Results of downhole microseismic monitoring at a pilot hydraulic fracturing site in Poland — Part 2: S-wave splitting analysis

Abstract

Observations of azimuthal seismic anisotropy provide useful information, notably on stress orientation and the presence of preexisting natural fracture systems, during hydraulic fracturing operations. Seismic anisotropy can be observed through the measurement of S-wave splitting (SWS) on waveforms generated by microseismic events and recorded on downhole geophone arrays. We have developed measurements of azimuthal anisotropy from a Lower Paleozoic shale play in northern Poland. The observed orthorhombic anisotropic symmetry system is dominated by a vertically transverse isotropy (VTI) fabric, produced by the alignment of anisotropic platy clay minerals and by thin horizontal layering and overprinted by a weak azimuthal anisotropy. Despite the dominating VTI fabric, we successfully identified a weaker horizontal-transverse isotropy fabric striking east–southeast. We do this by constraining the rock-physics model inversion with VTI background parameters incorporated from other geophysical methods: microseismic velocity model inversion, 3D reflection seismic, and borehole cross-dipole sonic logs. The obtained orientation is consistent with a preexisting natural fracture set that has been observed using X-ray micro-imaging (XRMI) image logs from a nearby vertical well. The present-day regional maximum horizontal stress direction differs from the observed fracture strike by approximately 45°. This implies that the SWS measurements recorded during hydraulic stimulation of a shale-gas reservoir are imaging the preexisting natural fracture set, which influences the treatment efficiency, instead of the present-day stress.

Rock physics modelling for determination of effective elastic properties of the lower Paleozoic shale formation, North Poland

“…The TIV anisotropy parameters in Fig. 4a and Table 2 are in accordance with the results of the Backus-averaged well log method applied by Gajek et al (2018), who obtained epsilon and gamma equal to 0.14 (14%). The anisotropy of the lower Paleozoic shale formations from the Baltic Basin was also estimated by Stadtmüller et al (2018).…”

Magnetic Anisotropy in Silurian Gas‐Bearing Shale Rocks From the Pomerania Region (Northern Poland)

“…Seismic anisotropy data (Cyz & Malinowski, ) confirm the crucial impact of the analyzed scale. The authors observed that the azimuthal anisotropy correlates with the natural fractures direction known from Bobek and Jarosiński () and corresponds to the preferred fracture strikes from the S wave splitting analysis based on microseismic data (Gajek et al, ). Analysis of the seismic data illustrated anisotropy at the scale of tens of meters, which cannot be detected in this investigation.…”

“…In the Baltic Basin, however, low‐density natural fractures seem to have a significant meaning. Their orientation corresponds with seismic anisotropy and has bearing on the preferred orientation of hydraulic fractures (Cyz & Malinowski, ; Gajek et al, ). Nevertheless, an interesting issue in a context of fracturing is the anisotropy at a smaller, centimeter, scale, where the origin and degree of anisotropy are poorly constrained.…”

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“…Furthermore, the velocity model also has relevance to mechanical properties, that can be used to investigate the geomechanical aspects of the hydraulic fracturing (e.g. Hudson 1981;Verdon and Wüstefeld 2013;Gajek et al 2018c).…”

“…, enhancing the precision of event locations (Grechka and Yaskevich 2014), investigation of fracture alignment (e.g. Miyazawa et al 2008;Gajek et al 2018c) and fracture connectivity (e.g. Baird et al 2013;Verdon and Wüstefeld 2013); and determining changing stress conditions generated by subsurface operations (e.g.…”