of this evidence is that these events occur at the termination of vertical hydraulic fracture growth or at layer boundaries between rocks with different mechanical properties. To investigate this hypothesis, we examined multiple data sets through moment tensor inversion using the horizontal-slip interpretation. The horizontal-slip interpreted events were then analyzed using a geomechanical model of the area. The magnitude and degree of layer varying mechanical properties along with the local geologic dip support the correlation of horizontal nodal plane orientation with bedding plane slip theory. Also observed in the data sets were strike-slip type events which correlated with known fracture orientations from geomechanical logs.

Introduction

Early attempts at characterizing source orientations of microseismic events assumed a double-couple mechanism due to limited observations, such as being constrained to a single vertical monitoring array. These results typically revealed strike-slip type mechanisms interpreted as slip on pre-existing fractures (Rutledge and Phillips 2003) or vertical dip-slip mechanisms that suggested the possibility of bedding plane slip (Rutledge et. al. 2014). With the prevalence of multiple arrays, improved characterization of these sources is now possible. Of the two data sets we examine, with nearly full focal sphere coverage, one reveals dominant strike-slip failure on a set of pre-existing natural fractures, and the other is dominated by events that we interpret as bedding plane slip.

Theory

Microseismic events have traditionally been associated with pore pressure diffusion and the associated failure resulting from the reduction in normal stress on fracture planes in a triaxial stress environment. In tight unconventional reservoirs with very low leak-off, direct communication of the hydraulic fracture with the natural fractures likely leads to slip along those fractures.

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