Microseismic Monitoring of a Vaca Muerta Hydraulic Stimulation: Evidence of a Subseismic Fault (Re)activation

Abstract

In this paper we report the analysis of a microseismic dataset acquired during the stimulation of a Vaca Muerta well. The data has been acquired with a surface, radial array as the treatment well was undergoing an 11 stages hydraulic stimulation. About half-way into the stimulation process we have observed the emergence of seismicity on a feature situated approx. 400m South-West to the treatment well in the underlying Tordillo Formation. This seismicity occurred independently of the stimulation and consistently showed normal faulting events dipping 57°. The delineated feature had a similar dip, which is compatible with the regional stress regime and published friction coefficients for the Tordillo Formation. Magnitudes were contained in the range M_w -3 to -2, until the occurrence of a M_w -0.75 event, the largest recorded in the sequence.

Analyzing this seismic cloud in further details, we found the following characteristics that clearly stand out from a frac’ related microseismic activity: (1) the seismicity located on the fault showed a 1.27 b-value, lower than the value found for fracturing-related events; (2) it exhibited a very peculiar spatio-temporal behavior, hinting to both a diffusion-like process and a subsequent poro-elastic static stress transfer along the fault. We hypothesize that the activated fault was close to criticality and triggered by an effective stress reduction likely induced by the nearby hydraulic stimulation; this was followed by a diffusion process along the fault plane and eventually lead to the occurrence of the M_w -0.75 event that itself induced an aftershock sequence with an Omori-Utsu p-value of 1.7.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018