Microseismicity induced by hydraulic stimulation is known to exhibit peculiar source mechanisms patterns -namely, dip-slip or strike-slip events with polarity reversal, regardless of the background stress. Such events typically strike parallel to the maximum horizontal stress, which also happens to be the direction of propagation for the hydraulic fracture. It has been proposed to explain these mechanisms as slip on the auxiliary plane instead, i.e. horizontal slip occurring on pre-existing features such as bedding planes or, considering strike-slip mechanisms, on the plane striking perpendicular to the hydraulic fracture. The physical source of such patterns remains however largely unknown, although several authors have proposed coherent models. In particular, it is still unclear whether the ruptured and slipping rock is in hydraulic connection with the hydraulic fracture, thus adding contact surface with the wellbore.
In the present paper, I have computed static, poro-elastic stress changes induced by a tensile fracture using a boundary element method, and projected it on either vertical or horizontal planes – or on planes striking parallel or perpendicular to the maximum horizontal stress, respectively. While the modeling shows that both mechanisms are as likely to occur in such a context, I discuss on the conditions required to induce slip on the auxiliary rather than on the principal nodal plane, and draw conclusions on what information is really carried by microseismicity in each case.
Although still an active field of research, the microseismicity associated with hydraulic stimulation remains one of the principal tools to monitor and assess the outcome of a reservoir (Maxwell, 2014). With the exponential development of so-called "unconventional" plays (i.e., reservoirs with ultra-low permeability), many groups have been looking for ways to link microseismic-derived parameters to reservoir parameters. While spatial (Fisher et al., 2002) or temporal distributions (Shapiro et al, 2002) are the most obvious ones to use, they have proven to be difficult to interpret because of an apparent ubiquity of microseismic events in both domains, i.e. the existence of microseismicity away from the stimulated zone, occurring either after shut in or at times too early for fluids to have propagated where they occur.